Biomedical subjects

Jordi Ignés-Mullol

Publications and source records attributed to Jordi Ignés-Mullol.

7 recordsLinked to original sources

PubMed2006

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.

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PubMed2006

Collective molecular precession induced by rotating illumination in photosensitive langmuir monolayers.

Langmuir monolayers of an amphiphilic azobenzene derivative exhibit easily perturbable mesophases with long-range orientational order that couples strongly to linearly polarized light, resulting in symmetric photoaligned textures. Controlled rotation of the polarization plane induces continuous collective precession of the molecular field, which reveals the shear-thinning nature of the surface rotational viscosity. The dynamics is controlled by a balance between the strength of the optical coupling and viscous dissipation so that an increase in the rotational velocity results in a dampening of the synchronous anharmonic oscillations of the orientational field, as revealed by a space-time analysis of the Brewster angle microscopy images.

Journal Article↗

PubMed2005

Backflow-induced asymmetric collapse of disclination lines in liquid crystals.

We present experiments where opposed pairs of planar parallel disclination lines of topological strength s=+/-1 move due to their mutual attraction. Our measurements show that their motion is clearly asymmetric, with +1 defects moving up to twice as fast as -1 ones. This is a clear indication of backflow, given the intrinsic isotropic elasticity of our system. A phenomenological model is able to account for the experimental observations by renormalizing the orientational diffusivity estimated from the velocity of each defect.

Cholesterol↗

PubMed2005

Photoinduced molecular reorientation dynamics in confined domains of Langmuir monolayers.

In a photoresponsive Langmuir monolayer comprised of smectic-C-like domains of mesogenic trans-azobenzene derivative embedded within an isotropic matrix of its cis isomer counterpart, several structurally differing circular droplets were irradiated with linearly polarized light. This report describes the structural rearrangements that occurred in these droplets upon illumination followed by Brewster angle microscopy analysis. Starting from initial well-characterized and symmetric states, final photoaligned situations were reached in which the azimuth angles of the rod-shaped elongated molecules were found to be perpendicular to the electric component of the excitation light. The dynamical aspects of the photoalignments, including their transient patterns, are captured by a theoretical model that couples a relaxational principle incorporating long-range elastic forces with a kinetic formalism presenting an anisotropic rate law.

Journal Article↗

PubMed2005

Photoswitchable orientational patterns of confined domains in monolayers.

We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.

Journal Article↗

PubMed2005

Texture changes inside smectic-C droplets in azobenzene Langmuir monolayers.

Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.

Journal Article↗

PubMed2004

Influence of temperature and composition on the mesoscopic textures of azobenzene Langmuir monolayers.

The monolayer behavior of the azobenzene derivative 8Az3COOH is shown to depend significatively on temperature and on the isomeric (trans-cis) composition. For pure trans monolayers, important temperature effects in the mesoscopic organization (as revealed by means of Brewster angle microscopy, BAM) are observed for the low-pressure phase in the studied temperature range (10 degrees C < T < 40 degrees C). Mixed trans-cis monolayers show that both isomers are virtually immiscible, leading to a phase segregation into birefringent, nearly pure trans droplets surrounded by an isotropic, nearly pure cis matrix. The existence of well-defined anchoring conditions at the droplet boundaries leads to highly symmetric textures, amenable of quantitative BAM image analysis, which helps to better visualize mesoscopic changes induced by variations in the control parameters (temperature, surface pressure or irradiation).

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