# Slope Form Of Normal To Ellipse Is Slope Form Of Normal To Ellipse Any Good? 1 Ways You Can Be Certain

At vanishing Reynolds numbers, additionally accepted as bit-by-bit breeze or the Stokes breeze regime, the breeze acreage about a advice apple absolutely absorbed in an abundant aqueous is one of the best characterized in aqueous mechanics (Leal Reference Leal2007). The analytic solutions of the breeze acreage about agee prolate and collapsed particles in an abundant aqueous with no-slip abuttals altitude on the particles are additionally accepted (Brenner Reference Brenner1963). In the case of a apple affective forth the interface amid two fluids, however, the botheration becomes added circuitous and cannot be apparent analytically (Chisholm & Stebe Reference Chisholm and Stebe2021). This implies that the annoyance force on an adsorbed atom affective tangentially to an interface cannot be afflicted analytically for almost acquaintance angles. Adding surfactant to the interface brings added complications (Manikantan & Squires Reference Manikantan and Squires2020). The antithesis of surfactant molecules leads to an interface viscosity, which increases the attrition of, and appropriately the annoyance on, the particle. A advice atom disturbs the apparent absorption of the surfactant at the interface. The aberration in the surfactant absorption causes a acclivity in interface astriction and gives acceleration to an added force, accepted as the Marangoni force, in the adverse administration of the atom motion. A aberration in the surfactant absorption additionally generates a agnate Marangoni flow, from the aerial absorption (low interface tension) arena to the low absorption (high interface tension) arena that counterbalances the Marangoni force (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021).

The aloft phenomena basic a circuitous alternation amid the advice particle, aggregate fluid, interface rheology and surfactant carriage are such that afterwards or alpha methods are adapted to acknowledge the attributes of this phenomena and actuate the frictional armament on particles affective forth an interface (Jaensson, Anderson & Vermant Reference Jaensson, Anderson and Vermant2021). The annoyance accessory is an important abundance in admiration and analysing interfacial rheological backdrop or trajectories acquired from atom tracking abstracts (Bonales et al. Reference Bonales, Ritacco, Rubio, Rubio, Monroy and Ortega2007; Maestro et al. Reference Maestro, Bonales, Ritacco, Fischer, Rubio and Ortega2011). Ability about the motion and circulation of an anisotropic atom at the interface is acute for the compassionate of biological systems (Ding, Warriner & Zasadzinski Reference Ding, Warriner and Zasadzinski2001), micro-organism motion (Lauga et al. Reference Lauga, DiLuzio, Whitesides and Stone2006; Masoud & Stone Reference Masoud and Stone2014; Shaik & Ardekani Reference Shaik and Ardekani2017), micrometre sized ‘Marangoni surfers’ (Dietrich et al. Reference Dietrich, Jaensson, Buttinoni, Volpe and Isa2020), and inclusions such as proteins, or added membrane-bound particles, in biological membranes, and the architecture of bogus membranes (Ally & Amirfazli Reference Ally and Amirfazli2010). Baby atom probes additionally accordance microrheological methods added acuteness compared with arresting methods (Samaniuk & Vermant Reference Samaniuk and Vermant2014).

A starting point for the assay and quantitative compassionate of these systems is the hydrodynamic archetypal for the adaptation of a annular atom in a slab of a viscous, incompressible blur with array $d$, which mimics the protein motion in bilayers presented by Saffman & Delbrück (Reference Saffman and Delbrück1975). According to Saffman’s model, the annoyance force should be a logarithmic action of atom size. The bulk-surface coupling is declared by a hydrodynamic breadth scale, the Saffman–Delbrück breadth $mathcal {L}$ (Saffman & Delbrück Reference Saffman and Delbrück1975; Saffman Reference Saffman1976),

where $eta ^s$ is the blur apparent viscosity, usually advised proportional to blur array $d$, and $eta _a$ and $eta$ are aggregate viscosities of the adjoining fluids, actuality air (vanishing $eta _a$) and a adhesive fluid. Accede a bearings breadth the atom arbor of acceding is in the administration accustomed to the blur and advice with a connected acceleration accustomed to its arbor of rotational symmetry. The archetypal predicts the afterward affiliation for the annoyance accessory $f$ for a non-protruding annular atom with ambit $R$ whose breadth $l$ is identical with the blur array (Saffman Reference Saffman1976; Dimova et al. Reference Dimova, Danov, Pouligny and Ivanov2000):

Here $C = 0.5772257$ is the Euler–Masceroni connected and the arrangement $mathcal {L}/R$ is alleged the Boussinesq number. Blueprint (1.2) assumes $mathcal {L}gg R$. In general, for a atom with adapted beeline admeasurement $a$, the Boussinesq cardinal $mathcal {L}/a$ quantifies the about accent of the interfacial microburst accent to aggregate stress. In the blow of this assignment we will denote this cardinal by ${textit {Bq}_{1}}$, as we are action to accede a dilatational apparent bendability as well, giving acceleration to an added dimensionless cardinal ${textit {Bq}_{2}}$.

Numerous biological studies accredit to Saffman’s continuum access and some of them appearance altercation with the aboriginal study. For example, Gambin et al. (Reference Gambin, Lopez-Esparza, Reffay, Sierecki, Gov, Genest, Hodges and Urbach2006) abstinent translational circulation coefficients $D=k_{B}T/f$ of annular peptides in a surfactant bilayer and showed that the annoyance accessory $f$ is proportional to the ambit $R$ of the diffusing object, $fpropto eta ^s R$, and, therefore, there is a aciculate qualitative altercation with the Saffman–Delbrück archetypal apropos the assurance on $R$. The furnishings of admittance admeasurement in the blur was advised by Levine & MacKintosh (Reference Levine and MacKintosh2002) and Levine, Liverpool & MacKintosh (Reference Levine, Liverpool and MacKintosh2004). They afflicted the annoyance accessory of a non-protruding adamant annular rod ($lll R$) by analytic the accompanying equations for in-plane and out-of-plane aqueous motions, bold incompressibility of both the aggregate and the membrane. In their work, the rod’s arbor of acceding is alongside to the interface. They showed that (i) for baby altar (specifically, $l ll mathcal {L}$ ), the annoyance coefficients become complete of both the rod acclimatization and aspect ratio; and (ii) for beyond rods ($l>mathcal {L}$), with aerial aspect ratio, the annoyance accessory in erect motion $f^{perp }$ becomes absolutely beeline in the rod breadth $l$. These after-effects are qualitatively altered from the motion of a rod in a three-dimensional aggregate aqueous with bendability $eta$. In the complete of low Reynolds number, the adhesive annoyance on a rod is anisotropic and exhibits a logarithmic breadth dependency,

where $f$ is the annoyance accessory in alongside motion (the particle’s centre moves in the administration of the particle’s acceding axis), and $A$ is a afterwards agency of adjustment of accordance (Kirkwood & Auer Reference Kirkwood and Auer1951; Klopp, Stannarius & Eremin Reference Klopp, Stannarius and Eremin2017). Comparing the after-effects in two and three ambit shows that the affiliation in (1.3) break bottomward in two dimensions.

Exploiting a agnate approach, Fischer (Reference Fischer2004b) acquired the annoyance force on an ideal aggravate of vanishing array affective in a apparent blur aloft a aqueous of abyss $H$. He showed that for a alongside motion at aerial Boussinesq numbers, at agnate viscosities and baptize depths, the annoyance on a aggravate equals that on a deejay if its breadth is 3.3 (for $H gg R$) or 10.9 (for $H ll R$) times best than the bore of the disk. For erect motion at aerial Boussinesq number, a aggravate adventures the aloft annoyance as for alongside motion, if it is beneath by the agency $1/e$ than the edge-on affective needle.

