Viscoelastic relaxation time of the monoatomic Lennard-Jones system
Y Wang and LL Zhao, ACTA PHYSICA SINICA, 69, 123101 (2020).
Viscoelastic relaxation time is an important concept to characterize the viscoelastic response of materials, which is directly related to the interactions among the microscopic atoms of materials. Few studies have focused on the methods of characterizing viscoelastic relaxation time. To investigate how to represent viscoelastic relaxation time effectively, the viscoelastic relaxation times of the monoatomic Lennard-Jones system on 22 conditions in a range of T* = 0.85-5, rho* = 0.85-1, epsilon = 0.97-1, and sigma = 0.8-1.3 are discussed from a microscopic perspective by the equilibrium molecular dynamics methods. Static viscoelasticity (viscosity eta*, high-frequency shear modulus G(infinity)*) is calculated by the Green-Kubo formula, and the Fourier transform is applied to the calculation of dynamic viscoelasticity (storage modulus G'* and loss modulus G ''*). On this basis, the viscoelastic characteristic relaxation time (tau(MD)*) Maxwell relaxation time (tau(Maxwell)*) and the lifetime of the state of local atomic connectivity (tau(LC)*) are calculated. The viscoelastic characteristic relaxation time tau(MD)*, defined when the two responses crossover, is the key measure of the period of such a stimulus when the storage modulus (elasticity) equals the loss modulus (viscosity). Maxwell relaxation time tau(Maxwell)* = eta*/G(infinity)*, where eta* is the static viscosity under infinitely low stimulus frequency (i.e., zero shear rate), G(infinity)* is the instantaneous shear modulus under infinitely high stimulus frequency, and tau(LC)* is the time it takes for an atom to lose or gain one nearest neighbor. The result is observed that tau(LC)* is closer to tau(MD)* than tau(Maxwell)*. But the calculation of tau(LC)* needs to take into count the trajectories of all atoms in a certain time range, which takes a lot of time and computing resources. Finally, in order to characterize viscoelastic relaxation time more easily, Kramers' rate theory is used to describe the dissociation and association of atoms, according to the radial distribution functions. And a method of predicting the viscoelasticity of the monoatomic Lennard-Jones system is proposed and established. The comparison of all the viscoelastic relaxation times obtained above shows that tau(Maxwell)* is quite different from tau(MD)* at low temperature in the monoatomic Lennard-Jones system. Compared with tau(Maxwell)*, tau(LC)* is close to tau(MD)* But the calculation of tau(LC)* requires a lot of time and computing resources. Most importantly, the relaxation time calculated by our proposed method is closer to tau(MD)*. The method of predicting the viscoelastic relaxation time of the monoatomic Lennard-Jones system is accurate and reliable, which provides a new idea for studying the viscoelastic relaxation time of materials.
Return to Publications page