Controlling self-assembly and interfacial mechanics of polymer spheres and ellipsoids at fluid interfaces with surface roughness

In this study, we describe the effect of surface topography on monolayer assembly and mechanics of spherical and ellipsoidal colloids at an air-water interface. Building off our prior work which directly measured the roughness-dependent capillary pinning of individual particles, we now show how the reduction in capillary interaction energy between rough ellipsoids, and increase in interaction energy between spheres, impacts their collective assembly. Two types of surface topography (convex/concave) and two degrees of roughness are compared with their smooth analogues. With increasing roughness, the measured surface pressure increases for spheres, in accordance with stronger capillary interactions, and decreases in ellipsoids, likewise confirming that individual particle attributes impacts their monolayer properties. However, the type of surface topography, not just the roughness magnitude, is shown to be a critical aspect of the assembly morphology as the interfaces approach their jammed state. In particular, concave rough ellipsoids are observed to form a complete unidirectional monolayer with high area fraction (~0.86), avoiding the kinetically arrested assemblies and low area fraction jamming (0.68) shown by smooth ellipsoids. Moreover, monolayers of concave rough ellipsoids demonstrate a two-dimensional interfacial isotropic–nematic phase transition with increasing particle areal density. The surface topography mediated capillary pinning and wetting behavior, coupled with the altered interparticle interactions and resultant interfacial microstruture further dictates the monolayer’s ability to resist compressive deformation and collapse mechanics. These findings open up opportunities to realize complex 2D ordered microstructures from anisotropic particles and manipulate fluid-fluid interface stability in emulsions and foams by leveraging particle topography and shape engineering.