Prewetting transition induced by an external bulk field

The Landau-de Gennes model of the boundary layer phase transition in nematic liquid crystals is extended to include electric field effects. Calculations are performed in the case of a semi-infinite nematic sample bounded by a solid wall. The phase diagrams presented show that the prewetting transition can be induced by the electric field. The transition moves to higher temperature with increasing field and disappears above the prewetting critical point. A system exhibiting partial wetting should transit to complete wetting under a high enough electric field. These predictions are confirmed within the actual experimental limits.     

Three-dimensional square water in the presence of an external electric field

In this work we study a tridimensional statistical model for the hydrogen-bond (HB) network formed in liquid water in the presence of an external electric field. This model is analogous to the so-called square water, whose ground state gives a good estimate for the residual entropy of the ice. In our case, each water molecule occupies one site of a cubic lattice, and no hole is allowed. The hydrogen atoms of water molecules are disposed at the lines connecting nearest-neighbor sites, in a way that each water can be found in 15 different states. We say that there is a hydrogen bond between two neighboring molecules when only one hydrogen is in the line connecting both molecules. Through Monte Carlo simulations with Metropolis and entropic sampling algorithms, and by exact calculations for small lattices, we determined the dependence of the number of molecules aligned to the field and the number of hydrogen bonds per molecule as a function of temperature and the intensity of the external field. The results for both approaches showed that, different of the two-dimensional case, there is no maximum in the number of HBs as a function of the electric field. However, we observed nonmonotonic behaviors as a function of the temperature of the quantities of interest. We also found the dependence of the entropy on the external electric field at very low temperatures. In this case, the entropy vanishes for the value of the external field for which the contributions to the total energy coming from the HBs and the field become the same.     

Relaxation of a liquid-crystalline order induced by an electric field

The influence of a strong electric field on isotropic melts of 6-cyanobiphenyl and comb-shaped polyacrylate with mesogenic groups in its side chains is investigated. It is established that the electric field induces the isotropic liquid-nematic liquid crystal phase transition in melts of these compounds. A relaxation process was discovered that destroys the nematic ordering induced in substances by electric fields. It was established that, for polymer liquid crystals, the time of transition from the ordered state to the isotropic phase is several orders longer then that for low-molecular liquid crystal.     

Structural transformations of carboxyl acids networks induced by concentration and oriented external electric field

The effects of concentration and an oriented external electric field on the transformations of hydrogenbonded structures of trimesic acid(TMA) and terephthalic acid(TPA) have been investigated at a liquidsolid interface by scanning tunneling microscopy(STM).The triangular periodic TMA framework can be transformed into a flower-like structure by changing the STM sample bias sign in situ.Networks of TMA and TPA are porous at a negative substrate bias,but typically change to relatively compact forms when the polarity of the applied bias is reversed.This change is reversible if the applied bias is reversed.The effects have potentials to locally control the capture and release of analytes in host-guest systems and the 2D morphology in multicomponent layers.     

Calculation of the Helmholtz potential of an elastic strand in an external electric field

We derive from statistical mechanics the Gibbs free energy of an elastic random-walk chain affected by the presence of an external electric field. Intrachain charge interactions are ignored. In addition, we find two approximations of the Helmholtz potential for this system analogous to the gaussian and Cohen-Pade? approximations for an elastic strand without the presence of an electric field. Our expressions agree well with exact numerical calculations of the potential in a wide range of conditions. Our analog of the gaussian approximation exhibits distortion of the monomer density due to the presence of the electric field, and our analog of the Cohen-Pade? approximation additionally includes finite chain extensibility effects. The Helmholtz potential may be used in modeling the dynamics of electrophoresis experiments.     

Theory of a helicoidal cholesteric phase induced by an external field

We present a mean field theory to describe a helicoidal cholesteric phase for mixtures of a chiral nematic liquid crystal (LC) and a polymer chain as well as for pure chiral nematic LC molecules in the presence of a longitudinal external field parallel to the pitch axis of a cholesteric (Ch) phase. The free energy of the helicoidal Ch phase (Ch_H) is derived as a function of a usual orientational order parameter and an order parameter of the Ch_H phase. On increasing the strength of the external field, we find that the Ch phase changes to the nematic (N) phase through the Ch_H phase. Depending on the temperature and the strength of the external field, we find the second-order N–Ch_H and Ch_H–Ch phase transitions and the first-order paranematic (pN)–N, pN–Ch_H and pN–Ch phase transitions. We also predict phase diagrams in mixtures of a flexible polymer and a Ch LC molecule under the external field.     

