Asymmetric Deposits and Crack Formation during Desiccation of a Blood Droplet on an Inclined Surface

We study the combined effect of varying droplet volume and inclination angles on the desiccation patterns left behind evaporating sessile droplets of human blood. We systematically varied the droplet volume in a range of [1-10 μL] and inclination angles between [0-70°]. Microstructural characterization of the deposits was performed using optical microscopy and surface profilometry. On a horizontal surface, typical deposits of toroidal shape with cracks oriented in radial and azimuthal directions were observed. With an increase in the droplet volume and inclination, the interplay between the gravitational and surface tension effects leads to an asymmetric liquid-vapor interface shape, resulting in a differential evaporative mass flux pattern across the interface. Subsequently, we observe elongation of the overall desiccation patterns along with asymmetric mass deposits between the advancing and receding fronts. As a consequence, the crack morphology on the two fronts exhibits pronounced differences. The distinct regimes of asymmetric mass deposits and crack morphology were quantitatively examined as a characteristic of varying droplet volume and inclinations, parametrized in terms of the mean radial crack spacing and width. These findings are qualitatively analyzed by a first-order theoretical model that is based on the energy conservation principle incorporating the release of mechanical stress energy by contraction of the deposit at the last stage of the desiccation process and the consumed surface energy upon formation of the new surfaces during crack evolution.