Self-organizing 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

Stirring competes with chemical induction in chiral selection of soft matter aggregates

Chirality, the absence of mirror symmetry, can be equally invoked in relation to physical forces and chemical induction processes, yet a competition between these two types of influence is rarely reported. Here we present a self-assembled soft matter system in which chiral selection is controlled by the combined independent action of a chiral dopant and vortical stirring, which are arbitrarily coupled, either constructively or destructively. In the latter case, perfect compensation, that is, absence of a net chiral effect, is realized. The induced enantiomorphic excess is measured in terms of the statistical imbalance of an ensemble of submillimetre domains, where achiral molecules self-assemble with a well-defined orientational chirality that is unambiguously resolved using optical microscopy. The possibility of combining top-down and bottom-up strategies to induce a chiral predominance in a supramolecular system of achiral components should be recognized as a new twist in the process of chiral recognition, selection and control

Chiral-symmetry selection in soft monolayers under vortical flow

Herein, we report and interpret a new chiral-selection phenomenon in the orientational ordering of soft-assembled domains that arise spontaneously in Langmuir monolayers of an azobenzene derivative at the air/water interface. The orientational chirality of isolated sub-millimeter domains was unambiguously assessed by optical microscopy. The selection process, quantified by using an enantiomeric excess parameter, was controlled by stirring the aqueous subphase. We have studied the dependence of this process on stirring rate and handedness, stirring time, temperature, and on the initial state of the monolayer. The influence of the concomitant cis/trans isomerization on the reported chiral-selection process is also discussed

Interfacial chiral selection by bulk species

A chiral selection process in a self-assembled soft monolayer of an achiral amphiphile as a consequence of its interaction with chiral species dissolved in the aqueous subphase, is reported. The extent of the chiral selection is statistically measured in terms of the enantiomorphic excess of self-assembled submillimeter domains endowed with well-defined orientational chirality that is unambiguously resolved using optical microscopy. Our results show that the emergence of chirality is mediated by electrostatic interactions and significantly enhanced by hydrophobic effects. This chiral chemical effect can be suppressed and even reversed by opposing a macroscopic physical influence, such as vortical stirring. This result gives evidence for the crucial role of hydrodynamic effects in supramolecular aggregation

Fractals en sistemes químico-físics: electrodeposició

Durant la passada dècada ha estat del tot reconegut pels científics en general, i els físics, químics, biòlegs, etc., en particular, que la majoria de les estructures que formen part del seu camp d'estudi posseeixen un tipus bastant especial de complexitat geomètrica. Aquest interès va sorgir amb el treball de B. Mandelbrot (Mandelbrot, 1977; 1982; 1989) el qual va recalcar les similituds de les propietats geomètriques particulars complexos amb les costes dels continents, les branques dels arbres o les superfícies dels núvols. El1 va introduir el nom fractal per a aquestes formes complexes per expressar que es poden caracteritzar per una dimensionalitat fraccionària (fractal). Amb el desenvolupament de la recerca en aquesta direcció, la llista dels exemples de fractals ha esdevingut extraordinàriament gran, i inclou tot tipus d'estructures, des d'agregats microscòpics fins a cúmuls de galàxies

Chiral selection by interfacial shearing of self-assembled achiral molecules

We report a novel phenomenon of chiral selection in self-assembled condensates of achiral amphiphiles. The handedness of chiral textures, reproducing the collective rotational component of the molecular orientation inside submillimeter circular domains, is correlated with the sign of a vortical stirring in the aqueous subphase. We propose an explanation based on the distinctive kinetics of topological defect annihilation during domain coalescence at the initial coarsening stage of a phase-separating monolayer

Healing of defects at the interface of nematic liquid crystals and structured Langmuir-Blodgett monolayers

We use Langmuir-Blodgett molecular monolayers and nematic liquid crystals as model two-and three-dimensional orientationally ordered systems to study the stability and healing of topological defects at their contact interfaces. Integer-strength defects at the monolayer induce disclinations of similar strength in the nematic that, however, do not propagate deep into the bulk, but rather form single- or double-split arch-shaped loops pinned to the interface. This behavior is qualitatively independent of the far-field director orientation and involves either half-integer singular or twist-escaped unity-strength nonsingular nematic disclinations. These two defect configurations can be selected by varying sample preparation given their comparable free energy, consistently with direct probing by use of laser tweezers

Quasi-two-dimensional electrodeposition under forced fluid flow

Experimental quasi-two-dimensional Zn electrodeposits are grown under forced convection conditions. Large-scale effects, with preferential growth towards the impinging flow, together with small-scale roughness suppression effects are evidenced and separately analyzed by using two different radial cell configurations. Interpretations are given in terms of primary concepts concerning current and concentration distributions