Figure 2. Experimental configurations.

Cartoon of the geometrical arrangement of the six optical tweezers (green conoids) and the colloids (blue spheres) during the experiment. The resulting harmonic optical traps are arranged as indicated beneath by the schematic orthogonal projection on the coverslip, with colloids and traps represented by numbered blue and dark green circles, respectively. The sizes of the particles are to scale with their distance, but not with the distance from the coverslip. At each temperature T, the interactions between the pairs 1–2, 1–3 and 2–3 of colloids are measured in the configurations a,b and c, respectively. The effective potential between colloids 1 and 2 in the presence of colloid 3 is then measured in the configuration d. While measuring the pair interactions in a–c, the remaining colloid is optically moved into the nearest external trap. All six optical traps are always switched on during the measurements in order not to alter the hologram on the spatial light modulator and the corresponding optical potentials.