The paradoxical behavior of rough colloids at fluid interfaces

Abstract

Colloidal particles adsorb and remain trapped at immiscible fluid interfaces due to strong interfacial adsorption energy, with a contact angle defined by the chemistry of the particle and fluid phases. An undulated contact line may appear due to either particle surface roughness or shape anisotropy, which results in a quadrupolar interfacial deformation and strong long range capillary interaction between neighboring particles. While each effect has been observed separately, here we report the paradoxical impact of surface roughness on spherical and anisotropic ellipsoidal polymer colloids. Using a seeded emulsion polymerization technique, we synthesize spherical and ellipsoidal particles with controlled roughness magnitude and topography (concave/convex). Via in situ measurement of the interfacial deformation around colloids at an air-water interface, we find that while surface roughness strengthens the quadrupolar deformation in spheres as expected by theory, in stark contrast, it weakens the same in ellipsoids. As roughness increases, particles of both shapes become more hydrophilic and their apparent contact angle decreases. Using numerical predictions, we show that this partially explains the decreased interfacial deformation and capillary interactions between ellipsoids. Therefore, particle surface engineering has the potential to decrease the capillary deformation by asymmetric particles via changing their capillary pinning as well as wetting behavior at fluid interfaces.