Experimental and numerical study on microbubble coalescence dynamics

Abstract

This thesis work aims to make a better knowledge on the insights of physics on microbubble coalescence process, using experimental and numerical approaches. The neck growth and bubble surface wave propagation at the early stage of coalescence, merging preference, as well as a reaction-channel modified microfluidic gas generator are presented in the thesis.

Coalescence of unequal-sized microbubbles captive on solid substrate was observed from cross-section view using synchrotron high speed imaging technique and a mi- crofluidic gas generation device. The bridging neck growth and surface wave propaga- tion at the early stage of coalescence were investigated by experimental and numerical methods. The results show that theoretical half power law of neck growth rate is still valid when viscous effect is neglected. However, the inertial-capillary time scale is based on the radius of smaller parent microbubble. The surface wave propagation rate is proportional to the inertial-capillary time scale, which is based on the radius of larger parent microbubble of a coalescence pair.

Meanwhile, the relationship of preference distance and size inequality microbub- bles were studied using the same micrfluidic gas generator and observation facilities. The size inequality of parent microbubbles would affect the preference distance of merged bubble in between. The merged bubble gets less closer to the larger parent bubble with an exponent of 1.82 as a reference, which largely affected by shear stress begotten on the solid interface. To express this phenomenon distinguished with free merging bubble pair, we propose the wall shear stress hinders the process of that parent bubbles move towards to each other during coalescence Our hypothesis was confirmed by identical coalescence simulation via ANSYS Fluent.

To address the multiple measurement, utilization of Java based photography pro- cessing software ImageJ was applied as a key point to the thesis work. To acquire more microbubble coalescence cases on experiment for study, we enhanced the perfor- mance of microfluidic gas generator with reaction channel optimization. An optimized design on increasing the number of parallel reaction channel from single to triple, was applied to obtain a higher gas generation rate. Also the gas vent shape was modified from triangle to rectangle to provide more information on reaction channel optimiza- tion. The gas generation rate and H2O2 conversion rate were provided to further discuss.