Interaction Potential Strength Research Articles

Effect of wall stiffness, mass and potential interaction strength on heat transfer characteristics of nanoscale-confined gas

The interactive thermal wall model is applied in three-dimensional molecular dynamics simulations to investigate the combined effect of the wall force field, the wall stiffness, the wall atom mass and the wall/gas interaction potential strength on the heat transfer characteristics of static rarefied argon gas within a nanochannel. By increasing the wall stiffness, a reduction in the heat flux through the gas medium occurs which leads to a higher temperature jump. As the wall atom mass is increased up to twice the argon atom mass, the heat flux is enhanced notably and a minimum temperature jump can be found at this point. Further increase in the wall atom mass results in reducing the heat flux and consequently increasing the temperature jump. The increment of the wall/gas interaction potential strength up to four times the one of gas/gas interactions is shown to enhance the heat flux and to reduce the temperature jump until it eventually vanishes. Furthermore, it is found that under such conditions, the density profile experiences a second peak. A further increase of this parameter is found to have a negligible effect on the heat flux through the gas medium and it only increases the second peak in the density profile.

Rheological behavior of colloidal suspension with long-range interactions

In this work, we study the constitutive behavior of interacting colloidal suspensions for moderate and high concentrations. Specifically, using a lattice Boltzmann solver, we numerically examine suspensions flowing through narrow channels, and explore the significance of the interaction potential strength on the system's macroscopic response. When only a short-range interaction potential is considered, a Newtonian behavior is always recovered and the system's effective viscosity mostly depends on the suspension concentration. However, when using a Lennard-Jones potential we identify two rheological responses depending on the interaction strength, the volume fraction, and the pressure drop. Exploiting a model proposed in the literature we rationalize the simulation data and propose scaling relations to identify the relevant energy scales involved in these transport processes. Moreover, we find that the spatial distribution of colloids in layers parallel to the flow direction does not correlate with changes in the system macroscopic response; but, interestingly, the rheology changes do correlate with the spatial distribution of colloids within individual layers. Namely, suspensions characterized by a Newtonian response display a cubiclike structure of the colloids within individual layers, whereas for suspensions with non-Newtonian response colloids organize in a hexagonal structure.

Dissipative particle dynamics study of velocity autocorrelation function and self-diffusion coefficient in terms of interaction potential strength

ABSTRACTThis research focuses on numerically investigating the self-diffusion coefficient and velocity autocorrelation function (VACF) of a dissipative particle dynamics (DPD) fluid as a function of the conservative interaction strength. Analytic solutions to VACF and self-diffusion coefficients in DPD were obtained by many researchers in some restricted cases including ideal gases, without the account of conservative force. As departure from the ideal gas conditions are accentuated with increasing the relative proportion of conservative force, it is anticipated that the VACF should gradually deviate from its normally expected exponentially decay. This trend is confirmed through numerical simulations and an expression in terms of the conservative force parameter, density and temperature is proposed for the self-diffusion coefficient. As it concerned the VACF, the equivalent Langevin equation describing Brownian motion of particles with a harmonic potential is adapted to the problem and reveals an exponentially decaying oscillatory pattern influenced by the conservative force parameter, dissipative parameter and temperature. Although the proposed model for obtaining the self-diffusion coefficient with consideration of the conservative force could not be verified due to computational complexities, nonetheless the Arrhenius dependency of the self-diffusion coefficient to temperature and pressure permits to certify our model over a definite range of DPD parameters.

Stresses in non-equilibrium fluids: Exact formulation and coarse-grained theory.

