PACS 47.63.Gd -Swimming of microorganisms PACS 87.85.gj -Movement and locomotion PACS 45.70.-n -Granular systemsAbstract. -The motility of the worm nematode Caenorhabditis elegans is investigated in shallow, wet granular media as a function of particle size dispersity and area density (φ). Surprisingly, we find that the nematode's propulsion speed is enhanced by the presence of particles in a fluid and is nearly independent of area density. The undulation speed, often used to differentiate locomotion gaits, is significantly affected by the bulk material properties of wet mono-and polydisperse granular media for φ ≥ 0.55. This difference is characterized by a change in the nematode's waveform from swimming to crawling in dense polydisperse media only. This change highlights the organism's adaptability to subtle differences in local structure and response between monodisperse and polydisperse media.

Dynamical systems theory has proven to be a successful approach to understanding mixing, with stretching and folding being the hallmark of chaotic mixing. Here we consider the mixing of a granular material in the context of a different mixing mechanism -cutting and shuffling-as a complementary viewpoint to that of traditional chaotic dynamics. Cutting and shuffling has a theoretical foundation in a relatively new area of mathematics called piecewise isometries (PWIs) with properties that are fundamentally different from the stretching and folding mechanism of chaotic advection. To demonstrate the effect of the cutting and shuffling combined with stretching and folding, we consider the mixing of granular materials of two different colors in a half-filled spherical tumbler that is rotated alternately about orthogonal axes. Mixing experiments using 1 mm particles in a 14 cm diameter tumbler are compared to PWI maps. The experiments are readily related to the PWI theory using continuum model simulations. By comparing experimental, simulation, and theoretical results, we demonstrate that mixing in a three-dimensional granular system can be viewed as mixing by traditional chaotic dynamics (stretching and folding) built on an underlying framework, or skeleton, of mixing due to cutting and shuffling. We further demonstrate that pure cutting and shuffling can generate a well-mixed system, depending on the angles through which the tumbler is rotated. We also explore the generation of interfacial area between the two colors of material resulting from both stretching in the flowing layer and cutting due to switching the axis of rotation.

Newtonian liquids that contain even small amounts ($ppm) of flexible polymers can exhibit viscoelastic behavior in extensional flows. Here, the effects of the presence of DNA molecules in viscous fluids on the dynamics of filament thinning and drop breakup are investigated experimentally in a cross-slot microchannel. Both bulk flow and single molecule experiments are presented. Suspensions of DNA molecules of different molecular weights (MW) are used, namely l-DNA (MW ¼ 3 Â 10 7 ) and T4 DNA (MW ¼ 1 Â 10 8 ). Results of both dilute (c/c * ¼ 0.5) and semi-dilute (c/c * ¼ 1) suspensions are compared to those of a viscous, Newtonian liquid. Results show that the dynamics of the high MW, semi-dilute suspension of T4 DNA are similar to viscoelastic fluids such as slow, exponential decay of the fluid thread and beads-on-a-string morphology. The exponential decay rate of the filament thickness is used to measure the steady extensional viscosity of all fluids. We find that the semi-dilute T4 DNA suspension exhibits extensional strain rate thinning extensional viscosity, while for all other fluids the extensional viscosity is independent of strain rate. Direct visualization of fluorescently labeled l-DNA molecules using high-speed imaging shows that the strong flow in the thinning fluid threads provide sufficient forces to stretch the majority of DNA molecules away from their equilibrium coiled state. The distribution of molecular stretch lengths, however, is very heterogeneous due to molecular individualism and initial conditions.

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Nanoscale electrochemical mapping techniques, e.g., scanning electrochemical cell microscopy (SECCM), have been increasingly used to study the local electrochemistry in electrocatalysis. Its capability of local electrochemistry mapping helps to reveal...