Anisotropy of spatiotemporal decorrelation in electrohydrodynamic turbulence.

Nonlinear straining and random sweeping spatiotemporal decorrelation properties, originally introduced as the main processes for turbulent fluctuations decorrelation in usual fluid flows, have been observed experimentally in anisotropic electroconvective turbulence generated in a nematic liquid crystal under the action of an external oscillating electric field. A transition between both processes occurs when the instability is driven toward states of increasing complexity, thus showing that decorrelation mechanisms in turbulent media are more universal than naively expected. A model for both decorrelation mechanisms is introduced, its comparison with experimental results providing an estimate of the characteristic sweeping velocity.

Inhomogeneous bulk nematic order reconstruction.

A two-dimensional model within the Q-tensor description of liquid crystals is used to describe the inhomogeneous order reconstruction in a nematic cell driven by tony modulation in the anchoring conditions. Homogeneous and inhomogeneous reconstruction are contrasted: the former is defectless, the latter is defect mediated. While the transition thresholds are comparable in both cases and in good agreement with experimental data, the biaxial wall breaking is considerably slower in the inhomogeneous transition than in the homogeneous one. The shape of the signal given by the electric current flowing through the cell allows us to distinguish the actual path followed by the transition.

Random lasing and weak localization of light in dye-doped nematic liquid crystals.

The first observation of random laser action in a partially ordered, optically anisotropic nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and the typical narrowing and explosion effect leads to discrete sharp peaks. The unexpected surviving of interference effects in recurrent multiple scattering provide the required optical feedback for lasing in nematics. Coherent backscattering of light waves in orientationally ordered nematic liquid crystals manifests a weak localization of light which strongly supports diffusive laser action in presence of gain medium. Intensity fluctuations of the speckle-like emission pattern indicate the typical spatio-temporal randomness of diffusive laser emission. A comparison of the laser action is reported for systems with different order degree: fully disordered semiconductor powders, self-ordered cholesterics and partially ordered nematic liquid crystals.

Virtual unrolling and deciphering of Herculaneum papyri by X-ray phase-contrast tomography.

A collection of more than 1800 carbonized papyri, discovered in the Roman 'Villa dei Papiri' at Herculaneum is the unique classical library survived from antiquity. These papyri were charred during 79 A.D. Vesuvius eruption, a circumstance which providentially preserved them until now. This magnificent collection contains an impressive amount of treatises by Greek philosophers and, especially, Philodemus of Gadara, an Epicurean thinker of 1st century BC. We read many portions of text hidden inside carbonized Herculaneum papyri using enhanced X-ray phase-contrast tomography non-destructive technique and a new set of numerical algorithms for 'virtual-unrolling'. Our success lies in revealing the largest portion of Greek text ever detected so far inside unopened scrolls, with unprecedented spatial resolution and contrast, all without damaging these precious historical manuscripts. Parts of text have been decoded and the 'voice' of the Epicurean philosopher Philodemus is brought back again after 2000 years from Herculaneum papyri.

Forces in nematic liquid crystals constrained to the nanometer scale under hybrid anchoring conditions.

Using a surface forces apparatus we have studied two thermotropic nematic liquid crystals (5CB and ME10.5) subjected to hybrid (homeotropic/planar) anchoring conditions. A film of nematic material is constrained between two curved smooth surfaces separated by less than 2500 A . The intersurface force is nonmonotonic with the separation, being repulsive for thicknesses larger than approximately 100 A and strongly adhesive at a shorter scale. While the repulsion can be qualitatively explained by an elastic model of director deformation, including anchoring deviation at the boundaries, the attraction cannot be explained either by elasticity or by dispersive forces. The expected confinement-induced anchoring transition has not been observed for a thickness as small as 200 A .

Liquid-crystal-electrochromic-material interface: a p-n-like electro-optic junction.

