Screening in Ionic Systems: Simulations for the Lebowitz Length

Simulations of the Lebowitz length, $\xi_{\text{L}}(T,\rho)$, are reported for t he restricted primitive model hard-core (diameter $a$) 1:1 electrolyte for densi ties $\rho\lesssim 4\rho_c$ and $T_c \lesssim T \lesssim 40T_c$. Finite-size eff ects are elucidated for the charge fluctuations in various subdomains that serve to evaluate $\xi_{\text{L}}$. On extrapolation to the bulk limit for $T\gtrsim 10T_c$ the low-density expansions (Bekiranov and Fisher, 1998) are seen to fail badly when $\rho > {1/10}\rho_c$ (with $\rho_c a^3 \simeq 0.08$). At highe r densities $\xi_{\text{L}}$ rises above the Debye length, \xi_{\text{D}} \prop to \sqrt{T/\rho}, by 10-30% (upto $\rho\simeq 1.3\rho_c$); the variation is portrayed fairly well by generalized Debye-H\"{u}ckel theory (Lee and Fisher, 19 96). On approaching criticality at fixed $\rho$ or fixed $T$, $\xi_{\text{L}}(T, \rho)$ remains finite with $\xi_{\text{L}}^c \simeq 0.30 a \simeq 1.3 \xi_{\text {D}}^c$ but displays a weak entropy-like singularity.Comment: 4 pages 5 figure

Universality of Ionic Criticality: Size- and Charge-Asymmetric Electrolytes

Grand canonical simulations designed to resolve critical universality classes are reported for $z$:1 hard-core electrolyte models with diameter ratios $\lambda {=} a_+/a_- {\lesssim} 6$. For $z {=} 1$ Ising-type behavior prevails. Unbiased estimates of $T_c(\lambda)$ are within 1% of previous (biased) estimates but the critical densities are $\sim$5 % lower. Ising character is also established for the 2:1 and 3:1 equisized models, along with critical amplitudes and improved $T_c$ estimates. For $z {=} 3$, however, strong finite-size effects reduce the confidence level although classical and O$(n {\geq} 3)$ criticality are excluded.Comment: 4 pages, 3 figure

Which technique for removing nevi is least scarring?

A shave biopsy with a razor blade or #15 scalpel is the best approach for a facial nevus, assuming malignancy is not suspected. The resulting scar is usually flat, smaller than the lesion, has no suture lines, and -- if shaved in mid or upper dermis -- has a low risk of producing a hypertrophic or hypotrophic scar (strength of recommendation: C, expert opinion, committee guidelines)

The influence of pre-experimental experience on social discrimination in rats (Rattus norvegicus)

The authors used laboratory rats (Rattus norvegicus) of known relatedness and contrasting familiarity to assess the potential effect of preexperimental social experience on subsequent social recognition. The authors used the habituation-discrimination technique, which assumes that multiple exposures to a social stimulus (e.g., soiled bedding) ensure a subject discriminates between the habituation stimulus and a novel stimulus when both are introduced simultaneously. The authors observed a strong discrimination if the subjects had different amounts of preexperimental experience with the donors of the 2 stimuli but a weak discrimination if the subjects had either equal amounts of preexperimental experience or no experience with the stimuli. Preexperimental social experience does, therefore, appear to influence decision making in subsequent social discriminations. Implications for recognition and memory research are discussed

Scaling for Interfacial Tensions near Critical Endpoints

Parametric scaling representations are obtained and studied for the asymptotic behavior of interfacial tensions in the \textit{full} neighborhood of a fluid (or Ising-type) critical endpoint, i.e., as a function \textit{both} of temperature \textit{and} of density/order parameter \textit{or} chemical potential/ordering field. Accurate \textit{nonclassical critical exponents} and reliable estimates for the \textit{universal amplitude ratios} are included naturally on the basis of the ``extended de Gennes-Fisher'' local-functional theory. Serious defects in previous scaling treatments are rectified and complete wetting behavior is represented; however, quantitatively small, but unphysical residual nonanalyticities on the wetting side of the critical isotherm are smoothed out ``manually.'' Comparisons with the limited available observations are presented elsewhere but the theory invites new, searching experiments and simulations, e.g., for the vapor-liquid interfacial tension on the two sides of the critical endpoint isotherm for which an amplitude ratio $-3.25 \pm 0.05$ is predicted.Comment: 42 pages, 6 figures, to appear in Physical Review

Lattice Models of Ionic Systems

A theoretical analysis of Coulomb systems on lattices in general dimensions is presented. The thermodynamics is developed using Debye-Huckel theory with ion-pairing and dipole-ion solvation, specific calculations being performed for 3D lattices. As for continuum electrolytes, low-density results for sc, bcc and fcc lattices indicate the existence of gas-liquid phase separation. The predicted critical densities have values comparable to those of continuum ionic systems, while the critical temperatures are 60-70% higher. However, when the possibility of sublattice ordering as well as Debye screening is taken into account systematically, order-disorder transitions and a tricritical point are found on sc and bcc lattices, and gas-liquid coexistence is suppressed. Our results agree with recent Monte Carlo simulations of lattice electrolytes.Comment: 25 pages, 3 figures, ReVTeX 4, Submitted to J. Chem. Phy

Vectorial Loading of Processive Motor Proteins: Implementing a Landscape Picture

Individual processive molecular motors, of which conventional kinesin is the most studied quantitatively, move along polar molecular tracks and, by exerting a force ${\bm F} = (F_x,F_y,F_z)$ on a tether, drag cellular cargoes, {\em in vivo}, or spherical beads, {\em in vitro}, taking up to hundreds of nanometer-scale steps. From observations of velocities and the dispersion of displacements with time, under measured forces and controlled fuel supply (typically ATP), one may hope to obtain insight into the molecular motions undergone in the individual steps. In the simplest situation, the load force ${\bm F}$ may be regarded as a scalar resisting force, $F_x < 0$, acting parallel to the track: however, experiments, originally by Gittes {\em et al.} (1996), have imposed perpendicular (or vertical) loads, $F_z > 0$, while more recently Block and coworkers (2002, 2003) and Carter and Cross (2005) have studied {\em assisting} (or reverse) loads, $F_x > 0$, and also sideways (or transverse) loads $F_y \neq 0$