Membrane Microfiltration by Recirculating the Surface Flow of a Suspension Droplet

We present a Hele-Shaw flow-based method (patent pending) for membrane filtration of a droplet of particulate suspension. The method features a sphere-on-flat, lifted Hele-Shaw cell (SoF-LHSC), where the suspension droplet is initially confined between two cell plates: one with a spherical cap and the other being flat. The fixed flat plate contains a filter membrane and a microfluidic filtrate collection channel. Oscillatory motion is imparted to the spherical plate to maintain the recirculating flow of the droplet over the membrane. Theoretical analysis of SoF-LHSC flow is used to arrive at experimental settings to get flow without fingering instabilities. The shadowgraphy technique demonstrated that shear due to surface flow induced a well-known particle migration inside the droplet/film and drives the otherwise settling particles away from the microporous membrane, thus enhancing filter efficiency. Filtrate is passively directed to a microchannel via a wedge-shaped cavity and collected there using capillary action. Parametric analysis shows that filter flux increases as particle size is increased, membrane resistance is reduced, and the optimum plate velocity is reached. Beyond this optimal value, the shear-induced migration effect starts diminishing. Shear diffusivity (D_shear) and estimated cake resistance are used as parameters to quantify enhanced filtration performance. For the experiments with a membrane pore size of 0.4 μm, an optimum filtrate volume of 12.98 ± 3.65 μL could be extracted from a 30 μL droplet of the suspension of 7 μm polystyrene beads at 9.5% v/v, corresponding to a yield of 47.98 ± 13.49%, after 5 min of operation. The proposed method promises enhancements in applications such as concentration of a suspension droplet, removal of impurities, extraction of a useful filtrate, and the like.