The kinetics of attachment and detachment at the interface provide further evidence of attractive pairwise interactions. Figure 2(b) shows the computer-measured trajectories  followed by the spheres photographed in Fig. 2(a). Spheres in the crystal simply rattle about their lattice positions with transit times comparable to the 1/30 second interval between video frames. For the most part, spheres in the neighboring fluid roam freely.
The track we have emphasized near the center of Fig. 2(b) follows a sphere which begins its trajectory as part of the fluid but joins the crystal interface for sec before resuming its random walk. In this time, the sphere wanders no further than from its lattice position. The probability that a Brownian sphere with the fluid's measured  diffusion coefficient of would remain so well localized is less than 0.02. Rather than being localized by chance, the sphere appears to be held in place by an energetic barrier to desorption over which it eventually escapes. If we estimate on the order of N = 100 unsuccessful attempts to leave the surface by thermal excitation, then the barrier height is at least . The existence of a surface activation energy also would explain why only one other sphere detaches from the crystal during the 15 seconds for which the trajectories are traced.
Crystals made from mutually repulsive spheres should put up no barrier to detachment. The intersphere attraction of around per neighbor deduced from the crystals' faceted structure, however, would account naturally for the estimated .