A liquid crystal (LC) cell has been made by using a standard sandwich configuration with one of the indium tin oxide (ITO) electrodes covered by a thin layer of tungsten trioxide (WO3). In this kind of cell optical polarization switching (observed under a crossed polarizer microscope) occurs for only one of the two polarities of an ac external applied electric field, while in the usual liquid crystal cells the electro-optic response does not depend on the sign of the field. The inhibiting switching configuration corresponds to anodic polarization of the tungsten trioxide film in which the deintercalation of cations occurs. Here we present the time behavior of charge and discharge for both the anodic and cathodic currents. A model based on charge carrier exchange between the ITO-WO3 and WO3-LC interfaces and also electrochemical processes is reported. Our model is also capable of explaining the electric and electro-optic asymmetric responses of the cell. Numerical calculations confirm the model.

Polarization-dependent optomechanics mediated by chiral microresonators.

Chirality is one of the most prominent and intriguing aspects of nature, from spiral galaxies down to aminoacids. Despite the wide range of living and non-living, natural and artificial chiral systems at different scales, the origin of chirality-induced phenomena is often puzzling. Here we assess the onset of chiral optomechanics, exploiting the control of the interaction between chiral entities. We perform an experimental and theoretical investigation of the simultaneous optical trapping and rotation of spherulite-like chiral microparticles. Due to their shell structure (Bragg dielectric resonator), the microparticles function as omnidirectional chiral mirrors yielding highly polarization-dependent optomechanical effects. The coupling of linear and angular momentum, mediated by the optical polarization and the microparticles chiral reflectance, allows for fine tuning of chirality-induced optical forces and torques. This offers tools for optomechanics, optical sorting and sensing and optofluidics.

Light scattered by a liquid crystal: a new quasi-thermal source.

The spatial and temporal coherence properties of laser light scattered by a liquid crystal cell under the application of an external dc field are studied. It is shown that the scattered field constitutes a source with very long coherence time. The coherence time can be changed by changing the value of the applied voltage, and the spatial coherence can be adjusted by varying the dimension of the illuminated spot on the cell. The effects produced when the length of correlation of fluctuations in the liquid crystal is not negligible with respect to the dimension of the spot in the cell are also studied. The scattered field is proposed as a new quasi-thermal source, alternative of the rotating ground glass disk.

Probing the inner surface of a capillary with the atomic force microscope.

The inner surface of a fused silica capillary for zone electrophoresis was probed by the atomic force microscope (AFM) in the contact mode. Only uncoated surfaces were analyzed, after a simple washing cycle with detergent, NaOH and HCl, and final equilibration in distilled water. Three progressively larger surface areas were probed: 0.5 x 0.5 micron, 2 x 2 microns and 4 x 4 microns. In all cases, it was found that the surface is remarkably smooth, with a median height increasing from 1.3 to 5.6 nm; mean height, from 1.3 to 5.8 nm; root-mean-squared roughness, from 0.35 to 1.5 nm; and average roughness, from 0.28 to 0.67 nm (the lower values referring to the smaller area, the larger to the largest area probed). The lowest "peaks" detected are of the order of 1-2 nm; some scattered peaks as high as 16 nm are occasionally found. It is concluded that the inner surface of a capillary is not a serrate or notchy structure, but is indeed quite smooth. Since the average roughness is comprised within the thickness of the diffuse double layer (> 10 nm) existing on the silica wall as a result of silanol ionization (and, in fact, it is on the average considerably smaller), it is concluded that it cannot possibly influence peak shape and contribute to peak decay in an electrophoretic run.

Coherence of Light Passing Through a Nematic Liquid Crystal Under a dc Electric Field.

A study of spatial coherence properties of light passing through a nematic liquid-crystal (MBBA) cell is given as a function of the external dc electric field applied to the cell and of initial molecular orientation and light polarization. Two different interferometric dispositions have been used. The molecular motion induced by the electric field is also studied.

Time resolved experimental analysis of the electric field induced biaxial order reconstruction in nematics.

