Over the years, nonwear friction with single asperity contact has been examined through experiments using the Surface Force Apparatus (SFA) and the Atomic Force Microscope (AFM). The contact radii in SFA and AFM friction experiments ranged in the order of tens of [mu]m (>10⁵ m) and several nm (< 10⁻⁸ m), respectively. In spite of the fact that the contact radii in these experiments differ by several orders of magnitude, the data from both experiments obey Bowden and Tabor's friction law F = [tau]A , where F is the friction force, [tau] is the frictional shear strength and A is the real contact area. However, there is a crucial difference between the results obtained with the two instruments. The shear strength from the SFA experiments in dry environment is in the tens of MPa, while the shear strength from the AFM measurement is several hundreds of MPa. In the intervening mesoscales, with contact radii ranging from 10⁻⁸ < a < 10⁻⁵, the frictional shear strength must be dependent on contact area in order to link these two extremes. Some models based on dislocation motions have recently been developed to bridge the gap (Hurtado and Kim, 1999a; b). Hitherto, no systematic mesoscale friction experiments to bridge the shear strengths obtained from AFM and SFA have been provided. In addition, this is precisely the range in which MEMS and potential NEMS devices are expected to operate. Therefore, apart from the fundamental challenges involved in resolving the scale dependence of friction, there is a strong technological motivation for studying friction at this scale. In the present work, this transition in shear strength is bridged using a newlydeveloped Mesoscale Friction Tester (MFT) over a wide range of contact radii and relative humidity levels. Since a nonwear and single asperity contact is of interest, novel procedures to fabricate tungsten probes with subnanometer (<0.3nm) surface roughness are initiated. In order to choose an appropriate contact mechanics theory in an ambient environment to obtain the true contact area, a modified Tabor parameter for JKR-DMT transition for capillary force dominant contact is employed. Results from friction experiments show that the transition in shear strength occurred over contact radii of only 20~30nm in both ambient and dry environments. It is hypothesized that shear strengths in the tens of MPa resulted from contact separated by a monolayer of interfacial molecules and shear strengths in hundreds of MPa resulted from intimate contact (no interfacial molecules inside the contact zone). It was the interfacial condition inside the contact zone that governed the transition. Furthermore, there is no continuous spectrum of shear strength, but a "quantized" behavior. A continuum analysis based on Lifshitz theory, which related the shear strength to the estimated strength of van der Waals bonds is proposed to explain the quantized shear strengths obtained from current experiments and both previous AFM and SFA friction experiments.

published_or_final_version / Industrial and Manufacturing Systems Engineering / Master / Master of Philosophy

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Research has shown the combination of strength training and motor imagery can increase isometric force production. This study explored the impact of motor imagery on dynamic strength using a 3RM bench press and back squat. Participants were randomized into either the treatment or placebo condition and engaged in an11 week training program (motor imagery: n=8; motivational music: n=7). Results for both the upper and lower body strength showed a significant overall main effect for time from baseline to post-test measure (upper body: motor imagery: M= 43.5 kg, SD= 18.65 kg to M= 60.7 kg, SD= 24.0 kg; placebo: M= 45.0 kg, SD= 15.54 kg to M=55.0 kg, SD=17.9 kg; p=.000) (lower body: motor imagery: M= 82.9 kg, SD= 29.72 kg to M=110.0 kg, SD= 23.4 kg; placebo: M= 84.6 kg, SD= 20.29 kg to M=119.3 kg, SD= 24.6 kg; p=.000). The upper body strength displayed a significant interaction effect (p=.001) between program type and time, lower body strength had an insignificant interaction effect (p=.162, ?p2=.162). Finally, there was no between group significant difference for overall main of upper (p=.870, ?p2=.002) and lower body (p=.818, ?p2=.004) strength. These results suggest that motor imagery may have an impact on the development of strength over an 11 week training program. However, further understanding of imagery use and how it impacts strength is needed.

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The effects of various antecedents to cognitive elaboration on the structural properties and subjective beliefs related to attitude strength were examined in 2 experiments. Participants read vaccine-related information and received arguments for and against the implementation of a vaccine program for a fictitious virus. Their ability and motivation to attend to the central merits of the arguments were manipulated using a 2 (high vs. low ability) x 2 (high vs. low motivation) between-participants design, and the effects on 21 attitude- and attitude-strength related outcome variables were observed. Ability was determined by time pressure (Study 1) and distraction (Study 2), and motivation was determined by personal responsibility (Study 1) and involvement (Study 2). A meta-analysis of the experiments revealed that whereas none of the structural properties related to attitude strength were affected, all of the subjective beliefs were affected, and ability and motivation had differential effects. Ability primarily affected beliefs that reflect the validity and consistency of the knowledge base as well as the intensity of the attitude; motivation affected beliefs that reflect the intensity of the attitude and the functional relevance of the attitude object. As well, ability and motivation interacted on a number of variables, with a consistent pattern emerging: High (vs. low) motivation was associated with increased perceived knowledge, clarity, and accessibility, and decreased ambivalence under low ability; the opposite pattern occurred under high ability. Overall, this research represents the first systematic and the most comprehensive attempt to determine which strength dimensions are affected by variations in elaboration. It also provides evidence that the subjective beliefs might operate independent of the structural properties, that various antecedents to elaboration might not be directly interchangeable, and that ability and motivation might have different strength-related outcomes. Finally, this research demonstrated that when used in tandem, the effects of ability and motivation do not necessarily operate in an additive fashion, but can interact in some previously unforeseen ways. / Thesis (Ph.D, Psychology) -- Queen's University, 2010-08-23 10:51:04.52

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