Effects of Defects on Friction for a Xe Film Sliding on Ag(111)

The effects of a step defect and a random array of point defects (such as vacancies or substitutional impurities) on the force of friction acting on a xenon monolayer film as it slides on a silver (111) substrate are studied by molecular dynamic simulations and compared with the results of lowest order perturbation theory in the substrate corrugation potential. For the case of a step, the magnitude and velocity dependence of the friction force are strongly dependent on the direction of sliding respect to the step and the corrugation strength. When the applied force F is perpendicular to the step, the film is pinned forF less than a critical force Fc. Motion of the film along the step, however, is not pinned. Fluctuations in the sliding velocity in time provide evidence of both stick-slip motion and thermally activated creep. Simulations done with a substrate containing a 5 percent concentration of random point defects for various directions of the applied force show that the film is pinned for the force below a critical value. The critical force, however, is still much lower than the effective inertial force exerted on the film by the oscillations of the substrate in experiments done with a quartz crystal microbalance (QCM). Lowest order perturbation theory in the substrate potential is shown to give results consistent with the simulations, and it is used to give a physical picture of what could be expected for real surfaces which contain many defects.Comment: 13 pages, 17 figures, latex plus postscript files for figure

Theory of a Possible Mechanism for Lubrication and Surface Protection by an Electrically Neutral Hydrogels

It is demonstrated that polymers sticking out of the surface of a neutral hydrogel are capable of preventing adhesive forces from pulling a hydrogel into close contact with a surface against which it is pressed. The proposed mechanism for lubrication or surface protection suggests a possible mechanism for protecting the cornea from a contact lens, which is held against the eye by Laplace pressure. This mechanism, however, is only able to keep a gel coated surface from sticking to a surface against which it is pressed, if the gel and surface are bathed in fluid. Expected optical properties of the gel-surface interface are discussed, in order to suggest possible ways to study the gel-solid interface experimentally

Investment in Fixed and Working Capital During Early Industrialization: Evidence From U.S. Manufacturing Firms

This paper utilizes a survey of the US manufacturing firms from 1832 to investigate the structure of manufacturing investment during early industrialization. Although several manufacturing industries, such as cotton textiles, depart from the pattern, most appear to have devoted the hulk of their investments to working capitaL This variation across industries in the composition of capital investmentsis indicative of a more general variation in factor intensities, and bears on the issues of why industries became concentrated in the regions they did, and the degrees to which they were adversely affected by the limited availability of long-term loans. Evidence that most manufacturing industries had quite modest investments in machinery and tools per unit of labor is also presented, serving to undercut the notion that the early period of industrialization was based on a proliferation of new, machinery-intensive technologies

The Heights of Americans in Three Centuries: Some Economic and Demographic Implications

This paper discusses the potential usefulness of anthropometric measurements in exploring the contributions of nutrition to American economic growth and demographic change. It argues that although the value of height-by-age data to economic historians will ultimately be resolved in the context of investigating specific issues, the early results of the NBER Projecton Long-term Trends in Nutrition, Labor Productivity, and Labor Welfare have been encouraging. Among the most significant findings to date are: (1)that by the time of the Revolution, Americans had attained a mean final height (and net nutritional status) that was very high, one that European populations did not generally reach until the twentieth century; (2) that the variation in stature across occupational classes was much less in the U.S. than in Europe; (3) that natives of the South have been taller than those from other regions of the U.S. since the middle of the eighteenth century, and that their absolute height increased during the antebellum period while mortality was declining there; and (4) that natives of large antebellum cities were much shorter than their country men born in rural areas or in small cities. The paper also examines, in a preliminary fashion, how a newly available data set bears on the hypothesis that a cycle in U.S. final heights began during the antebellum period. The theory might continue to be sustained, but a sample of U.S. Army recruits from 1850 to1855 does not seem to provide much support for it.