Biomedical subjects

I W Stewart

Publications and source records attributed to I W Stewart.

10 recordsLinked to original sources

PubMed2005

Surface anchoring and dynamics of jump-wise director reorientations in planar cholesteric layers.

A theoretical investigation is made into the dynamics of pitch jumps in cholesteric liquid-crystal layers having finite strength surface-anchoring conditions. A presentation is given of general formulations which connect the dynamics of pitch jumps with the key material parameters such as the viscosity, the specific form of the anchoring potential, and the dimensionless parameter S(d)=K(22)/Wd , where K22 is the elastic modulus, W is the depth of the anchoring potential, and d is the layer thickness. To illustrate the dependence of the pitch jump dynamics upon the shape and strength of the anchoring potential, we investigate two sets of different model surface-anchoring potentials for a jump mechanism that is connected with the slipping of the director at a surface over the barrier of the anchoring potential. Two types of "narrow" well potentials that are natural extensions of the more familiar "wide" potentials are considered: one type is based upon the well-known Rapini-Papoular potential and the other upon the B potential, introduced in Belyakov, Stewart, and Osipov, JETP 99, 73 (2004). Calculations for the unwinding (winding) of the helix in the process of the jump were performed to investigate the case of infinitely strong anchoring on one surface and finite anchoring on the other, which is important in applications. The results show that an experimental investigation of the dynamics of the pitch jumps will allow one to distinguish different shapes of the finite strength anchoring potential, and will, in particular, provide a means for determining whether or not the well-known Rapini-Papoular anchoring potential is the best suited potential relevant to the dynamics of pitch jumps in cholesteric layers with finite surface-anchoring strength.

Journal Article↗

PubMed2004

Traveling waves in ferroelectric smectic-C liquid crystals.

This article investigates, by means of Lie point symmetries, traveling wave solutions to a dynamic equation that frequently arises in the theory of ferroelectric smectic- C liquid crystals under the influence of an electric field. The equation considered has three sinusoidal nonlinearities and possible time-dependent solutions are discussed in the context of minima and maxima of the electric energy density for these liquid crystals: solutions travel between such constant equilibrium states. Implicit solutions to an approximation of the governing dynamic equation are determined. Nondimensional control parameters that characterize changes in the availability of equilibrium states as the magnitude of the field increases are also identified.

Journal Article↗

PubMed2003

Self-organized periodic photonic structure in a nonchiral liquid crystal.

A hybrid-aligned cell of the smectic A liquid material 8CB gives two stable director configurations, one of which is periodic and gives strong diffraction of light. This photonic lattice director profile arises from "frustration" caused by conflicting constraints imposed by the boundary conditions and the constant amplitude smectic density wave. A model is proposed which accords well with the experiment results, predicting correctly the dependence of the periodicity on the cell thickness and reproducing optical polarization microscopy results.

Journal Article↗

PubMed2001

Proof of factorization for B --> D pi.

We prove that the matrix elements of four fermion operators mediating the decays B macro (0)-->D(+)pi(-) and B--->D(0)pi(-) factor into the product of a form factor describing the B-->D transition and a convolution of a short distance coefficient with the nonperturbative pion light-cone wave function. This is shown to all orders in alpha(s), with corrections suppressed by factors of 1/m(b), 1/m(c), and 1/E(pi). It is not necessary to assume that the pion state is dominated by the qq macro Fock state.

Journal Article↗

PubMed2001

Renormalization-group improved calculation of top-quark production near threshold.

The top-quark cross section close to threshold in e(+)e(-) annihilation is computed including the summation of logarithms of the velocity at next-to-next-to-leading-logarithmic order in QCD. The remaining theoretical uncertainty in the normalization of the total cross section is at the few-percent level, an order of magnitude smaller than in previous next-to-next-to-leading order calculations. This uncertainty is smaller than the effects of a light standard-model Higgs boson.

Journal Article↗

PubMed2000

Iowa's pediatric low-vision services.

