Leveraging particle roughness and shape anisotropy to engineer particle-laden interfaces

Recently, we discovered the paradoxical effect of surface roughness and shape anisotropy on the pinning and capillary interactions of polymer colloids at fluid interfaces. For spheres, roughness strengthens the interfacial quadrupolar deformation and corresponding interparticle capillary attraction, while for ellipsoids, both are attenuated. Here, we discuss how this dilute interfacial behavior dictates monolayer mechanics and self-assembly. We measure surface pressure isotherms and microstructures of monolayers of particles with controlled shape, roughness and topography. Our prior discoveries manifest in concentrated monolayers: weakened capillarity in rough ellipsoids decreases monolayer surface pressure, in contrast with spheres. Increasing roughness also decreases the maximum static compression modulus. We show that interfacial mechanics are coupled with monolayer morphology. For ellipsoids, the decrease in capillary attraction with roughness enables ordered assembly at close packing, avoiding capillary driven disordered, kinetically arrested states. Our results shed light on the consequential role of particle surface and shape engineering on monolayer mechanics and assembly behavior that are paramount in designing Pickering emulsions and interfacially templated materials.