Effects of kinked linear defects on planar flux line arrays

In the hard core limit, interacting vortices in planar type II superconductors can be modeled as non-interacting one dimensional fermions propagating in imaginary time. We use this analogy to derive analytical expressions for the probability density and imaginary current of vortex lines interacting with an isolated bent line defect and to understand the pinning properties of such systems. When there is an abrupt change of the direction of the pinning defect, we find a sinusoidal modulation of the vortex density in directions both parallel and perpendicular to the defect.Comment: 13 figure

Operating Principles of Peristaltic Pumping through a Dense Array of Valves

Immersed nonlinear elements are prevalent in biological systems that require a preferential flow direction. A certain class of models is investigated where the fluid is driven by peristaltic pumping and the nonlinear elements are ideal valves that completely suppress backflow. This highly nonlinear system produces discontinuous solutions that are difficult to study. As the density of valves increases, the pressure and flow are well-approximated by a continuum of valves which can be analytically treated. Interestingly, two different pumping mechanisms emerge from this model. At low frequencies, diffusive transport pushes open all but one valve, and the radius takes the shape of the imposed force. At high frequencies, half of the valves open, and the flow is determined by the advective transport induced by peristalsis. In either case, the induced flow is linear in the amplitude of the peristaltic forces and is independent of pumping direction. Despite the continuum approximation used, the physical valve density is accounted for by modifying the resistance of the fluid appropriately. The suppression of backflow causes a net benefit in adding valves when the valve density is low, but once the density is high enough, the dominant valve effect is to suppress the forward flow, suggesting there is an optimum number of valves per wavelength.Comment: 11 pages, 7 figure

Elastic building blocks for confined sheets

We study the behavior of thin elastic sheets that are bent and strained under the influence of weak, smooth confinement. We show that the emerging shapes exhibit the coexistence of two types of domains that differ in their characteristic stress distributions and energies, and reflect different constraints. A focused-stress patch is subject to a geometric, piecewise-inextensibility constraint, whereas a diffuse-stress region is characterized by a mechanical constraint - the dominance of a single component of the stress tensor. We discuss the implications of our findings for the analysis of elastic sheets that are subject to various types of forcing

Threading the spindle: a geometric study of chiral liquid crystal polymer microparticles

Polymeric particles are strong candidates for designing artificial materials capable of emulating the complex twisting-based functionality observed in biological systems. In this letter, we provide the first detailed investigation of the swelling behavior of bipolar polymer liquid crystalline microparticles. Deswelling from the spherical bipolar configuration causes the microparticle to contract anisotropically and twist in the process, resulting in a twisted spindle shaped structure. We propose a model to describe the observed spiral patterns and twisting behavior