Direct observation of the entropic potential in a binary suspension

By tracking the Brownian trajectory of a sphere of 1 [mu]m radius in a suspension of 35 nm radius beads, we probe the depth of the entropic potential experienced by a large sphere close to the cell wall. This potential is caused by the pressure exerted by the fluid of small beads when they cannot fit in the gap between the large bead and the wall. We find that the potential depth is of order [ital kT]. Its dependence on the volume fraction of small spheres is significantly overestimated by a theory which treats the small beads as an ideal gas.