&nbsp; &nbsp; Hydrodynamic cavitation, the nucleation of vapor bubble within a liquid, is an extensively studied phenomenon. Cavitation is commonly triggered by a physical process that the fluid pressure is suddenly decreased below the saturated vapor pressure. Cavitation is usually accompanied by pernicious ramifications in a great range of biomedical and industrial processes, such as noise, erosion, surge stall, vibration, decreased power output and reduced efficiency in hydraulics machineries.&nbsp; &nbsp; To our knowledge, extensive investigations on the cavitation have been conducted in isotropic liquids (water, liquid hydrogen, et al.) at conventional scales. Few researchers pay attention to this phenomenon of anisotropic liquids in microchannels. Nematic liquid crystal is a classical anisotropic liquid that the molecules exhibit long-range order in their orientations or positions. Many building blocks in living organisms, such as amphiphilic lipids in cell membranes, cytoskeletal and so on, are the nematic liquid crystals. It should be pointed out that the hydrodynamic characteristics of nematic liquid crystals are determined not only by inertial and viscous parameters, but also by long-range elastic interactions between molecules and between molecules and confining boundaries. Therefore, the study of cavitation of nematic liquid crystals plays a crucial role in relevant biomedical sciences and industrial processes.&nbsp; &nbsp; In this work, the hydrodynamic cavitation of nematic liquid crystals in micochannels is studied by the experiments and theoretical analysis. The working fluid is nematic 5CB liquid crystal (4′-n-pentyl-4-cyanobiphenyl) and flows around circular micropillars with different diameters in rectangular microchannels. The microchannels’ width w is 150μm, depth d is 40μm, and the micropillars’ diameter, 2r, is between 40 and 100μm. Nematic phase 5CB liquid crystal is pumped into the microchannel by a micropump with a speed range of 800 to 3000μm/s. It is corresponding Reynolds number range is about 10-3 to 4×10-3. Shanghai Optical SG-37 microscope is used to observe the cavitation and flow phenomenon. The free energy theory is applied to analyse the cavitation inception conditions.Results show that the critical Reynolds number of the cavitation inception decreases with the increase of the micropillar diameter and order parameter of the nematic liquid crystal. The cavitation ensues in the Stokes flow of nematic liquid crystals, while it does not appear in isotropic liquids. The cavitation inception of nematic liquid crystals is much easier to occur at the cases with a lower surface tension and larger saturated vapor pressure. The aperiodic variation of the volume of the cavitation with time is observed. The results obtained by experiments and theoretical analysis are in a good agreement. A new micromixer based on nematic liquid crystals is proposed based on the cavitation and unsteady cavitation flows at very low Reynolds number.

Study on the Hydrodynamic Cavitation in Stokes Flows of Nematic Liquid Crystals in Microchannels

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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