Printing small dots from large drops.

Printing of droplets of pure solvents containing suspended solids typically leads to a ring stain due to convective transport of the particles toward the contact line during evaporation of the solvent. In mixtures of volatile solvents, recirculating cells driven by surface tension gradients are established that lead to migration of colloidal particles toward the center of the droplet. In favorable cases, a dense disk of particles forms with a diameter much smaller than that of the droplet. In the latter stages of drying, convective transport of the particles radially toward the contact line still occurs. Two strategies are described to fix the distribution of particles in a compact disk much smaller than the initial diameter of the drying droplet. First, a nanoparticulate clay is added to induce an evaporation-driven sol-gel transition that inhibits convective flow during the latter stages of drying. Second, a nonadsorbing polymer is added to induce depletion flocculation that restricts particle motion after the particles have been concentrated near the center of the droplet. The area of the resulting deposit can be as little as 10% of the footprint of the printed droplet.