What is it about?
A Janus particle has two faces with different surface properties -- one attractive, one repulsive. When given a self-propulsion force (modeling a catalytic motor), it moves directionally through its surroundings. Understanding how such particles navigate crowded environments matters for applications like targeted drug delivery and active matter physics. We used molecular dynamics simulations to study a Janus probe moving through a dense suspension of passive colloids. The simulations tracked both translational and rotational dynamics as a function of the suspension density and the probe's self-propulsion speed. At high densities, the probe's rotational diffusion increased with crowding, contrary to what simple hydrodynamic arguments predict. The effect arose from frequent collisions with neighboring particles that act as random torques. This mechanism does not require the surrounding fluid to be viscoelastic -- hard-sphere crowding alone is sufficient.
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Why is it important?
Experiments had observed that self-propelled particles rotate faster in complex fluids, but the mechanism was unclear. Viscoelasticity of the fluid was the leading explanation. Our simulations showed that the enhanced rotation occurs even in a simple hard-sphere suspension, pointing to a more general mechanism: steric collisions in crowded environments provide random angular kicks. This simplifies the physical picture and suggests that the effect should appear in any sufficiently dense environment, not only in polymer solutions. The results provide a baseline for interpreting experiments on active particles in biological and synthetic crowded media.
Perspectives
This paper came from my first research project, a summer program during my first year of undergrad. I ran the molecular dynamics simulations and analyzed the trajectories. The project took several years to reach publication. The main technical challenge was separating the effects of self-propulsion, crowding, and particle asymmetry on the dynamics. Each simulation had to run long enough to converge the rotational diffusion coefficient, which required careful equilibration. As my first exposure to computational physics research, this project shaped how I think about simulation studies: run enough statistics, check for finite-size effects, and compare to the simplest possible model before invoking complex explanations.
Rohit Goswami
University of Iceland
Read the Original
This page is a summary of: Translational and rotational dynamics of a self-propelled Janus probe in crowded environments, Soft Matter, January 2020, Royal Society of Chemistry,
DOI: 10.1039/d0sm00339e.
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