Microscale Dynamics of Oil Droplets at a Membrane Surface: Deformation, Reversibility, and Implications for Fouling

Despite their excellent capabilities, wide implementation of membranes for oil/water emulsion separation is limited due to severe fouling. To date, microscale dynamics of the oil–water–membrane system are poorly understood. The present study uses confocal microscopy at unprecedented resolution for direct observation of oil droplet deposition, deformation, and detachment during separation and cleaning, respectively. The 3D shape of the droplets was imaged as a function of the permeation rate, J, droplet radius, R, membrane permeance, k, water viscosity, μ, and the water/oil interfacial tension coefficient, σ. These parameters yield a modified capillary number, C a ̂ = μVR 1/2/σk 1/2, which accounts for the extra viscous “suction” at close proximity to the membrane surface. A clear correlation was observed between the degree of droplet deformation and an increasing C a ̂ . Furthermore, the reversibility of droplet deposition and membrane performance were assessed through microscopic surface coverage and flux recovery analysis. In general, operation at a low flux (3.9 μm/s) yields spherical droplets that are easily removed by crossflow cleaning, whereas a high flux (85 μm/s) leads to significant deformation and mostly irreversible deposition. These results shed important new insight on the influence of hydrodynamic conditions on fouling reversibility during emulsion separation, and may guide better design of surface-modified membranes.