Hitting the mark: probing at the initiation site allows for accurate prediction of a thin shell's buckling load.

Geometric imperfections are understood to play an essential part in the buckling of a thin shell, but how multiple defects interact to control the onset of failure remains unclear. Here, we examine the failure of real cylindrical shells by experimentally poking soda cans with a large imparted dimple. By high-speed imaging of the can's surface, the initiation of buckling from axial loading is directly observed, revealing that larger dimples tend to set the initial buckling location. However, the influence of the shell's background geometric imperfections can still occasionally dominate, causing initiation to occur far from the dimple. In this situation, probing at the dimple leads to an over-prediction of the axial capacity. Using finite-element simulations, we understand our experimental results as a competition between the large dimple and the shell's inherent defect structure. In our simulations, we empirically observe a deformation-based criterion that connects the ideal poking location to the initiation site. This article is part of the theme issue 'Probing and dynamics of shock sensitive shells'.

Effects of footwear cushioning on leg and longitudinal arch stiffness during running.

During running, humans increase leg stiffness on more compliant surfaces through an in-series spring relationship to maintain constant support mechanics. Following this notion, the compliant midsole material of standard footwear may cause individuals to increase leg stiffness while running, especially in footwear with very thick midsoles. Recently, researchers have also proposed that footwear stiffness can affect the stiffness of the foot's longitudinal arch (LA) via a similar mechanism. To test these ideas, we used 3D motion capture to record 20 participants running on a forceplate-instrumented treadmill while barefoot, and while wearing three types of sandals composed of materials ranging an order of magnitude in Young's modulus: ethylene vinyl acetate (EVA), and two varieties of polyurethane rubber (R30 and R60). We calculated leg stiffness using standard methods, and measured LA stiffness based on medial midfoot kinematics. While there was an overall significant effect of footwear on leg stiffness (P = 0.047), post-hoc tests revealed no significant differences among individual pairs of conditions, and there was no effect of footwear on LA stiffness. However, participants exhibited significantly greater LA compression when barefoot than when running in EVA (P = 0.004) or R30 (P = 0.036) sandals. These results indicate that standard footwear midsole materials are too stiff to appreciably affect leg stiffness during running, meaning that increasing midsole thickness is unlikely to cause individuals to alter their leg stiffness. However, use of footwear does cause individuals to restrict LA compression when compared to running barefoot, and further research is needed to understand why.

Transition to stress focusing for locally curved sheets.

A rectangular thin elastic sheet is deformed by forcing a contact between two points at the middle of its length. A transition to buckling with stress focusing is reported for the sheets sufficiently narrow with a critical width proportional to the sheet length with an exponent 2/3 in the small thickness limit. Additionally, a spring network model is solved to explore the thick sheet limit and to validate the scaling behavior of the transition in the thin sheet limit. The numerical results reveal that buckling does not exist for the thickest sheets, and a stability criterion is established for the buckling of a curved sheet.

Microchannel measurements of viscosity for both gases and liquids.

Quantifying the viscosity of a fluid is of great importance in determining its properties and can even be used to identify what the fluid is. While many techniques exist for measuring the viscosity of either gases or liquids, it is very challenging to probe both gases and liquids with a single approach because of the significant difference in their nature, and the vast difference in the values of their viscosities. We introduce a facile approach to measuring the viscosity of a Newtonian fluid, either a gas or a liquid, by flowing it through a deformable microchannel where the deformation depends on the pressure required to induce the flow, which, in turn, depends on the fluid viscosity. A strain gauge embedded just above and across the microchannel transduces the flow-induced deformation into strain. The strain is proportional to the square of the flow-induced deformation enabling us to precisely discriminate not only gases but also liquids based on their viscosities with the same device.

Sniffing speeds up chemical detection by controlling air-flows near sensors.

Most mammals sniff to detect odors, but little is known how the periodic inhale and exhale that make up a sniff helps to improve odor detection. In this combined experimental and theoretical study, we use fluid mechanics and machine olfaction to rationalize the benefits of sniffing at different rates. We design and build a bellows and sensor system to detect the change in current as a function of odor concentration. A fast sniff enables quick odor recognition, but too fast a sniff makes the amplitude of the signal comparable to noise. A slow sniff increases signal amplitude but delays its transmission. This trade-off may inspire the design of future devices that can actively modulate their sniffing frequency according to different odors.

Nondestructive Prediction of the Buckling Load of Imperfect Shells.

