Infiltration of impacting droplets into porous substrates

• Published in: Experiments in fluids Vol. 61; no. 11
• Main Authors: Bouchard, D. J., Chandra, S.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 2020; Springer Nature B.V
• Subjects: Beads; Capillary flow; Computed tomography; Diameters; Droplets; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Flow velocity; Fluid- and Aerodynamics; Glass; Glass plates; Heat and Mass Transfer; Image analysis; Impact velocity; Inertia; Infiltration; Mathematical models; Porous materials; Porous media; Research Article; Substrates; Thin films; Water drops
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-020-03056-9

The impact of water droplets on thin porous layers of glass beads held vertically between glass plates was photographed to observe simultaneous spreading and infiltrating of the liquid into the porous medium. Glass beads with diameters of either 100 µm or 500 µm were tested and micro-computed tomography was used to measure the porosities of the layers. The water drop diameter was kept constant at 2 mm while the impact velocity was varied from 0.06, to 1.9 m/s. High-speed videos of the advance of water into the porous structure were studied with image analysis software and the volume flow rate of liquid calculated. Increasing droplet impact velocity decreases droplet infiltration time because higher droplet inertia drives more liquid into the porous material and a larger droplet spreading diameter increases the area in contact with the substrate. Too high of an impact velocity results in droplet cleaving where a portion of the drop remains on the upper surface. A simple model is proposed to determine when inertia driven infiltration will be significant. Once droplet inertia is dissipated capillary forces draw liquid into the substrate and analytical models are compared to predict the rate of capillary driven flow.
Graphic abstract