title: Drop impact on natural porous stones.
abstract: Drop impact and spreading on three natural porous stones are experimentally determined using high-speed imaging and compared with spreading over an impermeable steel surface. The dynamic non-wetting behavior during spreading and the hydrophobic contact angle >90° is attributed to the presence of an air layer between the droplet and the porous substrate. As the contact line pins at maximum spreading on the porous stone, the maximum spreading determines the liquid contact area on such substrate. The droplet gets pinned when the air layer is broken at the contact line and capillary forces develop in fines pores at the droplet edge, pinning the droplet. Maximum spreading on porous stones increases with impact velocity but does not scale with Weber number at low impact velocity. It is demonstrated that dynamic wetting plays an important role in the spreading at low velocity and that the dynamic wetting as characterized by the dynamic contact angle θD has to be taken into account for predicting the maximum spreading. Correcting the maximum spreading ratio with the dynamic wetting behavior, all data for porous stones and non-porous substrate collapse onto a single curve.
authors.affiliation: Chair of Building Physics, ETH Zurich, Stefano-Franscini-Platz 5, CH-8093 Zürich, Switzerland.
authors.forename: J B
authors.initials: JB
authors.lastname: Lee
authors.affiliation: Laboratory for Multiscale Studies in Building Physics, Swiss Federal Laboratories for Materials Science and Technology, Empa, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Derome
authors.affiliation: Chair of Building Physics, ETH Zurich, 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. Electronic address: carmeliet@arch.ethz.ch.
authors.forename: J
authors.initials: J
authors.lastname: Carmeliet
keywords: Droplet; Impact; Pinning; Porous material