Fig. 1: Light-activated motion.
a Measured propulsion velocity v of the active particle (AP) vs. illumination intensity I. Above a threshold intensity I0 ~ 0.08 μW μm−2 the propulsion mechanism sets in, leading to self-propulsion. The vector n corresponds to the direction of propulsion and reverses its sign at the critical intensity Ir. b Example of measured trajectories of an AP in a static light gradient ∇I. Below Ir the AP moves opposite to ∇I (negative phototaxis, blue trajectory and background). Above Ir the particle moves along ∇I (positive phototaxis, red trajectory and background). Time evolution of the angle ϕ between the propulsion direction of the AP and the direction of the light gradient, after the AP is suddenly exposed to the light gradient for intensities c below Ir and d above Ir. Squares denote averaged data, and the full line shows the theoretical fits. In both cases, an identical light gradient of intensity \(\left| {\nabla I} \right|\) = 0.1 μW μm−3 was used. Inset: Sketch of an AP in a positive and negative light gradient together with the resulting angular velocity ω at which the AP rotates when the light gradient is switched on. Error bars denote the standard deviation.
