Rheological, thermodynamic, and structural studies of linear and branched alkanes under shear

Abstract

The rheological, thermodynamic, and structural behavior of linear and branched alkanes in simple shear is investigated using nonequilibrium molecular-dynamics simulation of united atom model fluids. Our results for the zero-shear viscosity of pure linear alkanes as well as mixtures of alkanes are in reasonable agreement with experiment. Simulation results for intermediate molecular weight linear alkanes indicate that the simple models employed here are capable of describing the pressure and temperature dependence of the viscosity. More importantly, our calculations indicate that addition of short, flexible branches to alkanes leads to a viscosity enhancement of a factor of 2 or more, thereby offering interesting possibilities for formulation of lubricants with specific properties.

Rheological and structural studies of liquid decane, hexadecane, and tetracosane under planar elongational flow using nonequilibrium molecular-dynamics simulations

“…All of our explanations seem to be physically plausible and consistent. Some physical properties, such as the hydrostatic pressure, appeared to be qualitatively similar to those observed under shear, 15,16 but not so for all the properties.…”

“…This same phenomenon is more obvious in the case of tetracosane, where we clearly see a minimum of the pressure at ˙͑m 2 / ͒ 1/2 = 0.02, below which it decreases with elongation rate, and above which it increases. This minimum behavior in the hydrostatic pressure has also been observed in NEMD simulations of shear flow for linear alkanes C 16 H 34 and C 28 H 58 by Khare et al, 16 and C 100 H 202 by Moore et al 26 Another interesting observation for tetracosane is that the pressure appears to reach a plateau value at very high elongation rates.…”

“…We explore three alkanes, C 10 H 22 ͑decane͒, C 16 H 34 ͑hexadecane͒, and C 24 H 50 ͑tetra-cosane͒, which previously have been studied under shear flow. [14][15][16] Thus, an advantage of choosing the three alkanes is to compare NEMD simulation results of PEF and those of shear flow, i.e., compare zero-elongation-rate and zero-shearrate viscosities. A NEMD simulation for these alkanes under shear flow was previously carried out by Cui et al; 15 therefore, it appears to be most appropriate to choose the same state points ͑see Sec.…”

Dynamics and rheology of a supercooled polymer melt in shear flow

“…[5][6][7] In chain systems without entanglements, on one hand, shear thinning occurs at sufficiently high (but sometimes unrealistically large) shear rates due to chain elongation. [8][9][10][11][12] In entangled polymers, on the other hand, shear thinning occurs at very small shear larger than τ −1 rep , where disentanglements are induced by shear. 3 Thus supercooled chain systems are most easily driven into a nonlinear response regime even by extremely small shear, though the crossover shear stress from linear to nonlinear regimes may not be very small.…”

“…In another application, nonequilibrium molecular dynamics (NEMD) simulations have been useful to investigate chain deformations and rheology in flow. [8][9][10][11][12]17,31 In particular, Kröger et al 17 studied the molecular mechanisms of the violations of stress-optical behavior for a melt consisting M = 260 chains with bead number N = 30 after application of elongational flow.…”