不同粒径Janus微球的自驱动：实验及驱动机制对比

Two differet self-propulsion types of Janus microspheres:from the comparative experiments and driving mechanisms

Janus颗粒是两侧具有不同性质的非均质颗粒的统称.利用Pt-SiO2型Janus微球的Pt一侧催化分解H_2O_2溶液,可以使得颗粒自驱运动.本文首先从实验角度对比了不同粒径、相同浓度下的两类自驱动现象,结果表明在d～O(1μm)时为扩散泳驱动,d～O(10μm)为气泡驱动,二者在运动轨迹、驱动速度上存在很大差异.随后,分析了主导的力学因素,并根据简化后的受力平衡关系建立了多场耦合的数值模型,重点研究了大粒径下浓度及速度场的分布,据此解释了气泡产生的位置及尺寸,并推断壁面滑移系数是数值模型中的关键匹配参数,及其在不同粒径下变化的可能机理.这一研究将为深入理解自驱动的机理及提高自驱动器件的驱动能力提供理论基础.

A Janus particle is a general term for a non-uniform particle that has different properties on different sides of particle.For a Pt-SiO_2 type of Janus microsphere,Pt side serves as the catalysis surface to decompose H_2O_2 solution,leading to the self-propulsion motion of particle.In this paper,the relevant experimental phenomena in two driven modes are compared first.The results show that under the same concentration of solution,the microsphere with a diameter of about 1 μm experiences self- diffusiophoresis propulsion;whereas,the one with an about 20 μm diameter experiences bubble self-propulsion.Significant differences in motional trajectory and propulsion velocity are found between them.Then,the dominated physical factors are analyzed and the multi-field coupling numerical model is constructed based on the simplified force balance analysis.Subsequently,the velocity field distribution and O_2 concentration distribution around Janus microsphere are also studied.According to these studies,we explain the position and size of the bubble generated.Further more,we infer that the wall slip coefficient is a key matching parameter in the numerical model,and two slip coefficients with a difference of an order of magnitude are given corresponding to the two types of self-propulsion modes.Then we explain the possible mechanism for the changes of wall slip coefficient under different particle sizes.The present study is beneficial to the in-depth exploration of the self-propulsion mechanism and also provides the theoretical foundation for improving the performance of self-propellant device.