Structure and Magnetization of Two-Dimensional Vortex Arrays in the Presence of Periodic Pinning

Ground-state properties of a two-dimensional system of superconducting vortices in the presence of a periodic array of strong pinning centers are studied analytically and numerically. The ground states of the vortex system at different filling ratios are found using a simple geometric argument under the assumption that the penetration depth is much smaller than the spacing of the pin lattice. The results of this calculation are confirmed by numerical studies in which simulated annealing is used to locate the ground states of the vortex system. The zero-temperature equilibrium magnetization as a function of the applied field is obtained by numerically calculating the energy of the ground state for a large number of closely spaced filling ratios. The results show interesting commensurability effects such as plateaus in the B-H diagram at simple fractional filling ratios.Comment: 12 pages, 19 figures, submitted for publicatio

Interaction model for magnetic holes in a ferrofluid layer

Nonmagnetic spheres confined in a ferrofluid layer (magnetic holes) present dipolar interactions when an external magnetic field is exerted. The interaction potential of a microsphere pair is derived analytically, with a precise care for the boundary conditions along the glass plates confining the system. Considering external fields consisting of a constant normal component and a high frequency rotating in-plane component, this interaction potential is averaged over time to exhibit the average interparticular forces acting when the imposed frequency exceeds the inverse of the viscous relaxation time of the system. The existence of an equilibrium configuration without contact between the particles is demonstrated for a whole range of exciting fields, and the equilibrium separation distance depending on the structure of the external field is established. The stability of the system under out-of-plane buckling is also studied. The dynamics of such a particle pair is simulated and validated by experiments.Comment: 15 pages, 11 figures (18 with subfigures). to appear in Phys. Rev.

Commensurate and Incommensurate Vortex Lattice Melting in Periodic Pinning Arrays

We examine the melting of commensurate and incommensurate vortex lattices interacting with square pinning arrays through the use of numerical simulations. For weak pinning strength in the commensurate case we observe an order-order transition from a commensurate square vortex lattice to a triangular floating solid phase as a function of temperature. This floating solid phase melts into a liquid at still higher temperature. For strong pinning there is only a single transition from the square pinned lattice to the liquid state. For strong pinning in the incommensurate case, we observe a multi-stage melting in which the interstitial vortices become mobile first, followed by the melting of the entire lattice, consistent with recent imaging experiments. The initial motion of vortices in the incommensurate phase occurs by an exchange process of interstitial vortices with vortices located at the pinning sites. We have also examined the vortex melting behavior for higher matching fields and find that a coexistence of a commensurate pinned vortex lattice with an interstitial vortex liquid occurs while at higher temperatures the entire vortex lattice melts. For triangular arrays at incommensurate fields higher than the first matching field we observe that the initial vortex motion can occur through a novel correlated ring excitation where a number of vortices can rotate around a pinned vortex. We also discuss the relevance of our results to recent experiments of colloidal particles interacting with periodic trap arrays.Comment: 8 figure

Drying nano particles solution on an oscillating tip at an air liquid interface: what we can learn, what we can do

Evaporation of fluid at micro and nanometer scale may be used to self-assemble nanometre-sized particles in suspension. Evaporating process can be used to gently control flow in micro and nanofluidics, thus providing a potential mean to design a fine pattern onto a surface or to functionalize a nanoprobe tip. In this paper, we present an original experimental approach to explore this open and rather virgin domain. We use an oscillating tip at an air liquid interface with a controlled dipping depth of the tip within the range of the micrometer. Also, very small dipping depths of a few ten nanometers were achieved with multi walls carbon nanotubes glued at the tip apex. The liquid is an aqueous solution of functionalized nanoparticles diluted in water. Evaporation of water is the driving force determining the arrangement of nanoparticles on the tip. The results show various nanoparticles deposition patterns, from which the deposits can be classified in two categories. The type of deposit is shown to be strongly dependent on whether or not the triple line is pinned and of the peptide coating of the gold nanoparticle. In order to assess the classification, companion dynamical studies of nanomeniscus and related dissipation processes involved with thinning effects are presented

Grasshoppers alter jumping biomechanics to enhance escape performance under chronic risk of spider predation

1. Prey can increase the probability of escaping a predator attack by adopting either behavioural tactics that provide more time or less distance to escape or by increasing their escape performances. It is assumed that the ability to improve escape performances is reserved for species that respond to chronic predation risk by substantially altering morphological traits related to locomotion. This thinking is at odds with fundamental predictions of physiological stress theory that suggest performance should be enhanced both independently of and in addition to morphological change. 2. The purpose of this study was to determine whether and how prey raised under chronic risk of predation can improve their escape performance independent of conspicuous morphological changes. 3. We reared herbivore grasshopper in field mesocosms with and without disarmed hunting spiders and measured their outdoor jumping performance. Grasshoppers were then transferred to the laboratory where we measured the biomechanics of their jumping to elucidate mechanisms that differentiate performance between rearing treatments. We collected second generation nymphal offspring from the field mesocosms and measured their morphology and escape performances to reveal costs associated with antipredator responses. 4. Chronically scared grasshoppers made adjustments in jumping technique that enabled them to take-off 1.2 times faster, which, in combination with additional in-flight behavioural adjustments, led to 2.6 times longer jump distances. These improvements were independent of conspicuous morphological changes but were accompanied by attendant costs: grasshoppers reared in risky environments produced smaller and less athletic offspring than grasshoppers from risk free environments. 5. The results suggest that prey do not need to undergo conspicuous morphological changes in locomotor traits before they can improve escape performance, revealing an adaptive and possibly common aspect of physiological stress reaction to predation. Keywords: escape behaviour, functional traits, inducible defenses, jumping technique, physiological stress, predation risk, predator–prey interactions, whole-animal performance

Ballistic motion of spirochete membrane proteins

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Probing Eukaryotic Chemotaxis with Optically Manipulated Biomimetic Microparticles

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Solid capillarity: when and how does surface tension deform soft solids?

Soft solids differ from stiff solids in an important way: their surface stresses can drive large deformations. Based on a topical workshop held in the Lorentz Center in Leiden, this Opinion highlights some recent advances in the growing field of solid capillarity and poses key questions for its advancement

Spatiotemporal control of chemical microenvironments of single cells

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