Abstract
The initial motivation for the study was to gain deeper understanding into the background of emulsion preparation by ultrasound (cavitation). In our previous work (Perdih et al., 2019) we observed rich phenomena occurring near the liquid-liquid interface which was exposed to ultrasonic cavitation. Although numerous studies of bubble dynamics in different environments (presence of free surface, solid body, shear flow and even variable gravity field) exist, one can find almost no reports on the interaction of a bubble with a liquid-liquid interface. In the present work we conducted a number of experiments where single cavitation bubble dynamics was observed on each side of the oil-water interface. These were accompanied by corresponding simulations. We investigated the details of bubble interface interaction (deformation, penetration). As predicted, by the anisotropy parameter the bubble always jets toward the interface if it grows in the lighter liquid and correspondingly away from the interface if it is initiated inside the denser liquid. We extended the analysis to the relationships of various bubble characteristics and the anisotropy parameter.Finally, based on the present and our previous study (Perdih et al., 2019), we offer new insights into the physics of ultrasonic emulsification process.
Highlights
Cavitation is a physical phenomenon accompanied by chemical processes that can occur in liquids
water in oil (W/O) droplets are later separated from the bulk oil phase and undergo further break down under the influence of ultrasonic waves, and after all these steps are repeated a few times a true oil in water (O/W) emulsion develops
In order to study cavitation bubble dynamics in the vicinity of water-oil interface, experimental setup was prepared, which enabled high-speed recording of this phenomenon for two cases: in the first case the cavitation bubble was produced in water and in the second case the bubble was produced in oil
Summary
Cavitation is a physical phenomenon accompanied by chemical processes that can occur in liquids. The phenomenon encompasses the growth and collapse of vaporous or gaseous cavities in a liquid as a consequence of the local pressure drop and its recovery, respectively. Despite new insight of the process was gained some open questions remained, which we were unable to answer when observing multiple acoustically generated bubbles. One such question is the physics behind the first step – formation of W/O emulsion – is it really the jetting of the bubble into the oil phase or is the process governed by the acoustic streaming [15,16]?
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