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Interfacial kinetics

The kinetics of attachment and detachment at the interface provide further evidence of attractive pairwise interactions. Figure 2(b) shows the computer-measured trajectories [17] 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 tex2html_wrap_inline497 sec before resuming its random walk. In this time, the sphere wanders no further than tex2html_wrap_inline499 from its lattice position. The probability tex2html_wrap_inline501 that a Brownian sphere with the fluid's measured [17] diffusion coefficient of tex2html_wrap_inline503 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 tex2html_wrap_inline507 . 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 tex2html_wrap_inline495 per neighbor deduced from the crystals' faceted structure, however, would account naturally for the estimated tex2html_wrap_inline511 .

David G. Grier
Mon Dec 2 14:09:59 CST 1996