Melting and freezing of two-dimensional colloidal crystallites with short-range attractive interactions
We present results on experiments performed to study the dynamics of melting and freezing in a model colloidal system with short-range tunable attraction mediated by depletion forces. In particular, we have created a two-dimensional system which forms crystallites and undergoes melting or freezing when a small temperature change is applied. To our knowledge, this is the first system that uses a tunable depletion potential and that allows tracking of individual particles at sufficiently short time scales.
We have discovered a cross-over in the melting kinetics of two-dimensional crystallites. Crystallites first sublimate until they reach a cross-over size, after which they quickly vaporize into the gas phase. This cross-over has not been predicted by any theory of which we are aware. In freezing experiments, we have found two qualitatively different mechanisms of nucleation. In one mechanism, highly ordered crystallites form directly from the fluid phase, as in classical nucleation theory. In the other, a meta- or unstable amorphous cluster is first formed, after which the crystallite nucleates from within. These results are likely to be generally applicable to systems with short-range centro-symmetric attractive interactions. The enhancement of kinetics by meta- or unstable phases may play a major role in the melting, freezing and annealing of crystals. In particular, our results may have profound consequences in the area of protein crystallization where the interactions are believed to be similar to ours.