Influence of Substrate Nature on the Evaporation of a Sessile Drop of Blood

Abstract

We fully characterize the natural evaporation of human drops of blood from substrates and substrate-dependent behavior. The heat flux adsorbed by the drops for evaporation is measured by means of a heat flux meter. A side-view measurement enables access to the drop contact angle, wetting diameter, and initial height. A top-view camera allows for the monitoring of the drying regime (deposition, gelation, and fracturation). This directly measured heat flux is related to the evaporative mass flux obtained from the mass of the drop, and the two show good agreement. Both types of measurements indicate that regardless of the substrate type, there is first a linearly decreasing regime of evaporation when the drop is mostly liquid and a second regime characterized by a sharp decrease. We show that the evaporation dynamics are influenced by the substrate’s wettability but not by the substrate’s thermal diffusivity. The different regimes of evaporation exhibited by glass and metallic substrates are explained in terms of evaporation fluxes at the drop surface. In the case of wetting drops (below 40 deg), the evaporation flux is very important along the drop periphery and decreases across the interface, whereas in the case of nonwetting drops (about 90 deg), the evaporation flux is almost uniform across the droplet’s surface. We show that these different evaporation fluxes strongly influence the drying behavior. In the case of metallic substrates, this enables the formation of a uniform "glassy skin" around the droplet surface and, in the case of glass substrates, the formation a skin along the drop periphery with an inward gelation front. This behavior is analyzed in terms of the competition between the drying time and the gel formation time. Unstable drop surfaces were observed at high initial contact angles and are very similar to those of polymer drops.