The Effects of Ambient Relative Humidity on the Final Shape of Evaporated Droplets of Aqueous Polymer Solution*

When a drop of aqueous polymer solution dries on a solid substrate, the polymer is deposited in a ring-like shape. This shape is due to the relatively high evaporation rate at the pinned three-phase contact line of the droplet that causes the aqueous polymer solution to move outward from the center of the drop in order to replenish the evaporated solvent at the edge. This phenomenon is called the coffee-ring effect, which prevents a uniform deposition of the polymer from forming over the contact area of the drop on the substrate. Decreasing the evaporation rate of the drop weakens the outward flow which decreases the coffee-ring effect and enhances the uniformity of the polymer film. This study investigates the different shapes the dried drops have on a glass substrate due to their different concentrations of the solute and various ambient relative humidities. We have found that the coffee-ring effect can be decreased as the ambient relative humidity is increased. When the ambient relative humidity reaches 100%, the coffee-ring effect is almost eliminated. Moreover, higher concentration of the solute helps to deposit a thicker and more uniform polymer.