Microbubble elevator induced buoyancy oscillations of reacting droplets.

Abstract

Autonomous droplet oscillations are intriguing and provide a strong inspiration for the development of active soft matter. However, they are often limited by their reliance on external gradients and slow transport. Here, we demonstrate a propulsion mechanism driven by confined interfacial reactions: the hydrogen microbubble elevator. We show that hydrogen evolution within a liquid organic hydrogen carrier (LOHC) droplet and the resulting bubble pinch off modulate the droplet's effective density, inducing periodic rising and sinking cycles in a stratified fluid. These droplets reach peak velocities of up to 25 mm/s over 2.5 cm trajectories - three orders of magnitude faster than typical Marangoni-driven bouncing - and sustain robust oscillatory motion for up to 25 minutes. The oscillation period and amplitude are programmable through the reaction kinetics, with motion persisting until the bubble-to-droplet size ratio exceeds a critical threshold. With a force balance model, we can quantitatively explain the experimental results and the interplay between gas evolution rates and hydrodynamic forces.