Thermal Roughening and Deroughening at Polymer Interfaces in Electrophoretic Deposition

Thermal scaling and relaxation of the interface width in an electrophoretic deposition of polymer chains is examined by a three-dimensional Monte Carlo simulation on a discrete lattice. Variation of the equilibrium interface width $W_r$ with the temperature $T$ shows deroughening $W_r \propto T^{-\delta}$, with $\delta \sim 1/4$, at low temperatures and roughening $W_r \propto T^{\nu}$, with $\nu \sim 0.4$ at high temperatures. The roughening-deroughening transition temperature $T_t$ increases with longer chain lengths and is reduced by using the slower segmental dynamics.Comment: This is a work of the U.S. Government and is not subject to copyright protection in the United States. Foreign copyrights may appl

Computer Simulation Study of Conductivity in a 2-Dimensional Binary Fluid Mixture

A computer-simulation model is introduced to study the transport properties of a binary fluid mixture in which the constituents of one fluid (the tracer particles) carry charges in a linear charge-density gradient in the background charge of the second fluid. In the steady-state equilibrium, an effective conductivity σc(p,r) is estimated as a function of the carrier concentration p and the range of interactions r. The conductivity is observed to vary nonmonotonically with the concentration p, and it exhibits a maximum at a characteristic value pc. The conductivity decreases when the range of interaction is increased until a characteristic value rc, beyond which (r \u3e rc) it begins to saturate as the onset of screening spans with r-rc

Correlated Response In a Driven Flow of Self-Organizing Particles Around a Slit in Porous Media By Interacting Lattice Gas

The flow of immiscible particles (A, B) through a porous medium with a vertical slit is studied by an interacting lattice-gas computer simulation on a discrete lattice. The source of the particles is connected to the bottom and particles are driven upward by concentration gradient and a pressure bias against gravity. Distribution of flowing particles around the slit is examined as a function of the slit width and bias at high and low porosity at a steady state. At the low bias, a sharp change in the densities (high in slit to low in adjacent porous media) of both constituents occurs as expected. Onset of an undershooting in the density and mobility of particle profiles appears at the interface of slit and the porous medium on increasing the bias, an unexpected correlated response. The competition between the faster flow in the slit and slower motion of the particles in the surrounding porous medium induces stronger correlations at higher bias; as a result, a well-defined density profile emerges with higher density in the porous matrix away from the slit interface. The range of correlation and therefore the response increases on increasing the bias. Lowering the porosity to near the percolation threshold leads to the onset of oscillation in the density profile and broadening of the mobility profile, a distinct difference from the response at high porosity

Driven Diffusion of Particles, First-Passage Front, and Interface Growth

We study a computer-simulation model for driven particles on a discrete lattice where a fraction p of the lattice sites is randomly occupied by frozen impurities (barriers), and an imposed bias governs the particles’ hopping through the lattice. These particles (the carriers) are initially released from a source of wetting fluid from one end of the lattice in order to wet and the dry lattice on their trails. We study the transport of particles, frontier of their trail, and the growth of the interface between the wet and dry regions as a function of the biased field and the number of carriers. The rms displacements of carriers (Rtr) and that of their center of mass (Rc.m.) show power-law behaviors with time t, with exponents depending on the biased field. At the impurity concentration p=0.30 in two dimensions, we find that the mean wetting front position Rf moves with a power law Rf∼t2/3 at low values of the biased field, whereas it becomes pinned at higher values. The interface width grows with time to a maximum value before relaxing to a saturation value

Driven Diffusion, Kawasaki Dynamics, Mixing, and Spatial Ordering in an Interacting Lattice-Gas

Kawasaki dynamics is used to study the transport properties of a nonequilibrium steady state system of interacting lattice gas of oppositely charged particles in a linear gradient field in two dimensions. The rms displacements show unusual nondiffusive transport. The effective conductivity varies with the temperature which deviates from the Arrhenius law and depends on the range of interaction. Density of a fully mixed state decays with temperature with a power law. Onset of spatial ordering occurs in a certain temperature range at a fixed range of interaction

Power Law Exponents for a Spreading Front and Growing Interface in an Irreversible Wetting

