| dc.description.abstract | Drying of a droplet containing non-volatile solutes, initially dispersed over the entire drop, is an everyday phenomenon and it commonly leaves a ring of solute deposit on a surface, rather than a uniform spot. Although droplet drying is often observed, it gives rise to surprisingly rich morphologies, which depend on the contact line geometry, solute size and chemistry, and substrate-solvent interaction.
The formation of the well-known coffee-ring pattern after the liquid drop evaporation has been successfully explained by Deegan et al. The radial outward flow is thus induced by the differential evaporation rates across the drop. Contact line pinning of the drying drop on the surface leads to liquid flow from the interior to replenish the liquid evaporating from the edge. The resulting edgeward flow can carry nearly all the dispersed solutes toward the edge and deposit them in the vicinity of the contact line to form a ring-like stain. Evidently, the appearance of the coffee-ring effect involves three ingredients for a drying droplet: non-volatile solute, outward flow, and contact line pinning.
Contact angle hysteresis dominates the formation of ring-like stain and there are two factors to influence the contact angle hysteresis: substrate and solutes. In our study, the drying process and drying patterns are observed on the substrate of low contact angle hysteresis with different sizes solutes.
From the observation of our experiments, we generalize some conclusions in the following:
(1) In order to acquire a ring-like stain on a hydrophilic substrate with weak CAH, the wetting property of the liquid drop on the substrate must be modified and it can be achieved by surface-active solute. The separation of a mixture can be achieved on evaporation stain based on their difference of surface-activity. The pattern of evaporation stain can be determined by the competition between contact line receding and small-sized solute precipitation.
(2) There are three types of solutes :
(i) No surface-activity. The addition of some solutes, such as Dextran and PDDC, has no influence on the surface tension of solution. Therefore, the dot pattern is observed.
(ii) Weak surface-activity. The surface tension will decrease slowly with increasing the concentration of some solutes such as NaPSS. Hence, the formation of dot pattern is found at low concentration while the ring-like pattern takes place at high concentration.
(iii) Strong surface-activity. The presence of a small amount of some solutes, such as PVP and PVA may result in a rapid decrement of surface tension of solution. As a result, the ring-like pattern is formed.
(3)By using nanoparticle solution, stick-slip pattern is observed and particle size may affect the evaporation stain. | en_US |