Colloids which adsorb to and straddle a fluid interface form monolayers that are paradigms of particle dynamics on a two dimensional fluid landscape. The dynamics is typically inertialess (Stokes flows) and dominated by interfacial tension so the interface is undeformed by the flow, and pairwise drag coefficients can be calculated. Here the hydrodynamic interaction between identical spherical colloids on a planar gas/liquid interface is calculated as a function of separation distance and immersion depth. Drag coefficients (normalized by the coefficient for an isolated particle on the surface) are computed numerically for the four canonical interactions. The first two are motions along the line of centres, either with the particles mutually approaching each other or moving in the same direction (in tandem). The second two are motions perpendicular to the line of centres, either oppositely directed (shear) or in the same direction (tandem). For mutual approach and shear, the normalized coefficients increase with a decrease in separation due to lubrication forces, and become infinite on contact when the particle is more than half immersed. However, they remain bounded at contact when the particles are less than half immersed because they do not contact underneath the liquid. For in-tandem motion, the normalizedmore »
Driven and Active Colloids at Fluid Interfaces
We derive expressions for the leading-order far-field flows generated by externally driven and active (swimming) colloids at planar fluid–fluid interfaces. We consider colloids adjacent to the interface or adhered to the interface with a pinned contact line. The Reynolds and capillary numbers are assumed much less than unity, in line with typical micron-scale colloids involving air– or alkane–aqueous interfaces. For driven colloids, the leading-order flow is given by the point-force (and/or torque) response of this system. For active colloids, the force-dipole (stresslet) response occurs at leading order. At clean (surfactant-free) interfaces, these hydrodynamic modes are essentially a restricted set of the usual Stokes multipoles in a bulk fluid. To leading order, driven colloids exert Stokeslets parallel to the interface, while active colloids drive differently oriented stresslets depending on the colloid's orientation. We then consider how these modes are altered by the presence of an incompressible interface, a typical circumstance for colloidal systems at small capillary numbers in the presence of surfactant. The leading-order modes for driven and active colloids are restructured dramatically. For driven colloids, interfacial incompressibility substantially weakens the far-field flow normal to the interface; the point-force response drives flow only parallel to the interface. However, Marangoni stresses induce more »
- Award ID(s):
- 1943394
- Publication Date:
- NSF-PAR ID:
- 10217486
- Journal Name:
- Journal of fluid mechanics
- Volume:
- 914
- ISSN:
- 1469-7645
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/: https://doi.org/10.1017/jfm.2020.708