There accept been attempts to break the Stokes blueprint for a bulging particle, at bound acquaintance angles. Danov et al. (Reference Danov, Aust, Durst and Lange1995) were the aboriginal to break the Stokes blueprint numerically for a three-dimensional atom that protrudes into the subphase. They modelled a compressible interface characterized by interface microburst and dilatational viscosity. They appear the after-effects for particles with acquaintance angles amid $20^{circ }$ and $90^{circ }$. In this model, the interface apparent astriction was afflicted to be a constant, therefore, the aftereffect of the Marangoni force was neglected. Dimova et al. (Reference Dimova, Danov, Pouligny and Ivanov2000) afterwards advised the Marangoni aftereffect and surfactant circulation by application the Gibbs animation of the interface in their adapted model, but they still abandoned a coupling amid interface and subflow. These calculations are abandoned accurate for baby deviations in the surfactant antithesis concentration, i.e. for baby Péclet numbers. Fischer, Dhar & Heinig (Reference Fischer, Dhar and Heinig2006) acclimated an access altered from Levine et al. (Reference Levine, Liverpool and MacKintosh2004), analytic for the stresses due to the subphase and at the acquaintance band abandoned for interfaces with a microburst viscosity, beneath the acceptance that the interface is incompressible. They presented solutions for acquaintance angles amid $0^{circ }$ and $180^{circ }$, as able-bodied as for absorbed particles in the aqueous abreast to the interface. Stone & Masoud (Reference Stone and Masoud2015) advised the appendage of collapsed particles at the interface. They acclimated a perturbation amplification for the acceleration and annoyance force on the atom as a action of atom protrusion. Application the Saffman annoyance force as a aught adjustment appellation in the amplification and applying the alternate assumption they afflicted the first-order appellation in the amplification which is due to the protrusion. We are not acquainted of antecedent works that advised a apple at a viscous, compressible interface that generates Marangoni flows.

To ample this gap of insight, we actuality investigate the aftereffect of acquaintance bend on the Marangoni breeze and the annoyance accessory of all-around and prolate spheroidal particles (ellipsoids of revolution) advice with a connected velocity, both alongside and borderline to their assumption axis, at the collapsed interface amid aqueous and air, while the interface is possibly accustomed baffling surfactant. The interface is characterized by both microburst and dilatational viscosities. This assignment extends our antecedent abstraction (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021) in two directions: to particles with altered acquaintance angles and to non-spherical particles.

For those readers mainly absorbed in real-world applications, it important to accent that the present assignment does not booty into annual any specific coupling amid apparent viscosities and surfactant concentration; all three are advised as complete parameters. This bogus ambience allows us to analyze the furnishings of the abandoned contributions to the breeze fields and annoyance coefficients. A majority of the accomplished abstract suggests that baffling surfactant monolayers are about inherently incompressible due to Marangoni breeze (Klingler & McConnell Reference Klingler and McConnell1993; Steffen et al. Reference Steffen, Heinig, Wurlitzer, Khattari and Fischer2001; Wurlitzer, Schmiedel & Fischer Reference Wurlitzer, Schmiedel and Fischer2002; Fischer Reference Fischer2004a; Fischer et al. Reference Fischer, Dhar and Heinig2006; Manikantan & Squires Reference Manikantan and Squires2020), while it has been afresh antiseptic that the interface can be incompressible by dilatational viscosity, Marangoni furnishings or a aggregate of both (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). The aloft assignment had apparent that the annoyance accessory of a all-around particle, symmetrically absorbed at an incompressible interface, aural the complete of vanishing interface microburst viscosity, apparent the aloft aggregate behindhand of the agent of incompressibility. The aloft empiric allegation appropriately do not acquiesce us to draw abstracts about the appliance of Marangoni effects. In fact, for the adapted case of inviscid interfaces accustomed surfactants, the artefact amid Marangoni (${textit {Ma}}$) and Péclet ($textit {Pe}_s$) numbers, but not aloof ${textit {Ma}}$ alone, plays a acute role in chargeless the addendum of the interface (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). At aerial ethics of ${textit {Ma}},textit {Pe}_s$ the interface is incompressible. A aerial aggregate of interface dilatational bendability makes the interface incompressible behindhand of ${textit {Ma}},textit {Pe}_s$. Back we are action to abstraction annoyance coefficients at incompressible interfaces we are appropriately chargeless to accept one out of the two routes. Treating the apparent viscosities as complete variables is along accurate by contempo abstracts breadth the rheological (extra) stresses are ample with annual to the thermodynamic ones (Peppicelli et al. Reference Peppicelli, Jaensson, Tregouet, Schroyen, Alicke, Tervoort, Monteux and Vermant2019). Since there are altered routes to interface (in)compressibility, answer the exact attributes of the interface application atom probes was apparent to possibly advance to inconsistent results, e.g. back alteration the atom admeasurement or aspect arrangement (Samaniuk & Vermant Reference Samaniuk and Vermant2014), giving added action to the accepted work.

After presenting the administering equations, definitions of dimensionless numbers like ${textit {Ma}}$ and $textit {Pe}_s$, and the afterwards arrangement in § 2, aural the after-effects § 3 we are action to investigate assorted extremal and beneath extremal situations, focus on the annoyance coefficient, altercate the aftereffect of the breeze and surfactant absorption fields, analyze with abstract results, for both all-around and spheroidal particles. Assumptions to be made, apropos the interfacial blueprint of accompaniment or the carelessness of the interface deformations are discussed in §§ 2.3 and 3.1, respectively. We actuality accept the breadth of the arbor of rotational acceding of the apple to ascertain dimensionless quantities. All after-effects to be presented again appropriately apply, afterwards acceptable ascent with $mathcal {D}$, to spheroids with connected aggregate or connected apparent area, as explained in § 3.4. Abstracts are provided in § 4.

A annual of the arrangement beneath abstraction is apparent in figure 1. A atom is advice with a connected acceleration $U$ in the $x$-direction at the interface amid a adhesive aqueous ($y<0$) and air ($y>0$). The atom appearance is authentic by the lengths $a$, $b$ and $c$ of the three arch semi-axes. Actuality we abstraction prolate spheroids with half-axes $age b=c$, which includes the apple as a adapted case ($a=b=c equiv R$). Spheroids construe either alongside or erect to their arbor of uniaxial symmetry, so that the arrangement is torque-free, and can ability a abiding state. The submergence of the atom is authentic by the alike of its centre at $y=-h$, while the interface is amid at $y=0$ and separates a adhesive aqueous with bendability $eta$ at $y < 0$ and the inviscid aqueous at $y > 0$ (figure 1).

Figure 1. Prolate apple at the interface amid air (white) and a adhesive aqueous (blue). The interface is eventually burdened with surfactant (not apparent here, but if so, we add a blubbery atramentous band apery surfactant). The atom geometry is authentic by the three arch semi-axes $a, b$ and $c$ with $age b=c$ for a prolate apple (and $ale b=c$ for an collapsed spheroid, so that $a$ is the breadth of the main, not necessarily longest, axis). Their arrangement is $mathcal {D}=a/bin [0,infty ]$ with $mathcal {D}>1$ for prolate spheroids, $mathcal {D}=1$ for a apple and $mathcal {D}in (0,1)$ for collapsed spheroids. The atom translates with connected acceleration $U$ in a complete $x$-direction, while its acceding arbor resides aural the interfacial $x$–$z$-plane. For alongside and erect motions, the arbor of uniaxial acceding is either accumbent in the $x$- or $z$-direction, respectively. The submergence of the atom is authentic by the $y$-coordinate of its centre, denoted by $h$, giving acceleration to dimensionless abrogating captivation abyss $mathcal {H}=h/b$, i.e. $mathcal {H}=-1$ if the atom is absolutely absorbed in water, in agriculture acquaintance with the interface. We are action to acquaint dimensionless quantities application the arch breadth $a$ as the breadth assemblage (§ 2.4). All after-effects again appropriately apply, afterwards acceptable ascent with $mathcal {D}$, to spheroids with connected aggregate or connected apparent breadth (§ 3.4).

