Airborne liquid marble: Evaporation dynamics of liquid marble in acoustic levitation

Abstract

Liquid marbles (LMs), droplets encapsulated by hydrophobic particles, allow for non-wetting manipulation and containerless handling, making them well-suited for applications such as microreactors. When integrated with acoustic levitation, the LMs serve as contact-free reaction platforms. However, the complex behaviours associated with acoustic fields, including deformation, internal flow, and evaporation, remain insufficiently understood. This study investigates the evaporation dynamics of acoustically levitated LMs. Simultaneous visualisation of the droplet morphology and surrounding vapour concentration fields was achieved using backlighting and the Background-oriented Schlieren (BOS) method. At intermediate relative humidity (RH = 40%), the evaporation rates and vapour distributions of LMs closely resembled those of pure water droplets, indicating that the particle shell exerts minimal influence under these conditions. Conversely, significant differences were observed at high- and low-humidity, attributable to the evaporation resistance and interfacial properties introduced by the hydrophobic particle layers. Furthermore, although pure water droplets maintained a quasi-spherical shape, LMs demonstrated progressive flattening over time, suggesting mechanical constraints imposed by the particle shell. These findings provide new insights into the coupled evaporation and deformation behaviours of LMs in acoustic fields, informing the design of contamination-free microfluidic and microreactor systems.