Mirau interferometry of fluid interfaces deformed by colloids under the influence of external fields

Abstract

The interfacial curvature surrounding colloidal particles pinned to fluid interfaces dictates their interparticle capillary interaction and assembly; however, it is a nontrivial function of particle anisotropy, surface roughness, external field conditions, macroscopic interfacial curvature, and the chemistry of each fluid phase. The prospect of dynamically modifying the pinning properties and interfacial organization of colloidal particles adhered to fluid interfaces via these approaches necessitates the development of experimental techniques capable of measuring changes in the interfacial deformation around particles in situ. Here, we describe a modified technique based on phase-shift Mirau interferometry to determine the relative height of the fluid interface surrounding adsorbed colloids while applying external electric fields. The technique is corrected for macroscopic curvature in the interface as well as in-plane motion of the particle in order to isolate the contribution of the particle to the interfacial deformation. Resultant height maps are produced with a maximum resolution of ±1 nm along the height axis. The measured topography of the interface is used to identify the contact line where the two fluids meet the particle, along with the maximal interfacial deformation (Δumax) of the undulating contact line and the three-phase contact angle, θc. The technique is calibrated using anisotropic polymer ellipsoids of varying aspect ratio before the effect of external AC electric fields on the pinned particle contact angle is demonstrated. The results show promise for this new technique to measure and quantify dynamic changes in interfacial height deformation, which dictate interparticle capillary energy and assembly of colloids at fluid interfaces.