Enhanced Brownian motion of the group self-propellant Janus particles based on dynamic light scattering technique

Abstract

Janus particles collectively refer to the non-uniform particles with different properties on two sides. Due to their asymmetry, asymmetric physical or chemical reactions shall be performed by the functionalized Janus particles by using “fuel” in the environment, thereby forming the gradient fields of different physical quantities, such as concentration, electric field or light intensity on both sides of the particles, so the autonomous motion shall be carried on by using the asymmetrical momentum distribution formed by the gradient. At present, the main methods to study the self-propulsion of Janus particles include single particle tracking (SPT) and dynamic light scattering (DLS). In this paper, the movement behavior of Pt-SiO2 Janus particles with a nominal diameter of 1 μm in pure water and in H2O2 solutions was analyzed, whose results were compared with that of the SPT. The experimental results show that the movement behavior of group Janus particles in H2O2 solution showed the enhanced Brownian motion. With the increase of the concentration of H2O2 solution, the effective diffusion coefficient of group Janus particles was increased by using the DLS method, which was less than the effective diffusion coefficient of single Janus particles measured by using the SPT method. In addition, in this paper, according to the inverse calculation of the self-correlation function measured by DLS, the mean square displacement of group Janus particles in the extremely short time interval was obtained. The results showed that at the same observation time interval, the mean square displacement shall be increased with the density enhancement of the H2O2 solutions, while in the H2O2 solutions with the same density, the mean square displacement shall be increased with the increase of the observation time interval. Finally, the characteristics and differences of SPT and DLS methods to study the Janus particle motion behavior were analyzed. It is shown that the observation interval is the key factor among the measurement of DLS and SPT. By SPT method, the movement characteristics of a single Janus particle at different stages can be obtained by recording the trajectory of Janus particle, while it is difficult for SPT to record the trajectory of Janus particle for small particles ( d In this case, SPT is not suitable for the study of Janus particles, while DLS is an effectively method to compensate this shortcoming of the SPT method. By DLS, the motion characteristics of Janus particles can be obtained by analyzing the light intensity fluctuations caused by particles motion, and the applicable range is 2 nm −3 μm, but he results of DLS are limited to the longest effective observation time (the time for the autocorrelation coefficient decreasing from 1 to 0). When the longest effective observation time is less than the characteristic time, the motion characteristics of the “directional driven diffusion” stage can be obtained, while when the longest effective observation time is greater than the characteristic time, the motion characteristics of the “Brownian-like motion” stage can be obtained. Due to the longest effective observation time, only the Janus particles’ movement characteristics at a certain stage in the above two phases can be obtain by DLS. DLS and SPT are two important methods to study the self-propelling movement of Janus particles. Although their mechanisms are different, the movement behavior of Janus particle can be studied more thoroughly by these two methods with supplementation mutually.