Bubble Dynamics
Acoustic cavitation generates gas bubbles that expand and contract under the influence of an alternating pressure field. Rectified diffusion describes the net growth of these bubbles over multiple cycles due to an asymmetry in mass transport. A bubble gains more gas during its expansion phase than it loses during the contraction phase, because the bubble surface area is larger when the radius increases.
This difference creates a time-averaged flux of dissolved gas into the bubble interior.
Gas Transport
Diffusion gradients exist between the liquid medium and the bubble boundary during every oscillation phase. Increased surface area during the expansion phase promotes gas diffusion into the cavity from the surrounding liquid. Contraction reduces the surface area, which decreases the outward flux of gas.
Chemical potential gradients drive this concentration change near the liquid-gas interface.
Threshold Pressure
Pressure amplitudes above a specific limit are required to sustain this growth against surface tension forces. Bubbles respond to these ultrasound frequencies by accumulating gas until they reach a resonant radius. Equilibrium occurs when the gas pressure inside the bubble matches the internal pressure exerted by the surrounding liquid and surface tension.
Instability sets in once the bubble exceeds its resonant size, leading to rapid expansion and potential collapse.
Stability Limit
Rectified diffusion stops if the bubble radius moves too far from the resonant frequency of the applied acoustic field. Higher ambient pressure inhibits this process by increasing the liquid density and the threshold required for gas entry. Small fluctuations in temperature change the solubility of the gas, which influences the rate at which equilibrium is reached.
Gas diffusion rates determine the maximum life of a cavitating bubble before it undergoes inertial collapse.
