Figure 1.
(a) One-dimensional model of oscillator. The colloidal particle is driven by two traps A and B switching on and off alternately (with harmonic potentials in this sketch: α = 2). The traps switch when the particle reaches a distance ξ from the centre of the active trap, leading to oscillations with a constant amplitude 2Rg. (b) The oscillator in (a) is modified to allow freedom in its direction of oscillation θ. When the particle is at a radial position greater than Rf, the trapping constant in the direction orthogonal to θ is set to 0. Furthermore, when the active trap is also in the same direction as θ, the trap will follow the angular position of the particle. The coloured track of the particle position over a few cycles shows a slow deviation of the angular position. The trap positions are indicated by the grey lines. (c) Evolution of the active trap's angle corresponding to the track in (b). The sharp jumps correspond to trap switches and the slow deviation occurs because of the allowed freedom between Rf and Rg, just before every trap switch. (Online version in colour.)