Experiment And Confinement Simulation Research For The Motion Of Self-propelled Janus Particles

Janus particles are composed of two parts which have different physical orchemical properties. With the different properties of two parts, there will create aconcentration gradient, temperature gradient or light intensity gradient around it andparticles will swim. This phenomenon is called self-propelled motion of Janus particles.The development of chemical synthesis method makes it possible to prepare differentkinds of Janus particle. The self-propelled motion of Janus particles has importantapplications in the fields of micro-system, biology, environment and medicine.This paper studies the motion characteristics of Janus particles from the aspect ofexperiment firstly. The autonomous movements of Janus particles immersed into waterand the H2O2solution with different concentrations (2.5%,5%,10%) are investigatedthrough Micro-PIV platform. The experiment results show that the values of effectivediffusion coefficient Deffof Janus particles are associated with the statistic time interval.In a short time interval, Deffincreases linearly as the time interval increases; however,for a long enough time interval, Defftends to a steady constant, which is about ten toone hundred times higher than that in pure water with the same particle size. Thisbehaviour is quiet different from the the classical Brownian motion, where diffusioncoefficient is unchanged. The characteristic time τR, from which the tendency of Deffturns to change from linear increase to steady platform, increases with the increase ofparticle size, but, the dependence of τRon the concentration of H2O2solution is not soobvious. Furthermore, the mean squared displacement L2and the directional angle γof Janus particles are investigated to obtain their statistic law. It is clear to show theconversion of Janus particles’ movement from self-propulsion to Brownian-like motion.In the graphs about the probability distribution of directional angle, the characteristic time τRcan be decided quantitatively.Through observing the experiment images, it is founded that for2m Janusparticles, the self-propelled motion has the feature of quasi-2D. In other words, particlesmove translationally within a plane near the substrate under the combination of gravity,diffusiophoretic force and hydrodynamic force and do not jump. On this basis, thecontinuum model based on the reference frame is adopted to simulate the self-propelledmotion of Janus particles near the wall by using COMSOL Multiphysics software. Thissimulation provides one possible mechanism to explain the self-propelled phenomenonfrom the perspective of diffusiophoresis force. This paper also studies the confinementeffect of Janus particles travelling on the substrate. Then, this paper gets thedistributions of the flow field and the concentration field and hence to evaluate thedifferent forces. According to the force balance principle, the gaps (2.1m,3m and3.2m) which corresponds to the H2O2solution whose concentrations are2.5%,5%and10%were calculated. Meanwhile, the fitting coefficients (147.9,51.8and27.2) tomatch the experimental and numerical results were suggested. This result may help usto get a better understanding of self-propulsion and is also beneficial for designing aDFP-based micro-device.