Starting from the stochastic equation for the density operator, we formulate the exact (instantaneous) stress tensor for interacting Brownian particles and show that its average value agrees with expressions derived previously. We analyze the relation between the stress tensor and forces due to external potentials and observe that, out of equilibrium, particle currents give rise to extra forces. Next, we derive the stress tensor for a Landau-Ginzburg theory in generic, non-equilibrium situations, finding an expression analogous to that of the exact microscopic stress tensor, and discuss the computation of out-of-equilibrium (classical) Casimir forces. Subsequently, we give a general form for the stress tensor which is valid for a large variety of energy functionals and which reproduces the two mentioned cases. We then use these relations to study the spatio-temporal correlations of the stress tensor in a Brownian fluid, which we compute to leading order in the interaction potential strength. We observe that, after integration over time, the spatial correlations generally decay as power laws in space. These are expected to be of importance for driven confined systems. We also show that divergence-free parts of the stress tensor do not contribute to the Green-Kubo relation for the viscosity.

Flexible star polymer chain adsorption by a flat surface: a molecular dynamics simulation

In this work, we have studied the adsorption of a single flexible star polymer chain by a flat surface. The molecular dynamics simulation has been validated for the case of a free star polymer chain using the diffusion experiment. The scaling laws found are in agreement with the Flory theory predictions and the Rouse model. For the adsorption, preliminary results were obtained for chains of different sizes N=31 to N=199 and different functionalities (f=3,4,6,8,10). For the case of semi-flexible star polymer chains, further investigation is needed to locate the critical point of adsorption when varying the potential interaction strength between the chain and the surface.

流道特性对流体压力影响的分子动力学研究

应用分子动力学方法研究纳米流道不同固–液间相互作用势能指数对液体流动特性的影响。研究结果显示，纳米尺度下流道出口高度及固液间相互作用势能不同会引起流体压力的改变，并出现射流现象。出口端距离较小时，流道内液体流动规律不再服从NS方程——按照分子动力学模拟与按NS或Renolds方程求解得到的压力分布结果存在较大的差异，此时射流现象比较明显，射流速度随着固液间势能指数的增强呈线性增大趋势。随着出口端高度的增加，纳米尺度效应减弱，压力分布逐渐趋近宏观流动下的压力分布曲线，固液间势能指数对于射流速度的大小不再产生明显的影响。 Molecular dynamics method is applied to study the influence of potential interaction strength be-tween the liquid and the solid on the properties of fluid film in nanochannels. The results indicate: the pressure distribution of fluid film is changed in Nanochannels with the change of the height of channel in the outlet end and the liquid-solid potential interaction strength. At the same time, the jet phenomenon can occur in the outlet end. The difference of the pressure distribution between the results obtained by molecular dynamics simulation and that by NS or Renolds equation is much bigger. At this point, the jet phenomenon is more obvious. The jet velocity increases linearly as the liquid-solid potential rises. With the increasing height of the outlet, the nanoscale effect becomes weaker and weaker. The pressure profile gradually approaches to that of the macro-flow. The liquid-solid potential has no significant effect on the jet velocity.

Molecular Dynamics of the Properties of Fluid Film Induced by Fluid-Solid Interaction Potential Strength in Wedge Nanochannel

Molecular dynamics method is applied to study the influence of fluid-solid interaction potential on the properties of fluid film in wedge nanochannel. The pressure and density are studied for a variety of potential interaction strength between the liquid and the solid. The impact of potential interaction strength between the liquid and the solid on the pressure is limitation. The density alongydirection is affected by the potential interaction strength. As the potential interaction strength is weak, the density of liquids can be affected easily.

Signature of a Nonharmonic Potential as Revealed from a Consistent Shape and Fluctuation Analysis of an Adherent Membrane

The interaction of fluid membranes with a scaffold, which can be a planar surface or a more complex structure, is intrinsic to a number of systems - from artificial supported bilayers and vesicles to cellular membranes. In principle, these interactions can be either discrete and protein mediated, or continuous. In the latter case, they emerge from ubiquitous intrinsic surface interaction potentials as well as nature-designed steric contributions of the fluctuating membrane or from the polymers of the glycocalyx. Despite the fact that these nonspecific potentials are omnipresent, their description has been a major challenge from experimental and theoretical points of view. Here we show that a full understanding of the implications of the continuous interactions can be achieved only by expanding the standard superposition models commonly used to treat these types of systems, beyond the usual harmonic level of description. Supported by this expanded theoretical framework, we present three independent, yet mutually consistent, experimental approaches to measure the interaction potential strength and the membrane tension. Upon explicitly taking into account the nature of shot noise as well as of finite experimental resolution, excellent agreement with the augmented theory is obtained, which finally provides a coherent view of the behavior of the membrane in a vicinity of a scaffold.

