RESUMEN
Relaxation following a temperature quench of two- (L(alpha) and L3) and three-phase (L(alpha), L3, and L1) samples has been studied in a sodium dodecyl sulfate-octanol-brine system. In the three-phase case we have observed samples that are initially mainly of the sponge phase, with lamellar and micellar phases on the top and bottom of the sample, respectively. Upon decreasing the temperature, most of the volume of the sponge phase is replaced by the lamellar phase. During equilibration we have observed three regimes of behavior within the sponge phase: (i) first there is disruption in the L3 texture, then (ii) after the sponge phase homogenizes there is a L(alpha) nucleation regime; finally (iii), a bizarre plume connects the L(alpha) phase with the L1 phase. The relaxation of the two-phase sample proceeds instead in two stages. First L(alpha) drops nucleate in L3, forming an onion "gel" structure. Over time the L(alpha) structure compacts while equilibrating into a two-phase L(alpha)-L3 sample. We offer possible explanations for some of these observations in the context of a general theory for phase kinetics in systems with one fast composition variable and one slow.
RESUMEN
The persistence exponent, straight theta, is defined by N(F) approximately t(-straight theta), where t is the time since the start of the coarsening process and the "no-flip fraction," N(F), is the number of points that have not seen a change of "color" since t=0. Here we investigate numerically the persistence exponent for a binary fluid system where the coarsening is dominated by hydrodynamic transport. We find that N(F) follows a power law decay (as opposed to exponential) with the value of straight theta somewhat dependent on the domain growth rate (L approximately t(alpha), where L is the average domain size), in the range straight theta=1.23+/-0.1 (alpha=2/3) to straight theta=1.37+/-0.2 (alpha=1). These alpha values correspond to the inertial and viscous hydrodynamic regimes, respectively.
RESUMEN
The interplay of topological constraints and the persistence length of ring polymers in their own melt is investigated by means of dynamical Monte Carlo simulations of a three-dimensional lattice model. We ask if the results are consistent with an asymptotically regime where the rings behave like (compact) lattice animals in a self-consistent network of topological constraints imposed by neighboring rings. Tuning the persistence length provides an efficient route to increase the ring overlap required for this mean-field picture to hold: The effective Flory exponent for the ring size decreases down to nu less, similar1/3 with increasing persistence length. Evidence is provided for the emergence of one additional characteristic length scale d(t) approximately N0, only weakly dependent on the persistence length and much larger than the excluded volume screening length xi. At distances larger than d(t) the conformational properties of the rings are governed by the topological interactions; at smaller distances rings and their linear chain counterparts become similar. (At distances smaller than xi both architectures are identical.) However, the crossover between both limits is intricate and broad, as a detailed discussion of the local fractal dimension (e.g., obtained from the static structure factor) reveals. This is due to various crossover effects which we are unable to separate even for the largest ring size (N=1024) presented here. The increased topological interactions also influence the dynamical properties. Mean-square displacements and their distributions depend crucially on the ring overlap, and show evidence of the existence of additional size and time scales. The diffusion constant of the rings goes down from effectively D(N) approximately N-1.22 for flexible rings with low overlap to D(N) approximately N-1.68 for strongly overlapping semiflexible rings.
RESUMEN
The structure of polydisperse hard sphere fluids, in the presence of a wall, is studied by the Rosenfeld density functional theory. Within this approach, the local excess free energy depends on only four combinations of the full set of density fields. The case of continuous polydispersity thereby becomes tractable. We predict, generically, an oscillatory size segregation close to the wall, and connect this, by a perturbation theory for narrow distributions, with the reversible work for changing the size of one particle in a monodisperse reference fluid.
RESUMEN
We present precise and reproducible mean pressure measurements at the bottom of a cylindrical granular column. If a constant overload is added, the pressure is linear in overload and nonmonotonic in the column height. The results are quantitatively consistent with a local, linear relation between stress components, as was recently proposed by some of us. They contradict the simplest classical (Janssen) approximation, and may rather severely test competing models.
