Nanoscale porosity in microellipsoids controls interparticle capillary attraction and assembly at fluid interfaces

Anisotropic particles pinned at fluid interfaces tend towards disordered multi-particle configurations due to large, orientationally-dependent, capillary forces, which is a significant barrier to exploiting these particles to create novel functional self-assembled materials. To create long-range ordered structures with complex configurations via interfacially trapped anisotropic particles, control over the interparticle interaction energy is necessary. In this presentation, we discuss the synthesis of colloidal ellipsoids with controlled nanoscale surface topography (roughness and porosity) and show how this surface topography attenuates the interparticle capillary attraction at a water-air interface. We show that porous particles exhibit a much shorter-range capillary interaction potential, with scaling intriguingly different than theory describing the behavior of smooth ellipsoids. Interferometry measurements of the fluid deformation surrounding a single particle shows that the interface around porous ellipsoids does not possess the characteristic quadrupolar symmetry of smooth ellipsoids, and quantitatively confirms the decrease in capillary interaction energy. Lastly, we show how this reduction in interparticle capillary attraction and alteration in interfacial pinning manifests in the overall 2D interfacial assembly of such particles.