Toward Controlling Evaporative Deposition: Effects of Substrate, Solvent, and Solute.

Understanding evaporative deposition from a colloidal suspension and on-demand control over it are important due to its industrial and biomedical applications. In particular, it is known that interactions among substrate, solute, and solvent have important consequences on evaporative depositions; however, how these are affecting the deposition patterns and at which conditions these interactions are prominent need detailed investigations. Here we report that the total time of deposition (td) and the geometric shape of the droplet (Lc = initial footprint diameter/height) have a significant role in determining the evaporative deposition patterns. We have identified four zones based on td and Lc, and found that with longer deposition time (high td) and larger available space for particle motion within a liquid droplet (high Lc), deposition patterns were governed by the interactions among the substrate, solute, and solvent. We also experimentally demonstrated that the pinned contact line is indispensable for the "coffee ring" effect by comparing the deposition on surfaces with and without hysteresis. The effect of the Marangoni flow is also discussed, and it is shown that by controlling Marangoni flow, one can manipulate the droplet deposition from uniform disk-like to coffee ring with a central deposition.