Three-Dimensional Imaging of Emulsion Separation through Liquid-Infused Membranes Using Confocal Laser Scanning Microscopy

Abstract

The removal of emulsified oils from water has always been a challenge due to the kinetic stability resulting from the small droplet size and the presence of stabilizing agents. Membrane technology can treat such mixtures, but fouling of the membrane leads to dramatic reductions in the process capacity. Liquid-infused membranes (LIMs) can potentially resolve the issue of fouling. However, their low permeate flux compared with conventional hydrophilic membranes remains a limitation. To gain insight into the mechanism of transport, we use 3D images acquired by confocal laser scanning microscopy (CLSM) to reconstruct the sequence of events occurring during startup and operation of the LIM for removal of dispersed oil from oil-in-water emulsions. We find evidence for coalescence of oil droplets on the surface of and formation of oil channels within the LIM. Using image analysis, we find that the rate at which oil channels are formed within the membrane and the number of channels ultimately govern the permeate flux of oil through the LIMs. Oil concentration in the feed affects the rate of coalescence of oil on the surface of the LIM, which, in turn, affects the channel formation dynamics. The channel formation dynamics also depend on the viscosity of the infused liquid and the operating pressure. A higher affinity to the pore wall for infused liquid than permeating liquid is essential to antifouling behavior. Overall, this work offers insight into the selective permeation of a dispersed liquid phase through a LIM.