The aqueous is advised incompressible. Its dynamics can appropriately be modelled with the Stokes equations

where $boldsymbol {u}$ is the aqueous acceleration field, $p$ is the burden acreage and $boldsymbol {{rm pi} } = -p boldsymbol{mathsf{I}} boldsymbol {tau }$ the accent tensor for a Newtonian aqueous modelled by $boldsymbol {tau } = 2 eta boldsymbol{mathsf{D}}$. Actuality $boldsymbol{mathsf{D}} = [boldsymbol {nabla }boldsymbol {u} (boldsymbol {nabla }boldsymbol {u})^textrm {T}]/2$ is the aggregate of anamorphosis tensor. All fields actualization in our equations are spatio-temporal fields that depend on $x$, $y$, $z$ and time $t$. A agnate atomization $boldsymbol {{rm pi} }^s = gamma boldsymbol{mathsf{I}}_s boldsymbol {tau }^s$ can additionally be accounting for the interface accent tensor, breadth $gamma$ is the apparent astriction of the interface, $boldsymbol{mathsf{I}}_s=boldsymbol{mathsf{I}}-boldsymbol {nn}$ the apparent or borderline bump tensor with apparent accustomed $boldsymbol {n}$ and $boldsymbol {tau }^s$ the added apparent accent tensor (Brenner Reference Brenner1991; Jaensson & Vermant Reference Jaensson and Vermant2018; Venerus & Öttinger Reference Venerus and Öttinger2018). It is afflicted to be accustomed by the Boussinesq–Scriven basal law (Boussinesq Reference Boussinesq1913; Scriven Reference Scriven1960)

where $eta ^s$ and $kappa ^s$ are the microburst and dilatational viscosities of the interface, respectively. The apparent aggregate of anamorphosis tensor $boldsymbol{mathsf{D}}_s$ actualization in (2.3) is authentic as $2boldsymbol{mathsf{D}}_s = (boldsymbol {nabla }_s boldsymbol {u}_s) boldsymbol {cdot } boldsymbol{mathsf{I}}_s boldsymbol{mathsf{I}}_s boldsymbol {cdot } (boldsymbol {nabla }_s boldsymbol {u}_s)^textrm {T}$, breadth $boldsymbol {u}_s$ is the acceleration $boldsymbol {u}$ evaluated at the interface, and $boldsymbol {nabla }_s=boldsymbol{mathsf{I}}_sboldsymbol {cdot }boldsymbol {nabla }$ is the apparent acclivity abettor (Brenner Reference Brenner1991).

The acceleration $boldsymbol {u}$ is afflicted to vanish on the simulation box surface. Abuttals altitude for $boldsymbol {u}$ at the aqueous interface are authentic by chain of acceleration in the borderline administration $boldsymbol {u} boldsymbol {cdot } boldsymbol {t} = boldsymbol {u}_s boldsymbol {cdot } boldsymbol {t}$ (Venerus & Öttinger Reference Venerus and Öttinger2018), breadth $boldsymbol {t}$ is a assemblage departure agent residing in the $x$–$z$-plane, and a vanishing accustomed acceleration $boldsymbol {u} boldsymbol {cdot } boldsymbol {n} = 0$. Moreover, attention of drive yields a drive jump antithesis in the borderline direction,

where the Gibbs–Marangoni modulus $K_{{rm pi} }=varGamma partial varPi ^s /partial varGamma$ allows one to chronicle the acclivity in apparent burden to the acclivity in surfactant absorption $varGamma$ as $boldsymbol {nabla }_s varPi ^s = K_{{rm pi} } boldsymbol {nabla }_s ln varGamma$. Apparent burden $varPi ^s$ is authentic as a aberration amid apparent astriction of a apple-pie interface $gamma _0$ and apparent astriction in the attendance of surfactant, $varPi ^s(varGamma ) = gamma _0-gamma (varGamma )$. The acceleration and burden fields appropriately accept their time annex through $varGamma$. The change of the surfactant absorption $varGamma$ is complete by the capricious apparent convection–diffusion (SCD) blueprint (Brenner & Leal Reference Brenner and Leal1978; Stone Reference Stone1990; Brenner Reference Brenner1991)

where $D_s$ is the apparent diffusivity of the surfactant at the collapsed interface. The Stokes equations supplemented by (2.5) are the administering equations for $boldsymbol {u}$ and $varGamma$ as a action of position and time. These equations are apparent with an antecedent action of $varGamma =varGamma _0$ and accountable to vanishing surfactant alteration from the interface boundary.

For a spheroidal atom advice with a connected acceleration $boldsymbol {U}$ at the interface, the affiliation amid the annoyance force on the atom $boldsymbol {F}$ and the velocity, in general, is a circuitous action of atom geometry, aggregate and interface rheological properties, the interfacial blueprint of accompaniment and the carriage backdrop of the surfactant. It additionally depends on the acclimatization of the anisotropic atom with annual to $boldsymbol {U}$. The annoyance force on the spheroidal atom anchored at the interface is the sum of three contributions: (i) the aggregate force

where $boldsymbol {n}_p$ is the assemblage accustomed agent to the apparent of the particle; (ii) the interface adhesive force

due to the added apparent accent tensor, breadth $partial S_p$ is the egg-shaped ambit of the atom at the interface; and (iii) the adaptable or Marangoni addition to the annoyance force

due to the non-uniform administration of the surfactant in the acquaintance line.

So far, we presented the accepted formulation. Because in our accoutrement the atom translates in the $x$-direction with its capital arbor accumbent in either the $x$- or $z$-direction, the force has abandoned an $x$-component, $F_x = -fU$, the actual two apparatus vanish for acceding reasons.

For an isothermal system, the apparent astriction is alone a (typically nonlinear) action of the surfactant absorption $varGamma$, i.e. $gamma = gamma (varGamma )$. In this assignment we accept a linearized blueprint of state: $gamma _L = gamma _* – varGamma k _{B}T_*$, breadth $gamma _*=gamma (varGamma _0) varGamma _0k_{B}T_*$ is a constant, and $k_{B}T_*$ represents the abrogating of the abruptness of $gamma$ with annual to $varGamma$, taken at $varGamma _0$, the calm surfactant apparent cardinal anatomy in the absence of the particle. The linearized Gibbs modulus is appropriately $K_{rm pi} = varGamma k_{B}T_*$. For the adapted case of abundantly baby $varGamma _0rightarrow 0$, one has $T_*rightarrow T$ and the blueprint of accompaniment reduces to the ‘ideal gas’ anatomy $gamma =gamma _0-varGamma k_{B}T$ due to actual alive contributions, and breadth $gamma _0=gamma (0)$ is the apparent astriction of the apple-pie interface.

Nonlinearities are acceptable to set in with accretion concentration, and nonlinear equations of accompaniment accept been discussed in the abstract (Lopez & Hirsa Reference Lopez and Hirsa2000; Manikantan & Squires Reference Manikantan and Squires2020). For cases breadth the absorption does not adapt decidedly beyond the interface, an blueprint of accompaniment linearized about a assertive $varGamma$ ability still serve as a acceptable approximation, so that $gamma _0$ and $k_{B}T$ aloof accept new interpretations. For this reason, the linearized anatomy does not bind this abstraction to systems at actual low surfactant concentrations. We are not acquainted of an accustomed and about parameter-free nonlinear interfacial blueprint of accompaniment that could accept been acclimated instead of the linearized one, for the purpose of the present study. Similarly, for the accepted analysis, apparent viscosities are afflicted to be arrangement parameters, and their assurance on $varGamma$ is abandoned (Scriven Reference Scriven1960; Ortega, Ritacco & Rubio Reference Ortega, Ritacco and Rubio2010). A clean, surfactant-free interface has $varGamma =0$. In best applied cases one needs some apparent alive breed at the interface to get cogent added (viscous) stresses, but apparent viscosities for apple-pie interfaces accept additionally been appear (Earnshaw Reference Earnshaw1981).

For the botheration at hand, we use the connected acceleration $U$, the semi-axis $a$ of the particle, the bendability $eta$ of the aqueous and the antecedent surfactant absorption $varGamma _0$ to acquaint bargain units, and to appear up with a cardinal of dimensionless ambit such as two Bousinessq numbers ${textit {Bq}_{1}}$ and ${textit {Bq}_{2}}$, arrangement of apparent microburst bendability to aggregate viscosity, and arrangement amid apparent addendum bendability to aggregate viscosity, and the apparent Péclet cardinal $textit {Pe}_s$, a arrangement of circulation time, $a^2/D_s$, to the adapted time for atom motion, $a/U$, i.e.

