Resistance Thermometer for Heat Transfer Measurements in a Shock Tube

This report describes a method for the application of the well-known principle of the resistance thermometer to the problem of measuring surface temperatures and heat transfer rates under highly transient conditions, such as are experienced in a shock tube. By using a thin platinum film sputtered on glass, a resistance thermometer gage is obtained which has a response lag of less than 1 µ sec, a linear output of 2-3 mv/°c, repeatability and durability. The gage preparation, including the sputtering technique, calibration method, and response characteristics are discussed, and some measurements of surface temperatures and heat transfer rates on models in the shock tube are presented in order to illustrate the performance that can be expected from this instrument

The distance upon contact: Determination from roughness profile

The point at which two random rough surfaces make contact takes place at the contact of the highest asperities. The distance upon contact d_0 in the limit of zero load has crucial importance for determination of dispersive forces. Using gold films as an example we demonstrate that for two parallel plates d_0 is a function of the nominal size of the contact area L and give a simple expression for d_0(L) via the surface roughness characteristics. In the case of a sphere of fixed radius R and a plate the scale dependence manifests itself as an additional uncertainty \delta d(L) in the separation, where the scale L is related with the separation d via the effective area of interaction L^2\sim\pi Rd. This uncertainty depends on the roughness of interacting bodies and disappears in the limit L\to \infty.Comment: 5 pages, 4 figures, to be published in PR

Resistance Thermometer for Transient High-Temperature Studies

Current interest in ultra-high-speed flight has stimulated the development of the shock tube as a device for simulating high stagnation temperatures in supersonic flow. The main difficulty in these tests is the extremely short duration of uniform flow, of the order of 100-500 µsec. This time interval dictates extremely fast response of any measuring instrument applied to shock tube research

Body-assisted van der Waals interaction between two atoms

Using fourth-order perturbation theory, a general formula for the van der Waals potential of two neutral, unpolarized, ground-state atoms in the presence of an arbitrary arrangement of dispersing and absorbing magnetodielectric bodies is derived. The theory is applied to two atoms in bulk material and in front of a planar multilayer system, with special emphasis on the cases of a perfectly reflecting plate and a semi-infinite half space. It is demonstrated that the enhancement and reduction of the two-atom interaction due to the presence of a perfectly reflecting plate can be understood, at least in the nonretarded limit, by using the method of image charges. For the semi-infinite half space, both analytical and numerical results are presented.Comment: 17 pages, 9 figure

Anomalous density dependence of static friction in sand

We measured experimentally the static friction force $F_s$ on the surface of a glass rod immersed in dry sand. We observed that $F_s$ is extremely sensitive to the closeness of packing of grains. A linear increase of the grain-density yields to an exponentially increasing friction force. We also report on a novel periodicity of $F_s$ during gradual pulling out of the rod. Our observations demonstrate the central role of grain bridges and arches in the macroscopic properties of granular packings.Comment: plain tex, 6 pages, to appear in Phys.Rev.

Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers

Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous imaging. A full study has been done for spherical particles with a +-20% size distribution. Stick-slip motion due to repetitive fluidization of the layer occurs for low driving velocities. Between major slip events, slight creep occurs that is variable from one event to the next. The effects of changing the stiffness k and velocity V of the driving system are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, but becomes irregular when k is large and V is relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period T and amplitude A of the relative motion are presented as a function of V. At a critical value Vc, a transition to continuous sliding motion occurs that is discontinuous for k not too large. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The force is a multi-valued function of the instantaneous velocity, with pronounced hysteresis and a sudden drop prior to resticking. Measurements of vertical displacement reveal a small dilation of the material (about one tenth of the mean particle size in a layer 20 particles deep) associated with each slip event. Finally, optical imaging reveals that localized microscopic rearrangements precede (and follow) each slip event. The behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.

The differential effects of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids on seizure frequency in patients with drug-resistant epilepsy – A Randomized, double-blind, placebo-controlled trial

Abstract The omega-3 (n-3) fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are known to play an important role in maintenance and modulation of neuronal functions. There is evidence that omega-3 fatty acids may have anticonvulsant effects. The effect of DHA and EPA on seizure rate in patients with DRE was investigated. Methods: A double-blind, randomized, placebo-controlled clinical trial included ninety-nine (n=99) DRE patients, aged 5-16 (n=85) and 17-45 (n=14). After randomization, patients were given two, four or six capsules per day of DHA (417.8 mg DHA and 50.8 mg EPA/capsule, n=33), EPA (385.6 mg EPA and 81.2 mg DHA/capsule, n=33) or placebo (high oleic acid sunflower oil, n=33) for one year. The primary endpoint was the effect of treatment on rate of seizure. Random-effects negative binomial regression models were fitted to model the patients’ total count of seizures per month. The treatment effects on seizure incidence rate ratio was tested after controlling for the covariate effects of gender, age, rate of seizure per week at enrollment, type of seizure and number of AEDs combinations used at enrollment. Results: Fifty-nine patients (n=59) completed the study (59.6%).The average number of seizures per month were 9.7 ± 1.2 in the EPA group, 11.7 ± 1.5 in the DHA group, and 16.6 ± 1.5 in the placebo group. Age, gender and seizure type adjusted seizure incidence rate ratios (IRRs) of the EPA and DHA groups compared with the placebo were 0.61 (CI= 0.42-0.88, p=0.008, 42% reduction) and 0.67 (CI = 0.46-1.0, p= 0.04, 39% reduction), respectively. There was no difference in IRR between the EPA and DHA groups (p=0.56). Both treatment groups had a significantly higher number of seizure-free days compared to placebo (p<0.05). Significance: This study demonstrates that EPA and DHA are effective in reducing seizure frequency in patients with DRE

Simulations of the Static Friction Due to Adsorbed Molecules

The static friction between crystalline surfaces separated by a molecularly thin layer of adsorbed molecules is calculated using molecular dynamics simulations. These molecules naturally lead to a finite static friction that is consistent with macroscopic friction laws. Crystalline alignment, sliding direction, and the number of adsorbed molecules are not controlled in most experiments and are shown to have little effect on the friction. Temperature, molecular geometry and interaction potentials can have larger effects on friction. The observed trends in friction can be understood in terms of a simple hard sphere model.Comment: 13 pages, 13 figure