Study on the mechanism of the interface evolution of dual-bubble coalescence driving micromotors in bulk phase

Abstract

Self-propelled micromotors serve as a bridge between the microfluidic environments and macroscopic control. They have broad application prospects in targeted drug delivery, biosensors, and other fields. The high driving speed of bubble micromotors is an irreplaceable advantage in practical applications. Bubble micromotors convert chemical energy in ambient solutions into mechanical energy through asymmetric surface catalytic reactions to drive their own motion. The energy conversion rate of bubble driving is used as an indicator to evaluate the driving force. The Pt catalytic layer of a tubular micromotor is located on the inner wall of the microtube. Bubbles form inside the tube. It is released from one end of the microtubule into the solution and self driven by bubble rebound, with an energy conversion rate of ~10-10. The Janus microsphere motor near the gas-liquid interface utilizes the energy of the bubble coalesced with the interface to drive the microsphere, with an energy conversion rate of ~10-7. In sum, the tubular bubble motor is suitable for complex scenarios but has low energy conversion rate. The Janus microsphere motor driven by bubbles has high efficiency but is only suitable near the gas-liquid interface. This paper combines the advantages of driving tubular micromotors in bulk solution and Janus microsphere motors utilizing interface energy to efficiently drive, proposing a new method of dual bubble coalescence and driving Janus microsphere motors. In the experiment, a high-speed camera was used to record the ~100μs of dual bubble coalescence and the process of driving micromotors. Then we investigates the initial kinetic energy conversion rate of micro motors driven by bubble coalescence. Three sets of different bubble/particle size ratios of Rb/Rp<1, Rb/Rp≈1, Rb/Rp>1 were presented for their propulsion effects on microspheres. The initial kinetic energy conversion rate was defined to characterize the contribution of bubble coalescence process to microsphere driving. After simulations with the pseudo potential lattice Boltzmann method, the mechanism of bubble coalescence driving the motion of microspheres was revealed. It is clarified that the interface oscillation caused by bubble coalescence is the main reason driving the micromotor, and its energy conversion rate is between the rebound driving of the tubular micromotor and the one-bubble coalescence driving with the freesurface. The research results revealed the details of bubble coalescence at different time periods, and provided the effects of factors such as bubble particle size ratio on microsphere displacement and initial kinetic energy conversion rate. It confirmed the efficient driving mechanism of dual bubble coalescence and release of surface energy.