The four vials contain suspensions of hard colloidal spheres in a refractive index matching solvent. Without index matching these samples of micron sized particles would be milky white. The volume fraction occupied by the particles in these samples is 0.44, 0.22, 0.125, and 0.041, going from left to right, respectively. The samples were initially homogenized by shaking and left to stand a few days. Notice that the settling rate increases with decreasing concentration. Crystal structures grow up as long columns from the bottom, and in some samples (the center two) there is an abrupt change in the disordered fluid phase volume fraction above the crystal (in addition to the abrupt drop to zero volume fraction in the clear supernatant). These abrupt changes in volume fraction are "shock" waves and are reasonably well understood using a theory by Kynch.

Above is a close up picture of "columnar crystals" growing in the bottom of a sample of colloidal hard spheres.

I was fortunate to advise Richard Rogers at NASA for his Ph.D. thesis. He measured the curvature between columnar crystals to determine surface energies.

Nonequilibrium Processes