Anomalous Damping of a Low Frequency Vibrating Wire in Superfluid 3He-B due to Vortex Shielding

We have investigated the behaviour of a large vibrating wire resonator in the B-phase of superfluid ³He at zero pressure and at temperatures below 200 μK. The vibrating wire has a low resonant frequency of around 60 Hz. At low velocities the motion of the wire is impeded by its intrinsic (vacuum) damping and by the scattering of thermal quasiparticle excitations. At higher velocities we would normally expect the motion to be further damped by the creation of quasiparticles from pair-breaking. However, for a range of temperatures, as we increase the driving force we observe a sudden decrease in the damping of the wire. This results from a reduction in the thermal damping arising from the presence of quantum vortex lines generated by the wire. These vortex lines Andreev-reflect low energy excitations and thus partially shield the wire from incident thermal quasiparticles.     

Measuring the Prong Velocity of Quartz Tuning Forks Used to Probe Quantum Fluids

Recently, quartz tuning forks have been used to probe the dynamics of quantum fluids. For many of these measurements it is important to know the velocity amplitude of the tips of the vibrating fork prongs. We have used different techniques to establish, with an accuracy of a few percent, the relationship between the electrical and mechanical properties of several commercial quartz tuning forks with fundamental resonant frequency ～32 kHz. The velocity is usually inferred from an electro-mechanical calibration that models a quartz prong as a clamped, rectangular cantilever beam. We have tested the accuracy of this calibration using three methods: measurement of the amplitude at which the fork prongs touch each other; direct optical measurement of the moving fork prongs using strobe microscopy; and a Michelson interferometry technique operating with a 670 nm laser. All three methods yield consistent results. The velocity so determined is found to be 10% lower than that of the standard electro-mechanical calibration.     

Family group cognitive–behavioral preventive intervention for families of depressed parents: 18- and 24-month outcomes.

Objective: In a long-term follow-up of a randomized controlled trial (Compas et al., 2009) to examine the effects at 18- and 24-month follow-ups of a family group cognitive–behavioral (FGCB) preventive intervention for mental health outcomes for children and parents from families (N = 111) of parents with a history of major depressive disorder (MDD). Method: Parents with a history of MDD and their 9- to 15-year-old children were randomly assigned to a FGCB intervention or a written information comparison condition. Children's internalizing, externalizing, anxiety/depression, and depressive symptoms; episodes of MDD and other psychiatric diagnoses; and parents' depressive symptoms and episodes of MDD were assessed at 18 and 24 months after randomization. Results: Children in the FGCB condition were significantly lower in self-reports of anxiety/depression and internalizing symptoms at 18 months and were significantly lower in self-reports of externalizing symptoms at 18 and 24 months. Rates of MDD were significantly lower for children in the FGCB intervention over the 24-month follow-up (odds ratio = 2.91). Marginal effects were found for parents' symptoms of depression at 18 and 24 months but not for episodes of MDD. Conclusions: Support was found for a FGCB preventive intervention for children of parents with a history of MDD significantly reducing children's episodes of MDD over a period of 2 years. Significant effects for the FGCB intervention were also found on internalizing and externalizing symptoms, with stronger effects at 18- than at 24-month follow-up. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

A New Device for Studying Low or Zero Frequency Mechanical Motion at Very Low Temperatures

We have developed a new ??floppy wire?? device for studying the motion through quantum fluids and solids at very low temperatures. The device is particularly well suited for producing large amplitudes of motion, for measuring drag forces at low frequency, and for studying ??zero?? frequency dynamics by measuring transient behavior. The device is very versatile and allows motion to be studied over a broad range of velocities and amplitudes. It generates negligible heat leaks and so is ideally suited for ultra low temperature experiments. The device has many potential applications in quantum fluids and solids research, including the study of drag forces at low frequencies in both the laminar and turbulent flow regimes, and the investigation of motion in (super)solid ⁴He. We discuss the principles and modes of operation of the device and present some preliminary measurements in vacuum, in normal liquid ³He and in superfluid ⁴He. We also present measurements of a ??floppy grid?? device, which could be used for generating large volumes of quantum turbulence in superfluids at low temperatures.     

Hysteresis,Switching and Anomalous Behaviour of a Quartz Tuning Fork in Superfluid 4He

We have been studying the behaviour of commercial quartz tuning forks immersed in superfluid ⁴He and driven at resonance. For one of the forks we have observed hysteresis and switching between linear and non-linear damping regimes at temperatures below 10 mK. We associate linear damping with pure potential flow around the prongs of the fork, and non-linear damping with the production of vortex lines in a turbulent regime. At appropriate prong velocities, we have observed metastability of both the linear and the turbulent flow states, and a region of intermittency where the flow switched back and forth between each state. For the same fork, we have also observed anomalous behaviour in the linear regime, with large excursions in both damping, resonant frequency, and the tip velocity as a function of driving force.     

