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 VJanus versus the observed time∆tobs. 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 φ. The deflection angle φ 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 φ 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 angleφand separation distanceδ 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 VJanus and the deflection angle φ, 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 distanceδ, while the higher concentration of H2O2 in the solution will result in a larger distanceδ. The predictedδ is 2–8 µm. With the obtained data, we further discuss the effect of near wall on the characteristic timeτR of rotational diffusion of the Janus microsphere. Because the predicted values ofδ 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 τR 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.
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