We present the time resolved experimental characterization of the biaxial switching between two topologically distinct textures of a nematic liquid crystal cell submitted to a strong electric field. This fast electro-optical effect is governed by the electric induced order reconstruction in the nematic bulk, which is an actual challenge for a complete theoretical description of the nematodynamics. The electric measurements across the cell are suitable to this purpose since they well discriminate among the dielectric, the ionic, and the order reconstruction contributions. A phenomenological model describes the experimental data, allowing the measurements of the order reconstruction characteristic time.

Probing soft polymeric coatings of a capillary by atomic force microscopy.

Atomic force microscopy (AFM) has been used to probe the surface of a capillary after coating with "soft" polymers, notably polyacrylamides. The aim was the investigation of the efficiency of coverage of the silica surface, so as to reduce or eliminate the electroosmotic flow (EOF), particularly noxious in the separation of macromolecules. The quality of such coating is strongly dependent on two variables: temperature and pH. In the first case, progressively higher temperatures produce open silica patches, where no polymer seems to be bound. The transition from coated to largely uncoated surfaces occurs at 50 degrees C. Also the pH of the polymerizing solution strongly affects the coating efficiency. Since in all coating procedures the monomer solution is not buffered, addition of accelerator (TEMED, N,N,N'N'-tetramethylethylendiamine) induces polymer growth at pH 10-11. These pH values generate hydrolysis of the siloxane bridge anchoring the bifunctional agent (Bind Silane, onto which the polymer chain should grow) to the wall. Thus, coating and de-coating occur simultaneously. Low temperatures during polymer growth (typically 10 degrees C) and buffered solutions (pH 7, titrated after TEMED addition) ensure a most efficient and thorough coating, with virtual elimination of EOF: well coated capillaries exhibit residual EOF values, at pH 10, of the order of 10(-7) cm2 V-1 s-1 vs. a standard value for uncoated capillaries of the order of 10(-4) cm2 V-1 s-1. The AFM data have been fully confirmed by direct measurement of EOF in coated and uncoated capillaries under an electric field.

Photopolarimetric characterization of the transition between two turbulent states in a nematic liquid crystal film.

This work was aimed at the photopolarimetric characterization of the transition between two dynamic scattering modes that take place in a planarly aligned nematic liquid crystal sample, under the effect of an external low-frequency electric field. The time evolution of the degree of polarization and the behavior of the radiation entropy of the transmitted light allow us to interpret the transition between two turbulent states, or dynamic scattering modes, as a decay from a two-dimensional (2D) to a (3D) turbulence.

Electrical and electro-optical investigations of liquid crystal cells containing WO3 thin films

An interesting application of the fast ion transport properties of tungsten trioxide is presented, when it is inserted as an electrode in nematic liquid crystal (NLC) cells. In a standard sandwichlike cell the nematic liquid crystal, confined between two transparent plane electrodes of purely electronic conductors [indium tin oxide (ITO)], undergoes a molecular reorientation under the action of an external electric field E. This electrically controlled birefringence (electro-optical switching) is proportional to E2, thus polarity insensitive [L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag, New York, 1994)]. When a thin film of tungsten trioxide is deposited by magnetron sputtering onto one of the transparent ITO electrodes, and a NLC cell is assembled with such asymmetry, the electro-optical response becomes polarity sensitive [G. Strangi et al., Appl. Phys. Lett. 74, 534 (1999)]. The analysis of this response suggests the occurrence of a reverse internal electric field, associated with the ionic diffusion process of protons always present in these sputtered WO3 films [E. Cazzanelli et al., Electrochim. Acta 44, 3101 (1999)]. By using an opportune voltage waveform it is possible to evaluate such an internal field. Impedance and cyclic voltammetry measurements were carried out on these cells, comparing "as-deposited" and "annealed" tungsten trioxide electrodes. These studies confirm that an important ionic diffusion process is involved in the establishment of an internal electric field, which modifies the electro-optical response of the nematic liquid crystal cell.