Evaporation Stains: Suppressing the Coffee-Ring Effect by Contact Angle Hysteresis

A ring-shaped stain is frequently left on a substrate by a drying drop containing colloids as a result of contact line pinning and outward flow. In this work, however, different patterns are observed for drying drops containing small solutes or polymers on various hydrophilic substrates. Depending on the surface activity of solutes and the contact angle hysteresis (CAH) of substrates, the pattern of the evaporation stain varies, including a concentrated stain, a ringlike deposit, and a combined structure. For small surface-inactive solutes, the concentrated stain is formed on substrates with weak CAR, for example, copper sulfate solution on silica glass. On the contrary, a ringlike deposit is developed on substrates with strong CAR, for example, a copper sulfate solution on graphite. For surface-active solutes, however, the wetting property can be significantly altered and the ringlike stain is always visible, for example, Brij-35 solution on polycarbonate. For a mixture of surface-active inactive solutes, a combined pattern of a ringlike and concentrated stain can appear. For various polymer solutions polycarbonate, similar results are observed. Concentrated stains are formed for weak CAR such as sodium polysulfonate, and ring-shaped patterns are developed for strong CAR such as poly(vinyl pyrrolidone). The stain pattern is actually determined by the competition between the time scales associated with contact line retreat and solute precipitation. The suppression of the coffee-ring effect can thus be acquired by the control of CAH.