We review the wetting and spreading properties of simple liquids on solid surfaces, putting emphasis on the role of the heterogeneities of the solid surface and on the spreading kinetics.In a situation of partial wetting, the liquid does not spread completely and shows a finite contact angle on a solid surface. The partial wetting behaviour on perfect solid surfaces is well described by classical capillarity. Heterogeneities of the solid surface lead to contact angle hysteresis.In a complete wetting situation, the liquid forms a film on a solid surface with a thickness in the mesoscopic range. The direct long range interaction between liquid and solid described by the so-called disjoining pressure governs the physics of these films. Films of mesoscopic thickness also appear in the spreading kinetics of liquids. These precursor films form ahead of macroscopic advancing liquid fronts. The spreading kinetics is extremely slow.

The fluctuation-dissipation theorem is a central result of statistical physics, which applies to any system at thermodynamic equilibrium. Its violation is a strong signature of nonequilibrium behavior. We show that for any system with Markovian dynamics, in a nonequilibrium steady state, a proper choice of observables restores a fluctuation-response theorem identical to a suitable version of the equilibrium fluctuation-dissipation theorem. This theorem applies to a broad class of dynamical systems. We illustrate it with linear stochastic dynamics and examples borrowed from the physics of molecular motors and Hopf bifurcations. Finally, we discuss general implications of the theorem.

We present a theoretical description of polymer adsorption from solution which is based on a mean field approximation but which goes beyond the standard ground state dominance approximation. The properties of the adsorbed polymer chains are described by two coupled order parameters. This allows a description of the chains in terms of tails and loops. When the bulk solution is dilute, the adsorbed polymer layer has a double layer structure with an inner layer dominated by loops and an outer layer dominated by tails. Explicit asymptotic forms are found for the monomer concentration profile and for the crossover distance between the loops and tail regions. The precise concentration profile is obtained by a numerical solution of two coupled differential equations. One of the surprising results is that the total polymer adsorbed amount has a nonmonotonic variation with molecular weight and decreases for large values of the molecular weight. The concentration profiles are also determined when the bulk solution is semidilute or concentrated. At any bulk concentration, the monomer concentration has a nonmonotonic variation with the distance to the adsorbing wall and shows a minimum at a finite distance. This depletion effect can be significant in the vicinity of the crossover between dilute and semidilute solutions. All the results are in agreement with the existing numerical solutions of the complete mean field theory of polymer adsorption. Excluded volume correlations are taken into account by constructing scaling laws for polymers in a good solvent both in dilute and in semidilute solutions.

We present a scaling theory for the modulus G of polyelectrolyte gels as a function of strand length between cross-links, monomer concentration c, salt concentration c s, and preparation conditions (monomer concentration c 0 and salt concentration c s°). The theory assumes affine displacement of the junction points when the concentration is changed. With no added salt (c s = c s° = 0), we predict a new concentration dependence of the modulus G ∼ c 5/6. In the high-salt limit, we predict the usual concentration dependence for uncharged polymers but a novel dependence on salt concentration, G ∼ c 7/12 c s 1/4. We also predict the modulus to decrease as charge is added to the gel strands. The predicted effects of added salt and charge on modulus have recently been observed by Candau and co-workers. At low concentrations, we discuss the strong stretching of network strands and its effect on modulus and equilibrium swelling.