PHENOMENOLOGY OF DROPLET IMPACT ONTO POROUS SURFACES

This paper presents a systematic study of droplet impact onto a porous substrate, with a focus on the combined effects of splash suppression and fluid penetration. Seven different sintered bronze plates are used with varying porosity, permeability, and pore size. Single droplets of water, ethanol, and isopropanol were released from various heights onto these surfaces under different conditions, namely, dry, saturated and soaked and tapped. The overarching goal is to recover water and soot from the exhaust gas, in order to mitigate the formation of persistent contrails. Hence, an important requirement is to attain the complete recovery of water through the porous substrate without generating secondary droplets. This is achieved by choosing porous media with high permeabilities and large pore sizes, which assure full penetration of the liquid droplet at all impact conditions. Droplet dynamics were recorded with a high-speed camera and analyzed using dimensionless parameters. The analysis shows that the splashing/deposition threshold is only weakly dependent upon the Darcy number. Instead, it is mainly controlled by the droplet fluid properties, impact parameters, and initial wetting state. Prompt splashing overall prevails, except for the saturated wetting state where corona splashing is observed. The impact outcome and spreading of the lamella are controlled by the interplay between dynamic pressure, capillary pressure at the pore scale and the Darcy number, which measures the relative effect of the permeability of the porous medium versus its cross-sectional area. The relative importance of these variables controls the impact dynamics in the transition regime.