High‐speed photography of water drop impacts on sand and soil

Abstract

SummaryThe way that water drops impact on soil is of widespread importance for soil science, particularly in the context of irrigation and erosion. With modern high‐speed photography techniques, impacts on granular materials are becoming more widely studied, but the variation in particle‐size distribution of soil presents particular challenges. Here, the impacts of individual water drops on two contrasting soil textures (pure sand and a loamy soil) have been studied with high‐speed photography, and compared at Weber numbers between 50 and 750. Granular samples with wider distributions of particle sizes produce larger variations in experimental outcomes. For the sand, which contained >98% particles between 60 μm and 2 mm in size, the typical outcome involved spreading and retraction of the drop to a circular profile with a raised rim. The loamy soil had greater surface roughness than the sand, with a wider distribution of particle sizes, so that the spread of the drop was more anisotropic, involved more movement of target particles and often involved droplet breakup into satellite drops. Overall outcomes were best described by a gravity‐dominated cratering mechanism, and the scaling exponent for the crater diameter with respect to impact energy was 0.22 ± 0.03. Dynamic penetration studies enabled visualization of granular motion below the surface, and both static and dynamic penetration results illustrated a transition between inertial and viscous regimes over time. As the water content of the soil was increased, droplet breakup was initially encouraged before capillary imbibition dominated at water contents close to 10%v/v. This research is useful for understanding how rain and irrigation, which comprise many millimetre‐scale drops, influence large‐scale hydraulic properties in soil, including surface sealing.Highlights Drop impact on to soil is important, but its systematic study is difficult because of variation in soil particle sizes. High‐speed photography was used to study drop impacts on to a pure sand and loamy soil, and analysed quantitatively. Results show how granular samples with wider distributions of particle sizes produce more variation in experimental outcomes. The maximum extent of drop spreading was best described by a scaling model developed for planetary cratering.