Suppression of Crystallisation in Saline Drop Evaporation on Pinning-Free Surfaces

For sessile droplets of pure liquid on a surface, evaporation depends on surface wettability, the surrounding environment, contact angle hysteresis (CAH) and surface roughness. For non-pure liquids the evaporation characteristics are further complicated by the constituents and impurities within the droplet. For saline solutions, this complication takes the form of a modified partial vapour pressure/water activity caused by the rising salt concentration as the aqueous solvent evaporates. It is generally thought that droplets on surfaces will crystallise when the saturation concentration is reached i.e. 26.3% for NaCl in water. This crystallisation is initiated by contact with the surface and is thus due to surface roughness and heterogeneities. Recently smooth, low contact angle hysteresis surfaces have been created by molecular grafting of polymer chains. In this work we hypothesise that by using these very smooth surfaces on which to evaporate saline droplets, we can suppress the crystallisation caused by the surface interactions and thus achieve constant volume droplets above the saturation concentration. In our experiments we used several different surfaces to examine the possibility of crystallisation suppression. We show that on polymer grafted surfaces, i.e. Slippery Omniphobic Covalently Attached Liquid-like (SOCAL) and Polyethylene Glycol surfaces, we can achieve stable droplets as low as 55% at 25 °C relative humidity with high reproducibility using NaCl in water solutions. We also show that it is possible to achieve stable droplets above the saturation concentration on other surfaces, including superhydrophobic surfaces. We present an analytical model, based on water activity, which accurately describes the final stable volume as a function of the initial salt concentration. These findings are important in for heat and mass transfer in relatively low humidity environments.