Using computer simulations, the power-law behavior of the interface growth of a spreading fluid is studied in a two-dimensional lattice model. The interface width exponent ν and the dynamical exponent k for the evolution of the front are consistent with their dynamical scaling relation. The magnitude of these exponents seems to depend upon the nature of the substrate and the concentration of the carriers of the wetting fluid

Transport Properties of an Interacting Lattice Gas Model in a Charge Density Gradient by Monte Carlo Simulation

A two-dimensional lattice is considered with a linear charge-density gradient produced by a charge source at one end and a sink at the opposite end. A fraction p of the lattice sites are occupied by mobile particles that interact only with neighboring particles and empty sites (the substrate) and carry charges from source to sink; the charge neutrality of the whole lattice is maintained. The root-mean-square (rms) displacement of the particles (i.e., the tracers) and their effective conductivity for the charge transport are studied as a function of temperature and concentration p. The rms displacement shows a crossover from diffusion (at short time) to driftlike behavior (in the long-time regime). The effective conductivity depends nonmonotonically on the carriers\u27 concentration, in which two maxima peaks are observed; the peak at the higher concentration seems to characterize the onset of static percolation. At a fixed concentration, the conductivity remains almost constant at low temperatures and increases before it saturates to a higher value in the high-temperature regime. In the intermediate-temperature range, an Arrhenius dependence seems valid at high concentrations; however, a deviation on varying the concentration cannot be ruled out at low concentration. We find that the activation energy depends on carrier concentration and temperature

Conformation of a Coarse-Grained Protein Chain (an Aspartic Acid Protease) Model In Effective Solvent By a Bond-Fluctuating Monte Carlo Simulation

In a coarse-grained description of a protein chain, all of the 20 amino acid residues can be broadly divided into three groups: Hydrophobic (H), polar (P), and electrostatic (E). A protein can be described by nodes tethered in a chain with a node representing an amino acid group. Aspartic acid protease consists of 99 residues in a well-defined sequence of H, P, and E nodes tethered together by fluctuating bonds. The protein chain is placed on a cubic lattice where empty lattice sites constitute an effective solvent medium. The amino groups (nodes) interact with the solvent (S) sites with appropriate attractive (PS) and repulsive (HS) interactions with the solvent and execute their stochastic movement with the Metropolis algorithm. Variations of the root mean square displacements of the center of mass and that of its center node of the protease chain and its gyration radius with the time steps are examined for different solvent strength. The structure of the protease swells on increasing the solvent interaction strength which tends to enhance the relaxation time to reach the diffusive behavior of the chain. Equilibrium radius of gyration increases linearly on increasing the solvent strength: A slow rate of increase in weak solvent regime is followed by a faster swelling in stronger solvent. Variation of the gyration radius with the time steps suggests that the protein chain moves via contraction and expansion in a somewhat quasiperiodic pattern particularly in strong solvent

Evidence of a Nondiffusive Transport in a Monodisperse Screened Coulomb System by a Molecular-Dynamics Simulation

A molecular-dynamics simulation is carried out for particles (tracers) interacting with a screened Coulomb potential in three dimensions. A phase transition is observed from a liquid to an fcc solid on reducing the temperature at a fixed density, consistent with previous studies. In the liquid phase, the variation of the rms displacement Rtr of the charged particles with time seems to depend on the density and the temperature. In the short-time regime, the effective exponent k for the subdiffusive power-law behavior, i.e., Rtr∼tk, decreases on increasing the density and lowering the temperature. A crossover from subdiffusive to a diffusive behavior is observed for a wide density regime in the liquid phase. At high temperatures and large densities, a single power law does not seem to describe the variation of Rtr with t

Relaxation to Native Conformation of a Bond-Fluctuating Protein Chain With Hydrophobic and Polar Nodes

The conformation and dynamics of a protein chain with hydrophobic and polar nodes are examined by the bond-fluctuation model using Monte Carlo simulations on a cubic lattice. The minimal (nearest neighbor) interaction leads to standard (self-avoiding walk) conformation, i.e., the scaling of the radius of gyration Rg with the molecular weight N Rg ∝ Nγ with γ ≃ 3/5/ Specific interactions with longer range and higher strength are needed to approach the native globular conformations with γ \u3c 3/5. Relaxation into the globular ground state shows a weak power-law decay, i.e., Rg ∝ t-α, α ~ 0.06-0.12