title: Absorption of impinging water droplet in porous stones.
abstract: This paper presents an experimental investigation and numerical analysis of the absorption of water droplets impacting porous stones. The absorption process of an impinging droplet is here fully characterized from spreading to evaporation in terms of absorbed mass during droplet depletion and moisture content distribution in a time-resolved manner for three different natural stones. High-speed imaging and neutron radiography are used to quantify moisture absorption in porous stones of varying moisture properties from deposition until depletion. During impact and spreading, the droplet exhibits a dynamic non-wetting behavior. At maximum spreading, the droplet undergoes pinning, resulting into the contact radius remaining constant until droplet depletion. Absorption undergoes two phases: initially, absorption is hindered due a contact resistance attributed to entrapped air; afterwards, a more perfect capillary contact occurs and absorption goes on until depletion, concurrently with evaporation and further redistribution. A finite-element numerical model for isothermal unsaturated moisture transport in porous media captures the phases of mass absorption in good agreement with the experimental data. Droplet spreading and absorption are highly determined by the impact velocity of the droplet, while moisture content redistribution after depletion is much less dependent on impact conditions.
authors.affiliation: Chair of Building Physics, ETH Zürich, Stefano-Franscini-Platz 5, CH-8093 Zürich, Switzerland.
authors.forename: J B
authors.initials: JB
authors.lastname: Lee
authors.affiliation: Chair of Building Physics, ETH Zürich, Stefano-Franscini-Platz 5, CH-8093 Zürich, Switzerland; Laboratory for Multiscale Studies in Building Physics, Swiss Federal Laboratories for Materials Science and Technology, Empa, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: A I
authors.initials: AI
authors.lastname: Radu
authors.affiliation: Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut (PSI), Villigen, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Vontobel
authors.affiliation: Laboratory for Multiscale Studies in Building Physics, Swiss Federal Laboratories for Materials Science and Technology, Empa, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland. Electronic address: dominique.derome@empa.ch.
authors.forename: D
authors.initials: D
authors.lastname: Derome
authors.affiliation: Chair of Building Physics, ETH Zürich, Stefano-Franscini-Platz 5, CH-8093 Zürich, Switzerland; Laboratory for Multiscale Studies in Building Physics, Swiss Federal Laboratories for Materials Science and Technology, Empa, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Carmeliet
keywords: Droplet absorption; Droplet impact; Neutron imaging; Porous natural stone; Transport modeling