The fine structure of stains

I blogged last year about the Chicago group's work on why coffee-stains are ring-shaped, with a sharp outer edge fading as one moves in. A quick reminder:

(In other words, the edge of a droplet is stuck where it is; as the droplet evaporates, more and more of the water must move from the center to the edge, so that most of the water gets to the edge before it evaporates, so most of the evaporation and hence the deposition happens at the edge.)

There's a nice new article in PRL today (ungated) that takes a much closer look at the structure of a stain:

(c) is a blow-up [optical microscope] of the red square in (b) and (d) is a blow-up [electron microscope] of the red square in (c). The solute particles at the outermost edge of the stain are arranged in precise crystalline patterns; as you move further in towards the (relatively sparse) middle of the stain, the particles become randomly distributed. The physics of this turns out to be fairly simple, given what's already known about evaporation. To quote the paper:

The [solute] particle velocity increases dramatically in the last moments of the droplet’s life. We refer to this sudden change in speed as ‘‘rush hour.’’ The particles that arrive early, at a low deposition speed, form an ordered (square or hexagonal) structure. In contrast, particles that arrive during rush hour have a high speed and form a jammed, disordered phase.

[NB you could ask why there's a tendency for things to crystallize at all. In this case I think that's just electrostatic repulsion -- particles would like to be as far from each other as possible, i.e. in a crystal, but might not have any way to get there.] The authors also claim to have a theory of why one sees both hexagonal and square crystals in the ordered region [see part (d)] but I don't have the time right now to follow up that paper trail.

Update Here is the Physics blurb about this.