Bubble-in-Bubble envelope in liquid environments: A new fluidic configuration*

Fluidic interactions in two-phase flow refers to the interaction of two phases of fluid (gas, liquid and two immiscible liquids) within a flow system resulting in an exchange of mass, momentum and energy between them which is bound to influence the flow dynamics. Typical two-phase systems involve a bubble and a drop. In this context, three phenomena occur at the interface separating the phases, namely coalescence, spreading and encapsulation. Coalescence is the merging of two single bubbles/drops resulting in the formation of a larger bubble/drop. Spreading is the increase in the surface contact area by spreading of a bubble/drop on a fluid surface. Encapsulation occurs when one phase is entirely surrounded by the other phase resulting in the formation of a distinct boundary. Recently, studies on the new combinations of fluids namely hollow droplet and drop encapsulated in a bubble reported that these fascinated phenomena do occur and require a precise framework in understanding the dynamics. When one bubble of lower surface tension spreads onto the other bubble and entirely covers it, this phenomenon is known as bubble-in-bubble envelope. This possible configuration can be resulted when the surrounding liquid is of self-rewetting in nature and it adds another complexity to the system due to its non-monotonic dependence of surface tension with temperature. This spreading and wetting characteristics helps to understand the controlled bubble interactions, the mechanisms governing the bubble-in-bubble envelopment in a surrounding liquid, thereby analyzing its potential applications in various fields of science. This nature of fluid finds its applications in improving heat transfer rates, thermal energy storage devices using phase change materials, microfluidics and enhanced oil recovery.