Biomedical subjects

G Morfill

Publications and source records attributed to G Morfill.

At least 19 recordsLinked to original sources

PubMed2006

Self-diffusion in a liquid complex plasma.

Self-diffusion has been experimentally studied in a two-dimensional underdamped liquid complex (dusty) plasma. It was found that the self-diffusion coefficient D increases linearly with the temperature T: D/omega(E)a2 = (0.019 +/- 0.007)(T/T(m) - 1), where T(m), omega(E), and a are the melting temperature, the Einstein frequency, and the mean particle separation, respectively. No superdiffusion was observed, whereas a subdiffusion occurred at temperatures close to melting.

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PubMed2005

Heat transfer in a two-dimensional crystalline complex (dusty) plasma.

Heating and heat transfer were studied in a two-dimensional crystalline complex plasma at the kinetic level. The lattice was formed of microspheres levitated in a plasma sheath. One half of the crystal was heated anisotropically to obtain higher kinetic temperatures in one direction and heat conduction was observed in real time. It was found that the longitudinal phonons conduct heat better than the transverse. The thermometric conductivity coefficient was measured to be 53 mm2/s for longitudinal heating and 30 mm2/s for transverse heating. Heat decay lengths and energy exchange times between the temperature components were determined.

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PubMed2005

Vertical wave packets observed in a crystallized hexagonal monolayer complex plasma.

Propagation of vertical wave packets was observed experimentally in a crystallized hexagonal monolayer complex plasma. It was found that the phase velocity exceeded the group velocity by a factor 65 and was directed into the opposite direction as expected for an inverse optical-like dispersion relation. The wave packets propagated keeping their width constant. The explanation of this behavior is based on three-dimensional equations of motion and uses a long-wavelength weak dispersion weak inhomogeneity approximation. While the wave dispersion causes the wave packet to spread, lattice inhomogeneity and neutral gas drag counteract spreading. A plasma diagnostic method was developed that is based on the ratio between vertical and dust-lattice wave speeds. This ratio is very sensitive to the lattice parameter kappa (ratio of the particle separation to the screening length) in a very useful range of kappa < or = 2 . It was found that only a two-dimensional lattice model can provide a quantitative description of the vertical waves, while a linear chain model gives only a qualitative agreement.

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PubMed2005

Wave spectra in solid and liquid complex (dusty) plasmas.

Spectra of longitudinal and transverse waves were obtained experimentally in liquid and solid two-dimensional complex (dusty) plasmas at different kinetic temperatures. As the temperature increased and the phase state of the plasma changed from solid to liquid, the phonon spectra of both longitudinal and transverse modes broadened (especially at high wave numbers), indicating increased damping. The transverse mode disappeared and a thermal (compressional) mode appeared.

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PubMed2004

Wave dispersion relation of two-dimensional plasma crystals in a magnetic field.

The wave dispersion relation in a two-dimensional strongly coupled plasma crystal is studied by theoretical analysis and molecular dynamics simulation taking into account a constant magnetic field parallel to the crystal normal. The expression for the wave dispersion relation clearly shows that high-frequency and low-frequency branches exist as a result of the coupling of longitudinal and transverse modes due to the Lorenz force acting on the dust particles. The high-frequency and the low-frequency branches are found to belong to right-hand and left-hand polarized waves, respectively.

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PubMed2003

Polarization of wave modes in a two-dimensional hexagonal lattice using a complex (dusty) plasma.

Wave spectra corresponding to the random particle motion in a monolayer Yukawa crystal were studied for various directions of propagation. It was found that there are two wave modes with a polarization alternating between the longitudinal and transverse. In the long-wavelength regime, the modes became purely longitudinal and transverse as was known before. In the short-wavelength regime the spectra strongly depended on the wavelength and the direction of propagation. The results obtained from the experiment, theory, and simulation agreed well with each other.

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PubMed2003

Melting of monolayer plasma crystals.

Melting of a monolayer plasma crystal in a radio-frequency discharge with no particles suspended above or below is studied. The experimental data are compared with results of molecular dynamics simulations and theory. It is shown that the melting is caused by the resonance coupling between the longitudinal and the transverse dust-lattice wave modes, due to the interaction of particles with the plasma wakes.

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PubMed2003

Rodlike particles in gas discharge plasmas: theoretical model.

Recently, complex plasmas with strongly asymmetric (rodlike) particles were investigated experimentally in rf and dc discharges [V. I. Molotkov et al., JETP Lett. 71, 102 (2000); B. M. Annaratone et al., Phys. Rev. E 63, 036406 (2001)]. In this paper, a theoretical model is proposed which describes the behavior of such systems. Major results of the proposed model are the following: Equilibrium charge is calculated for particles orientated perpendicular and parallel to the ion flux (electric field); equilibrium states of particles (orientation angle and levitation height) are obtained; energy of electrostatic interaction between rods is derived, depending on the mutual orientation. Comparison of experimental and theoretical results shows quite good agreement. In conclusion, some important theoretical issues as well as possible new experiments are discussed.

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PubMed2003

Weakly dissipative dust-ion-acoustic solitons.

We investigate the possibility for dust ion-acoustic solitons to exist. Compressive solitonlike perturbations are damped and slowed down, mainly due to the plasma absorption and ion scattering on microparticles. The perturbations are shown to possess the main properties of solitons. There is a principal possibility to study experimentally the role of trapped electrons in the soliton formation.

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PubMed2003

Kinetic measurements of shock wave propagation in a three-dimensional complex (dusty) plasma.

