Evaporation of water-ethanol binary sessile drop on fluoropolymer surfaces: Influence of relative humidity

Abstract

Experimental results on the diffusive drop evaporation behavior of 2 μL pure water, pure ethanol and ethanol-water binary droplets under constant relative humidity and temperature conditions were reported on two flat fluoropolymer surfaces of TEFLON-FEP and PTFE. Contact angles, droplet heights, contact radius were monitored with a camera as a function of time. A roughly linear relationship between the initial static contact angles on fluoropolymer surfaces and ethanol concentration was found similar to the previously published results. It was determined that constant contact angle mode dominated for most of the duration of drop evaporation process after the initial stages. Volumes of the binary droplets were found to decrease nonlinearly over time for all of the compositions. The most important effect of the RH increase of the medium was found to be the increase of the adsorption quantity of water vapor from the environment onto pure ethanol and also binary ethanol-water droplets. For pure ethanol droplet under high RH conditions, the rate of evaporation declined after the evaporation of most of the ethanol in 250 s, then the remaining droplet evaporated with a much slower rate which was close to, but not equal to the rate of pure water drop evaporation, indicating that ethanol was present in the droplet until to end. The same behavior was seen for ethanol-water binary droplets. Evaporation rate of the binary droplet containing 25% ethanol by wt. was observed to be close to that of pure water and 75% ethanol by wt. was found to be close to that of pure ethanol with minor differences. However, evaporation of a binary droplet containing 50% ethanol by wt. was more complicated and it is hard to distinguish ethanol and water evaporation zones for this composition and there was a long transition stage where ethanol and water evaporated together for long durations. It was found that droplet life-times were generally longer on flat TEFLON-FEP substrate than that of slightly rough PTFE due to the higher initial contact angles on the former. The change of V(2/3) values with time for pure water drop evaporation was determined to be linear under all RH conditions obeying the constant contact angle mode. The values of (dV(2/3)/dt) decreased linearly with the increase in RH for pure water. Similar, but a non-linear decrease of the (dV(2/3)/dt) slopes were determined for ethanol-water and pure ethonol droplet evaporations indicating the adsorption of water from the environment.