Sessile and Pendant Micro-Liter Drops Evaporate at Different Rates: An Experimental Approach

Abstract

Evaporation of micro-liter drops from solid surfaces at room condition is mainly governed by diffusion. Therefore, there should be no difference between evaporation rate of sessile and pendant drops. However, some studies indicate a difference and explain the difference using buoyancy. The objective here is to reconcile the inconsistency in the literature. For that, first, by comparing two identical suspended drops, one with a plate on top and the other underneath with a space between drop and plate, we showed the contribution of buoyancy in evaporation is at most less than 8%. When a plate was placed on top, water (its vapor is lighter than air) evaporated slower and hydrocarbons (their vapors are heavier than air) evaporated faster. Interestingly, it was observed when drops touch the plates (i.e. sessile and pendant drops), both water and hydrocarbon drops evaporated faster in sessile configuration. The observation for hydrocarbons is in contradiction with what buoyancy explains. To describe the difference, different scenarios were studied. It was found that sessile drops stay longer in the “constant wetted area" (CWA) mode, before switching to the CCA (constant contact angle) mode, e.g. a 4 µl sessile water drop on a Poly(methyl methacrylate) coated silicon stays in the CWA mode for 318 s whereas for a similar pendant drop this time is 274 s. Considering the fact that evaporation rate in the CWA mode is 30–40% higher compared to the CCA mode, the faster evaporation rate of sessile drops may be explained.