Granular impact cratering by liquid drops: Understanding raindrop imprints through an analogy to asteroid strikes

Abstract

When a granular material is impacted by a sphere, its surface deforms like a liquid yet it preserves a circular crater like a solid. Although the mechanism of granular impact cratering by solid spheres is well explored, our knowledge on granular impact cratering by liquid drops is still very limited. Here, by combining high-speed photography with high-precision laser profilometry, we investigate liquid-drop impact dynamics on granular surface and monitor the morphology of resulting impact craters. Surprisingly, we find that despite the enormous energy and length difference, granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we integrate the physical insight from planetary sciences, the liquid marble model from fluid mechanics, and the concept of jamming transition from granular physics into a simple theoretical framework that quantitatively describes all of the main features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals a remarkable analogy between familiar phenomena of raining and catastrophic asteroid strikes.liquid impacts | granular impact cratering | jamming | liquid marble G ranular impact cratering by liquid drops is likely familiar to all of us who have watched raindrops splashing in a backyard or on a beach. It is directly relevant to many important natural, agricultural, and industrial processes such as soil erosion (1, 2), drip irrigation (3), dispersion of microorganisms in soil (4), and spray-coating of particles and powders. The vestige of raindrop imprints in fossilized granular media has even been used to infer air density on Earth 2.7 billion years ago (5). Hence, understanding the dynamics of liquid-drop impacts on granular media and predicting the morphology of resulting impact craters are of great importance for a wide range of basic research and practical applications.Directly related to two long-standing problems in fluid and granular physics research, i.e., drop impact on solid/liquid surfaces (6-9) and granular impact cratering by solid spheres (10-16), liquid-drop impact on granular surfaces is surely more complicated. Although several recent experiments have been attempted to investigate the morphology of liquid-drop impact craters (17-21), a coherent picture for describing various features of the impact craters is still lacking. Even for the most straightforward impact-energy (E) dependence of the size of liquid-drop impact craters, the results remain controversial and incomplete (17,19,20). Katsuragi (17) and Delon et al. (19) reported that the diameter of liquid-drop impact craters D c scales as the 1/4 power of the Weber number of liquid drops, which yields D c ∼ E 1=4 , quantitatively similar to the energy scaling for low-speed solid-sphere impact cratering (10, 11). However, because the energy balance of liquid-drop impacts is different from that of solid...

Unifying Impacts in Granular Matter from Quicksand to Cornstarch

“…Along with its astrophysical [3] and ballistics applications [4], impact dynamics is an object of active research to understand the high-speed response of granular matter [5]. In dry granular media, impact by a solid object results in the formation of a corona of granular ejecta and a solid-like plastic deformation leading to a permanent crater [6][7][8][9][10][11]. For fine powders in air, granular jets and cavity collapse occur during impact [12,13].…”

The impact of raindrops on sand

“…Droplet impact on a granular substrate adds new challenges to the above: First, both the intruder and the target, not merely one of the two, deform during impact; second, the liquid composing the droplet may penetrate into the substrate during the impact and may, in the end, completely merge with the grains. These complex interactions between the droplet intruder and the granular target create various crater morphologies as reported in the literature [14,15,17,21,86], see figure 2.1 for examples. An appealing and natural question is by what mechanism craters are formed and how this leads to the observed rich morphological variation.…”

“…However, they are 'miscible' as well, i.e., the liquid that composes the droplet penetrates into the granular substrate. This penetration results in the formation of a liquid-grain mixture which needs to be taken into account in order to understand the crater morphology [17,18,21]. By changing impact speed U 0 and the initial packing fraction φ 0 the crater morphology is found to vary systematically.…”

“…The residue consists of a mixture of liquid and grains, which is referred to as 'liquid marble' in ref. 21. The discrimination between doughnut and truffle is vague, as while changing the impact parameters one shape seems to be continuously transformed into the other, whereas the transition from truffle to pancake is more abrupt.…”

“…It is only in the last decade, that drop impact on sand started to draw the attention of physicists, with studies focused on rain showers [12,13] or single drop impact [14][15][16][17][18][19][20][21].…”