Modulation of nanoparticle separation by initial contact angle in coffee ring effect

Abstract

The coffee ring effect occurs when a droplet of a suspension evaporates on a substrate; this process can separate suspended nanoparticles (NPs) by size as a result of geometric constraints at the contact line of the evaporating droplet. In the study, we used a polydimethylsiloxane (PDMS) stamp to make an even contact line, and we changed the contact angle θ of the droplet by selectively configuring hydrophilic and hydrophobic surfaces. In experiments, the temperature, relative humidity were held constant and glass was used as substrate. When the initial θ of the droplet was changed by using the PDMS stamp to coat the glass, NP separation was governed by θ, not by droplet volume VD. When droplets had different initial θ but the same VD, the NP separation in the droplet was ~ 8 µm at θ = 50°, ~ 10 µm at θ = 30°, and ~ 16 µm at θ = 14°. This ability to increase the separation between particles by changing the initial θ of the evaporating droplet may allow clear separation of NPs in evaporating droplets.