Conformational analysis of diphenylacetylene under the influence of an external electric field

Theoretical investigation of the torsional potentials of a molecular wire, diphenylacetylene, was carried out at the B3LYP/6-311+G** level by considering the influence of the external electric field (EF). It demonstrates that many molecular features are sensitive to the EF applied. In particular, the torsional barrier increases and the LUMO-HOMO gap decreases with the increase of EF. Quantitative correlations between these features and the external EF were revealed. The current-voltage behavior corresponding to different conformers was studied as well by non-equilibrium Green's function method combined with the density functional theory. Further, the evolution of the LUMO-HOMO gap and the spatial distribution of molecular orbital were used to analyze these structure-property relationships.     

Transient effects in electrolytic cells submitted to an external electric field

We analyse transient effects in an electrolytic cell submitted to an external voltage and determine the relaxation time of the redistribution of the ions and of the potential. We consider the case in which adsorption effects at the interface with the electrodes are present and show that the typical relaxation time, for applied voltage V ₀25 mV, is of the order of tens of seconds for commercial nematic liquid crystals. When V ₀>25 mV the linearized analysis is no longer valid. In this case, the relaxation time depends on the applied voltage. Increasing V ₀, but still remaining in the low amplitude limit, the relaxation time starts increasing. This indicates that the reduction of the actual field in the sample, for moderate values of V ₀, plays an important role. For large values of V ₀, the relaxation time is a decreasing function of V ₀. This result is interpreted in terms of a simple model, according to which the ionic charge is localized in a surface layer whose thickness depends on the amplitude of the applied voltage.     

Conformations of polymer chains in voids with an external electric field by Green's function methods

The statistical conformations of a length of polymer chain, such as DNA, trapped in a void within a gel under the influence of an external electric field, have been studied by the method of Green's functions. Based upon a rectangular box approximation for the void shape, the method gives an explicit analytical expression for the end-to-end distance (R_x) as a function of applied field strength, number of chain segments coiled within the void, and size of a chain segment. Results of calculations show that the field compresses the entrained coil into more compact configurations, as would be expected. Such compression is believed to affect the electrophoretic mobility of a long chain molecule like DNA in a dilute gel. © 1995 John Wiley & Sons, Inc.     

Nuclear magnetic relaxation of ionic nuclei moving in an external electric field

We have experimentally investigated nuclear magnetic relaxation of some quadrupolar ionic nuclei (²³Na⁺, ⁸⁷Rb⁺ and ³⁵Cl⁻ ) in electrolyte solutions in non-equilibrium states. The measurements of relaxation rates 1/T₁ in the presence of a direct electric field, and thus also an electric current, revealed that differences can occur in comparison with relaxation in the absence of the field. In some systems no change in the relaxation rate was observed, but in certain other (non-aqueous) systems there was a change in the quadrupolar relaxation rate in the presence of the field.     

Ferroelectric behavior and structural change of poly(trimethylene terephthalate) induced by external electric field

The ferroelectric behavior and structural change induced by the external electric field were observed for poly(trimethylene terephthalate) (PTT), which has nonpolar crystalline structure with carbonyl groups aligned antiparallel to each other. The displacement‐electric field hysteresis curves of PTT films showed hysteresis above 140 °C, which indicated the polarization reversal of PTT. The pyroelectric constant and thermally stimulated depolarization current suggest the depolarization of the polar structure of PTT induced by the external electric field above 150 °C. An endothermic peak caused by the depolarization was also observed in the differential scanning calorimetry for the PTT film poled at 30 MV/m. The change of the crystalline structure and the orientation of carbonyl groups induced by the electric filed were investigated by X‐ray diffraction patterns and infrared spectra. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1991–1996, 2009     

Orientation of liquid crystalline 5CB under an external electric field

The orientation characteristics of pre-aligned liquid crystalline 5CB (4-n-pentyl-4′-cyanobiphenyl) in a germanium cell with unidirectional rubbed polyimide-coated surfaces have been investigated. Orientation of 5CB molecules near the polyimide surface and those representing average properties of the system (i.e., the bulk) are compared. The orientation of the bulk is monitored by transmission Fourier transform infrared (FT-IR) spectroscopy while that of the molecules next to the surface is observed via attenuated total reflection (ATR) FT-IR spectroscopy. There are significant differences in orientation characteristics between the two groups of molecules. For molecules near the polyimide surface, there is an observable difference in orientation of the soft and hard segments of the liquid crystalline. Moreover, they show depth dependent orientation.     

