Abstract
Droplet impact on a granular layer results in various morphologies of the liquid–grain mixture. Some are concentrated and highly curved, some are extended and flatter. No matter how the morphology looks from the top, it is generally believed that its bottom is tightly connected to the concavely deformed granular target. In this paper we report the discovery of a hidden cavity below a droplet residual, formed upon impact on packings of hydrophilic grains and exposed by X-ray tomography. Its occurrence in the parameter space is explored. We elucidate the mechanism leading to this counterintuitive phenomenon using a dual-curvature model and an energy criterion. This research may shed new light onto the ongoing discussion about the origin of the so-called fossilized raindrop impressions.
Highlights
Observing marks of a brief rain shower on the beach or on mud is a common experience to many of us
Raindrop impressions are expected to be concave pits and, the presence of convex sedimentary features with underneath cavities has led to speculations on alternative causes, e.g. they could be air bubbles that dried in mud without bursting (Lyell 1851)
Even without retraction, the liquid–grain mixture could still undergo vertical deformation while its horizontal dimension is unchanged – the centre is lifted up, and in some cases even protrudes from the sandy surface. We discovered that this lift creates an unexpected feature associated with the last morphology, a cavity underneath, which is only observable with X-ray tomography
Summary
Observing marks of a brief rain shower on the beach or on mud is a common experience to many of us Some of these rain imprints may even have been preserved over geological time scales and are discovered as sedimentary structures that were first reported back in 1839 and are known as raindrop impressions (Cunningham 1839). These impressions were suggested as a means to determine the air density of remote eras (Lyell 1851), an idea which has been explored recently (Som et al 2012) As their name suggests, raindrop impressions are expected to be concave pits and, the presence of convex sedimentary features with underneath cavities has led to speculations on alternative causes, e.g. they could be air bubbles that dried in mud without bursting (Lyell 1851). We conduct a series of experiments varying the impacting liquid, the impact speed, the grain size and the wettability of individual grains
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