Interface evolution mechanism of dual-bubble coalescence driving micromotors in bulk phase
Authors: Wang Li-Na , Chen Li , Sheng Min-Jia , Wang Lei-Lei , Cui Hai-Hang , Zheng Xu , Huang Ming-Hua ,   School of Building Services Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China,   State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
Published: June 2023

<sec>Self-propelled micromotor serves as a bridge between the microfluidic environment and macroscopic control. It has broad application prospects in targeted drug delivery, biosensors, and other fields. The high driving speed of bubble micromotor is an irreplaceable advantage in practical applications. Bubble micromotor converts chemical energy in ambient solutions into mechanical energy through asymmetric surface catalytic reactions to drive its 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. They are released from one end of the microtubule into the solution and self driven by bubble rebound, with an energy conversion rate of <inline-formula><tex-math id="M1">\begin{document}$ \sim {10^{ - 10}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M1.png"/></alternatives></inline-formula>. The Janus microsphere motor near the gas-liquid interface utilizes the energy of the bubble coalesced with the interface to drive the microsphere, with an energy conversion rate of <inline-formula><tex-math id="M2">\begin{document}$ \sim {10^{ - 7}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M2.png"/></alternatives></inline-formula>. In sum, the tubular bubble motor is suitable for complex scenarios but has a low energy conversion rate. The Janus microsphere motor driven by bubbles has a high efficiency but is only suitable near the gas-liquid interface. This paper combines the advantages of driving tubular micromotors in bulk solution and Janus microsphere motors which can be driven efficiently by interface energy, proposes a new method of driving Janus microsphere motors by dual bubble coalescence.</sec><sec>In the experiment, a high-speed camera is used to record the ~100 μs of dual bubble coalescence and the process of driving micromotor. Then we investigate the initial kinetic energy conversion rate of micro motor driven by bubble coalescence. Three sets of different bubble/particle size ratios of <inline-formula><tex-math id="M3">\begin{document}${R_{\rm{b}}}/{R_{\rm{p}}} < 1$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M3.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M4">\begin{document}${R_{\rm{b}}}/{R_{\rm{p}}} \approx 1$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M4.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M5">\begin{document}${R_{\rm{b}}}/{R_{\rm{p}}} > 1$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230608_M5.png"/></alternatives></inline-formula> are adopted for their propulsion effects on microspheres. The initial kinetic energy conversion rate is defined to characterize the contribution of bubble coalescence process to microsphere driving.</sec><sec>After simulations with the pseudo potential lattice Boltzmann method, the mechanism of bubble coalescence driving the motion of microspheres is revealed. It is clarified that the interface oscillation caused by bubble coalescence is the main reason of 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 free surface. The research results reveal the details of bubble coalescence at different time periods, and provide the effects of factors such as bubble particle size ratio on microsphere displacement and initial kinetic energy conversion rate. Thus the efficient driving mechanism of dual bubble coalescence and release of surface energy are confirmed.</sec>