Molecular structural property and potential energy dependence on nonequilibrium-thermodynamic state point of liquid n-hexadecane under shear

Abstract

Extensive computer experiments have been conducted in order to shed light on the macroscopic shear flow behavior of liquid n-hexadecane fluid under isobaric-isothermal conditions through the nonequilibrium molecular dynamic methodology. With respect to shear rates, the accompanying variations in structural properties of the fluid span the microscopic range of understanding from the intrinsic to extrinsic characteristics. As drawn from the average value of bond length and bond angle, the distribution of dihedral angle, and the radius distribution function of intramolecular and intermolecular van der Waals distances, these intrinsic structures change with hardness, except in the situation of extreme shear rates. The shear-induced variation of thermodynamic state curve along with the shear rate studied is shown to consist of both the quasiequilibrium state plateau and the nonequilibrium-thermodynamic state slope. Significantly, the occurrence of nonequilibrium-thermodynamic state behavior is attributed to variations in molecular potential energies, which include bond stretching, bond bending, bond torsion, and intra- and intermolecular van der Waals interactions. To unfold the physical representation of extrinsic structural deformation, under the aggressive influence of a shear flow field, the molecular dimension and appearance can be directly described via the squared radius of gyration and the sphericity angle, R(g)(2) and ϕ, respectively. In addition, a specific orientational order S(x) defines the alignment of the molecules with the flow direction of the x-axis. As a result, at low shear rates, the overall molecules are slightly stretched and shaped in a manner that is increasingly ellipsoidal. Simultaneously, there is an obvious enhancement in the order. In contrast to high shear rates, the molecules spontaneously shrink themselves with a decreased value of R(g)(2), while their shape and order barely vary with an infinite value of ϕ and S(x). It is important to note that under different temperatures and pressures, these three parameters are integrated within a molecular description in response to thermodynamic state variable of density and rheological material function of shear viscosity.

Potential-dependent morphological change of n-hexadecane small droplets on a Au(1 1 1) electrode in aqueous solution: A voltammetric and electrochemical fluorescence microscopic study

“…Comparison of d with the squared radii of gyration of a HD molecule in neat liquid HD, ca. 0.25 nm 2[45,46] connotes that a thin limit of HD liquid covers the entire Au(1 1 1) surface or the Au surface is imperfectly wetted by liquid HD around pzc of the electrode. Therefore, it should be discarded to estimate the HD coverage only from the capacitance value.The CV with HD inFig.…”

Methyl branch effects on rheological behaviours of short-chain polypropylene under steady shear studied via nonequilibrium molecular dynamics simulations

“…In the case of bond bending, notably, the varying tendencies of the potential energy at shear rates higher than 10 10 s 21 are contrary in the two fluids; there is a decrease in the short-chain PP while an increase in n-hexadecane [9]. The larger cutoff allows more pair interactions with negative energy involved, which leads a quite low potential energy for nhexadecane.…”

“…Torsion (upper triangles) and nonbonding L -J (lower triangles) potential energies versus shear rate plot of the NPT-NEMD simulations for the short-chain PP (filled symbols) and nhexadecane[9] (open symbols) at 250 MPa and 450 K. The large gap in the nonbonding potentials between the fluids is due to different cutoffs used: 2 1/6 s for PP and 2.5 s for n-hexadecane.…”

“…The bending (upper triangles) and bond stretching (lower triangles) potential energies versus shear rate plot of the NPT-NEMD simulations for the short-chain PP (filled symbols) and n-hexadecane[9] (open symbols) at 250 MPa and 450 K.…”