Description
Antibubbles are fluid objects consisting of a liquid droplet encapsulated by a thin gaseous shell and fully immersed in a surrounding liquid. Their stability is typically limited by gas drainage and shell rupture, making them short-lived under terrestrial conditions. When the encapsulated droplet is composed of a volatile liquid and the surrounding medium is superheated, evaporation-driven gas production can inflate and sustain the shell, providing an alternative mechanism to delay antibubble collapse. This configuration can be seen as an encapsulated Leidenfrost droplet. However, in antibubbles, the thermalization of the initially cool inner droplet limits shell expansion, with evaporation rates highly sensitive to vapor shell thickness. Here, we report for the first time the formation and evolution of a thermal antibubble under microgravity conditions. The absence of gravity-driven drainage dramatically enhances antibubble stability, allowing us to isolate and investigate evaporation processes limited primarily by heat transfer. This unique environment enables a comparison with classical Leidenfrost systems while introducing new physical mechanisms specific to encapsulated geometries. Experiments are conducted in a temperature-controlled chamber filled with silicone oil heated to 120°C. Volatile hydrofluoroether (HFE) droplets, with a boiling temperature of 56°C, are injected into the superheated liquid. Upon immersion, rapid evaporation at the droplet interface generates a self-sustained gas shell, leading to the formation of a thermal antibubble. In microgravity, this system can be viewed as a drainage-free, encapsulated Leidenfrost droplet, where shell dynamics are governed by heat and mass transfer.
High-speed imaging is used to resolve the temporal evolution of both gas bubbles and antibubbles. In the absence of drainage, thermal antibubbles exhibit significantly increased lifetimes, exceeding 2s on average—approximately an order of magnitude longer than their terrestrial counterparts. Analysis of the antibubble volume evolution reveals two distinct regimes controlled by the evaporation timescale. An initial growth phase is driven by evaporation of the inner droplet, followed at longer times by a decrease in antibubble volume.
To explain the evaporation-driven growth regime, we develop a simplified one-dimensional, time-dependent heat transfer model. Contrary to intuitive expectations, convective heat transfer within the gas shell plays a dominant role, enhancing thermal transport despite the confined geometry. This mechanism accounts for the observed evaporation rates and volume increase during the early stages of antibubble evolution.
The subsequent volume decrease is elucidated by examining gas bubble dynamics and the associated mass transfer processes at the oil–vapor interface. Because HFE is partially miscible in silicone oil, vapor saturation within the gas shell enables the dissolution of HFE vapor into the surrounding superheated liquid, thus explaining the long-timescale shrinkage of the antibubble. Incorporating this effect allows us to fully describe the observed volume evolution.
What initially appeared as a model system dominated by diffusive heat transfer thus reveals a richer interplay between evaporation, convective heat transfer, and mass transfer in microgravity. These results provide new insights into evaporation-driven phase-change processes in encapsulated systems and highlight the importance of microgravity experiments for uncovering transport mechanisms usually neglected under normal gravity.