Sustaining Robust Cavities with Slippery Liquid-Liquid Interfaces.

The formation of a stable gas cavity on the surfaces of solid bodies is essential for many practical applications, such as drag reduction and energy savings, owing to the transformation of the originally sticky solid-liquid interface into a free-slip liquid-vapor interface by the creation of either liquid repellency or a Leidenfrost state on the surfaces. Here, it is shown that the simple infusion of a textured sphere with a smooth, slippery liquid layer can more easily create and sustain a stable gas cavity in a liquid at lower impact velocities compared to a dry solid sphere with the same contact angle. With a key parameter of curvature ratio, the early lamella dynamics during water entry of spheres and drops impact on planes are first unified. With the perspective of wetting transition, the unforeseen phenomenon of prone to cavity formation are successfully explained, which is the preferential lamella detachment from a slippery surface due to the higher viscosity of the lubricant relative to air. It is envisioned that the findings will provide an important and fundamental contribution to the quest for energy-efficient transport.

Numerical and experimental investigation on the oblique water entry of cylinder.

The study of water entry cavity and the analysis of load characteristics are hot topics in water entry research. The coupled Euler-Lagrange method is used to carry out simulation research on the water entry process of a cylinder. Aiming at the water vapor mixing phenomenon caused by structure slamming on the water at the initial time of water entry, the slamming load is further studied by correcting the sound speed in the water. The differences between the calculated results obtained by adjusting the number of units in the numerical simulation prove the convergence of the numerical method. Water entry experiments of a cylinder were carried out, and the results are in good agreement with the simulation data. The motion state simulation and analysis are carried out for the process of water entry with different initial speeds and angles. The changes in the structure's positions, air cavities, and slamming loads are obtained. The rule of slamming pressure with the water entry angle and the relationship between pressure and acceleration are determined.

Research on the water-entry attitude of a submersible aircraft.

BACKGROUND: The water entry of a submersible aircraft, which is transient, highly coupled, and nonlinear, is complicated. After analyzing the mechanics of this process, the change rate of every variable is considered. A dynamic model is build and employed to study vehicle attitude and overturn phenomenon during water entry. Experiments are carried out and a method to organize experiment data is proposed. The accuracy of the method is confirmed by comparing the results of simulation of dynamic model and experiment under the same condition. RESULTS: Based on the analysis of the experiment and simulation, the initial attack angle and angular velocity largely influence the water entry of vehicle. Simulations of water entry with different initial and angular velocities are completed, followed by an analysis, and the motion law of vehicle is obtained. To solve the problem of vehicle stability and control during water entry, an approach is proposed by which the vehicle sails with a zero attack angle after entering water by controlling the initial angular velocity. With the dynamic model and optimization research algorithm, calculation is performed, and the optimal initial angular velocity of water-entry is obtained. CONCLUSIONS: The outcome of simulations confirms that the effectiveness of the propose approach by which the initial water-entry angular velocity is controlled.

How seabirds plunge-dive without injuries.

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.

Water entry for the black locust (Robinia pseudoacacia L.) seeds observed by dedicated micro-magnetic resonance imaging.

Water entry at germination for black locust (Robinia pseudoacacia L.) seeds which are known as hard seeds with impermeable seed coat to water, was examined using micro-magnetic resonance imaging (MRI). The MRI apparatus equipped with a low-field (1 T; Tesla) permanent magnet was used, which is open access, easy maintenance, operable and transportable. The excellent point of the apparatus is that T 1-enhancement of water signals absorbed in dry seeds against steeping free water is stronger than the apparatuses with high-field superconducting magnets, which enabled clear detection of water entry. Water hardly penetrated into the seeds for more than 8 h but approximately 60 % of seeds germinated by incubating on wet filter papers for several days. Hot water treatments above 75 °C for 3 min effectively induced water gap; scarification was 70 % at 100 °C and 75 °C, declined to 15 % at 50 °C and decreased further at room temperature. Water entered into the scarified seeds exclusively through the lens, spread along the dorsal side of the seeds and reached the hypocotyl, whereas water migrated slowly through hilum side to radicle within 3 h.

Paths of water entry and structures involved in the breaking of seed dormancy of Lupinus.

Physical dormancy is the water impermeability of the seed coat caused by one or more palisade cell layer(s) called macrosclereids. The specialised structure for water entry sites is the water gap, which serves as a detector of environmental cues for germination. In Fabaceae, the water gap is the lens, although another seed structure for water entry could exist. In this study, we identified the initial site of water entry, observed the hydration of a cushion-like structure near the radicle, described the anatomy of the water gap, and analysed the association of anatomical seed traits with the initial site of water entry and the imbibition velocity of six species of Lupinus from the state of Jalisco, Mexico. Dye tracking with a toluidine blue solution was used to identify the initial site of water entry. The anatomical description was performed using conventional microtechnique and a light microscope. The entry of the toluidine solution into seeds of L. montanus was observed after 6h, followed by L. exaltatus and L. mexicanus after 18h and L. elegans, L. reflexus and L. rotundiflorus after 48h. The site of water entry was the lens in L. elegans, L. exaltatus, L. reflexus and L. rotundiflorus and the micropyle in L. mexicanus and L. montanus. The cushion-like structure was responsible for water accumulation in embryo imbibition. Significant differences among anatomical seed traits such as thickness in the hilar region, the counter-palisade layer, cushion-like structure, epidermis, hypodermis, and innermost parenchyma layer were found among the species.

On the water lapping of felines and the water running of lizards: A unifying physical perspective.

We consider two biological phenomena taking place at the air-water interface: the water lapping of felines and the water running of lizards. Although seemingly disparate motions, we show that they are intimately linked by their underlying hydrodynamics and belong to a broader class of processes called Froude mechanisms. We describe how both felines and lizards exploit inertia to defeat gravity, and discuss water lapping and water running in the broader context of water exit and water entry, respectively.

A Model Based on Facilitated Passive Diffusion is Needed to Describe Root Water Entry through Aquaporins.

Despite abundant evidence that water transfer from soil to xylem occurs along a pathway regulated by aquaporins (AQPs) water entry is still modeled using principles of ordinary passive diffusion. Problems with this model have been known for some time and include variable intrinsic properties of conductivity Lp, changing reflection coefficients, sigma, and an inability to accurately resolve osmotic differentials between the soil and xylem. Here we propose a model of water entry based on principles of facilitated passive diffusion and following Michaelis-Menten formalism. If one accepts that water entry is controlled, at least in part, by AQPs, then a model of ordinary passive diffusion is precluded, as it does not allow for facilitation kinetics. By contrast, recognition of facilitated water entry through protein channels could explain shortcomings of ordinary passive diffusion, such as diurnal variability in conductivity which we have recently shown is directly correlated to diurnal changes in PsPIP2-1 mRNA levels in Pisum sativum.