"Complex plasmas" consist of electrons, ions, and charged microparticles. The latter are individually observable, allowing kinetic measurements in plasmas. Using a sudden gas pulse, a traveling perturbation was initiated in such a complex plasma and its propagation, acceleration, and steepening-possibly into a shock was followed. The experiment was performed in the PKE-Nefedov laboratory under microgravity conditions on the international space station, i.e., in a complex plasma cloud with very little stored (potential or free) energy and thus free of, e.g., parametric instabilities. The perturbation front remained remarkably smooth, with a microroughness of the order of the interparticle distance. The observations are presented and interpreted.

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PubMed2003

Local 3D scaling properties for the analysis of trabecular bone extracted from high-resolution magnetic resonance imaging of human trabecular bone: comparison with bone mineral density in the prediction of biomechanical strength in vitro.

RATIONALE AND OBJECTIVES: A novel, nonlinear morphologic measure [DeltaP(alpha)] based on local 3D scaling properties was applied to high-resolution magnetic resonance images (HR-MRI) of human trabecular bone to predict biomechanical strength in vitro. METHODS: We extracted DeltaP(alpha) and traditional morphologic parameters (apparent trabecular volume fraction, apparent trabecular separation) from HR-MR images of 32 femoral and 13 spinal bone specimens. Furthermore, bone mineral density (BMD) and maximum compressive strength (MCS) were determined. The morphologic measures were compared with BMD in predicting the biomechanical strength. RESULTS: In the vertebral (femoral) specimens, R2 for MCS versus DeltaP(alpha) was 0.87 (0.61) (P < 0.001). Correlation between BMD and MCS was 0.53 (P = 0.05) (0.79 [P < 0.001]) for the vertebral (femoral) specimens. For the femoral specimens, prediction of MCS could be improved further by combining BMD and morphologic parameters by multiple regression (R2 = 0.88). CONCLUSIONS: Morphologic measures extracted from HR-MRI considering local 3D-scaling properties can be used to predict biomechanical properties of bone in vitro. They are superior to 2-dimensional standard linear morphometric measures and, depending on the anatomic location, more reliably predict bone strength as measured by MCS than does BMD.

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PubMed2002

Universal scaling in complex (dusty) plasmas.

The structural and dynamical properties of three-dimensional isotropic complex plasmas are investigated kinetically within the framework of a dissipative Yukawa model. A modified Coulomb coupling parameter is proposed whose value alone determines the location of the complex plasma melting line. This implies that the phase transition has a universal scaling at the kinetic level. In detail, our molecular dynamics investigations show that the system dynamics is universal (but different) in the limits of high as well as low-frictional dissipation, while in the intermediate case it depends considerably on the dissipation rate. Issues such as the influence of the interaction strength on the single particle diffusion constant and the applicability of dynamical criteria for freezing are discussed.

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PubMed2002

Dynamic Mach cone as a diagnostic method in reactive dusty plasma experiments.

We demonstrate how the observation of dynamic Mach cones in dusty plasma experiments allows one to follow the evolution of the dust acoustic wave velocity as charges on dust particles change, or as their mass changes. In experiments in which only the charge is changing due to, e.g., the application of a thin coating on UV-illuminated dust particles, the coating rate may be inferred through the analysis of the Mach cone pattern. In other experiments where the dust sizes are changing, also leading to some change in charge, the growth rate can be followed.

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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.

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PubMed2001

Interaction potential of microparticles in a plasma: role of collisions with plasma particles.

The interaction potential of two charged microparticles in a plasma is studied. Violation of the plasma equilibrium around the dust particles due to plasma-particle inelastic collisions results in three effects: long-range (non-Yukawa) electrostatic repulsion, attraction due to ion shadowing, and attraction or repulsion due to neutral shadowing (depending on the sign of the temperature difference between the particle surface and neutral gas). An analytical expression for the total potential is obtained and compared with previous theoretical results. The relative contribution of these effects is studied in two limiting cases-an isotropic bulk plasma and the plasma sheath region. The results obtained are compared with existing experimental results on pair particle interaction. The possibility of the so-called dust molecule formation is discussed.

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PubMed2001

Vertical pairing of identical particles suspended in the plasma sheath.

It is shown experimentally that vertical pairing of two identical microspheres suspended in the sheath of a radio-frequency (rf) discharge at low gas pressures (a few Pa) appears at a well-defined instability threshold of the rf power. The transition is reversible, but with significant hysteresis on the second stage. A simple model which uses measured microsphere resonance frequencies and takes into account, in addition to the Coulomb interaction between negatively charged microspheres, their interaction with positive-ion-wake charges, seems to explain the instability threshold quite well.

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PubMed2001

Dust acoustic solitons with variable particle charge: role of the ion distribution.

Dust-acoustic solitons of large amplitude with variable particle charge are studied using the Sagdeev quasipotential analysis. Two limiting cases of ion distribution are considered separately: Boltzmann and highly energetic cold ions. It is shown that in both cases only compressive (density) solitons are possible. The charge variation is not important in rarefied particle clouds, but becomes crucial if the particle number density is sufficiently high. Analytical expressions for the range of Mach numbers where solitons might exist are obtained. It is found that solitons are allowed in the supersonic regime, and that in dense clouds the width of the Mach number range remains finite for the Boltzmann ions, but tends to zero for highly energetic ions.

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PubMed2000

Anisotropic dust lattice modes.

Dust lattice (DL) wave modes in a one-dimensional plasma crystal suspended in the plasma sheath are studied. The ion flow in the sheath introduces an anisotropy, in particular "ion wakes" below the crystal particles. This leads to two types of transverse wave mode. It is shown that the "horizontal transverse mode" remains independent, but the "vertical transverse mode" and the longitudinal mode are coupled due to the particle-wake interaction. The coupling can drive an instability of the modes close to the point where their branches intersect. In addition, the particle-wake interaction might decrease the frequencies of the DL modes considerably.

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