Nonequilibrium orientational patterns in two-component Langmuir monolayers

Dynamics of the refractive index in Langmuir monolayers

We present theoretical description of dynamics of the refractive index in Langmuir monolayers subjected to continuous-wave (CW) laser radiation, using the mechanism of the local temperature fluctuations. Such mechanism can be applied to all monolayer systems that consist of rodlike molecules. The CW laser radiation induces conformational transitions of molecules in monolayer, which, after some time being in the excited state, come back into the ground state, thus renewing their initial shape and releasing energy into the medium, locally heating it. This leads to the local temperature change, which promotes new conformational transitions of molecules. Such nonlinear energy redistribution mechanism permits the conformational transition boundary to move. The existence of the transitions is seen from the change of the refractive index.

Nonequilibrium microstructures in reactive monolayers as soft matter systems

Chemical systems provide classical examples of nonequilibrium pattern formation. Reactions in weak aqueous solutions, such as the extensively investigated Belousov-Zhabotinsky reaction, demonstrate a rich variety of patterns, ranging from travelling fronts to rotating spiral waves and chemical turbulence. Pattern formation in such systems is based on interplay between the reactions and diffusion. Intrinsically, this puts a restriction on the minimum length scale of the developing structures, which cannot be shorter than the diffusion length of the reactants. However, much smaller nonequilibrium structures, with characteristic lengths reaching down to nanoscales, are also possible. They are found in reactive soft matter, where energetic interactions between molecules are present as well. In these systems, chemical reactions and diffusion interfere with phase transitions, yielding active, stationary or dynamic microstructures. Nonequilibrium soft-matter microstructures are of fundamental importance for biological cells and may have interesting engineering applications. In this Minireview, we focus on the microstructures found in reactive soft-matter monolayers at solid surfaces or liquid-air interfaces.

Nonequilibrium self-organization phenomena in active Langmuir monolayers

Langmuir monomolecular layers, formed by amphiphilic molecules at liquid-air interfaces and containing a fraction of chiral molecules, are theoretically investigated. These monolayers can be brought out of thermal equilibrium by applying a gradient of small molecules across the interface, resulting in the leakage flow. We show that, when splay coupling between the orientation field and the local concentration of chiral molecules in the monolayer is taken into account, this nonequilibrium soft matter system can show complex wave behavior, including the development of target wave patterns, spiral waves, and dense regions filled with inwardly propagating waves.

Lattice Boltzmann simulations of phase separation in chemically reactive binary fluids

We use a lattice Boltzmann method to study pattern formation in chemically reactive binary fluids in the regime where hydrodynamic effects are important. The coupled equations solved by the method are a Cahn-Hilliard equation, modified by the inclusion of a reactive source term, and the Navier-Stokes equations for conservation of mass and momentum. The coupling is twofold, resulting from the advection of the order parameter by the velocity field and the effect of fluid composition on pressure. We study the evolution of the system following a critical quench for a linear and for a quadratic reaction source term. Comparison is made between the high and low viscosity regimes to identify the influence of hydrodynamic flows. In both cases hydrodynamics is found to influence the pathways available for domain growth and the eventual steady states.

Self-organized propagation patterns from dynamic self-assembly in monolayers

Propagation of localized orientational waves, as imaged by Brewster angle microscopy, is induced by low intensity linearly polarized light inside axisymmetric smectic-C confined domains in a photosensitive molecular thin film at the air/water interface (Langmuir monolayer). Results from numerical simulations of a model that couples photoreorientational effects and long-range elastic forces are presented. Differences are stressed between our scenario and the paradigmatic wave phenomena in excitable chemical media.