Acoustically driven vertical coalescence of liquid marbles

Liquid Marbles (LMs) are liquid droplets coated with a shell of micro- or nanoparticles. These versatile structures hold promise for diverse applications, including microreactors, gas sensors, and fluid pumps. Consequently, various strategies have been explored to manipulate LMs in terms of transportation, coalescence, and separation. Among these strategies, acoustic levitation presents a unique opportunity for surface-free manipulation of LMs. This paper introduces a simple and cost-effective method to achieve the head-on coalescence of LMs using acoustic levitation. Coalescence was accomplished by superimposing a control signal on continuous acoustic fields to facilitate collision and merging. Five distinct outcomes were identified, which depended on the ability of the acoustic levitator to hold and merge the resultant liquid marble. The results revealed a significant influence of acoustic pressure in the major outcomes of complete coalescence and partial coalescence after the collision. Our experimental results provide deeper physical insights into contactless head-on LM coalescence while demonstrating its feasibility as an initial step toward advanced LM manipulation techniques using acoustic levitation.