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R A Quinn

Publications and source records attributed to R A Quinn.

8 recordsLinked to original sources
PubMed2004

Experimental determination of dust-particle charge in a discharge plasma at elevated pressures.

The charge of dust particles is determined experimentally in a bulk dc discharge plasma in the pressure range 20-100 Pa. The charge is obtained by two independent methods: one based on an analysis of the particle motion in a stable particle flow and another on an analysis of the transition of the flow to an unstable regime. Molecular-dynamics simulations of the particle charging for conditions similar to those of the experiment are also performed. The results of both experimental methods and the simulations demonstrate good agreement. The charge obtained is several times smaller than predicted by the collisionless orbital motion theory, and thus the results serve as an experimental indication that ion-neutral collisions significantly affect particle charging.

Journal Article↗
PubMed2004

Shock melting of a two-dimensional complex (dusty) plasma.

Shock waves with a linear front were experimentally studied in a monolayer hexagonal Yukawa lattice which was formed from charged monodisperse plastic microspheres and levitated in the sheath of a radio-frequency discharge. It was found that the shock can cause phase transitions from a crystalline to gaslike and liquidlike states. Melting occurred in two stages. First, the lattice was compressed in the direction of shock propagation and second, the particle velocities were randomized a few lattice lines downstream. The Mach number of the shock reached 2.7.

Journal Article↗
PubMed2002

Particle interaction measurements in a Coulomb crystal using caged-particle motion.

A technique for characterizing the particle interaction potential of a Coulomb crystal is developed. The mean-square displacement (MSD) is measured, showing both caged- and superdiffusive-particle motions. By subtracting the center of mass of neighboring particles in computing MSD, only short-wavelength particle motions are retained. This yields the lattice Einstein frequency, which contains information about the interparticle forces and potentials. Video measurements of particle motions in a complex (dusty) plasma are used to demonstrate the technique.

Journal Article↗
PubMed2002

Dissipative longitudinal solitons in a two-dimensional strongly coupled complex (dusty) plasma.

Solitary waves are experimentally studied in a monolayer hexagonal dust lattice which is formed from monodisperse plastic microspheres and levitated in the sheath of an rf discharge. It is found that the product of the soliton amplitude and the square of the soliton width is constant as the soliton propagates. The analytical theory describing the experiment is based on the equations of motion written for a linear chain. It takes into account damping, dispersion, and nonlinearity. The numerical simulation of a linear chain produces double solitons like those observed in the experiment.

Journal Article↗
PubMed2001

Experimental test of two-dimensional melting through disclination unbinding.

A two-dimensional (2D) melting transition has been studied in a nonequilibrium experimental model system. The system used was a complex or dusty plasma consisting of microspheres suspended in a glow-discharge plasma, where we have mapped the topological defects during the transition. The role of the defects in the melting transition is evaluated and the arrangement of the defects in the lattice is quantified in a new way. It is found that defect density increases dramatically during the melting; at all stages the defects tend to be clustered together rather than widely dispersed; the clustering tends to take the form of chain or string-like structures. We compare these results for the defect structure with the assumptions of the popular 2D melting theory of Halperin and Nelson, rather than the predictions, as is more common.

Journal Article↗
PubMed1994

The sleep patterns of normal children.

OBJECTIVES: To determine the range of sleep behaviour of normal children to age 38 months and to ascertain the level of parents' problems associated with their child's sleep behaviour. DESIGN AND SETTING: A cross-sectional survey by questionnaire of parents presenting with their children for routine well-child checks at child health centres, mobile clinics, flying doctor clinics and home visits throughout Queensland. Of 3383 questionnaires distributed 3269 (96.5%) were returned. MAIN OUTCOME MEASURES: 1. Sleep frequency and duration, settling procedures, time taken to settle at night, age when child first slept through the night and number of night-time wakenings requiring parental intervention. 2. Parents' problems with their child's sleep behaviour. RESULTS: There is a wide range of normal childhood sleep behaviour. Circadian rhythm is not well established until four months of age. Daytime sleep becomes less regular with increasing age. Frequent night-time wakening is common from four to 12 months. Night-time settling requires more parental input from 18 months. A large proportion of parents (28.6%) have a problem with their child's sleep behaviour. CONCLUSIONS: Parents require information from health care providers about the wide range of normal childhood sleep patterns. This information can help prevent misdiagnosis, inappropriate medication use, child abuse and parental depression when children's sleep patterns are a problem.

Child Behavior↗
PubMed1992

"Just a mother".

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Attitude↗