Self-Viscophoresis: Autonomous Motion by Biasing Thermal Fluctuations via Self-Generated Viscosity Asymmetry

TL;DR

This paper introduces self-viscophoresis, a novel mechanism where asymmetric viscosity fields generated by thermoresponsive particles bias thermal fluctuations to produce autonomous, directed motion without traditional propulsion methods.

Contribution

The study presents a minimal Langevin model and experimental evidence for self-viscophoresis, highlighting a new way to achieve controlled microscale motion through viscosity asymmetry.

Findings

Directed motion achieved via viscosity asymmetry.

Model reproduces motion without deterministic propulsion.

Reversible control of motion direction and dimensionality.

Abstract

Microscale transport often relies on ubiquitous yet intrinsically random thermal fluctuations. Understanding how such fluctuations can be biased into directed motion has long been a central theme of nonequilibrium physics. Here, we introduce self-viscophoresis, a mechanism of autonomous motion based on the rectification of thermal fluctuations in a self-generated nonequilibrium viscosity field. Asymmetric colloidal particles dispersed in a thermoresponsive polymer solution induce local heating under uniform illumination, producing a spatially asymmetric viscosity profile around the particle and resulting in persistent directed motion. To elucidate the physical origin of this behavior, we develop a minimal Langevin model coupling isotropic thermal fluctuations to a dynamically updating temperature-viscosity field. The model shows that viscosity asymmetry anisotropically damps stochastic dynamics, effectively biasing thermal fluctuations into a net drift. It thus reproduces the observed directed motion without invoking deterministic propulsion terms associated with effective potentials or environmental fluid flows. Our results distinguish self-viscophoresis from conventional self-propulsion mechanisms and establish it as a general framework enabling reversible control of both the direction and dimensionality of motion.