“Coffee ring” formation dynamics on molecularly smooth substrates with varying receding contact angles

A systematic study of “coffee-ring” formation dynamics was carried out using chemically and physically well-controlled substrates and colloids. Clean silicon wafers were reacted with tris(trimethylsiloxyl)chlorosilane in the vapor phase to yield molecularly smooth substrates with advancing and receding contact angles up to 84°/74° (θA/θR) and a constant hysteresis of 12 ± 2°. Monodisperse gold nanoparticles (AuNPs) of 15 nm in diameter were stabilized and functionalized with poly(vinyl alcohol) (PVOH) of 9 nm in thickness in aqueous solution. The evolution of contact angle and contact diameter of water and colloid-containing drops on clean and modified silicon wafers was assessed as sessile drops evaporated. PVOH-AuNPs in the colloid-containing drop increase the energy barrier for contact line depinning and the pinning force. There are up to four drying stages for colloid-containing drops, initial pinning, depinning, secondary pinning, and final evaporation. The contribution of each stage to evaporation depends on substrate receding contact angle. It is established in this study that the secondary pinning is unique for colloid-containing drops and is only observed on substrates with receding contact angles greater than 35°. “Coffee rings” are formed on all modified silicon wafers, but not on clean silicon wafers with contact angles of 0°/0°.