Metastable Crystallites

Can like-charged particles attract?

The monitor displays several unusual clusterings of micron-sized colloidal spheres in water. The clusterings are unusual in that the spheres are all negatively charged, and should be repelling each other strongly. For some reason, they aren't.

This strange behavior on the part of the spheres is of interest to MRSEC researchers Larsen and Grier, who have been using charge-stabilized colloidal suspensions to model changes of structural phase. To do so, they suspend the above-mentioned 650nm-diameter colloidal spheres in water and place the suspension between a glass slide and its cover slip, with thin film parallel plate gold electrodes bounding the glass. When the electrodes are turned on, a slowly oscillating electric field is formed, causing the spheres to compress against the glass and form polycrystalline layers. The whole apparatus is mounted on an inverted microscope which has a video camera attached, so that the behavior of the colloidal spheres can be easily observed and recorded. The apparatus used is pictured below; click on the picture to see a drawing with the parts labeled.

Every precaution is taken to prevent contamination. Mixed bed ion exchange resin is introduced to the suspension via glass reservoir tubes to reduce the concentration of stray ions from the surface of the glass plates, and the reservoir tubes themselves are continuously flushed with humidified Argon to prevent contamination from the air. No stray charges are in the system; the only reason the colloidal spheres do not repel each other is the presence of the electric field.

When that field is turned off, the suspension quickly returns to the equilibrium fluid state - at least, most of the time. Larsen and Grier, however, have seen colloidal crystals remain intact for up to an hour after most of the suspension has returned to a fluid state - even though the colloidal spheres have like charges and have no reason to cling together in the absence of an electric field.

How the metastable crystallites remain intact for so long is not yet understood. Their longevity is not caused by the inhomogeneities in the properties of the glass surfaces, because the crystals are observed to drift across the glass surfaces on which they form. Larsen and Grier are not the first to make these observations of the peculiarities of melting colloidal crystals; some recent experiments elsewhere have provided evidence for an attractive component in the long-range interaction between like-charged pairs of spheres confined by glass walls. The reason for the spheres' behavior, however, has yet to be explained.

(a) Video micrograph of the surface plane of a faceted colloidal crystal in contact with a dilute colloidal fluid. The dashed line demarks a (110) facet of the crystallite, while the arrow points out a sphere about to attach to the surface. (b) Trajectories of the spheres in (a) taken over 15 seconds. Dark trajectories are part of the crystal and light are in the fluid.

Reference:

  1. Like-Charge Attractions in Metastable Colloidal Crystallites" Amy E. Larsen and David G. Grier, Nature 385, 230 (1997).
  2. "Melting of Metastable Crystallites in Charge-Stabilized Colloidal Suspensions," Amy E. Larsen and David G. Grier, Physical Review Letters 76:20, 13 May 1996. Grier has a preprint of Larsen's and his findings available online.