The dynamics of self-propelled Janus microspheres near obstacles with different geometries
Authors: Zhang Hong , Zong Yi-Wu , Yang Ming-Cheng , Zhao Kun ,   Key Laboratory of Systems Bioengineering (Ministry of Education), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China,   Beijing National Laboratory for Condensed Matter Physics and CAS Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Self-propelled particles exhibit interesting behavior when approaching boundaries or obstacles, which has been drawn a lot of attention due to its potential applications in areas of cargo delivery, sensing and environmental remediation. However, our understanding on the mechanism of how they interact with boundaries or obstacles is still limited. Here, using video particle-tracking microscopy, we experimentally studied the dynamics of self-propelled Janus microsphere driven by H<sub>2</sub>O<sub>2</sub> near obstacles. The Janus particles used are sulfuric polystyrene (PS) microspheres (hydrodynamic diameter is 3.2 μm) with only half surface being sputter-coated with a five-micron-thick platinum layer. Two different types of obstacles are used. One is cylindrical post and the other is PS microsphere. To understand the size effect of obstacles, cylindrical posts with three different diameters (3 μm, 10 μm and 20 μm), and PS microspheres with four different diameters (1.0 μm, 1.8 μm, 2.4 μm and 7.2 μm) are tested, respectively. The results show that when obstacles are larger than a critical size, the self-propelled Janus microspheres will be captured and orbit around them. The retention time and the orbiting speed of the Janus particles increase with the concentration of H<sub>2</sub>O<sub>2</sub>, as well as with the diameter of obstacles no matter whether cylindrical posts or PS microspheres are used as obstacles. However, we found that under the same concentration of H<sub>2</sub>O<sub>2</sub>, compared with the case of PS microspheres as obstacles, when Janus particles orbit around cylindrical posts, the retention time is larger and the average speed is smaller. These results indicate that the self-propelled behavior of Janus particles near obstacles is closely dependent on the geometrical properties of obstacles. Our results of Janus spheres are different from earlier work on Au-Pt Janus rods [Takagi D, Palacci J, Braunschweig A B, Shelley M J, Zhang J 2014 <i>Soft Matter</i> <b>10</b> 1784]. By comparing the speed of Janus particles before and after they are captured by spherical obstacles, for our case, the speed of Janus spheres is reduced, while for the case of Au-Pt rods, the speed of Au-Pt rods doesn’t change much. Such discrepancies may originate from different driven mechanisms in these two systems (electropheoresis mechanism for Au-Pt micro-rods and diffusiophoresis mechanism for PS-Pt Janus microspheres), which are then resulted in different flow fields and different distributions of catalytic solutions. But to test this hypothesis, further work is needed. Our study provides us a better understanding on the dynamic behavior of self-propelled particles near obstacles, which will be helpful for applications in, for example, designing micro-structures to guide the motion of self-propelled particles.