Mesoscopic pinning forces in neutron star crusts

The crust of a neutron star is thought to be comprised of a lattice of nuclei immersed in a sea of free electrons and neutrons. As the neutrons are superfluid their angular momentum is carried by an array of quantized vortices. These vortices can pin to the nuclear lattice and prevent the neutron superfluid from spinning down, allowing it to store angular momentum which can then be released catastrophically, giving rise to a pulsar glitch. A crucial ingredient for this model is the maximum pinning force that the lattice can exert on the vortices, as this allows us to estimate the angular momentum that can be exchanged during a glitch. In this paper we perform, for the first time, a detailed and quantitative calculation of the pinning force \emph{per unit length} acting on a vortex immersed in the crust and resulting from the mesoscopic vortex-lattice interaction. We consider realistic vortex tensions, allow for displacement of the nuclei and average over all possible orientation of the crystal with respect to the vortex. We find that, as expected, the mesoscopic pinning force becomes weaker for longer vortices and is generally much smaller than previous estimates, based on vortices aligned with the crystal. Nevertheless the forces we obtain still have maximum values of order $f_{\rm{pin}}\approx 10^{15}$ dyn/cm, which would still allow for enough angular momentum to be stored in the crust to explain large Vela glitches, if part of the star is decoupled during the event.Comment: 17 pages, 16 figures, 5 table

Determining physical properties of the cell cortex

Actin and myosin assemble into a thin layer of a highly dynamic network underneath the membrane of eukaryotic cells. This network generates the forces that drive cell and tissue-scale morphogenetic processes. The effective material properties of this active network determine large-scale deformations and other morphogenetic events. For example,the characteristic time of stress relaxation (the Maxwell time)in the actomyosin sets the time scale of large-scale deformation of the cortex. Similarly, the characteristic length of stress propagation (the hydrodynamic length) sets the length scale of slow deformations, and a large hydrodynamic length is a prerequisite for long-ranged cortical flows. Here we introduce a method to determine physical parameters of the actomyosin cortical layer (in vivo). For this we investigate the relaxation dynamics of the cortex in response to laser ablation in the one-cell-stage {\it C. elegans} embryo and in the gastrulating zebrafish embryo. These responses can be interpreted using a coarse grained physical description of the cortex in terms of a two dimensional thin film of an active viscoelastic gel. To determine the Maxwell time, the hydrodynamic length and the ratio of active stress and per-area friction, we evaluated the response to laser ablation in two different ways: by quantifying flow and density fields as a function of space and time, and by determining the time evolution of the shape of the ablated region. Importantly, both methods provide best fit physical parameters that are in close agreement with each other and that are similar to previous estimates in the two systems. We provide an accurate and robust means for measuring physical parameters of the actomyosin cortical layer.It can be useful for investigations of actomyosin mechanics at the cellular-scale, but also for providing insights in the active mechanics processes that govern tissue-scale morphogenesis.Comment: 17 pages, 4 figure

ULTRASONIC MONITORING FOR THE EVALUATION OF CONDITIONING BY TRAINING SESSION FOR ATHLETES

Non-intrusive ultrasonic detection scheme has been implemented to monitor and quantify the loading effect of training sessions on athletes. The detection is obtained along a line between two acoustic transducers with similar size and shape as stick-on electrodes. All the data is derived from the transmission time-of-flight of the ultrasonic chirp signal passing through the muscle and the ultrasonic force sensor. Muscle dynamics and force generated due to maximum isometric contraction was synchronously detected with the aid of an arbitrary function generator and a two channels transient recorder. At least 16 performance deciding parameters of athletes are quantified. The achieved spatial and temporal resolutions are ± 0.01 mm and 0.01 ms respectively. Detected movement reaction time could be used as a potential indicator to identify false-start in athletics, swimming and other necessary fields

HIGH SPEED ULTRASONIC DETECTION SCHEME FOR SPORTS PERFORMANCE MONITORING

To observe muscle performance of athletes with high resolution a novel ultrasonic detection scheme has been developed. It is based on bulk waves passing the monitored muscle. The detection is obtained along a line between two acoustic transducers with similar size and shape as stick-on electrodes, mounted on the skin. The time-of-flight from which all the data is derived is observed with the aid of a computer controlled arbitrary function generator and a synchronized transient recorder. An available separate channel can be used for synchronous monitoring of the force or pressure or the EMG-signals. The demonstrated movement and time resolution is ± 0.02 mm and 0.01 ms respectively. The equipment of lap-top size is battery operated and suitable for on-field monitoring

Electronic structure of unidirectional superlattices in crossed electric and magnetic fields and related terahertz oscillations

We have studied Bloch electrons in a perfect unidirectional superlattice subject to crossed electric and magnetic fields, where the magnetic field is oriented ``in-plane'', i.e. in parallel to the sample plane. Two orientation of the electric field are considered. It is shown that the magnetic field suppresses the intersubband tunneling of the Zener type, but does not change the frequency of Bloch oscillations, if the electric field is oriented perpendicularly to both the sample plane and the magnetic field. The electric field applied in-plane (but perpendicularly to the magnetic field) yields the step-like electron energy spectrum, corresponding to the magnetic-field-tunable oscillations alternative to the Bloch ones.Comment: 7 pages, 1 figure, accepted for publication in Phys. Rev.

NON INVASIVE MONITORING OF IN-VIVO MUSCLE-TENDON MECHANICAL PROPERTIES OF ATHLETES WITH AN ULTRASONIC DETECTION SCHEME

A system has been developed and applied for monitoring where the lateral muscle extension is detected with the aid of an ultrasonic caliper. Ultrasonic monitoring is furthermore used to detect synchronously the force exerted by the activated limbs. The resolution for muscle extension is ± 0.01mm and that of force is ± 1.5N. The force-length relation is observed for the gastrocnemius muscle for rising voluntary isometric contraction up to maximum contraction and subsequent relaxation. The measurement principle is based on synchronous monitoring of variations of the time-of-flight of the ultrasound passing the muscle and synchronous monitoring with an ultrasonic force sensor that also serves to keep the flexion of the joint constant. The observed forcelength relation displays a hysteresis that is indicative of the athlete's training condition

Analysis of High-Perimeter Planar Electrodes for Efficient Neural Stimulation

Planar electrodes are used in epidural spinal cord stimulation and epidural cortical stimulation. Electrode geometry is one approach to increase the efficiency of neural stimulation and reduce the power required to produce the level of activation required for clinical efficacy. Our hypothesis was that electrode geometries that increased the variation of current density on the electrode surface would increase stimulation efficiency. High-perimeter planar disk electrodes were designed with sinuous (serpentine) variation in the perimeter. Prototypes were fabricated that had equal surface areas but perimeters equal to two, three or four times the perimeter of a circular disk electrode. The interface impedance of high-perimeter prototype electrodes measured in vitro did not differ significantly from that of the circular electrode over a wide range of frequencies. Finite element models indicated that the variation of current density was significantly higher on the surface of the high-perimeter electrodes. We quantified activation of 100 model axons randomly positioned around the electrodes. Input–output curves of the percentage of axons activated as a function of stimulation intensity indicated that the stimulation efficiency was dependent on the distance of the axons from the electrode. The high-perimeter planar electrodes were more efficient at activating axons a certain distance away from the electrode surface. These results demonstrate the feasibility of increasing stimulation efficiency through the design of novel electrode geometries