Finite Temperature QCD Interfaces Out of Equilibrium

The properties of interfaces in non-equilibrium situations are studied by constructing a density matrix with a space-dependent temperature. The temperature gradient gives rise to new terms in the equation for the order parameter. Surface terms induced in effective actions by abrupt temperature changes provide a natural theoretical framework for understanding the occurence of both continuous and discontinuous behavior in the order parameter. Monte Carlo simulation of pure QCD shows both kinds of interfacial behavior. Perturbation theory predicts a universal profile in the high temperature phase, which can be tested by Monte Carlo simulation.Comment: 3 pages, contribution to Lattice '94 conference, self-extracting (revised only to include heplat number in line below

Duality mapping and unbinding transitions of semiflexible and directed polymers

Directed polymers (strings) and semiflexible polymers (filaments) are one-dimensional objects governed by tension and bending energy, respectively. They undergo unbinding transitions in the presence of a short-range attractive potential. Using transfer matrix methods we establish a duality mapping for filaments and strings between the restricted partition sums in the absence and the presence of a short-range attraction. This allows us to obtain exact results for the critical exponents related to the unbinding transition, the transition point and transition order.Comment: 7 pages, eq. (20) corrected in this submissio

Bundles of Interacting Strings in Two Dimensions

Bundles of strings which interact via short-ranged pair potentials are studied in two dimensions. The corresponding transfer matrix problem is solved analytically for arbitrary string number N by Bethe ansatz methods. Bundles consisting of N identical strings exhibit a unique unbinding transition. If the string bundle interacts with a hard wall, the bundle may unbind from the wall via a unique transition or a sequence of N successive transitions. In all cases, the critical exponents are independent of N and the density profile of the strings exhibits a scaling form that approaches a mean-field profile in the limit of large N.Comment: 8 pages (latex) with two figure

Contact angles on heterogeneous surfaces; a new look at Cassie's and Wenzel's laws

We consider a three dimensional liquid drop sitting on a rough and chemically heterogeneous substrate. Using a novel minimization technique on the free energy of this system, a generalized Young's equation for the contact angle is found. In certain limits, the Cassie and Wenzel laws, and a new equivalent rule, applicable in general, are derived. We also propose an equation in the same spirit as these results but valid on a more `microscopic' level. Throughout we work under the presence of gravity and keep account of line tension terms.Comment: 10 pages RevTeX, 2 EPS figures. A few minor corrections mad

Wetting between structured surfaces: Liquid bridges and induced forces

Wetting phenomena are theoretically studied for a slab geometry consisting of a wetting phase confined between two chemically patterned substrates. Each of these is decorated by an array of stripes whose composition alternates between two different surface phases. For a single pair of opposing stripes, the wetting phase may either form a bridge spanning from one surface to the other or it may break up into two separate channels. The bridge state induces an effective interaction between the two substrates. This leads to the bridge itself having a preferred contact angle and the substrates having a preferred separation. In the case of many stripes, one has a whole sequence of morphological transitions with the number of bridges decreasing as the surface separation grows

Membrane adhesion and domain formation

We review theoretical results for the adhesion-induced phase behavior of biomembranes. The focus is on models in which the membranes are represented as discretized elastic sheets with embedded adhesion molecules. We present several mechanism that lead to the formation of domains during adhesion, and discuss the time-dependent evolution of domain patterns obtained in Monte-Carlo simulations. The simulated pattern dynamics has striking similarities to the pattern evolution observed during T cell adhesion.Comment: 68 pages, 29 figure

Active diffusion of motor particles

The movement of motor particles consisting of one or several molecular motors bound to a cargo particle is studied theoretically. The particles move on patterns of immobilized filaments. Several patterns are described for which the motor particles undergo non-directed but enhanced diffusion. Depending on the walking distance of the particles and the mesh size of the patterns, the active diffusion coefficient exhibits three different regimes. For micrometer-sized motor particles in water, e.g., this diffusion coefficient can be enhanced by two orders of magnitude.Comment: revtex, 4 pages, 4 figures, to appear in PR

Critical behavior of interacting surfaces with tension

Wetting phenomena, molecular protrusions of lipid bilayers and membrane stacks under lateral tension provide physical examples for interacting surfaces with tension. Such surfaces are studied theoretically using functional renormalization and Monte Carlo simulations. The critical behavior arising from thermally-excited shape fluctuations is determined both for global quantities such as the mean separation of these surfaces and for local quantities such as the probabilities for local contacts.Comment: 13 pages, 17 figures; accepted for publication in The European Physical Journa

Barrier crossing of semiflexible polymers

We consider the motion of semiflexible polymers in double-well potentials. We calculate shape, energy, and effective diffusion constant of kink excitations, and in particular their dependence on the bending rigidity of the semiflexible polymer. For symmetric potentials, the kink motion is purely diffusive whereas kink motion becomes directed in the presence of a driving force on the polymer. We determine the average velocity of the semiflexible polymer based on the kink dynamics. The Kramers escape over the potential barriers proceeds by nucleation and diffusive motion of kink-antikink pairs, the relaxation to the straight configuration by annihilation of kink-antikink pairs. Our results apply to the activated motion of biopolymers such as DNA and actin filaments or synthetic polyelectrolytes on structured substrates.Comment: 7 pages, 3 figure