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Membrane distillation (MD) can treat high-salinity brine. However, the system’s efficiency is hindered by obstacles, including salt scaling and temperature polarization. When properly implemented, surface patterns can improve the mass and heat transfer in the boundary layer, which leads to higher MD efficiency. In this work, the performance of direct contact membrane distillation (DCMD) using Sharklet-patterned poly (vinylidene fluoride) (PVDF) membranes is investigated. Both non-patterned and patterned PVDF membranes are prepared by lithographically templated thermally induced phase separation (lt-TIPS) process with optimized conditions. Sharklet patterns on the membranes improve the DCMD performance: up to 17 % higher water flux and 35 % increased brine-side heat transfer coefficient. The scaling resistance of the membranes during DCMD is tested by both saturated CaSO4 solution and hypersaline NaCl solutions. Patterned PVDF membranes show an average of 30 % higher water flux and up to 45 % lessened flux decline over time compared with non-patterned membranes when treating high-concentration brines. Post-mortem analysis reveals that Sharklet-patterned membranes display less salt-scaling on surfaces with smaller-sized CaSO4 and NaCl crystals, maintain a relatively cleaner surface, and exhibit better retention of hydrophobicity. 

Microstreaming of acoustically excited bubbles presents great potential to mitigate fouling for membrane technologies. However, the acoustic streaming in bulk fluids under membrane separation conditions is not well explored. In this work, we investigate the microstreaming of 3D printed Helmholtz-like bubble-trapping structures (BTSs) under no flow, pressurized, and crossflow conditions that are relevant to membrane applications. Trapped bubbles are shown to generate formidable microstreaming that spans millimeter distances with velocity as high as 125 mm/s in a bulk aqueous medium. However, complex mode shapes of the bubble oscillation and bubble growth were observed during the frequency sweep. As a result, the streaming velocity decreases by 76% over 30 min, under single frequency excitation. The BTS displayed effective microstreaming under hydrostatic pressure up to 9.0 kPa, and under a crossflow velocity up to 0.2 mm/s, where the microstreaming zone reduced to <1 mm. The results provide the operation window, as well as challenges, for future integration of the BTS into bulk membrane separation processes.