14 research outputs found
Pets as Sentinels of Human Exposure to Neurotoxic Metals
The idea that animals may be used as sentinels of environmental hazards
pending over humans and the associated public health implications is not a new one.
Nowadays pets are being used as bioindicators for the effects of environmental contaminants
in human populations. This is of paramount importance due to the large
increase in the worldwide distribution of synthetic chemicals, particularly in the
built environment. Companion animals share the habitat with humans being simultaneously
exposed to and suffering the same disease spectrum as their masters.
Moreover, their shorter latency periods (due to briefer lifespans) enable them to act
as early warning systems, allowing timely public health interventions. The rise on
ethical constraints on the use of animals and, consequently, on the sampling they
can be subjected to has led to the preferential use of noninvasive matrices, and in
this case we are looking into hair. This chapter focuses in three non-essential metals:
mercury, lead, and cadmium, due to their ubiquitous presence in the built environment
and their ability of affecting the mammal nervous system. There is a fairly
short amount of studies reporting the concentrations of these metals in petsâ hair,
particularly for cats. These studies are characterized, and the metal concentrations
corresponding to different parameters (e.g., age, sex, diet, rearing) are described in
order to provide the reader with a general vision on the use of this noninvasive
matrix on the studies conducted since the last two decades of the twentieth
century.publishe
Tilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect
The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals AFLCs are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circum- stances they contribute to the measured dielectric permittivity
Antiferroelectric liquid crystals with 45° tilt - a new class of promising electro-optic materials
Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmC/sub n/* phase in the interval between 27.4 degrees C and 121.6 degrees C, in which the tilt angle theta saturates at 45 degrees for T<or=80 degrees C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45 degrees AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level ``phase-only'' modulator
ANTIFERROELECTRIC LIQUID CRYSTALS WITH 45 ° TILT- A NEW CLASS OF PROMISING ELECTRO-OPTIC MATERIALS
Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far- perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmC a * phase in the interval between 27.4°C and 121.6°C, in which the tilt angle Ξ saturates at 45 degrees for Tâ€80°C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45 ° AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level âphase-onlyâ modulator. Keywords uniaxial antiferroelectric liquid crystal; 45 ° tilt; Sm Ca*; AFLC dielectric tensor; new electro-optic modes