Biomedical subjects
Publications and source records attributed to A M Cazabat.
The dynamics of evaporation of wetting droplets is investigated experimentally in an extended range of drop sizes to provide trends relevant for a theoretical analysis. A model is proposed, which generalizes Tanner's law in the presence of evaporation. A qualitative agreement is obtained, which represents a first step toward the solution of a very old, complex problem.
The spontaneous spreading of macroscopic drops of nematic liquid crystals on hydrophilic substrates has been investigated by interferometric techniques. There is a complex interplay between the elastic energy, due to antagonist anchoring at the interfaces, and the radial flow in the spreading drop. A relevant parameter appears to be the relative humidity of the atmosphere, because it controls the amount of water molecules adsorbed on the substrate and, therefore, the strength of anchoring defects. The spreading laws differ from the ones of simple wetting liquids, and contact line instabilities coupled to short- (anchoring) or large-scale (disclinations) defects of the nematic film are observed.
We study long-range morphological changes in atomic monolayers on solid substrates induced by different types of defects; e.g., by monoatomic steps in the surface, or by the tip of an atomic force microscope (AFM), placed at some distance above the substrate. Representing the monolayer in terms of a suitably extended Frenkel-Kontorova-type model, we calculate the defect-induced density profiles for several possible geometries. In case of an AFM tip, we also determine the extra force exerted on the tip due to the tip-induced dehomogenization of the monolayer.
Fingering instabilities are often observed at the contact line of drops of surfactant solutions spreading spontaneously on solid surfaces. It has been recognised recently that a usual linear stability analysis predicts stable behaviour in contrast to the observed instability. It now seems the instability arises from short-time transients, where the thickness of the film ahead of the main drop is a crucial parameter for amplification. We reconsidered previous experiments and performed new ones along these lines. The strengths and weaknesses of the available models were analysed.
The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl on silicon substrates is reported. In the vicinity of the nematic-isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h(2) law ( h is the film thickness). Microscope observations and small-angle x-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first-order phase transition where thickness plays the role of an order parameter.
We observe prewetting films of 8CB (4'-n-octyl-4-cyanobiphenyl) spreading at room temperature on silicon wafers by ellipsometry and x-ray reflectivity. Ellipsometry indicates the formation of a nondense monolayer spreading in front of a 45-A-thick film. X-ray reflectivity, performed using a ribbon geometry for the liquid crystal (LC) reservoir, allows us to determine the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar monolayer with tilted molecules. We show that the thickness of the bilayer is equal to the smectic periodicity in the bulk material and that the tilt angle of the molecules in contact with the solid surface is close to 60 degrees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using x-ray reflectivity because it induces a modulation of the electron density along the normal to the surface. Furthermore, a study of the ellispometric profile of a drop heated in the nematic phase, where we observe a complete spreading of the LC, shows the complex structuration of the LC close to the solid interface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific interaction between the trilayer and silicon wafer.
Using laser light scattering, photon correlation and spectral analysis, it was shown that cervical mucus is a non-Newtonian Hydrogel with large meshes (approximately 5 microns). The experimental results are in agreement with a model of hydrogel with weak linkages and are definitely not compatible with a model of entangled macromolecules. Large oscillations, induced by both thermal and mechanical excitation, have been observed in this medium, probably due to its non-Newtonian character.
The insertion of myelin basic protein into microemulsion droplets of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) has been studied by quasi-elastic light scattering. Measurements were made at both low and high molar ratios of water to surfactant, as a function of protein occupancy. The hydrodynamic radii of filled and empty droplets were experimentally evaluated. These were compared to values calculated using a water shell model of protein encapsulation, and excellent agreement was obtained. At low molar ratio of water to surfactant (w0 = 5.6), the hydrodynamic radius of filled droplets is significantly larger than the radius of empty ones. Under these conditions, about three empty (water-filled) droplets are required to build up a droplet of sufficient size to accommodate a single protein molecule. At maximum solubilization, which occurs at w0 = 5.6, a small fraction of droplets are found containing protein aggregates. In contrast, results at high values of w0 (22.4) reveal radii for empty and occupied droplets of comparable dimension, and the absence of aggregates. The results are discussed in terms of the model and the mechanism of interaction of this protein with the aqueous interfaces provided by these membrane-mimetic systems.
Explore the source record for details and available documents.