Chloroform transport in stress-whitened polypropylene

The transport property of chloroform in undeformed and stress-whitened polypropylenes was studied. The rate of penetration of liquid chloroform in whitened samples prepared by uniaxial stretching was slower than that of the undeformed material. In the whitened region, the process appeared to be Fickian diffusion, while in the undeformed material, it was a combination of Fickian and Case-II. The diffusivity (at 50°C) in the whitened region increased about 8 times as the strain rate of stretching decreased from 0.67 to 0.033/min. The temperature dependence of diffusivity (ε = 0.67/min) showed an activation energy of 10.16 Kcal/mole. The whitening effect was greatly reduced as chloroform was absorbed; the final residual whitening after desorption increased with the rate of stretching as determined by transmission densitometry. For the undeformed polypropylene, Case-II type of matrix relaxation contributed significantly to the total flux. The transport process was relaxation controlled at the surface: it gradually changed to Fickian diffusion behavior toward the middle plane. The flux contribution of relaxation across the thickness increased with the sorption time, and that of diffusion decreased. These observations seem to indicate that cold drawing impeded the deformation by swelling stress from occurring at the penetration front.     

A novel equivalent circuit model for gap-connected cells

Gap junctions connect neighbouring cells, providing the intercellular communication that is essential for cell growth regulation, for example. There is some evidence that gap communication changes upon exposure to electromagnetic (EM) fields. In previous work, we performed detailed finite element method (FEM) modelling of gap junction connected cells exposed to EM fields. For cell configurations, the presence of gap junctions influences the transmembrane potential and its frequency behaviour. The relaxation frequency cannot be accurately predicted by previously developed simplified models. We present a novel equivalent circuit model (ECM) that incorporates more detailed models of the gaps, and compare results obtained with this ECM to finite element and leaky cable (LC) model results. Our ECM provides more accurate estimates of the frequency behaviour of cells than the leaky cable model. Also, our ECM results suggest limitations of the application of simple models to gap-connected cells: with higher gap resistivity, the current flow in the cell interiors becomes increasingly complex and is not well represented by simple models. In this case, techniques such as the finite element method are required to model accurately cell behaviour.     

Biological cells with gap junctions in low-frequency electricfields

Biological effects have been observed from weak, low-frequency magnetic fields. It has been suggested that the observed effects are due to the induced currents and electric fields. The behavior of cells exposed to an electric field is investigated in this paper. The induced transmembrane potential (TMP) is examined in geometrically complex models of various cell configurations. The TMP is evaluated using the finite element method (FEM), a numerical technique that is well suited to complicated geometries. Because displacement currents can be neglected at very low frequencies, a FEM solver that considers only material conductivity is used. Therefore, our results apply only well below the relaxation frequency. Chains and clusters of gap-connected cells of various sizes are modeled. The conductivity and size of the gap junctions in the cell configurations are also varied. The results for small configurations are compared to models of ellipsoidal cells with shapes similar to those of the configurations. FEM estimates of TMPs in long, cylindrical cell chains are compared to the predictions of the leaky cable model. The FEM approach confirms that gap-junction-connected cells can be treated as a single similarly shaped cell. Gaps influence the potential in the interior of cell configurations, and these effects increase with gap size and conductivity. For configurations to which approximations such as the leaky cable model do not apply, the FEM approach can be used to estimate the TMP, if the model is adapted to fit within computational memory limits     

Modeling assemblies of biological cells exposed to electric fields

Gap junctions are channels through the cell membrane that electrically connect the interiors of neighboring cells. Most cells are connected by gap junctions, and gaps play an important role in local intercellular communication by allowing for the exchange of certain substances between cells. Gap communication has been observed to change when cells are exposed to electromagnetic (EM) fields. In this work, the authors examine the behavior of cells connected by gap junctions when exposed to electric fields, in order to better understand the influence of the presence of gap junctions on cell behavior. This may provide insights into the interactions between biological cells and weak, low-frequency EM fields. Specifically, the authors model gaps in greater detail than is usually the case, and use the finite element method (FEM) to solve the resulting geometrically complex cell models. The responses of gap-connected cell configurations to both dc and time harmonic fields are investigated and compared with those of similarly shaped (equivalent) cells. To further assess the influence of the gap junctions, properties such as gap size, shape, and conductivity are varied. The authors' findings indicate that simple models, such as equivalent cells, are sufficient for describing the behavior of small gap connected cell configurations exposed to dc electric fields. With larger configurations, some adjustments to the simple models are necessary to account for the presence of the gaps. The gap junctions complicate the frequency behavior of gap-connected cell assemblies. An equivalent cell exhibits lowpass behavior. Gaps effectively add a bandstop filter in series with the low-pass behavior, thus lowering the relaxation frequency. The characteristics of this bandstop filter change with changes to gap properties. Comparison of the FEM results to those obtained with simple models indicates that more complex models are required to represent gap-connected cells