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The scallop theorem posits that a two-link system immersed in a fluid at low Reynolds number cannot achieve any net translation via cyclic changes in its hinge angle. Here, we propose an approach to "breaking" this theorem, based on a static separation between the centers of mass and buoyancy in a net neutrally buoyant system. This separation gives the system a natural equilibrium orientation, allowing it to passively reorient without changing shape.

Swimming spermatozoa from diverse organisms often have very similar morphologies, yet different motilities as a result of differences in the flagellar waveforms used for propulsion. The origin of these differences has remained largely unknown. Using high-speed video microscopy and mathematical analysis of flagellar shape dynamics, we quantitatively compare sperm flagellar waveforms from marine invertebrates to humans by means of a novel phylokinematic tree. This new approach revealed that genetically dissimilar sperm can exhibit strikingly similar flagellar waveforms and identifies two dominant flagellar waveforms among the deuterostomes studied here, corresponding to internal and external fertilizers. The phylokinematic tree shows marked discordance from the phylogenetic tree, indicating that physical properties of the fluid environment, more than genetic relatedness, act as an important selective pressure in shaping the evolution of sperm motility. More broadly, this work provides a physical axis to complement morphological and genetic studies to understand evolutionary relationships.

We describe the inspiration, development, and deployment of a novel cocktail device modeled after a class of water-walking insects. Semi-aquatic insects like Microvelia and Velia evade predators by releasing a surfactant that quickly propels them across the water. We exploit an analogous propulsion mechanism in the design of an edible cocktail boat. We discuss how gradients in surface tension lead to motion across the water's surface, and detail the design considerations associated with the insect-inspired cocktail boat.

We present the results of a recent collaboration between scientists, engineers and chefs. Two particular devices are developed, both inspired by natural phenomena reliant on surface tension. The cocktail boat is a drink accessory, a self-propelled edible boat powered by alcohol-induced surface tension gradients, whose propulsion mechanism is analogous to that employed by a class of water-walking insects. The floral pipette is a novel means of serving small volumes of fluid in an elegant fashion, an example of capillary origami modeled after a class of floating flowers. The biological inspiration and mechanics of these two devices are detailed, along with the process that led to their development and deployment.

We consider the role of flexibility in the weight-bearing characteristics of bodies floating at an interface. Specifically, we develop a theoretical model for a twodimensional thin floating plate that yields the maximum stable plate load and optimal stiffness for weight support. Plates small relative to the capillary length are primarily supported by surface tension, and their weight-bearing potential does not benefit from flexibility. Above a critical size comparable to the capillary length, flexibility assists interfacial flotation. For plates on the order of and larger than the capillary length, deflection from an initially flat shape increases the force resulting from hydrostatic pressure, allowing the plate to support a greater load. In this large plate limit, the shape that bears the most weight is a semicircle, which displaces the most fluid above the plate for a fixed plate length. Exact results for maximum weight-bearing plate shapes are compared to analytic approximations made in the limits of large and small plate sizes. The value of flexibility for floating to a number of biological organisms is discussed in light of our study. C 2012 American Institute of Physics.

No abstract

Sound propagation of quasi-one-dimensional waves through a uniform duct partially filled with porous material has been studied theoretically and experimentally. The porous material makes the effective propagation wave number in the duct complex. A fairly simple theory based on cross-sectional averaging is derived and tested and found to work extremely well up to fairly high frequency. Interestingly, the basic theory depends only on the ratio of cross-sectional areas and the properties of the individual propagation media, but not on the specific configuration of material in a cross section. A higher order correction is developed to achieve excellent accuracy to very high frequency. This correction includes a coefficient that does depend on the specific cross-sectional configuration. Results are compared to exact solutions for layered and annular configurations, and also to experimental measurements with open cell foam as the porous material. An interesting application is to use measured wave numbers to predict the complex effective density and sound speed of porous media samples partially filling the duct. Other applications include fairly simple improved predictions of the behavior of sound in ducts lined with, or partially filled with, bulk reacting absorbing material.