Session A08: Bubbles: Cavitation I

Drops inside elastic media: cavitation through diffusion

Plants have found an elegant way to combine fluid mechanics and elasticity to generate rapid motion. By making use of the elasticity of their tissue structure and the pressure differences in their internal liquid conducts, plants can easily achieve surprisingly fast movements, useful for instance in spore dispersion. 
In this work, we study this conversion from elastic to kinetic energy using a model experiment: an evaporating water drop trapped in the bulk of a poro-elastic gel (Polydimethylsiloxane). The droplet shrinks due to evaporation, which follows a slow diffusive-limited process and can be modeled accordingly. During the droplet shrinkage, the pressure in the liquid decreases as a consequence of the build up of elastic stress at its interface. This underpressure eventually leads to the birth of a cavitation bubble. Using high speed imaging, we access the dynamics of the bubble growth and subsequent oscillations. We show that these observations can be understood through a modified Rayleigh-Plesset equation to account for elasticity.   

Measurements of the temperature variations during the growth and collapse of cavitation bubbles

The present work focuses on the analysis of the extreme conditions encountered during the process of collapse of cavitation bubbles. The objective is to characterize the temperature variations inside the vapor/gas bubble, and also in the surrounding liquid. This information is especially relevant to address the issues of cavitation erosion and develop appropriate models that take into account the thermal effects in cavitation. The work is based on an experimental approach where temperature measurements are performed with cold wire thermometers. This technique is characterized by a very small time response, and can be applied to both phases during the growth and collapse of the bubbles. Temperature peaks have been measured between 50°C and 400°C, at the end of the collapse, depending on the bubble distance to the wall and bubble size. Large temperature gradients inside the bubble have also been detected.   

Direct visualization of temperature evolution of interfacial cavitation

Cavitation at the air/water interface, unlike the bulk cavitation embedded inside liquids, is produced by CO₂ laser impacting on water. By using thermal camera, direct visualization of temperature evolution of interfacial cavitation is achieved. Gaussian distribution is observed when none cavitation occurs due to the Gaussian distribution of laser intensity, while an interesting ring-shape distribution is observed with cavitation formation. Theoretically, we attribute the ring-shape temperature distribution to the coupling of heat transfer and liquid outflow driven by cavitation expansion at large Peclet number (Pe~10³) associated with the dominate convection. This novel temperature profiles is validated by our numerical simulation for the cavitation formation and heat transfer. Moreover, we identify that superheating is required for interfacial cavitation, and propose a classical nucleation model consistent with experiments excellently. These results might have implications in microfluidic manipulation, nanomaterial synthesis and medical treatment.   

Bubbly cavitating flow in a 200-kHz ultrasonic cleaning bath

Recent studies on ultrasonic cleaning suggest that acoustic cavitation bubbles play a dominant role in physical cleaning. Inside cleaning baths, standing-wave-like acoustic fields are formed due to repeated reflections of underwater ultrasound at the bath walls and the free surface. In these acoustic fields, however, acoustic intensity gradients can appear due to dissipative effects of the fluid and cavitation bubbles; in this case, cavitation bubbles can translate by the acoustic radiation force that appears under such acoustic intensity gradients, thereby entraining the surrounding liquid. Here, we experimentally examine bubbly cavitating flow that appears in an ultrasonic cleaning bath. We perform PIV measurement of streaming flow of both cavitation bubbles and water in a 200-kHz ultrasonic cleaning bath in order to quantify the liquid flow entrainment by the translating cavitation bubbles. We also perform a cleaning test using a glass slides on which small silica particles are spin-coated. SEM images of the cleaned samples show that the cleaned patterns agree with the direction of the bubbles' translation, supporting the fact that cavitation bubbles play a dominant contribution to particle removal.