What is it about?
Imagine a tiny, motorized particle trying to navigate through a thick, crowded soup—similar to how a nanobot might move through a living cell. In this work, we used computer simulations to study a "Janus particle"—a tiny sphere with two different faces (one sticky, one not) and a built-in motor. We placed it in a crowded environment made of either long, gooey polymer chains or simple disconnected beads. We made a key discovery: the particle's motor doesn't just make it move forward faster; it also makes it tumble and rotate faster when in a crowd. This is because the motor leads to more frequent and forceful collisions with the surrounding obstacles, which imparts an extra spin. We showed that this effect is caused by the physical crowding itself, not by any special "gooey" properties of the environment.
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Why is it important?
This work provided a clear, physics-based explanation for recent experimental observations where self-propelled particles were seen to rotate faster in complex fluids. Our simulations showed that the "gooeyness" (viscoelasticity) of the fluid wasn't necessary and that the effect is more general, arising simply from collisions in any crowded space. This insight contributes to the fundamental understanding of "active matter"—a major field in physics. It also has practical implications for nanotechnology. Understanding how to control the movement and rotation of tiny motorized particles is crucial for designing nanobots for applications like targeted drug delivery inside the crowded environment of the human body.
Perspectives
This paper was based on my very first research project, which I started during a summer program in my first year of undergrad. It took a long time to finally get published, but it was my introduction to the world of computational physics. As the most junior person on the team, my main task was a crucial piece of the setup: figuring out how to actually build the two-faced Janus particle in the simulation software, which was a challenging modeling problem. This was where I first got my hands dirty with large-scale molecular dynamics simulations. It was a steep learning curve, but it was exciting to see our model produce complex, non-intuitive behavior—like the decoupling of movement and rotation—that helped explain real-world experiments. Even though I was a junior author, this project was incredibly formative. It was my first taste of the full research cycle, from the nitty-gritty of building a computational model to the big-picture goal of understanding a complex physical phenomenon, and it set the stage for my future work.
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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