Paired states of interacting electrons in a two-dimensional lattice

We show that two tight binding electrons that repel may form a bounded pair in two dimensions. The paired states form a band with energies that scale like the strength of the interaction potential. By applying an electric field we show that the dynamics of such states is that of a composite particle of charge 2e. The system still sustains Bloch-like states, so that if the two bands overlap single and paired states might coexist allowing for a bosonic fluid component that, if condensed, would decrease the resistance at low temperatures. The presence of two bands allows for new oscillations whose experimental detection would permit a direct measurement of the interaction potential strength.

A systematically coarse-grained model for DNA and its predictions for persistence length, stacking, twist, and chirality

We introduce a coarse-grained model of DNA with bases modeled as rigid-body ellipsoids to capture their anisotropic stereochemistry. Interaction potentials are all physicochemical and generated from all-atom simulation/parameterization with minimal phenomenology. Persistence length, degree of stacking, and twist are studied by molecular dynamics simulation as functions of temperature, salt concentration, sequence, interaction potential strength, and local position along the chain for both single- and double-stranded DNA where appropriate. The model of DNA shows several phase transitions and crossover regimes in addition to dehybridization, including unstacking, untwisting, and collapse, which affect mechanical properties such as rigidity and persistence length. The model also exhibits chirality with a stable right-handed and metastable left-handed helix.

Velocity slip of liquid flow in nanochannels

Molecular dynamics simulations are carried out to investigate the velocity slip phenomenom of liquid flow in nanochannels. The liquid is argon, and the wall is taken to be platinum or its model solids. The effect of the surface wettability on the velocity slip is obtained through varying the potential interaction strength between the liquid and the wall. The simulation results show that the liquid adjacent to a hydrophilic surface is solid-like and has high density and orderliness. However, the liquid near a hydrophobic surface forms a gap of low density. The velocity slip of the liquid flow over solids decreases with the increasing interaction strength between the liquid and the channel surface. The slip, the no-slip and the negative slip are all possible to take place for different liquid-surface wettabilities. We find that the apparent slip of the liquid flow over solids is affected by the integrated factors of boundary slip, liquid sticking and liquid-internal slip.

Clusters in a magnetic toy model for binary granular piles.

Results on a generalized magnetically controlled ballistic deposition model of granular piles are reported in order to search for the effect of "spin flip" probability q in building a granular pile. Two different regimes of spin cluster site distributions have been identified, a borderline q(c) (betaJ) where J is the interaction potential strength.

The Horvath–Kawazoe method revisited

A modified Horvath–Kawazoe (HK) pore size analysis method is presented in this paper. The new HK model is generalized so as to account for the effect of the temperature and the solid–fluid interaction potential strength on the adsorbed fluid density within a slit-shaped pore. Two representations of the local adsorbate density profile are considered in the modified HK model, an ‘unweighted’ version similar to the original HK method in which a uniform density profile is assumed; and a ‘weighted’ version in which a structured density profile is adopted. The pore filling correlations predicted by the two HK models are compared with density functional theory (DFT) results calculated using the same solid–fluid potential and potential parameters. It is found that the pore filling correlation of the unweighted HK model agrees surprisingly well with the DFT correlation for argon and nitrogen adsorption at 77 K. Interestingly, the weighted HK pore filling correlation is in poorer agreement with DFT results, even though the weighted HK model more realistically represents the local density profile than does the unweighted HK approach. As in the original HK method, the modified HK models do not describe the pore wall wetting that occurs prior to capillary condensation in mesopores. In this respect, they remain inferior to DFT or molecular simulation adsorption models. The modified HK method is, however, much more computationally efficient than either DFT or molecular simulation, and it can be conveniently implemented on a spreadsheet for interpretation of adsorbent PSDs in gas–solid systems for which DFT model isotherms are not available.

