Daichi Torii

Molecular dynamics study of thermal phenomena in an ultrathin liquid film sheared between solid surfaces: the influence of the crystal plane on energy and momentum transfer at solid-liquid interfaces.

A molecular dynamics study has been performed on a liquid film sheared between moving solid walls. Thermal phenomena that occur in the Couette-like flow were examined, including energy conversion from macroscopic flow energy to thermal energy, i.e., viscous heating in the macroscopic sense, and heat conduction from the liquid film to the solid wall via liquid-solid interfaces. Four types of crystal planes of fcc lattice were assumed for the surface of the solid wall. The jumps in velocity and temperature at the interface resulting from deteriorated transfer characteristics of thermal energy and momentum at the interface were observed. It was found that the transfer characteristics of thermal energy and momentum at the interfaces are greatly influenced by the types of crystal plane of the solid wall surface which contacts the liquid film. The mechanism by which such a molecular scale structure influences the energy transfer at the interface was examined by analyzing the molecular motion and its contribution to energy transfer at the solid-liquid interface.

Molecular-Scale Mechanism of Thermal Resistance at the Solid-Liquid Interfaces: Influence of Interaction Parameters Between Solid and Liquid Molecules

The solid-liquid interfacial thermal resistance is getting more and more important as various solid-liquid systems are utilized in nanoscale, such as micro electro-mechanical systems/nano electro-mechanical systems (MEMS/NEMS) with liquids and nanoparticle suspension in liquids. The present paper deals with the transport of thermal energy through the solid-liquid interfaces, and the goal is to find a molecular-scale mechanism that determines the macroscopic characteristics of the transport phenomena. Nonequilibrium molecular dynamics simulations have been performed for systems of a liquid film confined between atomistic solid walls. The two solid walls have different temperatures to generate a steady thermal energy flux in the system, which is the element of macroscopic heat conduction flux. Three kinds of liquid molecules and three kinds of solid walls are examined, and the thermal energy flux is measured at the control surfaces in the liquid film and at the solid-liquid interfaces. The concept of boundary thermal resistance is extended, and it is defined for each degree of freedom of translational motion of the molecules. It is found that the interaction strength between solid and liquid molecules uniformly affects all boundary thermal resistances defined for each degree of freedom; the weaker interaction increases all the resistances at the same rate and vice versa. The boundary thermal resistances also increase when the solid and liquid molecules are incommensurate, but the incommensurability has a greater influence on the boundary thermal resistances corresponding to the molecular motion parallel to the interface than that for the normal component. From these findings it is confirmed that the thermal resistance for the components parallel to the interface is associated with the molecular-scale corrugation of the surface of the solid wall, and that the thermal resistance for the component normal to the interface is governed by the number density of the solid molecules that are in contact with the liquid.

Contribution of inter- and intramolecular energy transfers to heat conduction in liquids

The molecular dynamics expression of heat flux, originally derived by Irving and Kirkwood [J. Chem. Phys. 18, 817 (1950)] for pairwise potentials, is generalized in this paper for systems with many-body potentials. The original formula consists of a kinetic part and a potential part, and the latter term is found in the present study to be expressible as a summation of contributions from all the many-body potentials defined in the system. The energy transfer among a set of sites for which a many-body potential is defined is discussed and evaluated by the rate of increase in the kinetic energy of each site due to the potential, and its accumulation over all the potentials in the system is shown to make up the potential part of the generalized expression. A molecular dynamics simulation for liquid n-octane was performed to demonstrate the applicability of the new expression obtained in this study to measure the heat flux and to elucidate the contributions of inter- and intramolecular potentials to heat conduction.

Molecular dynamics study on ultrathin liquid water film sheared between platinum solid walls: Liquid structure and energy and momentum transfer

Molecular dynamics simulation has been performed on a liquid film that is sheared in between solid surfaces. As a shear is given to the liquid film, a Couette-like flow is generated in the liquid and energy conversion occurs from the macroscopic flow to the thermal energy, which is discharged back to the solid walls. In such a way, momentum and thermal energy fluxes are present simultaneously. And all these thermal and fluid phenomena take place in highly nonequilibrium state where thermal energy is not distributed equally to each degree of freedom of molecular motion in the vicinities of the solid-liquid interface. In the present paper, platinum and water are employed as solid and liquid, respectively. First, the structure and orientation of water molecules in the vicinities of the solid surfaces are analyzed and how these structure and orientation are influenced by the shear is considered. Based on this result, momentum and thermal energy transfer in the vicinities of and at the solid-liquid interfaces are investigated in detail. Results are compared with those of our previous study, in which monatomic and diatomic molecules are employed as liquid.

