Advances in antibubble formation and potential applications.

Antibubbles are unusual physical objects consisting of a liquid core(s) surrounded by a thin air film/shell while in a bulk liquid. Antibubbles carry two air-liquid interfaces, i.e., one with the inner liquid and the other with the outer liquid. The distinct structure of antibubbles makes them quite attractive for drug and therapeutic delivery, although their potential applications have not been realized so far. The major challenge in this regard is a short-lived span of antibubbles, which is usually in the order of a few minutes to a few hours based on the stabilization mechanism used. We present a critical overview of different techniques that can be used to generate antibubbles. This includes a more commonly applied conventional approach in which the air-film is created through surface entrapment when a liquid jet/drop falls on a bulk liquid. The other available options rely on entirely different mechanisms for antibubble formation, for instance, through drop encapsulation by a submerged air bubble, or through evaporation/sublimation of volatile oil from a W/O/W double emulsion. Furthermore, the mechanisms of antibubble formation and collapse, and the factors affecting their stability have been discussed explicitly; and wherever required, the concept is correlated to other allied physical objects such as bubbles, liquid marbles, etc. Finally, the potential applications, research gaps in the existing knowledge, and some directions for future research are provided towards the end of this article.