Drying of a colloidal suspension

How does a colloidal suspension dry? Is evaporation the only factor we need to consider in this process? By watching the drying of an index matched colloidal suspension with a confocal microscope, we found that drying is a two-stage process: in the first stage, due to faster evaporation rate at edge, colloidal particles accumulate and compact at the edge, forming a porous medium filled by solvent (figure 1); after all the particles are packed, the second stage starts: air invades this porous medium and solvent retreats, eventually all liquid is replaced by air.

We found rather surprising behavior in the air invasion stage: instead of continuous evaporation, the drying proceeds in a jumping manner: for some time interval nothing happens, and suddenly a large volume (hundreds to thousands of particle size) is invaded by air in less than 0.07 second (figure 2)!

We believe this process is dominated by surface tension: In the beginning, the liquid-air interface is smooth over the whole drop. But as solvent evaporates, the interface is trapped between particles, forming small menisci. These small menisci make very low pressure inside the liquid (figure 3), and when air breaks into the system, the large pressure difference pushes the interface advancing in big steps.