Napolitano S.,Soft Science |
Capponi S.,University College Dublin |
Vanroy B.,Catholic University of Leuven
European Physical Journal E | Year: 2013
The structural dynamics of polymers and simple liquids confined at the nanometer scale has been intensively investigated in the last two decades in order to test the validity of theories on the glass transition predicting a characteristic length scale of a few nanometers. Although this goal has not yet been reached, the anomalous behavior displayed by some systems - e.g. thin films of polystyrene exhibit reductions of Tg exceeding 70K and a tremendous increase in the elastic modulus - has attracted a broad community of researchers, and provided astonishing advancement of both theoretical and experimental soft matter physics. 1D confinement is achieved in thin films, which are commonly treated as systems at thermodynamic equilibrium where free surfaces and solid interfaces introduce monotonous mobility gradients, extending for several molecular sizes. Limiting the discussion to finite-size and interfacial effects implies that film thickness and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. On the contrary, such an oversimplified picture, although intuitive, cannot explain phenomena like the enhancement of segmental mobility in proximity of an adsorbing interface, or the presence of long-lasting metastable states in the liquid state. Based on our recent work, we propose a new picture on the dynamics of soft matter confined in ultrathin films, focusing on non-equilibrium and on the impact of irreversibly chain adsorption on the structural relaxation. We describe the enhancement of dynamics in terms of the excess in interfacial free volume, originating from packing frustration in the adsorbed layer (Guiselin brush) at t < 1, where t* is the ratio between the annealing time and the time scale of adsorption. Prolonged annealing at times exceeding the reptation time (usually t* ≫ 1 induces densification, and thus reduces the deviation from bulk behavior. In this Colloquium, after reviewing the experimental approaches permitting to investigate the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy, we propose several methods to determine gradients of mobility in thin films, and then discuss on the unexploited potential of analyses based on the time, temperature and thickness dependence of the orientational polarization via the dielectric strength. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
Krishnamurthy K.S.,Soft Science
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2014
The Bobylev-Pikin striped-pattern state induced by a homogeneous electric field is a volume flexoelectric instability, originating in the midregion of a planarly aligned nematic liquid crystal layer. We find that the instability acquires a spatiotemporal character upon excitation by a low frequency (<0.5 Hz) square wave field. This is demonstrated using a bent-core liquid crystal, initially in the 90°-twisted planar configuration. The flexoelectric modulation appears close to the cathode at each polarity reversal and, at low voltage amplitudes, decays completely as the field becomes steady. Correspondingly, at successive polarity changes, the stripe direction switches between the alignment directions at the two substrates. For large voltages, the stripes formed nearly along the alignment direction at the cathode gradually reorient toward the midplane director. These observations are generally attributed to inhomogeneous and time-dependent field conditions that come to exist after each polarity reversal. Polarity dependence of the instability is attributed to the formation of intrinsic double layers that bring about an asymmetry in surface fields. Momentary field elevation near the cathode following a voltage sign reversal and concomitant gradient flexoelectric polarization are considered the key factors in accounting for the surfacelike modulation observed at low voltages. © 2014 American Physical Society.
Krishnamurthy K.S.,Soft Science
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2015
The electric Freedericksz transition is a second-order quadratic effect, which, in a planarly aligned nematic liquid crystal layer, manifests above a threshold field as a homogeneous symmetric distortion with maximum director-tilt in the midplane. We find that, upon excitation by a low frequency (<0.2Hz) square-wave field, the instability becomes spatially and temporally varying. This is demonstrated using calamitic liquid crystals, initially in the 90°-twisted planar configuration. The distortion occurs close to the negative electrode following each polarity switch and, for low-voltage amplitudes, decays completely in time. We use the elastically favorable geometry of Brochard-Leger walls to establish the location of maximum distortion. Thus, at successive polarity changes, the direction of extension of both annular and open walls switches between the alignment directions at the two substrates. For high voltages, this direction is largely along the midplane director, while remaining marginally oscillatory. These results are broadly understood by taking into account the time-varying and inhomogeneous field conditions that prevail soon after the polarity reverses. Polarity dependence of the instability is traced to the formation of intrinsic double layers that lead to an asymmetry in field distribution in the presence of an external bias. Momentary field elevation near the negative electrode following a voltage sign reversal leads to locally enhanced dielectric and gradient flexoelectric torques, which accounts for the surface-like phenomenon observed at low voltages. These spatiotemporal effects, also found earlier for other instabilities, are generic in nature. © 2015 American Physical Society. ©2015 American Physical Society.
Kikuchi M.,Japan Atomic Energy Agency |
Azumi M.,Soft Science
Reviews of Modern Physics | Year: 2012
Tokamaks have demonstrated excellent plasma confinement capability because of their symmetry but has an intrinsic drawback because of their pulsed inductive operation. Efforts have been made in the past 20 years to realize steady-state operation, the most successful utilizing a bootstrap current. In this review, progress in understanding tokamak physics related to steady-state operation is described to investigate the scientific feasibility of a steady-state tokamak fusion power system. © 2012 American Physical Society.
Rajalakshmi R.,Soft Science |
Angappane S.,Soft Science
Journal of Alloys and Compounds | Year: 2014
We have deposited ZnO and 3% Mn doped ZnO (ZnO:Mn) thin films of different thickness by RF magnetron sputtering and studied the structural and optical properties. The deposited films were characterized by a host of characterization techniques, such as, X-ray diffraction, scanning electron microscopy, UV-visible transmittance and photoluminescence. The X-ray diffraction measurements on all the films show a preferential growth along c axis and the intensity of (0 0 2) peak is found to increase with increase of thickness up to 80 and 90 nm for ZnO and ZnO:Mn films respectively and decreases thereafter. The FESEM images of the films illustrate a hexagonal granular surface morphology for lower thickness and a growth of pyramidal nanostructures for higher thickness. The calculated values of the optical band gaps are found to decrease upon increasing the film thickness. Markedly, the band edge emission is large for 80 and 90 nm films of ZnO and ZnO:Mn respectively. The obtained optimized sputtering growth conditions will facilitate to exploit these ZnO and ZnO:Mn thin films for various device applications. © 2014 Elsevier B.V. All rights reserved.