Dynamics of Micro-Particles in Complex Environment
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Micro-particles are ubiquitous in microsystems. The effective manipulation of micro-particles is often crucial for achieving the desired functionality of microsystems and requires a fundamental understanding of the particle dynamics. In this dissertation, the dynamics of two types of micro-particles, Janus catalytic micromotors (JCMs) and magnetic clusters, in complex environment are studied using numerical simulations. The self-diffusiophoresis of JCMs in a confined environment is studied first. Overall, the translocation of a JCM through a short pore is slowed down by pore walls, although the slowdown is far weaker than the transport of particles through similar pores driven by other mechanisms. A JCM entering a pore with its axis not aligned with the pore axis can execute a self-alignment process and similar phenomenon is found for JCMs already inside the pore. Both hydrodynamic effect and 'chemical effect', i.e., the modification of the concentration of chemical species around JCMs by walls and other JCMs, play a key role in the observed dynamics of JCMs in confined and crowded environment. The dynamics of bubbles and JCMs in liquid films covering solid substrates are studied next. A simple criterion for the formation of bubbles on isolated JCMs is developed and validated. The anomalous bubble growth law (r~t^0.7) is rationalized by considering the relative motion of growing bubbles and their surrounding JCMs. The experimentally observed ultra-fast collapse of bubbles is attributed to the coalescence of the bubble with the liquid film-air interface. It is shown that the collective motion of JCMs toward a bubble growing on a solid substrate is caused by the evaporation-induced Marangoni flow near the bubble. The actuation of magnetic clusters using non-uniform alternating magnetic fields is studied next. It is discovered that the clusters' clockwise, out-of-plane rotation is a synergistic effect of the magnetophoresis force, the externally imposed magnetic torque and the hydrodynamic interactions between the cluster and the substrate. Such a rotation enables the cluster to move as a surface walker and leads to unique dynamics, e.g., the cluster moves away from the magnetic source and its trajectory exhibits a periodic fluctuation with a frequency twice of the field frequency.
- Doctoral Dissertations