Biomedical subjects

Rosa Albalat

Publications and source records attributed to Rosa Albalat.

4 recordsLinked to original sources

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.

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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.

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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.

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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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