Science 338, 87–90 (2012 )

Credit: © 2012 AAAS

It has been commonly assumed that a crystal heated above the melting point starts to melt through the formation of defects, which diffuse and coalesce to form liquid nuclei (nuclei that reach a critical size grow spontaneously until the whole crystal is melted). But this is not what Ziren Wang and collaborators see in their experiments with crystals of thermoresponsive colloidal gel microspheres, which shrink with increasing temperature. After slowly heating the interior of face-centred cubic crystals with a focused laser, the researchers found with video microscopy that the precursors to the liquid nuclei are instead coordinated circular motions of spheres within regions where the spheres are highly mobile, as had been predicted from recent computer simulations. Interestingly, they also observed that stronger superheating (which caused the colloids to shrink faster) leads to the coalescence of liquid nuclei, which quickly acquire a spherical shape to minimize the liquid–crystal surface tension. The researchers' set-up should lead to a better understanding of melting dynamics and further challenge classical nucleation theory.