next
Next: Stable Facets Up: Like-Charge Attractions in Metastable Previous: Colloidal interactions and phase

Superheated crystals

We take advantage of forces which arise during low frequency electrophoresis to compress a colloidal fluid past its freezing point [20]. The latent heat of freezing is absorbed by the surrounding water whose heat capacity is comparatively so enormous that the suspension's temperature does not change. When the electric field is turned off, the suspension is free to return to its equilibrium fluid state so that any remaining crystals are superheated with respect to the fluid.

Parallel plate gold electrodes patterned onto one of the sample volume's glass surfaces provide electrical contact to the suspension. A 60 Hz 10 tex2html_wrap_inline479 signal applied across the 1 mm gap then drives the spheres through electrophoresis. The interplay of this force and electro-osmotically driven bulk fluid flow engenders a Magnus force which pushes the spheres toward the walls and away from the sample volume's mid-plane. Similar forces drive spheres settling under gravity to the walls of fluidized beds [21]. The resulting increase in density induces freezing at the walls and produces several polycrystalline layers.

As we have previously described [20], superheated colloidal crystals in suspensions of moderate ionic strength melt at a rate limited only by diffusion of spheres away from the crystal surface. All traces of order in such suspensions typically disappear within 10 sec. More thoroughly deionized suspensions at lower ionic strength display dramatically different behavior, retaining residual metastable crystallites drifting through the fluid for as long as an hour. The examples in Fig. 1 are in a nonequilibrium ordered state 10 minutes after compression was stopped. The largest of these crystallites consist of five layers of spheres in a face-centered cubic packing. Individual spheres appear either light or dark depending on their distance from the focal plane of the microscope's tex2html_wrap_inline481 N.A. 1.4 oil immersion objective. Dark spheres in Figs. 1 and 2(a) constitute the layer closest to the glass wall, the brightest are in focus in the second layer, and the third layer is barely visible as light smudges. Nearest-neighbors are separated by tex2html_wrap_inline483 or roughly 3 diameters.

That the melting rate changes dramatically with changing ionic concentration is not necessarily surprising. Increasing the screening length could push the ensemble of spheres close to their crystal-fluid coexistence line. In that case, the free energy difference driving spheres from the crystal back into the fluid could be vanishingly small and the melting process correspondingly slow. Analysis of the crystals' structure, however, shows that other forces are at work.


next
Next: Stable Facets Up: Like-Charge Attractions in Metastable Previous: Colloidal interactions and phase

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