Guillermo Ramirez-Santiago

Interfacial tension behavior of binary and ternary mixtures of partially miscible Lennard-Jones fluids: A molecular dynamics simulation

By means of extensive equilibrium molecular dynamics simulations we have investigated the behavior of the interfacial tension γ of two immiscible symmetrical Lennard-Jones fluids. This quantity is studied as function of reduced temperature T*=kBT/e in the range 0.6⩽T*⩽3.0. We find that, unlike the monotonic decay obtained for the liquid-vapor interfacial tension, for the liquid–liquid interface, γ(T) has a maximum at a specific temperature. We also investigate the effect that surfactantlike particles have on the thermodynamic as well as the structural properties of the liquid–liquid interface. It is found that γ decays monotonically as the concentration of the surfactantlike particles increases.

Phase and interfacial behavior of partially miscible symmetric Lennard-Jones binary mixtures

We have carried out extensive equilibrium molecular-dynamics simulations to study quantitatively the topology of the temperature versus density phase diagrams and related interfacial phenomena in a partially miscible symmetric Lennard-Jones binary mixture. The topological features are studied as a function of miscibility parameter, alpha = epsilonAB/epsilonAA. Here epsilonAA = epsilonBB and epsilonAB stand for the parameters related to the attractive part of the intermolecular interactions for similar and dissimilar particles, respectively. When the miscibility varies in the range 0 < alpha < 1, a continuous critical line of consolute points Tcons(rho)--critical demixing transition line--appears. This line intersects the liquid-vapor coexistence curve at different positions depending on the values of alpha, yielding mainly three different topologies for the phase diagrams. These results are in qualitative agreement to those found previously for square-well and hard-core Yukawa binary mixtures. The main contributions of the present paper are (i) a quantitative analysis of the phase behavior and (ii) a detailed study of the liquid-liquid interfacial and liquid-vapor surface tensions, as function of temperature and miscibility as well as its relationship to the topological features of the phase diagrams.

Wetting phenomenon in the liquid-vapor phase coexistence of a partially miscible Lennard-Jones binary mixture.

We have carried out extensive equilibrium molecular dynamics simulations to study the structure and the interfacial properties in the liquid-vapor phase coexistence of partially miscible binary Lennard-Jones mixtures. By analyzing the structural properties as a function of the miscibility parameter, alpha, we found that at relatively low temperatures the system separates forming a liquid A-liquid B interface in coexistence with the vapor phase. At higher temperatures and, 0< alpha < or =0.5 , we found a temperature range, T*w (alpha) < or =T*< T*Cons (alpha) , where the liquid phases are wet by the vapor phase. Here, T*w (alpha) represents the wetting transition temperature and T*Cons (alpha) is the consolute temperature of the mixture. However, for 0.5< alpha <1 , no wetting phenomenon occurs. For the particular value, alpha=0.25 , we analyzed quantitatively the T* versus rho* , and P* versus T* phase diagrams and found, T*c approximately 1.25 , and T*Cons approximately 1.25 . We also studied quantitatively, as a function of temperature, the surface tension and the adsorption of molecules at the liquid-liquid interface. It was found that the adsorption shows a jump from a finite negative value up to minus infinity, when the vapor wets the liquid phases, suggesting that the wetting transition is of first order. The calculated phase diagram, together with the wetting phenomenon, strongly suggests the existence of a tricritical point. These results agree well with some experiments carried out in fluid binary mixtures.

Comment on "Two Phase Transitions in the Fully Frustrated XY Model"

The conclusions of a recent paper by Olsson (Phys. Rev. Lett. 75, 2758 (1995), cond-mat/9506082) about the fully frustrated XY model in two dimensions are questioned. In particular, the evidence presented for having two separate chiral and U(1) phase transitions are critically considered.

Metastable liquid lamellar structures in binary and ternary mixtures of Lennard-Jones fluids.

We have carried out extensive equilibrium molecular dynamics simulations to investigate the liquid-vapor coexistence in partially miscible binary and ternary mixtures of Lennard-Jones fluids. We have studied in detail the time evolution of the density profiles and the interfacial properties in a temperature region of the phase diagram where the condensed phase is demixed. The composition of the mixtures is fixed, 50% for the binary mixture and 33.33% for the ternary mixture. The results of the simulations clearly indicate that in the range of temperatures 78<T<102 K-in the scale of argon-the system evolves towards a metastable alternated liquid-liquid lamellar state in coexistence with its vapor phase. These states can be achieved if the initial configuration is fully disordered-that is, when the particles of the fluids are randomly placed on the sites of an fcc crystal or the system is completely mixed. As temperature decreases these states become very well defined and more stables in time. We find that below 90 K, the alternated liquid-liquid lamellar state remains alive for 80 ns, in the scale of argon, the longest simulation we have carried out. Nonetheless, we believe that in this temperature region these states will be alive for even much longer times.

Growth laws and spinodal decomposition type of scaling in fractal aggregation of colloids

By means of extensive numerical simulations we have studied the time evolution and the scaling properties of the structure factor S(q,t) of diffusion-limited (DLCA) and reaction-limited (RLCA) colloid aggregation in three dimensions. We found that in DLCA S(q, t) scales in a similar way as in the spinodal decomposition process. The exponents a′ and a″ that relate the position and the height of the maximum of S(q, t) versus time, respectively, were however found to be different from those corresponding to the spinodal decomposition exponents. Unlike in the DLCA case, the S(q, t) for RLCA neither shows a pronounced maximum for the earlier times nor presents scaling. Nonetheless, the earlier broad peak eventually stretches and becomes higher than in DLCA.

Critical exponents of frustrated josephson junction arrays (JJA)

Abstract We present a reanalysis of the experimental data obtained at Delft in niobium and aluminum square J J A in magnetic fields B = 0 f p 2 , with f 0 = h/ 2e , lattice spacing p∼10μm,f=0, 1 2 , and the “irrational” 5−3 2 ∼ 0.38 . Above the critical temperature τc the linear normal state resistance is fitted, using different statistical tests, to the form R R N = Ae −B (τ−τ c ) ν . For τ f = 1/ 2 we can not distinguish between ν = 1 2 or 1 3 . The jump in α(τ = τc) gives 2η = [a(τ c ) − 1] −1 of 1 4 and 0.19, for f = 0 and 1 2 , respectively. The results for f = 1 2 compare well with recent theoretical predictions. For (f = 0.38), a(τ) has no jump and decreases monotonically as τ → τc.

Spatio-temporal dynamics of a cell signal pathway with negative feedbacks: the MAPK/ERK pathway

Abstract.We studied the spatio-temporal dynamics of a cell signal cascade with negative feedback that quantitatively emulates the regulative process that occurs in the Mitogen Activated Protein Kinase/Extracellular Regulated Kinase (MAPK/ERK) pathway. The model consists of a set of six coupled reaction-diffusion equations that describes the dynamics of the six-module pathway. In the basic module the active form of the protein transmits the signal to the next pathway’s module. As suggested by experiments, the model considers that the fifth module’s kinase down-regulates the first and third modules. The feedback parameter is defined as,

Number of arm selection in two-dimensional diffusion processes

\mu^{r}_{j}

Critical properties of the fully frustrated 2‐D XY model

= kkin5/kkinj, (j = 1, 3). We analysed the pathway’s dynamics for