Shear behavior of squalane and tetracosane under extreme confinement. III. Effect of confinement on viscosity

Journal Article · Mon Dec 01 04:00:00 EST 1997 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.474173· OSTI ID:565114

Gupta, S A; Cochran, H D; Cummings, P T ^[1]

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1. Department of Chemical Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200 (United States)

This study uses nonequilibrium molecular dynamics simulation to explore the rheology of confined liquid alkanes. Two alkanes that differ in molecular structural complexity are examined: tetracosane (C{sub 24}H{sub 50}), which is a linear alkane, and squalane (C{sub 30}H{sub 62}), which has six symmetrically placed methyl branches along a 24 carbon backbone. These model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. This paper, the third of a three part series, compares the viscosities of the confined fluids to those of the bulk fluids. The alkanes are described by a well-documented potential model that has been shown to reproduce bulk experimental viscosity and phase equilibria measurements. Details of the simulation method, and structural information can be found in the preceding two papers of this series. The measured strain rates in these simulations range between 10{sup 8} and 10{sup 11} s{sup {minus}1}, which is typical of a number of practical applications. The confined fluids undergo extensive shear thinning, showing a power-law behavior. Comparison of results for the confined fluid to those for the bulk fluid reveal that, for the conditions examined, there is no difference between the bulk and confined viscosities for these alkanes. This observation is in contrast to experimental results at much lower strain rates (10{endash}10{sup 5} s{sup {minus}1}), which indicate the viscosities of the confined fluid to be much larger than the bulk viscosities. In making the comparison, we have carefully accounted for slip at the wall and have performed simulations of the bulk fluid at the same conditions of strain rate, temperature, and pressure as for the corresponding confined fluid. The viscosity is found to be independent of the wall spacing. The calculated power-law exponents are similar to experimentally observed values. We also note that the exponent increases with increasing density of the fluid. {copyright} {ital 1997 American Institute of Physics.}

• Research Organization: Oak Ridge National Laboratory
• DOE Contract Number: AC05-96OR22464
• OSTI ID: 565114
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 23 Vol. 107; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
ALKANES
DENSITY
LIQUIDS
LUBRICANTS
MOLECULAR DYNAMICS METHOD
RHEOLOGY
SHEAR PROPERTIES
SIMULATION
SLIP FLOW
SQUALANE
THIN FILMS
VISCOSITY