Physics Department - Direct Numerical Simulations of Active Particles in Complex Environments 

Abstract
My research focuses on the computational simulation of complex fluids, particularly colloidal dispersions. Colloidal dispersions are systems in which fine particles are suspended in a host solvent; their macroscopic properties evolve from liquid-like to gel-like as the particle volume fraction increases. In many cases, the dispersed particles are charged, and the solvent is an electrolyte containing ions. As a result, hydrodynamic interactions between particles play a crucial role, making their accurate treatment a central challenge.

Traditional simulation methods for particle dispersions, such as Brownian Dynamics and Stokesian Dynamics, do not explicitly resolve fluid motion and have therefore been widely used due to their computational efficiency. However, extending these approaches to non-Newtonian fluids is inherently difficult. Direct numerical simulation (DNS), which solves the fluid dynamics fully coupled with particle motion, overcomes this limitation and provides a powerful framework for studying complex dispersions.

In this presentation, I will introduce recent studies carried out using our DNS approach for particle dispersions, focusing on the following topics:

1. Motion of a microswimmer in a Newtonian fluid
   - N. Oyama, J. J. Molina, and R. Yamamoto, “Purely hydrodynamic origin for swarming of swimming particles,” Phys. Rev. E 93, 043114, 2016.
2. Motion of a microswimmer in a viscoelastic fluid
   - T. Kobayashi, J. J. Molina, and R. Yamamoto, “Propulsion of a chiral swimmer in viscoelastic fluids,” Phys. Rev. Res. 6, 033304, 2024.
3. Motion of a microswimmer in a liquid–liquid phase-separated fluid
   - C. Feng, J. J. Molina, M. S. Turner, and R. Yamamoto, “Dynamics of microswimmers near a liquid–liquid interface with viscosity difference,” Phys. Fluids 35, 051903, 2023.
4. Autonomous navigation of a microswimmer in a non-uniform flow
   - K. Sankaewtong, J. J. Molina, and R. Yamamoto, “Autonomous navigation of smart microswimmers in non-uniform flow fields,” Phys. Fluids 36, 041902, 2024.