Static Normal Force Research Articles

Representation of moving tactile stimuli in the somatic sensory cortex of awake monkeys

1. We have studied how neurons of primary somatic sensory (SI) cortex encode the direction and speed of moving tactile stimuli delivered to the glabrous skin of the contralateral hand. 2. From a total of 178 neurons recorded in SI cortex of 3 awake Macaca mulatta monkeys, 103 were selected for quantitative analysis. Forty-six neurons had slowly adapting (SA) responses, 43 quickly adapting (QA), and 14 mixed SA-QA properties. All possessed cutaneous receptive fields on the distal segments of digits 2, 3, or 4. Receptive fields were scanned with a metal probe (2 mm diam, hemispheric tip) in four different directions (0, 90, 180, and 270 degrees), over a fixed traverse distance of 6 mm, at a variety of speeds (4-100 mm/s), and with a static normal force of 20 g by means of a computer-controlled tactile stimulator. 3. Most neurons gave statistically significant differences in mean impulse rate during the moving stimuli (Wilcoxon, P < 0.01), in at least one of the four directions, compared with the control (nonstimulus) period. The Kruskal-Wallis test (P < 0.01) and the direction index (DI > 35%) determined that about one-half of SI cortical neurons showed significant differences in mean impulse rates associated with the direction of the stimuli at the speeds of 23, 50, and 100 mm/s, and about one-third at 4 mm/s. 4. We determined how the temporal covariance of the neural activity was associated with the parameters of the moving stimuli by calculating the coefficients of the Karhunen-Loève (KL) transform for each set of stimulus responses. Decomposition of the neural activity into principal components indicated that approximately 85% of the impulse train variance during the stimulus responses was contained in the 1st 10 coefficients of the KL transform for the speeds of 23, 50, and 100 mm/s, and approximately 75% at the speed of 4 mm/s. The line spectra calculated from the coefficients of the KL transform showed that the variance contained in the impulse trains in about one-half of the neurons is related to the stimuli. 5. We investigated how the temporal covariance of the neuronal activity was correlated with the direction of the stimulus, by fitting the first coefficient of the KL transform to a weighting function model. This analysis showed that the first coefficient of the KL transform varied as an orderly function of the direction of the moving stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Nanorheology of Confined Polymer Melts. 1. Linear Shear Response at Strongly Adsorbing Surfaces

The linear-response effective shear moduli of polymer melts confined between strongly adsorbing surfaces (parallel plates of mica) was studied as a function of the excitation frequency. Linear response (achieved with shear amplitudes of e2 A) implies that measurements did not perturb the film structure. The measurements employed a surface forces apparatus modified for dynamic mechanical shear oscillation. The polymers (atactic poly(phenylmethylsiloxane), PPMS, chain length from 31 to 153 skeletal bonds) were selected to be glass-forming with a low glass transition temperature, in order to avoid issues of possible surface-induced crystallization which have been much discussed. Three principal conclusions emerged. First, in a comparison of polymers of different molecular weight, both the shear moduli (measured at fxed film thickness) and the static normal forces (to compress the polymers to this film thickness) scaled approximately with the estimated unperturbed radius of gyration of the chain (RG). Second, a transition from fluid- to solidlike response occurred when the film thickness was less than 5-6 RG: the longest relaxation time slowed to the point that the storage shear modulus (G) exceeded the loss shear modulus (G) over the entire frequency spectrum. This contrasts strongly with the behavior of the bulk samples, for which Rouse dynamics would be expected. Third, the magnitude of G under conditions of strong confinement was characteristic of rubberlike elasticity, approximately lo5 N mF2 for the range of chain length that was studied. This indicates enhanced entanglement interactions in thin polymer films. Two accompanying papers analyze the nonlinear rheology and the respective influences of surface adsorption and of geometrical confinement.

Using bridging analogies and anchoring intuitions to deal with students' preconceptions in physics

AbstractLessons were designed to deal with students' alternative conceptions in three areas of mechanics: static normal forces, frictional forces, and Newton's third law for moving objects. Instructional techniques such as class discussions of the validity of an analogy between a target problem and an intuitive anchoring example, and forming a structured chain of intermediate bridging analogies were used. There were large differences in pre–posttest gains in favor of the experimental group. In formulating a model of learning processes that can explain these results, it is argued that (a) the lessons have a more complex structure than a simple model of analogy use; (b) rational methods using analogy and other plausible reasoning processes that are neither proof based nor directly empirical can play a very important role in science instruction; (c) much more effort than is usually allocated should be focused on helping students to make sense of an analogy; and (d) researchers and curriculum developers should be focusing at least as much attention on students' useful prior knowledge as they are on students' alternative conceptions.

In situ auger electron spectroscopy characterization of wet-CO 2-lubricated sliding copper electrical contact

The electrical contact resistance, elemental surface composition and friction of an OFHC copper slip ring rotating in contact with two high purity copper wire brushes on different tracks were investigated in situ for heavy and light normal contact forces under a wet CO 2 environment at atmospheric pressure. Scanning electron microscopy was also used to characterize the slip ring and brush surfaces. Previous work in ultrahigh vacuum showed that, as rotation proceeded, interfacial impurities were almost totally removed and the electrical contact resistance decreased until cold welding occurred. In the present work, the slip ring surface was sputter cleaned (more than 95% Cu) before contact rotation and was only slightly contaminated after rotating in wet CO 2. Both the contact resistance and the friction decreased quickly and reached steady state values almost simultaneously in the early stages of rotation. Also, cold welding phenomena did not occur. Scanning electron micrographs taken after each experiment showed that the surfaces of the slip ring tracks and the brush wire ends were much rougher when heavy normal contact forces were used than in the light normal force condition. All these results confirm that wet CO 2 is an effective lubricant for Cu-Cu electrical sliding contacts.