1,720,984 research outputs found
Les vertus des défauts: The scientific works of the late Mr Maurice Kleman analysed, discussed and placed in historical context, with particular stress on dislocation, disclination and other manner of local material disbehaviour
No full textIn memory of Maurice Kleman: Over the last half century, Maurice Kleman was the High Priest of the science of defects in condensed matter. I discuss some aspects of the history of dislocations, disclinations, and defects in liquid crystals, together with some of Maurice's other work. In so doing, I combine intellectual strands coming from pure and applied mathematics, physics, material science, and biology
Longitudinal study of Web-related patents
No full textOriginal dataset and analysis scripts for our patent-based study into the history of corporate Web innovation. The dataset contains 20,493 Web-related patents filed between years 1990 through 2013 in the US. From this dataset we extracted temporal patenting rates which are used in the analysis. The accumulation of patents over time is fitted using regressions to examine the shape of the growth curve in Web technology. We also analyse the relationship between monthly patenting rates and NASDAQ stock market movements using a VAR model.</span
Jerry Ericksen: liquid crystal pioneer
No full textIn the 1960s Jerry Ericksen made major contributions to the construction of the continuum theory of nematic liquid crystals. This paper gives a brief summary of his work and the consequent giant impact on the field
Molecular field theory for biaxial smectic A liquid crystals
No full textThermotropic biaxial nematic phases seem to be rare, but biaxial smectic A phases less so. Here we use molecular field theory to study a simple two-parameter model, with one parameter promoting a biaxial phase and the second promoting smecticity. The theory combines the biaxial Maier-Saupe and McMillan models. We use alternatively the Sonnet-Virga-Durand (SVD) and geometric mean approximations (GMA) to characterize molecular biaxiality by a single parameter. For non-zero smecticity and biaxiality, the model always predicts a ground state biaxial smectic A phase. For a low degree of smectic order, the phase diagram is very rich, predicting uniaxial and biaxial nematic and smectic phases, with the addition of a variety of tricritical and tetracritical points. For higher degrees of smecticity, the region of stability of the biaxial nematic phase is restricted and eventually disappears, yielding to the biaxial smectic phase. Phase diagrams from the two alternative approximations for molecular biaxiality are similar, except inasmuch that SVD allows for a first-order isotropic-biaxial nematic transition, whereas GMA predicts a Landau point separating isotropic and biaxial nematic phases. We speculate that the rarity of thermotropic biaxial nematic phases is partly a consequence of the presence of stabler analogous smectic phase
Biaxiality-induced magnetic field effects in bent-core nematics: molecular field and Landau theory
No full textNematic liquid crystals composed of bent-core molecules exhibit unusual properties, including an enhanced Cotton-Mouton effect and an increasing isotropic (paranematic)-nematic phase transition temperature as a function of magnetic field. These systems are thought to be good candidate biaxial liquid crystals. Prompted by these e xperiments, we investigate theoretically the effect of molecular biaxiality on magnetic field-induced phenomena for nematic liquid crystals, using both molecular field and Landau theory. The geometric mean approximation is used in order to specify the degree of molecular biaxiality using a single parameter. We reproduce experimental field-induced phenomena, and predict also an experimentally accessible magnetic critical point. The Cotton-Mouton effect and temperature dependence of the paranematic-nematic phase transition are more pronounced with increased molecular biaxiality. We compare our theoretical approaches and make contact with recent relevant experimental results on bent-core molecular systems
Theory of dielectric and optical properties of PDLC films
No full textWe present results of the light scattering properties of PDLC films in the Rayleigh-Gans long wavelength régime. The calculations take into account dependent scattering effects using an effective medium theory. The Percus-Yevick hard-sphere approximation is used to introduce droplet correlation effects. We discuss the light scattering properties of radial and bipolar partially ordered droplets, making detailed comparisons with the literature where appropriate
The effect of defects on magnetic droplet nucleation
Full text linkDefects and impurities strongly affect the timing and the character of the (re)ordering or disordering transitions of thermodynamic systems captured in metastable states. In this paper we analyse the case of two-dimensional magnetic systems. We adapt the classical JMAK theory to account for the effects of defects on the free energy barriers, the critical droplet area and the associated metastable time. The resulting predictions are successfully tested against the Monte-Carlo simulations performed by adopting Glauber dynamics, to obtain reliable time-dependent results during the out-of-equilibrium transformations. We also focus on finite-size effects, and study how the spinodal line (separating the single-droplet from the multi-droplet regime) depends on the system size, the defect fraction, and the external field
Field-induced motion of nematic disclinations
No full textAn individual defect in a nematic liquid crystal moves not only in response to its interaction with other defects but also in response to external fields. We analyze the motion of a wedge disclination in the presence of an applied field of strength H. We neglect backflow and seek steadily traveling patterns. The stationary picture yields a semi-infinite wall of strength ?, bounded by the defect line. We find that the disclination advances into the region containing the wall at velocity v(H), where v scales as H/|log H| as long as the magnetic coherence length is greater than the core radius. When the external field is applied in the presence of a pair of disclinations, their dynamics is strongly influenced. We compute the expected relative velocity of the disclinations as a function of distance and field. The natural tendency for the disclinations to annihilate each other can be overcome by a sufficiently strong field suitably directed
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