Figure 2. Light-activated self-propulsion mechanism.

(a) Sketch of the sample cell with a capped particle suspended in a binary critical mixture of water–2,6-lutidine. Illumination with light leads to heating of the cap and thus to local demixing resulting in active motion. (b) Experimental set-up for creation of periodic illumination landscapes by a scanned line focus of a laser beam (Methods). (c) Propulsion velocity v_p versus illumination intensity I. Symbols (with error bars representing the s.d.) correspond to homogeneous illumination of the sample cell, while coloured lines were obtained in the presence of light profiles with different gradients (|∇I|=2 × 10⁻³ μW μm⁻³ (pink), 0.027 μW μm⁻³ (green), 0.037 μW μm⁻³ (red), 0.115 μW μm⁻³ (blue) and 0.156 μW μm⁻³ (grey)). The agreement between the data for different intensity gradients demonstrates that v_p is only determined by the local intensity incident on the particle.