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Colloid-sized particles (10 nm–10 μm in characteristic size) adsorb onto fluid interfaces, where they minimize their interfacial energy by straddling the surface, immersing themselves partly in each phase bounding the interface. The energy minimum achieved by relocation to the surface can be orders of magnitude greater than the thermal energy, effectively trapping the particles into monolayers, allowing them freedom only to translate and rotate along the surface. Particles adsorbed at interfaces are models for the understanding of the dynamics and assembly of particles in two dimensions and have broad technological applications, importantly in foam and emulsion science and in the bottom-up fabrication of new materials based on their monolayer assemblies. In this review, the hydrodynamics of the colloid motion along the surface is examined from both continuum and molecular dynamics frameworks. The interfacial energies of adsorbed particles is discussed first, followed by the hydrodynamics, starting with isolated particles followed by pairwise and multiple particle interactions. The effect of particle shape is emphasized, and the role played by the immersion depth and the surface rheology is discussed; experiments illustrating the applicability of the hydrodynamic studies are also examined. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 54 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

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 normalized coefficients decrease with a decrease in separation; they collapse, for all immersion depths, to the dependence of the drag coefficient on separation for two particles moving in tandem in an infinite medium. The coefficients are used to compute separation against time for colloids driven together by capillary attraction.