From soda cans to space rockets, thin-walled cylindrical shells are abundant, offering exceptional load carrying capacity at relatively low weight. However, the actual load at which any shell buckles and collapses is very sensitive to imperceptible defects and cannot be predicted, which challenges the of such structures. Consequently, probabilistic descriptions in terms of empirical design rules are used and designing reliable structures requires the use of conservative strength estimates. We introduce a nonlinear description where finite-amplitude perturbations trigger buckling. Drawing from the analogy between imperfect shells which buckle and imperfect pipe flow which becomes turbulent, we experimentally show that lateral probing of cylindrical shells reveals their strength nondestructively. A new ridge-tracking method is applied to commercial cylinders with a hole showing that when the location where buckling nucleates is known we can accurately predict the buckling load of each individual shell, within ±5%. Our study provides a new promising framework to understand shell buckling, and more generally, imperfection-sensitive instabilities.

How localized force spreads on elastic contour feathers.

Birds can experience localized forces against their bodies due to impact against solid objects (like a branch or another bird) or water (during plunge-dives or landings). In this study, we hypothesize that densely packed contour feathers around the bird body would spread localized impact force while diving and maintaining plumage integrity. To test the hypothesis, we performed experiments with individual feathers and elastic beams, and developed a theoretical model to determine the response of feathers during the dive. First, we used a micro computed tomography scanner to characterize the internal structure of the contour feather from a northern gannet and calculate Young's modulus of feathers sampled from different parts of the body. This value was found to be of the order of 109 Pa for feathers from chest and belly. Second, we model the feathers as elastic beams taking into account their pre-curvature and non-uniform cross-section. Results from our experiments with polycarbonate beams suggest that the interaction of feathers on the skin patch redistributes the force, thereby reducing the impact on any particular area of the skin. Finally, a theoretical model of multiple overlaying feathers is proposed to quantify the spreading of impact force on the skin of the bird body which shows that the pressure on the skin at the impact point can be reduced by as much as three times the pressure if feathers had been absent.

Jumping dynamics of aquatic animals.

Jumping out of water is a phenomenon exhibited by a variety of aquatic and semi-aquatic animals. Yet, there is no common groundwork that clarifies the physical constraints required to jump out of water. In this study, we elucidate the physical conditions required for an animal to jump out of water. More than 100 jumps are analysed over five taxonomic groups. By balancing the power produced by animals with drag-induced dissipation, we expect that maximum jumping height, H, scales with body length, L, as H/ L ∼ L-1/3 ∼ Fr2, where the Froude number, Fr, is a ratio of inertia to gravity. To identify jumping regimes, simplified experiments are conducted by shooting axisymmetric bodies through the water surface. Here, we see a transition in which partial exits scale as H/ L ∼ Fr and complete exits scale as H/ L ∼ Fr2. A bioinspired robotic flapping mechanism was designed to mimic the fast motion of impulsive jumping animals. When exiting water, the robot carries a large volume of fluid referred to as an entrained mass. A theoretical model is developed to predict the jumping height of various water-exiting bodies, which shows that the mass of the entrained fluid relative to the mass of the body limits the maximum jumping height. We conclude that the lack of entrained fluid allows animals to reach extraordinary heights compared to our water-exiting robots.

Stability Landscape of Shell Buckling.

We measure the response of cylindrical shells to poking and identify a stability landscape, which fully characterizes the stability of perfect shells and imperfect ones in the case where a single defect dominates. We show that the landscape of stability is independent of the loading protocol and the poker geometry. Our results suggest that the complex stability of shells reduces to a low dimensional description. Tracking ridges and valleys of this landscape defines a natural phase-space coordinates for describing the stability of shells.

Physics of chewing in terrestrial mammals.

Previous studies on chewing frequency across animal species have focused on finding a single universal scaling law. Controversy between the different models has been aroused without elucidating the variations in chewing frequency. In the present study we show that vigorous chewing is limited by the maximum force of muscle, so that the upper chewing frequency scales as the -1/3 power of body mass for large animals and as a constant frequency for small animals. On the other hand, gentle chewing to mix food uniformly without excess of saliva describes the lower limit of chewing frequency, scaling approximately as the -1/6 power of body mass. These physical constraints frame the -1/4 power law classically inferred from allometry of animal metabolic rates. All of our experimental data stay within these physical boundaries over six orders of magnitude of body mass regardless of food types.

Popcorn: critical temperature, jump and sound.

Popcorn bursts open, jumps and emits a 'pop' sound in some hundredths of a second. The physical origin of these three observations remains unclear in the literature. We show that the critical temperature 180°C at which almost all of popcorn pops is consistent with an elementary pressure vessel scenario. We observe that popcorn jumps with a 'leg' of starch which is compressed on the ground. As a result, popcorn is midway between two categories of moving systems: explosive plants using fracture mechanisms and jumping animals using muscles. By synchronizing video recordings with acoustic recordings, we propose that the familiar 'pop' sound of the popcorn is caused by the release of water vapour.