Janus球形微马达的自驱动机理研究：自扩散泳动与微气泡推进

The mechanisms of the self-propelled spherical Janus micromotor: Self-diffusiophoresis and microbubble propulsion

微纳马达的研究是一个多学科交叉的新兴领域.其中,Janus微马达利用自身两面异性导致的局部梯度场而产生自驱动现象,引起了学界的普遍关注.本文主要基于目前已开展的工作并借鉴国内外的一些最新成果,以期对Janus球形微马达的物理特征给出全面的描述.针对铂-二氧化硅(Pt-SiO_2)型Janus微球在过氧化氢溶液中发生的自驱动,通过实验和数值模拟分析了其两种自驱动形式(自扩散泳动和微气泡推进)的物理机制和运动特征.直径小于5mm的Janus微球发生自扩散泳动,通过无量纲均方位移随时间的变化揭示了微球平动经历由纯布朗运动、扩散泳动到类布朗运动的过程,给出了特征时间及不同阶段的主导物理因素.位移概率分布可以表征非高斯性,并分析布朗力矩主导的旋转特性并讨论壁面限制及剪切流的影响.直径20~50mm微球可观测到微气泡推进,微球位移揭示了随气泡尺寸增长,微球经过自扩散泳、气泡生长和气泡溃灭推进3个阶段组成的周期运动.Rayleigh-Plesset(R-P)方程则揭示了依次由黏性力、表面张力及气泡周围流体压力控制下的气泡生长标度率.本文还从应用角度介绍了交变电场下,利用介电泳操控Janus微球的微穿梭输运(microshuttle)技术,并讨论了自扩散泳与自电泳差别及微气泡推进型微马达效率提高等问题.

Micro/nano-motor is a new and interdisciplinary field. The Janus micromotor that utilizes the heterogeneity between its two hemispheres to generate self-propulsion has attracted great research interests. This paper focuses on the selfpropulsion of platinum-silica （Pt-SiO2） Janus microsphere suspended in hydrogen peroxide solution （H202）. We systematically review the experimental and numerical results of the two instinct mechanism： Self-diffusiophoresis and microbubble propulsion. The motion of Janus microsphere whose size is smaller than 5 μm is dominated by self- diffusiophoresis. In this regime, the Janus microsphere experiences simple Brownian motion at short times, self- diffusiophoresis at intermedia times and Brownian like motion at long times as indicated by the time variation of dimensionless mean square displacement. The non-Gaussianity due to propulsion is revealed by the probability distribution of displacement. The Janus microsphere＇s rotation is found to be governed by Brownian torque, and the influences of solid wall and shear flow are discussed. When the Janus microsphere＇s size is about 20-50 μm, the microbubble propulsion is observed. Three typical stages, self-diffusiophoresis, microbubble growth and bubble collapse, are identified based on the Janus microsphere＇s movement. Three different scaling laws are also established to describe that the microbubble growth in one period is respectively governed by viscous force, capillary force, and ambient fluid pressure, as bubble size increass. It is the first time to observe that a microjet occurs after the microbubble＇s asymmetric collapse, and it will prople the Janus microsphere with a speed up to 0.1 m/s. In view of application, this paper introduces a new microshuttle technique which uses dieletrophoresis under AC electric field to manipulate Janus microsphere＇s motion, and disscuses some important issues like the difference between self-electrophoresis and self-diffusiophoresis and improving the efficiency of Janus micromotor. A thorough understanding of the physics of Janus microsphere＇s self-propulsion will shed light on designing better micromotors.