Figure 1. Experimental set-up.

(a,b) Schematics of the experimental set-up for the controlled superposition of two orthogonal standing waves in a fluid tank. Waves are created using two computer controlled electrodynamic shakers. The amplitudes, frequencies and relative phase of the two waves are adjusted with high accuracy. Both the wave field and the surface flow can be measured (see ‘Methods' section for details). (c) A photo of the laboratory set-up showing the time-averaged streaks of drifting imaging particles. (d) Zoom into spatially resolved, small-scale particle drifting orbits.