High pressure cells, filled with water, incorporate two parallel thin membranes which hold a layer of microbubbles tightly in place in order to reduce diffusion and movement of bubbles during analysis. (a) and (b) For acoustic experiments, the cell windows are made of PMMA, which is relatively transparent to sound at the frequencies tested, and a piston type ultrasound transducer is coupled through the top PMMA window. (c) and (d) For optical imaging, sapphire windows are used as they are stronger/thinner and clearer for better image quality, and a long working distance microscope objective views the microbubbles through the sapphire window.
Optical images of microbubbles as they underwent hydrostatic compression from 0 MPa to 13 MPa. Scale bars are 100 μm. Glass bubbles (a), with average diameter of 30 μm, showed no conformational change with increase in pressure, but were observed to break as pressures reached 2 MPa. At 13 MPa, there was still a significant population of bubbles left, while large glass fragments accumulated. (b) Hard shell polymer microbubbles, with average diameter of 40 μm, deformed by buckling under elevated pressure. At 2 MPa, almost all shells had fractured. (c) Surfactant bubbles, with average diameter of 50 μm, also deformed under elevated pressure, but instead of buckling, these surfactant bubbles gradually decreased in diameter as they were compressed, eventually irreversibly dissolving.
Acoustic reflections from three types of microbubbles, normalized for the maximum amplitude and the baseline of each sample. The plot shows the relative decay of the acoustic reflection as hydrostatic pressure was increased. Glass shelled microbubbles were the most resistant to pressure induced collapse, and surfactant shelled microbubbles were the most susceptible to hydrostatic pressure. The polymer shelled microbubbles were slightly more resistant to pressure than the surfactant microbubbles.
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