The atom aspect arrangement $mathcal {D}$ is authentic by the arrangement amid half-axes $a$ and $b$, and its dimensionless captivation $mathcal {H}$ is authentic by the arrangement amid $h$ and $b$, i.e.

so that $mathcal {H}le -1$ at complete captivation and $mathcal {H}=0$ for a atom symmetrically amid at the interface. Making the interfacial blueprint of accompaniment dimensionless, we access $gamma (varGamma )/eta U = {textit {Ca}}^{-1} – {textit {Ma}} , varGamma /varGamma _0$, giving acceleration to the dimensionless capillary cardinal ${textit {Ca}}=eta U/gamma _*$ (ratio amid aggregate accent and interface astriction force) and Marangoni number

the arrangement amid the force disposed to batter the interface and apparent animation which tends to restore the aboriginal appearance of the film.

Dimensionless variables, apparent by a tilde (only in the accepted sentence), are accordingly authentic abnormally in acceding of their dimensional counterparts, such as $tilde {gamma }=gamma /eta U$, $tilde {t}=t U/a$, $tilde {x}=x/a$, $tilde {boldsymbol {u}}=boldsymbol {u}/U$, $tilde {varGamma }=varGamma /varGamma _0$, $tilde {boldsymbol {{rm pi} }}=boldsymbol {{rm pi} } a/(eta U)$, $tilde {boldsymbol {F}}=boldsymbol {F}/(eta U a)$ and $tilde {f}=f/(eta a)$. For the blow of this work, variables are meant to be dimensionless, and we omit asterisks for brevity. The dimensionless blueprint of accompaniment appropriately reads as $gamma ={textit {Ca}}^{-1}-{textit {Ma}},varGamma$. Similarly, $varPi ^s=K_{{rm pi} }={textit {Ma}},varGamma$ in the dimensionless form, which shows that the Marangoni addition to the force is proportional to ${textit {Ma}}$. The Stokes equations are replaced by $-boldsymbol {nabla } p nabla ^2 boldsymbol {u}=0$ and $boldsymbol {nabla } boldsymbol {cdot } boldsymbol {u} = 0$, the dimensionless added apparent accent tensor (2.3) now involves the Bousinessq numbers,

and the SCD blueprint (2.5) becomes

featuring the Péclet cardinal $textit {Pe}_s$. The borderline drive jump balance, (2.4), charcoal unchanged.

We use the bound aspect adjustment (FEM), implemented in an centralized code, to break the abounding arrangement of equations, breadth the dimensionless ambit ${textit {Bq}_ {1}}$, ${textit {Bq}_ {2}}$, $textit {Pe}_s$, $mathcal {D}$, $mathcal {H}$ and ${textit {Ma}}$ authentic by (2.9a–c)–(2.11) absolutely call the problem. The Marangoni cardinal enters abandoned in the attendance of surfactant. In the complete of $textit {Pe}_s to 0$, the surfactant absorption charcoal accordant and regaining the accordant administration is instantaneous, therefore, the apparent burden charcoal constant. In this apparent viscosity-dominated administration both the ${textit {Ma}}$ and $textit {Pe}_s$ numbers do not access as parameters.

The afterwards accomplishing is agnate to the one begin in Pourali et al. (Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021), with the notable aberration that we absolutely clue the motion of the atom in the accepted work. In adjustment to abate the abuttals furnishings a large, cubic simulation box is acclimated with box admeasurement $L=400$ (figure 2). An all-encompassing abstraction on the access of the admeasurement of the bonds box for a all-around atom at an incompressible interface was conducted in our antecedent assignment (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). There, we showed that a box admeasurement of $L=400$ yields an absurdity of beneath than 1 % for the annoyance accessory (compared with a abundant beyond box). Moreover, due to acceding of the botheration in the $x$–$y$-plane, abandoned bisected of the breadth is meshed, and adapted acceding altitude are employed. As in this antecedent work, we denote the simulation breadth complete aqueous and air by $varOmega$, the box abuttals by $partial varOmega$, the air–liquid interface by $S_I$, the atom apparent by $S_p$ and the circle of the atom and the air–liquid interface by $partial S_p$. The anchored administration is accomplished in anniversary case at $t=30$, as we will authenticate in § 3.6.

The (2.15)–(2.17) are apparent application the Galerkin FEM on meshes that are abuttals adapted to the atom and the interface, and which are confused in time to clue the motion of the particle. Tetrahedral $P_2P_1$ elements (Taylor & Hood Reference Taylor and Hood1973) are acclimated for $boldsymbol {u}$ and $p$ admitting triangular $P_2$ elements are acclimated for $varGamma$. Cobweb bearing is done application Gmsh (Geuzaine & Remacle Reference Geuzaine and Remacle2009), which allows for abundant ascendancy over the bounded aspect size.

Time affiliation commences by breeding a cobweb with nodal coordinates $boldsymbol {x}_m = [x_m,y_m,z_m]^textrm {T}$, based on the antecedent particle’s centre position $boldsymbol {X}_0 = [X_0,0,0]^textrm {T}$. Then, at the alpha of a footfall at time $n Delta t Delta t$ (for which we use the short-hand characters $n 1$, e.g. $X(n Delta t Delta t)=X_{n 1}$), the exact new atom position is bent from $X_{n 1} = X_n Delta t , U$, breadth we agenda that abandoned the $X$ breadth has to be updated. The atom displacement is appropriately accustomed by $Delta X = X_{n 1} – X_n = Delta t, U$. To ensure a bland anamorphosis of the mesh, the new atom position is acclimated to amend the nodal coordinates $boldsymbol {x}_m$ of the cobweb by analytic the afterward Laplace equation:

Here $K_e$ is a circulation accessory which varies per aspect and which is alleged as the changed of the aspect size. This access ensures that best of the cobweb anamorphosis takes abode in beyond elements, aspersing cobweb baloney (Hu et al. Reference Hu, Patankar and Zhu2001). The cobweb displacement acreage is again acclimated to amend the $x$-components of the bulge coordinates via $(x_m)_{n 1} =(x_m)_{n} Delta x_m$, while the $y$- and $z$-components abide unchanged.

With the cobweb coordinates accepted at the new time step, the cobweb acceleration is begin application a first-order astern differencing arrangement for the aboriginal step,

whereas a second-order astern differencing arrangement is acclimated for consecutive accomplish $nge 1$,

On the adapted mesh, the anemic anatomy of the administering equations, (2.15)–(2.17), are apparent afterward an complete scheme, as will be explained next. For comparison, we accept additionally implemented an complete scheme, as declared in Appendix B. The allegory shows that after-effects are basically artless by the best of scheme.

In the complete arrangement the anemic anatomy of the antithesis equations, (2.15) and (2.16), is apparent aboriginal to access the acceleration and burden at footfall $n 1$. For the apparent absorption $varGamma$, which is bare to appraise the apparent astriction $gamma =gamma (varGamma _{n 1})$ at footfall $n 1$, a first-order extrapolation is acclimated for the aboriginal footfall $gamma (varGamma _{n 1})=gamma (varGamma _0)$ with $varGamma _0=1$, admitting a second-order extrapolation is acclimated for consecutive accomplish $gamma (varGamma _{n 1})=gamma (2varGamma _n – varGamma _{n-1})$.

After analytic this arrangement for $boldsymbol {u}_{n 1}$ and $p_{n 1}$, the apparent acceleration $(boldsymbol {u}_s)_{n 1}$ is readily extracted, and acclimated in the anemic anatomy of the SCD blueprint (2.17). Application a first-order semi-implicit Euler arrangement for the aboriginal time step, we obtain

For consecutive time steps, a second-order, semi-implicit Gear arrangement is acclimated to appraise $varGamma _{n 1}$,

After analytic the consistent arrangement for $varGamma _{n 1}$, all variables are now accepted at footfall $n 1$, and time affiliation can abide to the abutting time step.

Within the afterward sections we are action to investigate (i) a all-around atom anchor a assertive acquaintance bend with a clean, i.e. surfactant-free, absolutely compressible and inviscid interface; (ii) a all-around atom at a adhesive interface bedeviled by apparent viscosities; (iii) a prolate spheroidal atom symmetrically amid at a surfactant-free, absolutely compressible and inviscid interface; (iv) a prolate apple at a apparent viscosity-dominated regime; and (v) a prolate apple at a sufactant-laden, partially compressible and adhesive interface. While in (i)–(iii) the focus is on the annoyance accessory and allegory with complete abstract results, in (iv) and (v) we are action to added investigate the breeze and surfactant absorption fields at the interface, for both all-around and spheroidal particles. All after-effects to be presented are acquired for particles affective at connected speed, abysmal in the anchored regime. The case of incompressible interfaces, accomplished at ample ${textit {Bq}_ {2}}$, or ample ${textit {Ma}},textit {Pe}_s$, is captured by (ii) for the case of a sphere, and by (iv) for the case of a prolate spheroid.

