Water striders are impervious to raindrop collision forces and submerged by collapsing craters

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA(2024)

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摘要
Water striders are abundant in areas with high humidity and rainfall. Raindrops can weigh more than 40 times the adult water strider and some pelagic species spend their entire lives at sea, never contacting ground. Until now, researchers have not systematically investigated the survival of water striders when impacted by raindrops. In this experimental study, we use high-speed videography to film drop impacts on water striders. Drops force the insects subsurface upon direct contact. As the ensuing crater rebounds upward, the water strider is propelled airborne by a Worthington jet, herein called the first jet. We show the water strider's locomotive responses, low density, resistance to wetting when briefly submerged, and ability to regain a supersurface rest state, rendering it impervious to the initial impact. When pulled subsurface during a second crater formation caused by the collapsing first jet, water striders face the possibility of ejection above the surface or submersion below the surface, a fate determined by their position in the second crater. We identify a critical crater collapse acceleration threshold similar to 5.7 gravities for the collapsing second crater which determines the ejection and submersion of passive water striders. Entrapment by submersion makes the water strider poised to penetrate the air-water interface from below, which appears impossible without the aid of a plastron and proper locomotive techniques. Our study is likely the first to consider second crater dynamics and our results translate to the submersion dynamics of other passively floating particles such as millimetric microplastics atop the world's oceans. Significance The familiar water strider has received intrigue for its unique locomotion; this study considers their fate in the face of energetic raindrop impacts. We identify a seemingly ignored drop impact feature, a second crater, formed following descent of the rebounding jet, whose retraction produces a water surface acceleration strong enough to leave the water strider below the surface, submerged. We reveal the morphological features that enable striders to survive the onslaught of violent rainfall that, in addition to superhydrophobicity, include passivity and the ability to resurface by swimming. Over the timescales of impact, striders and plastics of comparable size are nearly indistinguishable. Our results lay a foundation for the transport of other floating debris subsurface.
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关键词
impact craters,jets,rainfall,insects,splash
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