To live and clamber about in an arboreal habitat, tree frogs have evolved adhesive pads on their toes. In addition, they often have long and slender legs to facilitate not only long jumps, but also to bridge gaps between leaves when climbing. Both adhesive pads and long limbs are used in conjunction, as we will show in this study. Previous research has shown that tree frogs change from a crouched posture (where the limbs are close to the body) to a sprawled posture with extended limbs when clinging on to steeper inclines such as vertical or overhanging slopes. We investigated this change in posture in White's tree frogs (Litoria caerulea) by challenging the frogs to cling onto a tiltable platform. The platform consisted of an array of 24 three-dimensional force transducers, which allowed us to measure the ground reaction forces of the frogs during a tilt. Starting from a crouched resting position, the normal forces on the forelimbs changed sign and became increasingly negative with increasing slope angle of the platform. At about 1068 + 128, tilt of the platform the frogs reacted by extending one or two of their limbs outwards. At a steeper angle (1318 + 118), the frogs spread out all their limbs sideways, with the hindlimbs stretched out to their maximum reach. Although the extension was strongest in the lateral direction, limbs were significantly extended in the fore-aft direction as well. With the extension of the limbs, the lateral forces increased relative to the normal forces. The large contribution of the in-plane forces helped to keep the angle between the force vector and the platform small. The Kendall theory for the peeling of adhesive tape predicts that smaller peel angles lead to higher attachment forces. We compare our data with the predictions of the Kendall model and discuss possible implications of the sliding of the pads on the surface. The forces were indeed much larger for smaller angles and thus can be explained by peeling theory.

Anurans are well known for their jumping abilities, making use of their strong hindlimbs. In contrast, the function of the forelimbs during take-off has rarely been studied. We measured the ground reaction forces exerted by forelimbs and hindlimbs during short jumps in the Dybowski's frog Rana dybowskii. Take-off occurred in two phases. Phase one (from the initial time until the forelimbs took off), which lasts a relatively long time (63.2 ± 4.1% of the total take-off phase, N = 20), provides sufficient time for the forelimbs to elevate the body to a suitable posture to deliver the best take-off angle. Phase two (from the forelimbs lift-off until hindlimbs lift-off) was dominated by the hindlimbs which provided a constant and fast elevation. The force angle (angle of the resultant vector from fore-aft and normal force components towards the plane of the substrate) of the hindlimbs and body trajectory was variable before the forelimbs lifted off of the substrate and then primarily followed the direction of the line from the foot-substrate point to the center of mass (COM). The preparation angle adopted when the forelimbs lifted off of the substrate was a good predictor of the take-off angle. The total normal force oscillated around body weight (BW) before the forelimb normal force peaked. The BW shifted from the hindlimbs to the forelimbs during the initial phase of take-off. A simple lever model suggests that the forelimbs are responsible for raising the COM, thus influencing the take-off angle in short jumps.

The mean square size of topologically interlocked molecules (TIMs) is presented as a linear combination of contributions from the backbone and subcomponents. Using scaling analyses and extensive molecular dynamics simulations of polycatenanes as a typical example of TIMs, we show that the effective exponent ν­(m) for the size dependence of the backbone on the monomer number of subcomponent m is asymptotic to a value ν (∼0.588 in good solvents) with a correction of m –0.47, which is the same as for the covalently linked polymer. However, the effective exponent for the size dependence of subcomponents on m is asymptotic to the same value ν but with a new correction of m –1.0. The different corrections to the scaling on the backbone and subcomponent structure induce a surprising double asymptotic behavior for the architecture of the TIMs. The scaling model that takes into account the double asymptotic behavior is in good quantitative agreement with the simulation result that the effective exponent for the size dependence of TIMs on m increases with the subcomponent number n. The full scaling functional form of the size dependence on m and n for polycatenanes in a good solvent is well described by a simple sum of two limiting behaviors with different corrections.

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Although aluminum (Al) powder is a potential fuel that has been widely used in many important fields, Al powders are dangerous because of its flammable and explosive characteristics. From this point of view, the investigation of the explosion characteristics of Al powders is of critical importance. The characteristics of a dust explosion are closely related to the dispersion behavior of the powders. The purpose of this research is to analyze the dispersion behavior of Al powder and the uniformity of dust cloud. The dispersion process of Al powders was studied using a high‐speed camera. Image processing technology was applied to obtain the transmission value of the light beam passing through the Al dust cloud at different locations within a vessel. The images obtained by the high‐speed camera indicated that the dispersion process had three stages, that is, a fast injection stage, diffusion stage, and stabilization stage. By examining the transmission‐time histories at 13 different locations in the chamber and at various Al concentrations (from 100 to 800 g/m3), the decay behavior of transmission under different concentrations was obtained, and the decay time fluctuated from 44 to 68 ms. By comparing the deviation of transmission at different locations and for various concentrations in the stabilization stage, the maximum SD obtained was 8.2%, that meant the dust clouds were relatively uniform. Finally, based on the experimental results, the ignition delay time (after which the powder/air mixture is considered homogeneous) corresponding to different dust concentrations should be different to ensure the accuracy of explosion characteristics test.