For a all-around atom with ambit $R$ and, thus, $mathcal {D}=1$ at a apple-pie interface, we break the aloft equations in the absence of surfactant, $varGamma =0$, at vanishing Bousinessq numbers ${textit {Bq}_ {1}}={textit {Bq}_ {2}} = 0$ for a accustomed dimensionless captivation akin $mathcal {H}=h/R$. This akin $mathcal {H}$ is bent by three circuitous interfacial tensions, and the annoyance accessory is alone bent by the aggregate force $boldsymbol {F}^b$.

The apparent action of particles does not resemble surfactant molecules, due to their amphiphilic nature. A aqueous at blow intersects a solid atom at a altered angle, which is alleged the acquaintance angle, and is a key connected back ambidextrous with solid particles at the aqueous interfaces. This abstraction has affinity with wetting phenomena. In wetting phenomena a atom spreads on the apparent of a solid surface, in acquaintance with air. In the wetting action a new apparent (between aqueous and solid or aqueous and air) is formed. Creating a new apparent changes the activity of the system. Apparent (or interface) activity is the assignment per assemblage breadth bare (or generated) to actualize a new surface. In the wetting action the antithesis of the interfacial energies amid the solid, the aqueous and the air, determines whether the aqueous spreads or not. Paraphrasing from Zanini & Isa (Reference Zanini and Isa2016), the admeasurement by which a atom spreads is bent by the point at which the activity acquired in abbreviation the interface amid the solid and air equals the activity amends paid in creating new liquid–solid and liquid–air interfaces. This translates into the automated calm of the three interfacial tensions at the three-phase acquaintance line. Similarly, we can ascertain the acquaintance bend of a solid atom at a aqueous interface. The attendance of the atom at the interface is agilely favourable if this acceding has a lower activity than the bearings breadth the atom is absolutely absorbed in one of the fluids. Amount 3 shows a all-around atom with ambit $R$ at the interface amid air and a adhesive fluid.

Figure 3. Schematic representation of a distinct all-around atom at the interface amid air and a adhesive fluid, at a arresting acquaintance bend $theta$.

The complete activity of this arrangement in the absence of flow, for the case of $|mathcal {H}|le 1$, can be accounting as (Davies et al. Reference Davies, Krüger, Coveney and Harting2014)

so that $E=4{rm pi} R^2 gamma _{pf}$ if the atom is absolutely attenuated in the aqueous ($mathcal {H}<-1$), and breadth $gamma _{pa}$, $gamma _{pf}$ and $gamma$ denote the interfacial tensions amid particle–air, particle–fluid and fluid–air respectively. The calm position of the atom is acquired from the action $partial E/partial mathcal {H} = 0$. This yields

where $mathcal {H}$ is from now on the dimensionless calm position. From trigonometry, as continued as $|bar {mathcal {H}}|le 1$, the atom prefers to abide at the interface and its calm acquaintance bend $theta$ is accustomed by Young’s equation

Using this analogue of the acquaintance angle, the activity of the calm acceding is accustomed by

These activity considerations, which are based on the ethics of apparent tensions of the altered phases, accordingly actuate whether a amphibian all-around atom at the interface is agilely favourable or not. While the vertical atom motion is acerb suppressed or bedfast about the bargain calm position $bar {mathcal {H}}$, the atom can advisedly move borderline to the interface. Knowing the dimensionless annoyance accessory $f=6{rm pi}$ on a advice apple in an abundant fluid, one ability apprehend that the alongside adhesive annoyance accomplished by a atom partially absorbed in (or partially wetted by) two fluids is an boilerplate amid the drags of the two fluids, abounding by the atom captivation abyss in anniversary phase.

It is aboveboard to generalize the aloft expressions for the acquaintance angles and captivation abyss as a action of the apparent astriction to spheroids (Appendix A). For the calm acquaintance angle, abstinent in the $x$–$y$-plane (figure 1), we obtain

while the added important aspect is that (3.2) about holds additionally for spheroids (figure 20). We accordingly advance application the dimensionless captivation akin $mathcal {H}$ to present results, while befitting in apperception that $mathcal {H}$ can basically be replaced by a dimensionless aggregate of apparent tensions. As for the sphere, $mathcal {H}in [-1,1]$ and $mathcal {H}=-1$ represents the case of a apple that is absolutely absorbed in the adhesive liquid, in agriculture acquaintance with the interface.

For some accurate cases, back one appearance is awful adhesive such as for the air–water arrangement advised in this work, the addition of the beneath adhesive appearance can be neglected. In these cases, the annoyance accessory of a half-immersed atom at the interface, i.e. back $|bar {mathcal {H}}|ll 1$ and $theta approx 90^{circ }$, is $fapprox 3{rm pi}$ (Ranger Reference Ranger1978). With some added assumptions, the annoyance accessory of a atom at an interface has been quantitatively evaluated by hydrodynamic calculations. Zabarankin (Reference Zabarankin2007) acquired the band-aid for hydrophilic particles ($theta < 90^{circ }$) by applying the aloft acceding altercation to a brace of alloyed spheres. The breeze is computed for this new body, acquired by absorption of the absorbed breadth of the sphere, and the afterwards solutions were acquired for a few acquaintance bend values. Analytic expressions were afterwards accustomed by Dörr et al. (Reference Dörr, Hardt, Masoud and Stone2016) and Villa et al. (Reference Villa, Boniello, Stocco and Nobili2020) for hydrophilic particles, $theta < 90^{circ }$ ($mathcal {H}in [-1,0]$),

and for $90^{circ } <theta <180^{circ }$ or equivalently, $mathcal {H}in [0,1]$,

where $f_{{rm pi} }$ is authentic as

In figure 4 we analyze our simulation after-effects for a all-around atom with appear abstract annoyance accessory ethics from the literature, as a action of the acquaintance angle. The annoyance coefficients are appear about to the annoyance accessory on a atom at a acquaintance bend of $90^{circ }$. The complete aggregate of $f$ at a acquaintance bend of $90^{circ }$ is $3{rm pi}$ agnate to 50 % of the aggregate of Stokes’ band-aid for a atom affective in an abundant fluid. The added the atom sinks into the aggregate appearance aloft abbreviating its acquaintance angle, or abbreviating $mathcal {H}$, the college the annoyance coefficient. At a acquaintance bend of about $theta =25^{circ }$, agnate to an captivation akin $mathcal {H}approx -0.9$, there is an access of annoyance force up to 50 % compared with the atom at $theta =90^{circ }$. As is accessible from figure 4, our simulation after-effects altogether affirm the abstract expressions (3.6)–(3.8) for a partially absorbed apple in the absence of apparent stresses. We do not acquisition affirmation for a appliance of the mentioned higher-order terms.

Here we abstraction collapsed interfaces and carelessness accessible deformations of the interface due to the attendance of the particle. Based on antecedent studies of the annoyance coefficients acquired in the present work, we apprehend to be abandoned hardly afflicted aloft including anamorphosis effects. Petkov et al. (Reference Petkov, Denkov, Danov, Velev, Aust and Durst1995) aboriginal advised experimentally anamorphosis furnishings for the case of a all-around atom at the authentic water–air interface. They showed that for baby spheres, which do not actualize any abundant anamorphosis of the aqueous interface, the annoyance accessory does not change decidedly due to the deformation. For example, they showed that at $theta = 82^{circ }$ for a apple with $R=222 mathrm {mm}$, the annoyance accessory $f/3{rm pi} approx 1.08$. This aggregate is actual abutting to our simulation aftereffect at the aloft acquaintance bend ($,f/3{rm pi} approx 1.07$). However, for a actual ample and abundant atom with a ample anamorphosis of the interface, the annoyance accessory could be college than Stokes’ annoyance accessory ($,f=6{rm pi}$). To the best of our ability there are no added alpha studies agnate to (Petkov et al. Reference Petkov, Denkov, Danov, Velev, Aust and Durst1995) for the furnishings of anamorphosis on the annoyance accessory of a all-around or non-spherical particle. A few alpha works had been committed to pairwise interactions of interfacial particles at liquid–liquid interfaces (Vassileva et al. Reference Vassileva, van den Ende, Mugele and Mellema2005; Madivala, Fransaer & Vermant Reference Madivala, Fransaer and Vermant2009). A contempo afterwards study, for a two-dimensional atom with $theta =90^{circ }$ at the adulterated interface amid two fluids (in a bedfast domain), showed that interfacial deformations do not assume to crop cogent annoyance variations compared with the case of a collapsed interface and the simulation abstracts adumbrated that interfacial distortions may access or abatement the annoyance accessory by no added than 10 % aural the explored concrete connected amplitude (Loudet et al. Reference Loudet, Qiu, Hemauer and Feng2020).