BACKGROUND: Reports in the literature concerning best practice for the evaluation and management of children with visual impairments are limited, with a resulting lack of information concerning the potential for optimizing vision to enhance general development and assist with the educational needs of this population. METHOD: The development of a multidisciplinary approach to provide low-vision services for children with visual impairment has occurred over the past 18 years in Iowa. In that time, 1,348 children from around the state of Iowa have been evaluated through an itinerant low-vision service program, coordinated by the Iowa Braille School. RESULTS: A low-vision clinic model--designed to provide services (primarily) for academic students--was not meeting the needs of the pediatric low-vision population in the state. After a statewide review of the program, changes were made that have resulted in low-vision services being provided to a greater and more diverse number of students. The roles of the various members of the multidisciplinary team will be reviewed. Changes in large-print orders and use by special education teachers in the state as a direct result of the low-vision services will also be discussed. CONCLUSION: Ongoing, comprehensive multidisciplinary low-vision services--including optometric low-vision care as a key component--are necessary to help children with visual impairments meet their educational, vocational, and avocational needs. With ongoing low-vision services, unnecessary costs such as those associated with large-print materials can be reduced, thereby creating significant savings to local, state, and federal special educational services.

Child↗

PubMed1999

Layer undulations induced by a magnetic or electric field in concentric cylindrical layers of smectic-A liquid crystals.

This paper derives theoretical results for the onset of Helfrich-Hurault-type transitions in samples of smectic-A liquid crystals arranged in concentric cylindrical layers [J. Chem. Phys. 55, 839 (1971); 59, 2068 (1973)]. A magnetic field is applied in the azimuthal direction, parallel to the layers. The governing equilibrium equation is solved in order to derive the critical magnitude H(c) of the magnetic field above which the onset of periodic smectic layer distortions is expected. The distortion solution is shown to be energetically favorable for H>H(c). Some examples of critical thresholds are given, by means of figures, for relevant physical parameters. The qualitative features of the solution at H(c) are displayed via the plot of a typical variable solution to the equilibrium equation in a particular case. The consequences of the results, especially the relationship between H(c) and the radial depth, are explored, and comparisons are drawn with known results for infinite planar aligned smectic-A samples. The results are also interpreted for the case of an electric field which follows the plane of the layers in the usual wedge geometry. Critical applied voltage magnitudes U(c) are derived for various radial sample depths and wedge angles.

Journal Article↗

PubMed1999

Dynamics of solitary waves in smectic-C* liquid crystals.

The dynamics of solitary waves in ferroelectric liquid crystals is considered in the context of an overdamped double sine-Gordon model. Various possible types of single waves are studied. The dependence of wave velocity and scale on the system parameters and initial conditions is found. Sharp initial profiles lead to waves with a universal form. The parameters of the wave arising from smooth profiles are determined also by the scale of the initial conditions. Numerical calculations show good agreement with analytical results. The possibility of using such waves to improve the characteristics of liquid crystal devices is also discussed.

Journal Article↗

PubMed1998

The adaptation of existing personal inhalable aerosol samplers for bioaerosol sampling.

Health-related monitoring of bioaerosol exposures in the workplace should ideally be carried out using size-selective personal samplers that separate the aerosol into biologically relevant size fractions and allow both quantification and identification of the microorganisms present in each fraction. As a first stage in the development of personal bioaerosol samplers a number of collection substrates were assessed for their ability to maintain the viability of the collected microorganisms, so that subsequent culturing and species identification may be carried out. The substrates were tested with bioaerosols of varying robustness, consisting of Saccharomyces cerevisiae cells, Escherichia coli cells, and Penicillium expansum spores, aerosolized under controlled environmental conditions. The survival of microorganisms on each test substrate, assessed on the basis of the culturable fractions of cells recovered, was compared with that of microorganisms collected in a reference glass cyclone sampler. These bioefficiency tests identified the substrate combinations with the potential to fulfill personal sampler design criteria. The substrates were then subjected to further development to evaluate and optimize their particle size selection characteristics. The outcome of this work is two prototype personal bioaerosol samplers in which size-selective substrates are adapted for use in existing designs of personal inhalable sampler. This offers an effective and low-cost solution to personal monitoring of bioaerosol exposures in the workplace.

Aerosols↗

PubMed1996

Iowa's pediatric low vision services.

BACKGROUND: Reports in the literature concerning the evaluation and low vision management of visually impaired children are limited, resulting in a lack of information about the characteristics and needs of this population. METHODS: A review was performed of 762 children evaluated over a 14-year period in a low vision clinic program coordinated by the Iowa Braille and Sight Saving School. RESULTS: The age, sex, ocular condition, best corrected visual acuity, habitual near point working distance, and optical devices recommended for use by these children were analyzed. CONCLUSIONS: Ongoing, comprehensive multidisciplinary low vision services are necessary to help children with visual impairments meet their educational, vocational and avocational needs. With ongoing low vision care unnecessary costs-such as those for large print material-can be avoided, therefore creating a significant savings to local or state educational services.

Adolescent↗