Local order and shape of a polymer chain in an external electric field

A solution of polymer chains in the presence of an external electric field is considered. Dipole-like interactions between polymer chain segment-vectors and the electric field are assumed to be proportional to the cosine of the angle between the segment-vector and the direction of the electric field. Parameters characterizing the shape of the chain (i.e., the chain end-to-end distance and cross-section per chain), and parameters describing the local order at the segmental and chain level (i.e., moments of the first and second Legendre polynomials 〈P₁〉 and 〈P₂〉 are calculated. The optical anisotropy and the molecular shape change induced by the external electric field are discussed.     

Thermal desorption of cations from polysilicates and zeolites in an external electric field

The desorption of cations from crystalline polysilicates and polysilicic acids, A and Y zeolites, and natural mordenite in the NH₄, H, and dealuminized forms under the action of a strong electrostatic field at temperatures ranging from 20 to 600°C has been studied. The desorption of sodium and potassium cations was detected above a temperature of 150–200°C, and magnesium, calcium, and aluminum cations were not detected. Potassium cations are desorbed at lower temperatures than are sodium cations. Zeolite NaA retains cations more strongly than-does zeolite NaY. The observed spectra are consistent with the concentrations of the cations and the structural features of the samples studied.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 626–629, September–October, 1988.     

Rotational diffusion and alignment of short gold nanorods in an external electric field

We present measurements of the polarized extinction of gold nanorod suspensions exposed to an external electric field. By employing an amplitude modulated field in combination with lock-in detection we resolve changes in the optical density as low as 10(-6) in an integration time of 10 s. This sensitivity allows us to probe the partial alignment of small gold nanorods with an aspect ratio of 2.5 and a width ranging from 13 nm to 28 nm. The degree of orientation scales as the square of the electric field strength, as expected for an induced dipole moment in an external field. By varying the modulation frequency we measure the rotation diffusion constant of different samples, which are in excellent agreement with the calculated values for a short cylinder.     

Pattern formation in PMMA film induced by electric field

Micro‐sized patterns were created on thin poly(methyl methacrylate) (PMMA) films by the effect of external field, perpendicular to the film surface. The PMMA film, prepared by spin‐coating onto Si wafer, was heated to the fluid temperature (275 °C) and a linear pattern was created by the effect of electric field produced by a strip electrode. In another experiment, a round pattern was created as a result of local laser heating of the PMMA film under homogenous electrical field. The created patterns were analyzed by optical microscopy and profile meter. The dependence of the form and size of the created patterns on the intensity of the electric field, exposure time, and initial film thickness was examined. Wave guiding property of a linear pattern, produced by the above technique, was examined in a simple experiment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1131–1135, 2009     

Effect of the overall rotation on the cis-trans isomerization of HONO induced by an external field

Rovibrational eigenenergies of HONO are computed and compared to experimental energies available in the literature. For their computation, we use a previously developed potential energy surface (PES) and a newly derived exact kinetic energy operator (KEO) including the overall rotation for a tetra-atomic molecule in non-orthogonal coordinates. In addition, we use the Heidelberg Multi-Configuration Time-Dependent Hartree (MCTDH) package. We compare the experimental rovibrational eigenvalues of HONO available in the literature with those obtained with MCTDH and a previously developed potential energy surface (PES) [F. Richter et al., J. Chem. Phys., 2004, 120, 1306.] for the cis geometry. The effect of the overall rotation on the process studied in our previous work on HONO [F. Richter et al., J. Chem. Phys., 2007, 127, 164315.] leading to the cis→trans isomerization of HONO is investigated. This effect on this process is found to be weak.