Ion Motion in Electrolytic Cells: Anomalous Diffusion Evidences

In this study, we argue that ion motion in electrolytic cells containing Milli-Q water, weak electrolytes, or liquid crystals may exhibit unusual diffusive regimes that deviate from the expected behavior, leading the system to present an anomalous diffusion. Our arguments lie on the investigation of the electrical conductivity and its relationship with the mean square displacement, which may be used to characterize the ionic motion. In our analysis, the Poisson-Nernst-Planck diffusional model is used with extended boundary conditions to simulate the charge transfer, accumulation, and/or adsorption-desorption at the electrode surfaces.

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.

Probing cavitand-organosilane hybrid bilayers via sum-frequency vibrational spectroscopy

Quinoxaline cavitands (QxCav) are transferred by Langmuir-Schaefer method on self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) and N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP) on fused silica substrates. The molecular architectures of both the hydrophobic SAMs templates and the hybrid cavitand-organosilanes bilayers at the solid-air interface are investigated and correlated by sum-frequency vibrational spectroscopy. The results show that QxCav are always in the closed vase configuration and orient with their principal axis normal to the substrates. The role of the alkyl chains density in the SAM templates on the QxCav transfer ratio is pointed out.

Non-Debye relaxation in the dielectric response of nematic liquid crystals: surface and memory effects in the adsorption-desorption process of ionic impurities

We demonstrate theoretically that the presence of ions in insulating materials such as nematic liquid crystals may be responsible for the dielectric spectroscopy behavior observed experimentally. It is shown that, at low frequencies, an essentially non-Debye relaxation process takes place due to surface effects. This is accomplished by investigating the effects of the adsorption-desorption process on the electrical response of an electrolytic cell when the generation and recombination of ions is present. The adsorption-desorption is governed by a non-usual kinetic equation in order to incorporate memory effects related to a non-Debye relaxation and the roughness of the surface. The analysis is carried out by searching for solutions to the drift-diffusion equation that satisfy the Poisson equation relating the effective electric field to the net charge density. We also discuss the effect of the mobility of the ions, i.e., situations with equal and different diffusion coefficients for positive and negative ions, on the impedance and obtain an exact expression for the admittance. The model is compared with experimental results measured for the impedance of a nematic liquid crystal sample and a very good agreement is obtained.

Fractal aggregates evolution of methyl red in liquid crystal

The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layered system consisting of a film composed of liquid crystal, dye and solvent cast above a polymer substrate. The observed aggregates are promoted by phase separation processes induced by dye diffusion and solvent evaporation. The growth properties of the aggregates are studied through the temporal evolution of their topological properties (surface, perimeter, fractal dimension). The fractal dimension of the completely formed structures, when they are coexistent with different types of structures, is consistent with theoretical and experimental values obtained for Diffusion-Limited Aggregates. Under different experimental conditions (temperature and local dye concentration) the structure forms without interactions with other kinds of structures, and its equilibrium fractal dimension is smaller. The fractal dimension is thus not a universal property of the observed structures, but rather depends on the experimental conditions.

Langmuir-Blodgett film of hydrophobin protein from Pleurotus ostreatus at the air-water interface

We present results concerning the formation of Langmuir-Blodgett (LB) films of a class I hydrophobin from Pleurotus ostreatus at the air-water interface, and their structure as Langmuir-Blodgett (LB) films when deposited on silicon substrates. LB films of the hydrophobin were investigated by atomic force microscopy (AFM). We observed that the compressed film at the air-water interface exhibits a molecular depletion even at low surface pressure. In order to estimate the surface molecular concentration, we fit the experimental isotherm with Volmer's equation describing the equation of state for molecular monolayers. We found that about (1)/ 10 of the molecules contribute to the surface film formation. When transferred on silicon substrates, compact and uniform monomolecular layers about 2.5 nm thick, comparable to a typical molecular size, were observed. The monolayers coexist with protein aggregates, under the typical rodlet form with a uniform thickness of about 5.0 nm. The observed rodlets appear to be a hydrophilic bilayer and can then be responsible for the surface molecular depletion.

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.

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.

Electric field induced order reconstruction in a nematic cell

We have experimentally observed the biaxial switching between two topologically distinct textures of a nematic liquid crystal cell submitted to a strong electric field. The effect is deduced from optical and electrical measurements across the cell. Above a static threshold, a bulk order reconstruction is observed, where the final nematic orientation in the centre becomes perpendicular to its initial one, inducing a total pi change of orientation across the cell. Using short electric field pulses, a higher dynamical threshold is observed. These experiments are explained by a Landau-de Gennes-Khalatnikov model. The threshold implies the local exchange of two eigenvalues of the nematic order tensor through intermediate biaxial states. The onset of the effect in a thin splay-bend wall decreases the static threshold by almost an order of magnitude. The model explains reasonably well the static and dynamic measurements within the present description of nematic biaxiality.

Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell

We present an optical study of the influence of both the flexoelectric effect and surface polarization on a hybrid-aligned nematic cell using the half-leaky guided mode technique. Tilt angle profiles, obtained from fits of experimental data (reflectivity curves) taken under applied voltages, are compared with the ones derived by a complete theoretical model. Measurements with an applied alternating voltage allow the evaluation of the anchoring energy by solving the torque balance equation at the planar surface. From measurements with static fields, the sum of flexoelectric coefficients and the surface polarization are determined by numerical solution of Euler-Lagrange equations.

Helix-specific interactions induce condensation of guanosine four-stranded helices in concentrated salt solutions

Deoxyguanosine-5'-monophosphate in water self-associates into stable structures, which include liquid-crystalline hexagonal and cholesteric phases. The structural unit is a four-stranded helix, composed of stacked Hoogsteen-bonded guanosine quartets. By using the osmotic stress method, we recently measured the force between helices in KCl solutions up to 2 M. In addition to the long-range electrostatic force, a short-range hydration repulsive contribution was recognized. The hydration repulsion is exponential, and shows a decay length independent from the ionic strength of the solution. Here, we report that more concentrated KCl solutions cause condensation of the guanosine helix in a hexagonal phase with constant equilibrium separation of approximately 7 A between helix surfaces. Long-range attraction, which induces the self-assembly, and short-range repulsion, which prevents the contact between the helices, are implied. By using osmotic stress, the force needed to push helices closer from the spontaneously assumed position has been measured. The attractive force was then estimated as a difference between the net force and the repulsive contribution, revealing an exponential decay length about two times larger than that of the short-range repulsion. The agreement with the helix interaction theory introduced recently by Kornyshev and Leikin (Kornyshev, A. A., and S. Leikin, 1997. Theory of interaction between helical molecules. J. Phys. Chem. 107:3656-3674) suggests that the repulsive and attractive forces originate from helix-specific interactions.