Previous studies have shown that air pollution and wind erosion, which damage a leaf's epicuticular wax layer, can change leaf surface properties from hydrophobic to hydrophilic. However, the dynamic response of a damaged leaf to a raindrop impact has not been investigated and could clarify the direct influence of changes in wettability on early leaf abscission. In this study, we investigate how leaves with different surface properties respond to falling raindrops, viewing this as a unique system of coupled elasticity and drop dynamics. An elastic beam with tunable surface wettability properties is used as a simple leaf model. We find that wettable beams experience much higher torque and bending energy than non-wettable beams. This is because a drop sticks to a wettable beam, while a drop falls off a non-wettable beam. An analytical model using momentum balance and simple cantilever beam theory quantifies the bending energy and torque experienced by wettable and non-wettable beams. The results elucidate the potential damage caused by raindrops impacting a leaf as a function of its surface wettability, and are correlated with environmental factors contributing to premature changes of leaf surface properties.

In nature, several seabirds (e.g., gannets and boobies) dive into water at up to 24 m/s as a hunting mechanism; furthermore, gannets and boobies have a slender neck, which is potentially the weakest part of the body under compression during high-speed impact. In this work, we investigate the stability of the bird's neck during plunge-diving by understanding the interaction between the fluid forces acting on the head and the flexibility of the neck. First, we use a salvaged bird to identify plunge-diving phases. Anatomical features of the skull and neck were acquired to quantify the effect of beak geometry and neck musculature on the stability during a plunge-dive. Second, physical experiments using an elastic beam as a model for the neck attached to a skull-like cone revealed the limits for the stability of the neck during the bird's dive as a function of impact velocity and geometric factors. We find that the neck length, neck muscles, and diving speed of the bird predominantly reduce the likelihood of injury during the plunge-dive. Finally, we use our results to discuss maximum diving speeds for humans to avoid injury.diving | seabirds | buckling | injury | water entry N ature contains several species of creatures that interact with the air-water interface (1). A number of bird species are able to dive into water from the air as a hunting mechanism (e.g., kingfishers, terns, and gannets), a behavior known as plunge-diving (2, 3). Some seabirds, like the northern gannet, are highly specialized plunge-divers, making 20-100 dives per foraging trip, diving from heights of 5-45 m, and attaining speeds of more than 20 m/s (4-7). Thus, the bird's structure and behavior have presumably evolved to withstand a variety of high dynamic stresses, because no injuries have been reported in plunge-diving seabirds. Biologists have previously focused on the diving behavior in terms of ecological factors, such as diving depths, prey species, and hunting success rate (8-10), and physiological features, such as the role of vision while crossing the air-water interface (11,12). Unique kinematic and morphological features during the dive have also been observed, such as having a sharp, arrow-like body posture and a straight, long, and slender neck (13,14). However, a mechanical understanding of plunge-diving birds is not well-established.To study such a phenomenon, Morus bassanus (hereafter gannets) and Sula leucogaster (hereafter boobies), from the Sulidae family, are used as a model species due to their highly specialized diving characteristics (5, 13). First, they plunge-dive at very high speeds, using that momentum to carry them to some depth. Then, they use their webbed feet and/or wings to propel themselves further underwater, like penguins and cormorants (15, 16). Although plunge-diving at high speeds allows the bird to dive deeper, it induces much larger stresses on the seabird's body than pursuit diving alone (13). The two main forms of plunge-diving observed are known as the V-shaped dive and the U-shaped dive (5). During ...

It is well known that animals can use neural and sensory feedback via vision, tactile sensing, and echolocation to negotiate obstacles. Similarly, most robots use deliberate or reactive planning to avoid obstacles, which relies on prior knowledge or high-fidelity sensing of the environment. However, during dynamic locomotion in complex, novel, 3D terrains, such as a forest floor and building rubble, sensing and planning suffer bandwidth limitation and large noise and are sometimes even impossible. Here, we study rapid locomotion over a large gap-a simple, ubiquitous obstacle-to begin to discover the general principles of the dynamic traversal of large 3D obstacles. We challenged the discoid cockroach and an open-loop six-legged robot to traverse a large gap of varying length. Both the animal and the robot could dynamically traverse a gap as large as one body length by bridging the gap with its head, but traversal probability decreased with gap length. Based on these observations, we developed a template that accurately captured body dynamics and quantitatively predicted traversal performance. Our template revealed that a high approach speed, initial body pitch, and initial body pitch angular velocity facilitated dynamic traversal, and successfully predicted a new strategy for using body pitch control that increased the robot's maximal traversal gap length by 50%. Our study established the first template of dynamic locomotion beyond planar surfaces, and is an important step in expanding terradynamics into complex 3D terrains.

STATEMENTGeneralist snakes divide their body into sections, each using distinct movement patterns, to get over large step-like obstacles. Such body partitioning may be generally useful for diverse, complex 3-D terrain. ABSTRACTMany snakes live in deserts, forests, and river valleys and traverse challenging 3-D terrain like rocks, felled trees, and rubble, with obstacles as large as themselves and variable surface properties. By contrast, apart from branch cantilevering, burrowing, swimming, and gliding, laboratory studies of snake locomotion focused on that on simple flat surfaces. Here, to begin to understand snake locomotion in complex 3-D terrain, we study how the variable kingsnake, a terrestrial generalist, traversed a large step of variable surface friction and step height (up to 30% snout-vent length). The snake traversed by partitioning its body into three sections with distinct functions. Body sections below and above the step oscillated laterally on horizontal surfaces for propulsion, while the body section in between cantilevered in a vertical plane to bridge the large height increase. As the animal progressed, these three sections traveled down its body, †

Small animals and robots must often rapidly traverse large bump-like obstacles when moving through complex 3D terrains, during which, in addition to leg-ground contact, their body inevitably comes into physical contact with the obstacles. However, we know little about the performance limits of large bump traversal and how body-terrain interaction affects traversal. To address these, we challenged the discoid cockroach and an open-loop six-legged robot to dynamically run into a large bump of varying height to discover the maximal traversal performance, and studied how locomotor modes and traversal performance are affected by body-terrain interaction. Remarkably, during rapid running, both the animal and the robot were capable of dynamically traversing a bump much higher than its hip height (up to 4 times the hip height for the animal and 3 times for the robot, respectively) at traversal speeds typical of running, with decreasing traversal probability with increasing bump height. A stability analysis using a novel locomotion energy landscape model explained why traversal was more likely when the animal or robot approached the bump with a low initial body yaw and a high initial body pitch, and why deflection was more likely otherwise. Inspired by these principles, we demonstrated a novel control strategy of active body pitching that increased the robot's maximal traversable bump height by 75%. Our study is a major step in establishing the framework of locomotion energy landscapes to understand locomotion in complex 3D terrains.