Biomedical subjects
Publications and source records attributed to Janka Petravic.
We propose a phenomenological model for shear-induced melting aimed at assisting the design of experimental studies of this phenomenon. For increasing strain rates, the model predicts the changes in liquid fraction and shear stress as a function of interfacial supercooling. We discuss the experimental conditions under which shear-induced melting could be observed in a range of materials.
We present a method for the direct calculation at equilibrium of the shear viscosity and thermal conductivity over distances as short as one molecular diameter. The method is directly applicable to the calculation of viscosity and thermal conductivity in inhomogeneities such as the interface between coexisting phases. The method makes use of a novel extension of our recently developed boundary fluctuation theory.
In this paper we present, for the first time, a linear-response theory of transport coefficients-shear viscosity and thermal conductivity-involving thermal, as opposed to mechanical, fields. The theory involves the explicit treatment of the boundaries and the constraints that are applied to them. Expressions for the shear viscosity and thermal conductivity are obtained in terms of the fluctuations at the boundaries of the variable conjugate to that which is constrained. We explain how the choice of ensemble, as defined by the boundary constraints, determines the form in which the transport coefficients are evaluated.
We present a factorization of the Ewald sum permitting efficient computation of the reciprocal space part of the molecular representation for the heat flux vector. We use the derived expression to evaluate thermal conductivity of a model of ethanol at several near-ambient state points.
We use nonequilibrium molecular-dynamics (NEMD) simulations to characterize the response of a fluid subjected to an electric field. We focus on the response for very weak fields. Fields accessible by conventional NEMD methods are typically of the order of 10(9) V m(-1), i.e., several orders of magnitude larger than those typically used in experiments. Using the transient time-correlation function, we show how NEMD simulations can be extended to study systems subjected to a realistic dc electric field. We then apply this approach to study the response of molten sodium chloride for a wide range of dc electric fields.
Boundary driven shear flows and wall thermostats are being used in computer simulations of materials with increasing frequency. One attraction is that such boundary constraints offer a more realistic representation of the physical constraints imposed experimentally than the widely employed homogeneous constraints. In this paper we derive the linear response expressions for shear viscosity and thermal conductivity based on the fluctuations associated with boundary constraints. We demonstrate that our approach provides an effective method of describing the rheology in interfaces as well as bulk samples.
We study the dependence of viscosity of ethanol on shear rate using constant volume and constant pressure nonequilibrium molecular dynamics simulations, with the emphasis of the interrelationship between breaking, stability, and alignment of hydrogen bonds and shear thinning at high shear rates. We find that although the majority of hydrogen bond breakings occur at low shear rates, we do not observe shear thinning until there is some shear-induced alignment of the hydrogen bonds with the direction of shear.
We study the periodic time dependence of shear stress that occurs in a low- and a high-density fluid in a molecular dynamics algorithm for simulation of constant shear flow. We present a generalization of the linear response theory for a case when the equilibrium relaxation function and the equilibrium shear stress depend on strain. The predictions of this generalization reveal that the time dependence at low densities is a completely nonlinear effect. At high densities the amplitude of equilibrium oscillations of shear stress with strain is modified by strain-dependent viscosity, causing a decrease in amplitude proportional to shear rate in the linear response regime.
We systematically investigate the cooperative effects in shear stress relaxation using equilibrium molecular-dynamics simulations in periodic boundary conditions containing a variable degree of strain. We show that, even in simple liquids, shear stress relaxation is a cooperative effect associated with a correlation length that increases with isobaric decrease in temperature. If the system size is less than the correlation length, shear stress in the system is determined by the boundary strain. Transport, however, does not depend on the boundary conditions. We relate these two effects to the number and properties of the configurations accessible to the system.
We show that at high densities, as the system size decreases, liquid becomes able to permanently sustain increasing internal shear stress after a constant deformation, although the other characteristic liquid properties, such as the pair distribution function and diffusion coefficient do not change under strain. The system size necessary for observation of this effect increases with the decrease in temperature, and it is stronger in pair potentials with steeper repulsive part. We relate this result to the size of the "cooperatively rearranging regions" of the Adam-Gibbs theory of glass transition.
We study the shear stress relaxation and temperature dependence of the diffusion coefficient, viscosity, and thermal conductivity along a high-density Lennard-Jones isochore of the reduced density of 1.0, as it crosses the freezing and melting lines, in equilibrium and under constant strain.
We investigate evolution of an isolated system in an external field, and compare the ensemble averages of the response on successive constant internal energy surfaces to the ensemble averages of steady-state responses constrained to the same energy. We find that the two ensemble averages converge for sufficiently high energies, irrespective of the field strength and the initial energy from which the adiabatic evolution starts. This rule is satisfied for any phase-space distribution on the initial energy surface that can relax to equilibrium. At sufficiently high energies transport coefficients converge to their equilibrium values, because the effect of a constant field on the behavior of a system decreases with its temperature.
Recently, a solution for collision-free trajectories in an N particle thermostatted hard-sphere system undergoing homogeneous shear (the so-called "Sllod" equations of motion) led to a kinetic theory of dilute hard-sphere gases under shear. However, a solution for collisions, necessary for a complete theory at higher densities, has been missing. We present an analytic solution to this problem, which provides surprising insights into the mechanical aspects of thermostatting a system in an external field. The equivalence of constant temperature and constant energy ensembles in the thermodynamic limit in equilibrium, the conditions for the nature of heat exchange with the environment (entropy creation and reduction) in the system, and the condition for appearance of the artificial string phase follow from our solution.