Electrohydrodynamic convection in the nematic liquid crystal MBBA in the presence of a competing magnetic field produces disclination loops in the two dynamic scattering states DSM 1 and DSM 2. The convection and magnetic field bias this initial state so that the subsequent decay of the disclination loops does not obey dynamic scaling. We experimentally confirm this breakdown of dynamic scaling. [S0031-9007(97)04577-8] PACS numbers: 61.30.Gd, 47.65. + a, 61.30.Jf, 64.60.HtSystems quenched from a disordered to an ordered state often evolve patterns of some characteristic size as they return to equilibrium. The challenge is to describe the temporal evolution of these patterns, which can often be described by a scaling relation. The best known example of such a transition is phase separation in binary mixtures undergoing spinodal decomposition in which domains grow with a power law dependence. In this example the system has discrete symmetry and a conserved order parameter. More recently, attention has focused on systems with continuous symmetry and nonconserved order parameters in which line defects (strings or disclinations) form as a result of the quench. Such systems have been studied theoretically [1-4], numerically [5], and with computer simulations [6] which predict that the defect density scales as t 2f , where the exponent depends on details of the order parameter and the spatial dimensionality. Experimental realizations include liquid crystals undergoing a pressure quench [7-10] from the isotropic to nematic phase and coarsening of domains in twisted nematic films [11,12] under temperature quenches in which defect density exhibits the predicted dynamic scaling.Recent theoretical work [13] and computer simulations [14] have considered the evolution of defects resulting from a quench in which the disordered state is biased by some field which favors a particular state in the ordered phase. The bias causes the system's evolution to no longer be described by a simple power law. The length of the defects is instead predicted to behave as l͑t͒ ϳ ͑t͒ 2f exp͓2g͑t͒ d ͔, where the exponent f depends on the nature of the order parameter and d depends on the spatial dimensionality. The decay constant g is a nontrivial function of the bias field such that power law behavior is recovered when the bias is removed. An experiment [15] involving the relaxation of biased twist regions in nematic liquid crystals showed agreement with the functional form of the above equation but no values for exponents were reported.In this Letter, we report on an experimental study of the decay of disclination loops formed by a nematic liquid crystal undergoing electrohydrodynamic convection [16] with the convecting flow field acting as the bias field. These disclination loops, which are topological defects whose circumference is a disclination line that separates regions of opposite twist, do not obey dynamic scaling as they decay, in contrast to loops formed by pressure or temperature quenches from the isotropic to the nematic phase.These ...
The activity concentrations of naturally occurring radionuclides ((238)U, (226)Ra, (228)Ra, (210)Pb and (40)K) in Jordanian phosphate ore, fertilizer material and phosphogypsum piles were investigated. The results show the partitioning of radionuclides in fertilizer products and phosphogypsum piles. The outcome of this study will enrich the Jordanian radiological map database, and will be useful for an estimation of the radiological impact of this industrial complex on the immediate environment. The activity concentration of (210)Pb was found to vary from 95 +/- 8 to 129 +/- 8 Bq kg(-1) with a mean value of 111 +/- 14 Bq kg(-1) in fertilizer samples, and from 364 +/- 8 to 428 +/- 10 Bq kg(-1) with a mean value of 391 +/- 30 Bq kg(-1) in phosphogypsum samples; while in phosphate wet rock samples, it was found to vary between 621 +/- 9 and 637 +/- 10 Bq kg(-1), with a mean value of 628 +/- 7 Bq kg(-1). The activity concentration of (226)Ra in fertilizer samples (between 31 +/- 4 and 42 +/- 5 Bq kg(-1) with a mean value of 37 +/- 6 Bq kg(-1)) was found to be much smaller than the activity concentration of (226)Ra in phosphogypsum samples (between 302 +/- 8 and 442 +/- 8 Bq kg(-1) with a mean value of 376 +/- 62 Bq kg(-1)). In contrast, the activity concentration of (238)U in fertilizer samples (between 1011 +/- 13 and 1061 +/- 14 Bq kg(-1) with a mean value of 1033 +/- 22 Bq kg(-1)) was found to be much higher than the activity concentration of (238)U in phosphogypsum samples (between 14 +/- 5 and 37 +/- 7 Bq kg(-1) with a mean value of 22 +/- 11 Bq kg(-1)). This indicates that (210)Pb and (226)Ra show similar behaviour, and are concentrated in phosphogypsum piles. In addition, both isotopes enhanced the activity concentration in phosphogypsum piles, while (238)U enhanced the activity concentration in the fertilizer. Due to the radioactivity released from the phosphate rock processing plants into the environment, the highest collective dose commitment for the lungs was found to be 1.02 person nGy t(-1). Lung tissue also shows the highest effect due the presence of (226)Ra in the radioactive cloud (0.087 person nGy t(-1)).
We have studied electrohydrodynamic convection in the nematic liquid crystal MMBA in the conduction regime in the presence of a competing magnetic eld. This eld substantially alters the behaviour of the system, causing a metastable surface deformation and travelling waves. The magnetic eld also alters the transition between the two dynamic scattering modes so that both states retain anisotropic ordering. A scaling relation is found describing this hysteretic transition. We report the existence of a stable mixed state of DSM 1 and DSM 2.
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