In-air bubble phobicity and bubble philicity depending on the interfacial air cushion: toward bubbles manipulation using superhydrophilic substrates

Bubbles, a typical air-in-liquid system including both underwater and in-air bubbles, are important phenomena involved in our daily life and engineering fields. So far, the fundamental and dynamics for in-air bubbles largely remain unclear. Here, two typical in-air wetting states of the bubble are revealed: the bubble-phobic state in which a bubble with a perfect spherical shape stays on the liquid surface but without coalescence with the liquid; and the bubble-philic state in which a bubble on the liquid substrate gives a quasi-hemispherical shape. It is proposed that the bubble phobicity is a typical high-energy state where the air cushion at the bubble/liquid interface plays a crucial role in preventing their coalescence by generating an energy barrier. By applying an external force to overcome the energy barrier, the bubble will distort and finally be stabilized in a bubble-philic state. It is demonstrated that superhydrophilic substrates enable manipulating bubbles in air: a superhydrophilic surface with microscale hairy textures shows robust in-air repellence to the bubbles, while a hemispherical bubble is capable of transport directionally in air on a superhydrophilic conical fiber. Here, the concept of superwetting into air-in-air system (liquid films) is promoted, which may open a new perspective in surface and interface chemistry.