Thermophoretic microswimmers: Interplay of phoresis, geometry and hydrodynamics

• Main Author: Roca Bonet, Sergi
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2021
• Subjects: ddc:530; Blue whale
• Online Access: https://kups.ub.uni-koeln.de/53829/; https://kups.ub.uni-koeln.de/53829/1/dissertation_roca_bonet_sergi.pdf

The term swimmer refers to biological or artificial structures that are capable of self-propel by drawing energy from the surrounding environment. The typical size of a swimmer ranges orders of magnitude, from the macroscopic world of a blue whale in the ocean, to the microscopic of a bacteria. Microscopic swimmers, or microswimmers, live in an environment where the viscosity of the fluid dominates their motion, suppressing the inertia that we are so familiar with. Phoresis refers to the physical mechanism in which colloidal particles migrate due to the presence of a solvent gradient, such as thermal, chemical or magnetic. Phoretic colloids have recently emerged as a promising avenue for the design of artificial microswimmers. Thermophoretic colloids are partially coated with a high heat conductivity material, such as gold, which heats faster under laser illumination, creating then a local thermal gradient. The non-coated surface reacts to the difference in temperatures and displays the thermophoretic response to it, driving the motion of the swimmer. The motion of colloids immersed in fluid produce long-ranged flows, which can infere in the motion of further colloids. These fluid-mediated interactions are known as hydrodynamic interactions. Since the colloid is found in solvent, phoresis and propulsion are linked to a hydrodynamic flow field. These fluid-mediated interactions are deeply influenced by the geometry and surface properties of the colloid, and play a major role in the interaction between swimmers. This dissertation addresses the study of self-thermophoretic dimeric and trimeric colloidal swimmers by means of mesoscale computer simulations. In order to precisely understand the debated role of hydrodynamic interactions in these systems, two computational approaches are hereby presented. We use a full hydrodynamic approach, which includes thermophoresis, and a second method which neglects fluid-mediated effects while accounting for thermophoretic interactions. Hydrodynamic simulations are performed via ...