Pore size heterogeneity and the carbon slit pore: a density functional theory model

We present model isotherms predicted by nonlocal density functional theory for adsorption of simple fluids in carbon slit pores. The effects of pore size, temperature, and solid-fluid potential interaction strength are examined. Our results are summarized into a classification scheme based upon regimes of continuous pore filling, capillary condensation, and 0 1 layering transitions. The descriptions we have devised depart from the IUPAC convention in that the filling behavior, rather than the physical width of the pore, is used as a guide to classification. Our results suggest that while the magnitude of the solid-fluid interactions dictates the pressure at which pore filling occurs, the type of filling depends primarily upon the ratio of pore width to adsorbate molecular diameter. The critical pore widths that denote the boundaries between various regimes of filling behavior are strongly dependent upon the temperature. To confirm the accuracy of the theoretical results, we compare adsorption isotherms and density profiles calculated from nonlocal theory and Gibbs ensemble simulation. The results from theory and simulation are found to be in good agreement. We conclude with a discussion of the problems associated with estimating solid-fluid potential parameters from experiment for use in theoretical computations. A comparison of nonlocal theory model isotherms and experimental nitrogen uptake measurements on nonporous carbon is presented.

Computer simulation of relaxation process of deposited films

The relaxation process for deposited films has been simulated by the molecular dynamics (MD) method. The system calculated was that involving a monatomic layer of fcc(111) on a bcc(110) substrate. For simplicity, the inter-atomic potential was assumed to be a morse function and only the lattice constants were fitted to the experimental values. The systems with lattice constant ratios of 2 √3 , √ 3 2 , 4 3 and √2 were constructed and relaxed by the MD method. The kinetic and potential energies of the system were monitored during the calculation. The relaxed structures strongly depended upon the depths of the interaction potentials. Some systems showed coherent interfaces under the strong effects of the substrate potential. These systems showed some definite epitaxial relationship when the strength of interaction potentials of the deposit-substrate and that of the deposit-deposit were comparable.

Transition between fitting laws for collision rotational transfers in molecules with internal electronic structure (CdH A 2Π [formula omitted

Rotational collision cross sections σ J i → J f for CdH A 2Π 1 2 ν′ = 0, J′ i = 0.5–20.5, with He or Ar are fitted with known angular-momentum(IOS or ECS)- or energy(EG or PG)-based fitting laws. With He, the collision is sudden and the IOS law fits most of the results. It accounts for the observed large index conservation propensity. At larger J i, there appear constraints for energy transfers which reduce the IOS cross sections, by a factor of two for ¦Δ E¦ ≈ 2 × kT. Sign conservation propensity is observed for lower J i and ¦Δ J¦ only, strongly attenuated with ¦Δ E¦, by a factor of two for ¦Δ E¦ ≈ 0.2 × kT. These results may be understood by assuming the interaction potential strength depends on the symmetry, i.e. great dipole, weak quadrupole. With Ar, the index and sign conservation propensities are weak. The fit with the ECS law is poor and restricted to lower J i. More satisfactory fits are obtained with laws depending on energy only, showing that the collision is governed essentially by the constraints for energy transfer.

Grundzustandskorrelationen bei kollektiven Kernanregungen

Applying the Fermimodel ground state correlations are calculated in the quasi-particle formalism. Their dependence on the interaction potential strength, distance of energy levels and pairing is investigated. By means of the normalization constant of the ground state we criticize the quasi-boson approximation.