Generation of microbubbles at high gas concentrations via cavitation

A novel method of continuous microbubble generation is proposed in which an air–water mixture is pressurized in a chamber (up to 47 atm), leading to complete gas dissolution, before being expelled through a restriction to induce bubble nucleation. This approach differs from existing methods for small-scale bubble generation due to the high pressure gas–liquid mixture, nucleation approach, and large gas concentrations, which range from saturation ratios of 2.85 to 9.56 at normal temperature and pressure. The flow of bubbles produced by the system is characterized through optical microscopy to determine the influence of various operating parameters on the volumetric bubble size distributions (VBSDs) at a fixed flow rate. In particular, the gas concentration, compressor pressure, restriction diameter, and restriction length are varied for two classes of restrictions: capillaries with large length-to-diameter ratios, and orifices with comparatively low ratios. VBSDs are modestly polydisperse under all conditions, but with tunable distributions in the 10–70 μm diameter range. The VBSDs for each configuration collapse when non-dimensionalized by the Sauter mean diameter, which provides an opportunity to describe bubble size distributions using a predictor model for this reduced order quantity. The coefficients from the predictor model reveal a low sensitivity of the Sauter mean diameter to the gas concentration and a reduction of bubble diameters when the residence time of solution in the restriction is decreased. Analysis of the technique of bubble generation yields an energy balance to quantify the relative energy ratio of the bubble generation process, which is presently less than 0.2%. Insight from the energy balance suggests that the process could be optimized to generate smaller bubbles.