Tuning the velocity of thermophoretic microswimmers with thermo-sensitive polymers

Understanding and controlling the motion of self-propelled particles in complex fluids is crucial for applications in targeted drug delivery, microfluidic transport, and the broader field of active matter. Here, we investigate the thermophoretic self-propulsion of partially gold-coated polystyrene Janus particles (Au-PS) in temperature-responsive linear poly(N-isopropyl acrylamide) (PNIPAM) solutions across various concentrations and temperatures. Particle velocities are examined at three representative temperatures: room temperature ((21 ± 0.2) °C), (28 ± 1) °C (just below the LCST), and (34 ± 1) °C (above the LCST). Viscosity values of the PNIPAM solutions were found to be close to those of pure water, with no significant shear thinning or other viscoelastic effects observed under relevant experimental conditions. In pure water, Au-PS Janus particles propel with the PS hemisphere leading, driven by their intrinsic thermophoretic response. At low polymer concentrations (0.05 wt%), experiments and theoretical calculations reveal a non-monotonic dependence of particle velocity on temperature, with a maximum near the LCST. In this regime, the positive Soret coefficient of PNIPAM causes the polymer to accumulate near the cooler PS hemisphere, generating a diffusiophoretic drift that can dominate the intrinsic thermophoretic motion and reverse the propulsion direction. Experimentally, the propulsion direction switches from PS-forward to Au-forward between 0.04 wt% and 0.05 wt%, and within the 0.05 wt% solution, a secondary reversal back to PS-forward is observed at higher temperatures, consistent with the weakening of the depletion-induced drift above the LCST. At higher concentrations (0.5 wt% and 1 wt%), the increased polymer content leads to stronger adsorption onto the entire particle surface, which suppresses propulsion by reducing local asymmetry. At 34 °C, thermophoretic propulsion stops, leaving only Brownian motion. Additionally, the diffusion coefficient increases due to temperature raise. These results highlight the potential of thermo-responsive polymers to control microswimmer dynamics, offering tunable transport properties for applications in active matter and targeted delivery systems.