Witnessing transition at a crystal/liquid interface

At the liquid/crystal interface of colloid particles at equilibrium, the yellow-orange area along the bumpy line is liquid, but almost crystal. The light blue area is crystal, but almost liquid.
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Researchers have now captured images of the fuzzy interface between crystal and liquid at equilibrium. Such interfaces occur where ice meets the liquid in a glass of ice water, for example. A better understanding of how these interfaces work could be useful in industrial applications such as investigating the use of colloidal crystals as optical switches.

Water molecules are too small to study during a transition in state, so Eric Weeks and Jessica Hernández-Guzmán at Emory University upped the scale by modeling molecules with colloidal PMMA particles (~2.3 μm average diameter) suspended in a mixture of cyclohexyl bromide and decalin.

The solvent closely matched the density and refractive index of the particles, but the researchers added extra decalin. The resulting slight density mismatch let them use gravity to create a crystal in coexistence with the liquid inside glass microscope slides. They stored the slides vertically and allowed them to equilibrate for more than one month. Then the researchers imaged the samples using a confocal microscope with a base turned 90° to keep the slides oriented in the same way that they were stored.

The researchers acquired 3D images every 25 seconds and followed the particle movement with standard tracking methods. Algorithms were used to classify the degree of organization of each particle. The particles were digitally labeled with a color scale according to their state: dark blue represented the most crystalline and dark red the most liquid.

The images revealed some surprising results. Spatially, the colloids transitioned very quickly from completely organized to completely disorganized. The fuzzy region between the two states was not much thicker than the particle size—much smaller than scientists previously thought. In the other dimension, the researchers observed the capillary waves—which are known to occur in a transition between two states of matter—and found that the waves seemed to broaden the surface of the interface. (Proc. Natl. Acad. Sci. U.S.A. 2009, DOI 10.1073/pnas.0904682106)