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

Methods of data analysis for the micro-scale abrasion test on coated substrates

The micro-scale abrasive wear test is attractive for coated substrates because it is simple, only small samples are required, and the specific wear rates for both coating and substrate kappa(c) and kappa(s) can be determined simultaneously. This paper reviews and critically discusses the methods available for data analysis in this test and proposes some new approaches. The wear volumes of the coating and the substrate can be described by two parameters chosen from among the inner and outer crater diameters, the coating thickness, and the penetration depth. The inner crater diameter can usually be measured more accurately than the outer crater diameter since it is more clearly defined. It is recommended to obtain an accurate value for coating thickness, e.g. by creating and measuring a sharply defined crater, and then to calculate the wear volumes in terms of the inner crater diameter and the coating thickness. In general the errors in kappa(c) and kappa(s) are determined by the ratio of the specific wear rates kappa(c)/kappa(s), the non-dimensional parameter a(2)/Rt (where a is the inner crater diameter, R is the ball radius and t is the coating thickness), and the relative measurement errors of the inner crater diameter and the coating thickness. As these relative errors decrease, the errors in both kappa(c) and kappa(s) decrease. In addition a decrease in kappa(c)/kappa(s) and/or a(2) /Rt will decrease the error in K-c, but increase that in K-s. For a typical case where R= 12.5 mm, t= 3 +/- 0.15 mum, and a = 1 +/- 0.01 mm, the errors in kappa(c) and kappa(s) will be < 50% for 0.08 < kappa(c)/kappa(s) < 10. A lower limit to the inner crater diameter is proposed, determined by the ball radius and the abrasive particle size, in order to achieve reasonable accuracy in the data. A new method is proposed for plotting the experimental results, termed the double intercept method, which provides a clear graphical representation of the data and usually gives reliable values for kappa(c) and kappa(s). However, for the analysis of typical experimental data to obtain values for the specific wear rates another method, termed the KVH plot, is shown to be somewhat more consistently accurate. Detailed guidelines are proposed for analysing the data by this method. (C) 2003 Elsevier B.V. All rights reserved.status: publishe

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

Objectives: 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 drug-resistant epilepsy (DRE) was investigated. Methods: A double-blind, randomized, placebo-controlled clinical trial included ninety-nine (n = 99) subjects with DRE, aged 5?16 years (n = 85) and 17?45 years (n = 14). After randomization, subjects 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 (IRR) were tested after controlling for the covariate effects of gender, age, rate of seizure per week at enrollment, type of seizure, and number of antiepileptic drug (AED) combinations used at enrollment. Results: Fifty-nine subjects (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 IRRs of the EPA and DHA groups compared with the placebo group 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 with the placebo group (p < 0.05). Significance: This study demonstrates that EPA and DHA are effective in reducing seizure frequency in patients with DRE