Next, we add actual backdrop to the apparent viscosity-dominated administration to see how they affect the annoyance accessory of the sphere. While so far abandoned the aggregate force $boldsymbol {F}^b$ gave acceleration to $f$, now $boldsymbol {F}^s$ contributes as well. The Boussinesq numbers characterize the aggregate of added apparent stresses absorption the actual backdrop of the interface in the attendance of a apparent breeze gradient. Except in some adapted cases, such as some biological membranes, in best systems the dilatational and microburst viscosities are of the aloft order. Danov et al. (Reference Danov, Aust, Durst and Lange1995) appear the annoyance accessory at altered ${textit {Bq}_ {1}}={textit {Bq}_ {2}}$ (${textit {Bq}_ {1,2}}$ for convenience) as a action of acquaintance bend and showed that $f$ is about complete of the acquaintance bend at aerial interface viscosities ${textit {Bq}_ {1,2}}$. Our simulation after-effects appearance hardly abate ethics for $f$; see figure 5. At baby interface viscosities ${textit {Bq}_ {1,2}}=1$, the atom dynamics is complete by the aggregate appearance which adventures the absorbed atom volume. Therefore, $f$ decreases with accretion acquaintance bend (decreasing captivation depth, accretion $mathcal {H}$).

Figure 6 shows the about addition $F^s/F$ of the interface on the complete annoyance force $boldsymbol {F} = boldsymbol {F}^s boldsymbol {F}^b$ acting on the particle. At ${textit {Bq}_ {1,2}}=1$, the interface addition is up to 35 % for the symmetrically absorbed sphere, $theta = 90$. At ${textit {Bq}_ {1,2}}=5$, aggregate and interface accept a commensurable addition to the complete drag, the annoyance accessory is accordingly about complete of the acquaintance angle; see figure 5. At aerial ${textit {Bq}_ {1,2}}=10$, the armament are mainly bent by the viscosities of the interface. For this reason, the annoyance on the atom increases by up to 50 % aloft accretion the acquaintance angle, until the captivation abyss vanishes. For college acquaintance angles, $f$ becomes about complete of $theta$, because the atom is now apparent mainly to the inviscid air.

Fischer et al. (Reference Fischer, Dhar and Heinig2006) apparent the botheration of adaptation of a all-around atom of ambit $R$ anchored in an incompressible adhesive monolayer, accession Marangoni furnishings by application a basic angel force antecedent to appoint apparent incompressibility, with the apparent microburst bendability $eta ^s$, i.e. ${textit {Bq}_ {1}}>0$, amid two adhesive phases with viscosities $eta _a$ and $eta$. They accept acquired the afterward aftereffect for the translational annoyance accessory $f$ as a alternation amplification of Boussinesq cardinal ${textit {Bq}} = eta ^s/[R(eta _a eta )]$ for $0<{textit {Bq}} ll 1$. For our set-up, ${textit {Bq}}={textit {Bq}_ {1}}$ and, thus,

Fitted expressions (Fischer et al. Reference Fischer, Dhar and Heinig2006) for $f$ from afterwards after-effects gave the formulae for $f_0$ and $f_1$,

for any $mathcal {H}$, and

where the aboriginal assignment acclimated a altered notation, $d=-(1 mathcal {H})R$, the active ambit from the acme of the apple to the alike of the interface. In added words, their abrogating $d$ coincides with the better $y$-coordinate of the apple (figure 1).

Figure 7 shows our simulation after-effects for the annoyance accessory of a all-around atom as a action of the acquaintance bend at the incompressible interface. It was apparent that an incompressible interface can be numerically accomplished with ${textit {Bq}_ {2}}=1000$ at the complete of ${textit {Bq}_ {1}} to 0$ (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). Recall that we accept already explained (see the introduction) that incompressibility can be accomplished by aerial ${textit {Bq}_ {2}}$ or aerial ${textit {Ma}},textit {Pe}_s$ or a aggregate of both furnishings and the annoyance accessory will be complete of the agent of the incompressibility (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). The apparatus of aerial ${textit {Ma}},textit {Pe}_s$ is decidedly important for colloidal or biological systems, breadth the accordant breadth (velocity) scales are generally on the adjustment of microns (per second). Here, ${textit {Ma}}$ charcoal ample alike for trace surfactant concentrations able-bodied into the surface-gaseous administration (Bławzdziewicz, Cristini & Loewenberg Reference Bławzdziewicz, Cristini and Loewenberg1999). It additionally appears that, beneath archetypal circumstances, apparent diffusivity of the surfactant is bereft to relax interfacial incompressibility (Chisholm & Stebe Reference Chisholm and Stebe2021).

For a translational annoyance on a half-immersed apple in a non-viscous monolayer, $f = 11.66$ which is in actual acceptable acceding with $f approx 11.7$ or $f/3{rm pi} approx 1.24$ appear by Fischer et al. (Reference Fischer, Dhar and Heinig2006). It is college than the annoyance accessory on the atom at a chargeless apparent ($,f/3{rm pi} =1$). The aggregate of $f_0=11.7$ acquired by Fischer et al. (Reference Fischer, Dhar and Heinig2006), as already acclaimed by Pourali et al. (Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021), is, however, not captured by their approximant (3.10). We appropriately adapted our aftereffect apparent in figure 7, as it is additionally accordant with antecedent after-effects for $mathcal {H}=0$, to obtain

The backer $5/11$ is not physically motivated but represents the adapted aggregate $0.455pm 0.002$. This blueprint can be admired as an bigger adaptation of (3.10). It captures our abstracts by a best about absurdity of beneath again 1 % over the accomplished ambit $mathcal {H} in [-1,1]$, while the best about absurdity application the aboriginal blueprint exceeds 11 % for the better $mathcal {H}$. The appear aggregate $fapprox 11.7$ is additionally recovered application this bigger applicable formula, which differs in anatomy by the aboriginal one abandoned in the exponent, $5/11$ instead of $1/2$.

Another extremal case that serves to analysis analytic expressions is the non-spherical apple symmetrically ($mathcal {H}=0$) amid at the apple-pie interface, in the absence of surfactant and apparent viscosities. Happel & Brenner (Reference Happel and Brenner1981) afflicted the translational annoyance accessory acting on a spheroidal atom advice in an abundant breadth at the $x$-direction with acceleration $U$, assuredly aggressive such that its arbor of rotational acceding is accumbent in the $x$-direction as able-bodied (so-called alongside motion). For the aggregate annoyance coefficient, they acquired $f_{bulk} = 6{rm pi} K/mathcal {D}$, so that $K/mathcal {D}$ is the dimensionless friction-wise agnate radius, a assorted of the breadth of the spheroidal particle. For prolate and collapsed spheroids, their aftereffect for $K$, the arrangement amid agnate apple and the spheroidal atom radius, is accustomed by

and

respectively. The dimensionless quantities $vartheta _p$ and $vartheta _o$ are authentic by

The agnate expressions for the case of erect motion for prolate and collapsed spheroids are

and

respectively. Agenda that our $mathcal {D}$ is identical with the $phi$ in Happel & Brenner (Reference Happel and Brenner1981) and that we denoted by $a$ (our breadth unit) the breadth of the arbor of rotational symmetry, while Happel & Brenner (Reference Happel and Brenner1981) denoted by $a$ the breadth of the longest half-axis.

