(141e) Capillary Migration of Microparticles on Isotropic and Complex Curved Fluid Interfaces

Microparticles trapped at fluid interfaces interact by capillarity to migrate, form structures and find preferred locations.  These phenomena are exploited to organize colloids at fluid interfaces, impacting emulsion stabilization and forming the basis for advanced materials.  Interface curvature plays a strong role in microparticle behavior by acting as a field which directs microparticle migration. Here, we first discuss the behavior of microparticles at simple fluid interfaces with well-defined curvature fields. For a particle on a fluid interface, there are two conditions that can apply at the three phase contact line: either the contact line adopts an equilibrium contact angle, or it can be pinned by kinetic trapping, e.g. at chemical heterogeneities, asperities or other pinning sites on the particle surface. We formulate the curvature capillary energy for both scenarios. For particles with equilibrium contact angles, we find that the capillary energy is negligible, with the first contribution bounded to fourth order in the deviatoric curvature. For pinned contact lines, we find curvature capillary energies that are finite, with a linear functional form with respect to the host interface deviatoric curvature. We compare the energy dissipated by particle migration in experiment to prediction, finding excellent agreement with the expression for pinned contact lines. These results imply that contact line pinning occurs for microparticles at these curved fluid interfaces with dramatic implications in their dynamics at interfaces. We finally discuss the extension of our analysis for interactions of microparticles on interfaces with bending energy and tensions.