自驱动Janus微球近壁运动特性实验与数值模拟研究

自驱动Janus微球是形状规则但表面构成不同的特殊活性颗粒. 针对微米级Pt-SiO₂型Janus 微球近壁面自驱动现象, 实验测得了微球的自驱动速度V_Janus, 并观察到微球运动过程中与垂直方向存在一偏转仰角ψ, 且ψ角随H₂O₂溶液浓度的增大呈减小趋势. 在此基础上, 建立自驱动Janus微球的数值模型, 通过模拟得到了微球在不同浓度H₂O₂溶液中的偏转仰角ψ及距底面的高度δ, 模拟与实验一致. 利用这些数据进一步讨论了壁面效应对微球旋转特征时间τ_R的影响. 这一工作对于理解Janus 微球的运动机理及发展相关应用具有重要意义.

Experiment and numerical study on the characteristics of self-propellant Janus microspheres near the wall

Self-propellant Janus microsphere is a special class of active particles with a regular shape and irregular surface characteristic. With the self-propulsion of 2 μm diameter Pt-SiO₂ Janus microsphere near the wall, we have measured the relationship of self-propellant velocity V_Janus versus the observed time Δt_obs. A diffusiophoretic force-dominated motion, which can be deemed as a quasi-1 D motion with the characteristics of both force free and torque free, is distinguished from the entire motion process. At the same time, it is also observed that the Janus microsphere is deflected about the vertical direction with an angle ψ. The deflection angle ψ is found to decrease with the increase of H₂O₂ concentration in the solution. For the 2.5%-10% H₂O₂ solution in this experiment, the angle ψ ranges from 20° to 7° approximately. A numerical model, involving viscous force, diffusiophoretic force and the effective gravity, is created with a reference frame, this quasi-1 D self-propellant motion can be solved to satisfy the conditions of the force and torque balance simultaneously. We have studied the changes of angle ψ and separation distance δ of the microsphere from the substrate under different conditions, including the concentrations of H₂O₂ solution, the material density, and the diameter of the microsphere. For the self-propulsion velocity V_Janus and the deflection angle ψ, numerical results show good agreement with the published experimental observation results. Moreover, it is found that the lower density or the smaller diameter of the microsphere will generate the smaller distance δ, while the higher concentration of H₂O₂ in the solution will result in a larger distance δ. The predicted δ is 2-8 μm. With the obtained data, we further discuss the effect of near wall on the characteristic time τ_R of rotational diffusion of the Janus microsphere. Because the predicted values of δ are relative high, the near wall effect can be neglected, indicating that this effect should not be a significant factor to cause a big discrepancy of τ_R in different references. The present work will be beneficial to the understanding of the mechanism of self-propulsion and the development in its potential applications.