Experimental analysis of number concentration and disruption rate of two-dimensionally distributed microcapsules

Abstract

Disruption of gas-filled microcapsules under pulsed ultrasound irradiation is experimentally studied. The goal of this study is to evaluate the disruption rate of the polymer-shelled microcapsules as a function of the mean number concentration of the microcapsule population. In the present study, we defined the disruption of a microcapsule as decapsulation of the shell layer followed by exposure of an internal gas bubble, growing to visually detectable size through mass diffusion. An experimental setup has been designed to sonicate two-dimensionally dispersed microcapsules in a planar microchamber. Pulsed ultrasound at 1 MHz is irradiated normal to the chamber from a planar ultrasound transducer. Pure water and glycerol aqueous solution of 30 wt. % were used as the dispersion media. Image processing of the recorded microscopic images yields capsule size distribution and local concentration distribution defined as an inverse of the occupied area of the capsule position (Voronoi area). The mean number concentration is controlled over a wide range from 5 to 200 capsules/mm², while the mean capsule diameter is controlled between 25 and 40 μm regardless of the concentration. The disruption rate is evaluated comparing the number of disrupted microcapsules (gas bubbles) and the total number of microcapsules in the target area of the ultrasound irradiation. The disruption rate turned out to decrease with the mean concentration and is inversely proportional to the mean concentration for the particular case of the two-dimensional spatial distribution.