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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. 

Liquid emulsion droplet evaporation is of importance for various sensing and imaging applications. The liquid-to-gas phase transformation is typically triggered thermally or acoustically by low–boiling point liquids, or by inclusion of solid structures that pin the vapor/liquid contact line to facilitate heterogeneous nucleation. However, these approaches lack precise tunability in vaporization behavior. Here, we describe a previously unused approach to control vaporization behavior through an endoskeleton that can melt and blend into the liquid core to either enhance or disrupt cohesive intermolecular forces. This effect is demonstrated using perfluoropentane (C 5 F 12 ) droplets encapsulating a fluorocarbon (FC) or hydrocarbon (HC) endoskeleton. FC skeletons inhibit vaporization, whereas HC skeletons trigger vaporization near the rotator melting transition. Our findings highlight the importance of skeletal interfacial mixing for initiating droplet vaporization. Tuning molecular interactions between the endoskeleton and droplet phase is generalizable for achieving emulsion or other secondary phase transitions, in emulsions.