Fig. 3: Response of the isotropic TiO₂ particles under the illumination of UV light.

a–h Time-lapsed bright field micrographs show the pushing behaviour of individual swimmers against the neighbouring particles. These repulsions are isotropic. Overlaid trajectories reveal the dynamic response of the particles. i The average velocity over all particles decays with time as V∝t^{−0.69±0.04} and follows the predicted far-field scaling (t^−0.67). The blue line is the prediction of the theoretical model obtained by numerical simulations. The error bars in i represent the standard deviation of the average velocity over all particles. j-o Dynamic response of the passive particles to an immobile active particle under the illumination of UV light. j–o Time-lapsed overlaid trajectories show the tracer passive particles experience isotropic repulsions and push the tracers radially. p An empirical fit of the experimental velocity data shows the predictions of the theoretical model for the dynamics of tracer particles (solid line). The experimental average velocity over the all particles decays with time and follows the numerically obtained flow field produced by an immobilised active particle, modelled as a near-wall Stokeslet. Broken blue line is the far-field prediction which is expected to be valid at much longer times. The error bars in p represent the standard deviation of the average velocity over all particles. Scale bar is 5.0 μm.