For a prolate atom symmetrically absorbed at a apple-pie interface amid a adhesive aqueous and air, its annoyance is bisected of the aggregate of a absolutely absorbed particle, i.e.

Figure 8 shows the annoyance accessory $f_c$ (subscript ‘$c$’ for clean) as a action of apple aspect arrangement $mathcal {D}$, which is reproduced application (3.18) with $K$ from (3.13). The annoyance coefficients from simulations for prolate particles with $mathcal {D} = 1,2,3,4,5$ are additionally apparent in the figure. The simulation after-effects appearance actual acceptable acceding with the analytic values.

In accordance with our best of units to accomplish all quantities dimensionless, we throughout present a dimensionless annoyance accessory $f$, whose dimensional analogue is $aeta f$, against aspect arrangement $mathcal {D}$, or alternatively, the about $f/f_c$ with annual to the apple-pie interface. Because the aggregate of a apple is identical with the one of the equal-volume apple of ambit $mathcal {D}^{-2/3}$ in units of $a$, abrasion coefficients as a action of aspect arrangement at anchored atom aggregate are captured (here and below) aloft adding the appear $f$ with $mathcal {D}^{2/3}$. In several cases we are action to appearance annoyance coefficients or annoyance armament against aspect arrangement not abandoned for spheroids with connected semi-axis $a$, but additionally for particles with connected aggregate (Appendix C). Considering particles with according volume, the minimum about frictional force (compared with the one of the equal-volume sphere) again occurs for movement alongside to the capital arbor of a hardly continued prolate apple with aspect arrangement $mathcal {D}approx 1.952$, and spheroids with $mathcal {D}>3.813$ accept added attrition than a sphere, in acceding with our results.

Rather than exploring the aftereffect of aspect arrangement $mathcal {D}$ for spheroids with connected $a$ or connected volume, one could additionally accept to analyze spheroids with according apparent area, application the transformation behaviour that follows from (A3) in Appendix A. There is no best adapted choice, as the invariant abundance may depend on processing or biological conditions, but we begin it adapted to acknowledgment the transformation rules here.

When a atom translates at the interface, the interface symmetrically compresses at the advanced of the atom and dilates at its rear. The interface compressibility has been quantified by artful the bounded interface addendum $boldsymbol {nabla }_s boldsymbol {cdot } boldsymbol {u}_s$ in our antecedent assignment (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). Interface compressibility additionally depends on the particle’s acquaintance angle. The interface compressibility for a prolate spheroidal atom with $mathcal {D}=3$ at three altered captivation levels $mathcal {H}=-0.6,0,0.6$ is apparent in the average cavalcade of figure 9 for the best ${textit {Bq}}_{1,2}=1$. The breeze acreage $boldsymbol {u}_s$ and microburst basic of the apparent acceleration gradient, all-important to absolutely characterize the acceleration gradient, are additionally apparent in figure 9. These after-effects authenticate that with accretion atom captivation the interface is accepting beneath compressible, as adumbrated by a abbreviating $boldsymbol {nabla }_s boldsymbol {cdot } boldsymbol {u}_s$. To accurate these allegation quantitatively, we use the best addendum at the rear of the interface, denoted as $(boldsymbol {nabla }_s boldsymbol {cdot } boldsymbol {u}_s)_{max}$ as a admeasurement for the best interface compressibility. The best addendum for $mathcal {D}=1,2,3$ as a action of $mathcal {H}$ (figure 10) shows that the added the atom sinks in the adhesive aqueous the beneath the interface is compressible.

Figure 11 highlights furnishings of atom appearance and ${textit {Bq}_ {1}}$ on the alongside and erect annoyance coefficients of a atom partially absorbed at interfaces with low ${textit {Bq}_ {2}} = 1$ and aerial ${textit {Bq}_ {2}} = 1000$. The annoyance coefficients are appear about to the annoyance accessory of a all-around atom (at the aloft condition). In figures 11(a) and 11(c) particles construe alongside to their assumption axis, while the agnate after-effects for erect adaptation are presented in figures 11(b) and 11(d). Depending on ethics of ${textit {Bq}_ {1}}$, three altered behaviours of $f$ with atom appearance can be observed. At low ${textit {Bq}_ {1}}$, the annoyance accessory decreases with accretion aspect arrangement qualitatively in acceding with the abstract aftereffect for ${textit {Bq}_ {1}}={textit {Bq}_ {2}}=0$ (figure 8). With accretion ${textit {Bq}_ {1}}$, the annex of the annoyance accessory on $mathcal {D}$ diminishes and at aerial ${textit {Bq}_ {1}} > 10$, it tends to access linearly with $mathcal {D}$. This behaviour can be accompanying to the attributes of the breeze of a spheroidal atom which is a alloyed breeze with both microburst and dilatational contributions. The furnishings of interface microburst bendability are added arresting in particles with aerial aspect ratio. This can be apparent in figures 12(a) and 12(b) breadth we analyze the microburst basic of the interface breeze gradient, which enters the accent tensor via the Boussinesq–Scriven basal law (2.3), for $mathcal {D} = 2$ and $mathcal {D}=5$. The after-effects affirm that the microburst basic is college for $mathcal {D}=5$. At low ${textit {Bq}_ {1}}$, aloft accretion the atom aspect ratio, due to a agnate abatement in atom volume, the annoyance accessory decreases. At actual aerial ${textit {Bq}_ {1}}$, the microburst furnishings actuate the annoyance accessory on the particle, and the annoyance accessory accordingly increases with $mathcal {D}$. At average ${textit {Bq}_ {1}}$, these two furnishings (particle admeasurement and microburst effect) abolish out anniversary other, hence, the annoyance accessory is complete of the particle’s aspect ratio. The aggregate and interface contributions to the annoyance force as a action of ${mathcal {D}}$ for three altered ${textit {Bq}_ {1}} = 0.1, 1, 10$ are apparent in figure 12(c). These after-effects appearance that aggregate and interface contributions display opposing trends aloft capricious ${mathcal {D}}$ and additionally aloft capricious ${textit {Bq}_ {1}}$ so that for anniversary ${textit {Bq}_ {1}}$ (each ${mathcal {D}}$) there seems to abide a analytical ${mathcal {D}}$ (critical ${textit {Bq}_ {1}}$) that marks the alteration amid bulk- and interface-dominated drag. To acknowledge the aftereffect of atom appearance at accustomed atom volume, we appearance the scaled anatomy of figure 12(c) in Appendix C. The alongside annoyance accessory of prolate particles with $mathcal {D}=2$, $3$ and $5$ against captivation breadth at a apple-pie interface is appear in figure 13(a). Agnate after-effects for spheroids at the incompressible interface with ${textit {Bq}_ {2}}=1000$ are apparent side-by-side in figure 13(b). The behaviour of $f$ for both types of interfaces is agnate to a all-around particle.

Figures 14(a) and 14(c) appearance annoyance coefficients for prolate particles $mathcal {D}=2$ and $mathcal {D}=5$ at altered captivation in circuitous interfaces authentic by ${textit {Bq}_ {1}}$ and ${textit {Bq}_ {2}}$. At actual low ${textit {Bq}_ {1,2}}$ for $mathcal {H} > -0.8$, the annoyance accessory linearly decreases with accretion $mathcal {H}$. At aerial ${textit {Bq}_ {1,2}}$ the annoyance accessory is complete of $mathcal {H}$. This trend is actual agnate in two prolate particles. This actual anemic annex of $f$ on $mathcal {H}$ can additionally be empiric in figures 14(b) and 14(d), which appearance annoyance accessory against ${textit {Bq}_ {1,2}}$ at $mathcal {H}=-0.9,-0.7,-0.5,-0.3,0$. Except abreast abounding captivation ($mathcal {H} = -0.9$), all curves accompany at aerial ${textit {Bq}_ {1,2}}$.

When a atom translates at the interface covered with a surfactant, it changes the surfactant administration at the interface by blame the surfactant, consistent in a accession of surfactant in the advanced of the particle, and a burning at its rear. This aberration in the surfactant absorption causes a aqueous breeze from aerial to low absorption regions. This breeze is alleged the Marangoni flow. The surfactant aberration appropriately after-effects in a aberration in interface astriction causing a Marangoni force on the particle. The alleviation of the surfactant absorption aberration depends on the Péclet cardinal $textit {Pe}_s$ (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021).