Molecular thermal phenomena in an ultrathin lubrication liquid film of linear molecules between solid surfaces

Molecular dynamics simulation has been performed on an ultra-thin lubrication liquid film, where the liquid film of diatomic molecules having a thickness of molecular scale (several nanometers) is sheared by two parallel solid walls moving at different speeds. The Couette-like flow is generated and energy conversion from the macroscopic flow energy to thermal energy, which is the viscous heating in a macroscopic sense, occurs in the liquid film. It was observed in the present simulations that the thermal phenomena in the liquid film are far from the macroscopically expected ones; thermal energy is not distributed evenly to each degree of freedom of molecular motion, and energy transfer in the liquid adjacent to the solid surface is contributed by molecular motion in a manner different from those in a bulk liquid.

Suppression of the secondary flow in a suction channel of a large centrifugal pump

The suction channel configuration of a large centrifugal pump with a 90-degree bend was studied in detail to suppress the secondary flow at the impeller inlet for improving suction performance. Design of experiments (DOE) and computational fluid dynamics (CFD) were used to evaluate the sensitivity of several primary design parameters of the suction channel. A DOE is a powerful tool to clarify the sensitivity of objective functions to design parameters with a minimum of trials. An L9 orthogonal array was adopted in this study and nine suction channels were designed, through which the flow was predicted by steady state calculation. The results indicate that a smaller bend radius with a longer straight nozzle, distributed between the bend and the impeller, suppresses the secondary flow at the impeller inlet. An optimum ratio of the cross sectional areas at the bend inlet and outlet was also confirmed in relationship to the contraction rate of the downstream straight nozzle. These findings were obtained by CFD and verified by experiments. The results will aid the design of large centrifugal pumps with better suction performance and higher reliability.

Ion Transport By The Thermally Rectified Brownian Ratchet

The Brownian ratchet is a mechanism that is effective in nanoscale for ion transport and molecular motor, in which isotropic thermal diffusion of charged particles such as ions is rectified by some asymmetric ratcheting and net mass flux in a certain direction is generated. The principle is important for future design of novel devices for particles separation, molecular motors, and ion pumps. In the present article, it is demonstrated that the combination of a nonuniform temperature field and a spatially symmetric ratchet, either of which induces no net mass transport when being applied alone, works to rectify the thermal mass diffusion and generate net mass flux of charged particles. This finding is important in the following two points: It adds a new mode of the Brownian ratchet to rectify the diffusion of particles and suggests a new method to transport particles utilizing a preexisting temperature gradient.

Heat Conduction in Liquids Involving Many-Body Potentials : Contribution of Inter- and Intramolecular Energy Transfer(Thermal Engineering)

The molecular dynamics expression of heat conduction flux, which was originally derived by Irving and Kirkwood (1950) for pair-wise potentials, is generalized to systems with many-body potentials defined. The formula, newly derived in the present report, consists of kinetic part and potential part, and the latter term is expressed as a summation of thermal energy transfer in each group of interaction sites for which a many-body potential is defined. Next, the mechanism of the thermal energy transfer involving many-body potentials is clarified by extending the concept of intermolecular energy transfer previously proposed by one of the authors, and the accumulation of the thermal energy transfer over all the potentials and interaction sites in the system is proved to make up the potential part of the expression. A molecular dynamics simulation is conducted for liquid n-octane to examine the applicability of the expression newly obtained, and the contributions of the inter- and intramolecular potentials to heat conduction are elucidated.

Molecular Scale Mechanism of Thermal Resistance at Solid-Liquid Interfaces (Influence of Interaction Parameters Between Solid and Liquid Molecules)

Nonequilibrium molecular dynamics simulations have been performed for systems of a liquid film confined between atomistic solid walls. The two solid walls have different temperatures to generate a steady thermal energy flux in the system, which is the element of macroscopic heat conduction flux. Three kinds of liquid molecules and three kinds of solid walls are examined, and the thermal energy flux is measured at control surfaces in the liquid film and at the solid-liquid interfaces. By analyzing the thermal energy flux in detail by decomposing it into several molecular-scale contributions, influence of interaction parameters between solid and liquid molecules and the spacing of molecular alignment on the surface of the solid wall are clarified, and the molecular-scale mechanisms that govern the thermal resistance at a solid-liquid interface are elucidated.Copyright

Molecular Dynamics Study on Ultra-Thin Liquid Film Sheared between Solid Surfaces (Influence of the Crystal Plane to Energy and Momentum Transfer at the Solid-Liquid Interfaces)

Molecular dynamics simulation has been performed on a liquid film sheared in between solid walls. As a shear is given to the liquid film by moving the solid walls, the Couette-like flow is generated in the liquid film and energy conversion occurs from the macroscopic flow to the thermal one, i.e., viscous heating in the macroscopic sense. In such a way, momentum transfer and thermal energy transfer are present simultaneously. At the solid-liquid interfaces, large jumps in velocity and temperature according to the large momentum and heat flux are observed. It has been revealed in the present study that the characteristics of the energy and momentum transfer at the interfaces are greatly influenced by the crystal plane of the solid walls which contact the liquid film.