We study the capillary-induced interactions and configuration of spherical and non-spherical Janus particles adsorbed at flat liquid-fluid interfaces. For Janus spheres, the equilibrium orientation results in each hemisphere being exposed to its more favored fluid. However, experimental observations suggest that some of these particles may take a tilted orientation at the interface, giving rise to a deformed interface. On the other hand, Janus ellipsoids with a large aspect ratio or a small difference in the wettability of the two regions tend to tilt even at equilibrium. The overlap of deformed menisci results in energetic interactions between neighboring particles. We numerically calculate the interface shape around the particles by minimizing the total surface energy of the system comprising of the interface and particle-fluid regions. We quantify these interactions through evaluation of capillary energy variation as a function of the orientation and separation distance between the particles. We find that Janus spheres with similar orientations undergo a relative realignment in the interface plane in order to minimize the capillary energy. In case of ellipsoidal particles, the particles assemble in a preferred side-by-side configuration. We evaluate the role of anisotropy and degree of amphiphilicity on the inter-particle force and the capillary torque. The results can be used to predict the migration and oriented assembly of Janus particles with various geometrical and surface properties at liquid-fluid interfaces.
Assembling Ellipsoidal Particles at Fluid Interfaces Using Switchable Dipolar Capillary Interactions
