Effects of Large Load and Shear Rate Variations on the Friction of a Branched Hydrocarbon Liquid
- PDF / 194,079 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 10 Downloads / 165 Views
Y8.33.1
Effects of Large Load and Shear Rate Variations on the Friction of a Branched Hydrocarbon Liquid Delphine Gourdon and Jacob Israelachvili Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, U.S.A. ABSTRACT Shear measurements were performed on mica surfaces with molecularly thin films of squalane confined between them. Squalane is a branched hydrocarbon liquid that can be in the liquid, glassy or liquid-crystalline state under confinement. The friction forces, especially the transitions between smooth and intermittent (e.g., stick-slip) sliding, were measured over a wide range of applied loads and sliding velocities. The results reveal that, depending on the conditions, qualitatively different behavior can arise in the same system. These include both abrupt and continuous transitions, short and very long transient effects, and chaotic or saw-tooth stick-slip. The differences between these branched and simpler molecules are compared, and the results are analyzed in terms of rate-and-state models traditionally used in the analysis of seismic phenomena. INTRODUCTION One of the most important questions regarding the motion of rubbing surfaces is what happens when one of them is pushed at a constant driving velocity or force relative to the other. Often, the surfaces themselves do not move at a constant velocity, i.e., ‘smoothly’, but ‘intermittently’, and when this occurs the measured friction force is also not constant. Intermittent motion can be regular, as in the case of regular stick-slip, or irregular, when both the amplitude, frequency and shapes of the ‘friction spikes’ can vary in a chaotic or random way. (Mathematically, chaotic and random are not synonymous: chaotic motion may appear to be random but is deterministic; random motion is totally unpredictable.) Such phenomena are now recognized as being common and a central issue in many different systems and disciplines [1,2], including tribology (friction), materials science (fracture, failure), geology (seismic motion, earthquakes), the flow of granular materials (avalanches), and the generation of sound (as in the case of a violin), to name a few. Much experimental and numerical work has already been done on the shearing properties of molecularly thin liquid films – including both ‘good’ lubricating fluids such as linear or branched chained hydrocarbons or polymer melts, and simple liquids that are not normally employed as lubricants [3,4]. In the case of certain polymer surfaces and branched chained hydrocarbon liquids, which are commonly used in ‘lube oils’, shear measurements on ‘single asperity’ molecularly smooth micron-sized contacts have revealed surprisingly long relaxation times and large characteristic ‘memory distances’ [5]. Here we extend our study over a larger range of experimental variables, especially to higher loads (pressures), which are closer to those commonly encountered in engineering and seismic systems.
Y8.33.2
EXPERIMENTAL DETAILS AND RESULTS A Surface Forces Apparatus (SFA),
Data Loading...
