Crystal nucleation beyond classical nucleation theory

5 problems

• Why simulated and measured hard-sphere nucleation rates disagree

  Contested

  Hard-sphere colloids are the simplest crystallizing system, yet computer simulations predict crystal formation many orders of magnitude slower than experiments observe at modest supercooling, by up to roughly 10 to 20 powers of ten depending on the estimate. The cause of this gap is disputed.

• Homogeneous ice nucleation rate of deeply supercooled water

  Partially resolved

  Pure water droplets can stay liquid well below 0 C because ice needs a seed to start. How fast ice appears on its own at very low temperature, and which form of ice appears first, is still uncertain.

• Do stable prenucleation clusters exist in calcium carbonate solutions?

  Contested

  Experiments reported small, stable ion clusters in calcium carbonate solutions even before the solution is saturated, which classical theory forbids. Whether these clusters are truly stable or are short-lived precursors of a dense liquid phase is disputed.

• Curvature correction to the surface tension of small nuclei

  Contested

  Classical nucleation theory uses the surface tension of a flat interface, but critical nuclei are only a few molecules across, where the curvature should change the tension. The size and even the sign of this correction is not agreed on.

• When does crystal nucleation proceed in two steps?

  Open

  Some crystals, for example of proteins, appear to form first as dense liquid-like blobs that later order inside. When this two-step route beats direct crystal formation, and what rate it gives, is not known.