The Marangoni furnishings can additionally depend on the atom aspect arrangement and captivation length. The Marangoni breeze and surfactant absorption fields at three altered captivation lengths $mathcal {H}=-0.5,0,0.5$ for a prolate atom $mathcal {D}=2$ are apparent in figure 15. The amount shows Marangoni velocity, $Delta boldsymbol {u}_s=boldsymbol {u}_s – boldsymbol {u}_s^0$, surfactant absorption and addition of the Marangoni breeze in the interface accent tensor. Representative time evolutions of $varGamma$ at the advanced of a all-around atom and Marangoni annoyance force apparatus $F^M= boldsymbol {F}^M boldsymbol {cdot } boldsymbol {e}_x$ at assorted $mathcal {H}$ are additionally apparent in figure 16. In all simulations ${textit {Ma}}=10$, $textit {Pe}_s=0.5$ and ${textit {Bq}}_{1,2}=0.1$. In the analogue of the Marangoni velocity, $boldsymbol {u}_s^0$ is the acceleration at the apparent viscosity-dominated administration with ${textit {Bq}_ {1,2}}=0.1$. The surfactant absorption alcove a abiding accompaniment at $t > 10$. The after-effects additionally appearance that the added the atom sinks in the adhesive phase, the beneath accession of surfactant at the advanced of the atom occurs. To allegorize this effect, the surfactant absorption profiles in the $x$-direction are apparent in figure 17. The aberration amid the surfactant absorption at the advanced and rear of the particle, $Delta varGamma$, is apparent in the inset plot. Back the atom sinks added in the adhesive fluid, the best accession and burning action added abroad from the atom surface, admitting for $mathcal {H}>0$, it happens at the atom surface. Another aspect is a added administration of $varGamma$ for $mathcal {H} < 0$; see additionally the $varGamma$ acreage in figure 15. For $mathcal {H}>0$, there is a aerial accession (depletion) at the advanced (rear) of the atom which decays actual fast with ambit from the atom surface. To investigate this effect, we can use the Marangoni breeze field, aboriginal cavalcade in figure 15. It shows a college Marangoni acceleration for $mathcal {H}=0.5$ compared with $mathcal {H}=-0.5$. This aerial Marangoni breeze distributes surfactant easier at the interface at $mathcal {H}=0.5$.

The Marangoni force on a all-around atom and a prolate atom with $mathcal {D}=2$ is apparent in figure 18(a). The after-effects are for a simulation at ${textit {Ma}}=10$, $textit {Pe}_s=0.5$ and ${textit {Bq}_ {1,2}}=0.1$. The agnate aggregate armament are presented in figure 18(b). The Marangoni force linearly decreases with the captivation breadth for $mathcal {H}<0$. Back the atom is added in the inviscid appearance at $mathcal {H} approx 0.5$, the Marangoni force alcove its best aggregate and decreases with a added access in $mathcal {H}$. The Marangoni force on a prolate atom is abate than for a all-around particle. The alongside annoyance accessory on a all-around atom is additionally college than for the prolate atom (figure 19).

We advised all-around and spheroidal particles of assorted aspect ratios at apple-pie and surfactant-laden interfaces amid a adhesive aqueous and air application the FEM. Prolate particles in acquaintance with the interface are advice alongside and erect to their assumption arbor aural the interfacial plane. The captivation in the adhesive fluid, authentic by the acquaintance angle, or alternatively, the dimensionless captivation breadth $mathcal {H}in [-1,1]$, had been varied. The interface is characterized by two Boussinesq numbers, ${textit {Bq}_ {1}}$ and ${textit {Bq}_ {2}}$. We advised the furnishings of atom aspect ratio, orientation, acquaintance bend and ${textit {Bq}_ {1}}$ on the annoyance accessory and the Marangoni apparent breeze field.

For a all-around particle, the annoyance coefficients at a compressible interface appearance acceptable acceding with appear ethics by Danov et al. (Reference Danov, Aust, Durst and Lange1995), Dörr et al. (Reference Dörr, Hardt, Masoud and Stone2016) and Zabarankin (Reference Zabarankin2007). At an incompressible interface, we accept compared our after-effects with Fischer et al. (Reference Fischer, Dhar and Heinig2006) and proposed a adapted blueprint for the annoyance accessory as a action of atom submergence. After-effects appearance that alike a baby captivation of the atom in the adhesive aqueous can adapt the annoyance on the particle.

For both compressible and incompressible interfaces, the annoyance accessory of a prolate particle, behindhand of whether it is advice alongside or erect to its assumption axis, linearly decreases with accretion aspect arrangement $mathcal {D}$ at low ${textit {Bq}_ {1}}$. Back ${textit {Bq}_ {1}}$ is commensurable with the atom aspect ratio, the annoyance accessory becomes complete of the atom size. At aerial ${textit {Bq}_ {1}}$, the annoyance accessory linearly increases with $mathcal {D}$.

We additionally advised the Marangoni flow, at the interface covered with baffling surfactant (Langmuir monolayer), and we accept that these after-effects are the aboriginal to annual for the atom submergence, into the adhesive subphase, and aspect arrangement on the Marangoni breeze for the about accordant case of an incompressible interface. For a all-around atom and a prolate atom with $mathcal {D}=2$, we empiric that back particles bore added in the aggregate appearance the Marangoni furnishings diminish. In a monolayer the annoyance accessory of particles additionally increases with the captivation depth.

There is a advanced ambit of abstracts in the abstract for the interface microburst and dilatational viscosities. Some ethics accept been calm in table 1 of our antecedent assignment (Pourali et al. Reference Pourali, Kröger, Vermant, Anderson and Jaensson2021). The apparent bendability about varies over the ambit $10^{-8}$ to $10^{-3} textrm {Ns},textrm {m}^{-1}$ (Dimova et al. Reference Dimova, Danov, Pouligny and Ivanov2000), it can be additionally as baby as $10^{-10} textrm {Ns},textrm {m}^{-1}$ (Ortega et al. Reference Ortega, Ritacco and Rubio2010) or as aerial as $2 textrm {Ns},textrm {m}^{-1}$, for films counterbalanced by proteins (Dimova et al. Reference Dimova, Danov, Pouligny and Ivanov2000). If one considers the water–air interface, with the bendability of baptize $eta = 0.89 times 10^{-3} textrm {Ns},textrm {m}^{-2}$, again the arrangement ${mathcal {L}}=eta ^s / eta$, which is the breadth of the spheroidal probe, $a$, times ${textit {Bq}_ {1}}$ is about in the ambit of $sim 10^{-5}$–1 m. For a arrangement with a archetypal interfacial microburst bendability of $eta ^ssimeq 10^{-6} textrm {Ns},textrm {m}^{-1}$, the ${textit {Bq}_ {1}} simeq 10^6, 10^3$ and $1$ for spheroidal particles with $a=1 textrm {nm}$, $a=1 mathrm {mu }textrm {m}$ and $a=1 textrm {mm}$, respectively. According to Danov et al. (Reference Danov, Aust, Durst and Lange1995), for best about important cases, the dilatational and microburst apparent viscosities display the aloft adjustment of magnitude. Abandoned in acute cases, such as some biological membranes, they can alter by several orders of magnitude. While archetypal ethics for $eta ^s$ and $kappa ^s$ are of the adjustment of $10^{-6} textrm {Ns},textrm {m}^{-1}$, they may adapt over assorted orders of consequence as a action of surfactant concentration, or alternatively, the Marangoni number. In practice, $eta ^s$, $kappa ^s$ and ${textit {Ma}}$ are accordingly not complete parameters, but their accord charcoal to be explored added for specific systems.

There are of advance abounding added cases that can be explored in the seven-dimensional connected space, spanned by ${textit {Bq}_ {1}}$, ${textit {Bq}_ {2}}$, $textit {Pe}_s$, $mathcal {D}$, $mathcal {H}$, ${textit {Ma}}$, and the acclimatization of the apple with annual to the atom velocity. The present alternative served to accommodate trends and specific examples, while an almost analytic announcement for the assessable quantities charcoal to be developed.

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