Spherical Dust Grains Research Articles

Direct observation of dynamics of single spinning dust grains in weakly magnetized complex plasma

The rotational dynamics of single dust grains in a weak magnetic field is investigated on a kinetic level. Experiments reveal spin-up of spherical dust grains and alignment of their magnetic moments parallel to the magnetic induction vector. The angular velocity of spinning prolate grains varies as magnetic induction increases to 250 G. Spinning dust grains are found to flip over only when the magnetic field magnitude is changing. The results demonstrate that dusty plasma has paramagnetic properties. Qualitative interpretations are proposed to explain newly discovered phenomena.

On the thermal force acting on dust grain in fully ionized plasma

The expressions for ion and electron thermal forces and electron friction force acting on a spherical dust grain from plasma embedded into a magnetic field are found. The electron and ion distribution functions were considered by using 21 moments Grad approximation. Dust grain with a radius much smaller than ion and electron gyroradii was assumed to be either negatively or relatively weakly positively charged. The forces found in this paper can be important, particularly for dust dynamics in fusion plasmas with stagnated plasma flow.

The cold origin of the warm dust aroundε Eridani

Context: The K2V star eps Eri hosts one known inner planet, an outer Kuiper belt analog, and an inner disk of warm dust. Spitzer/IRS measurements indicate that the warm dust is present at distances as close as a few AU from the star. Its origin is puzzling, since an "asteroid belt" that could produce this dust would be unstable because of the known inner planet. Aims: Here we test the hypothesis that the observed warm dust is generated by collisions in the outer belt and is transported inward by Poynting-Robertson (P-R) drag and strong stellar winds. Methods: We simulated a steady-state distribution of dust particles outside 10AU with a collisional code and in the inner region (r<10AU) with single-particle numerical integrations. By assuming homogeneous spherical dust grains composed of water ice and silicate, we calculated the thermal emission of the dust and compared it with observations. We investigated two different orbital configurations for the inner planet inferred from RV measurements, one with a highly eccentric orbit of e=0.7 and another one with a moderate one of e=0.25. We also produced a simulation without a planet. Results: Our models can reproduce the shape and magnitude of the observed SED from mid-IR to sub-mm wavelengths, as well as the Spitzer/MIPS radial brightness profiles. The best-fit dust composition includes both ice and silicates. The results are similar for the two possible planetary orbits and without a planet. Conclusions: The observed warm dust in the system can indeed stem from the outer belt and be transported inward by P-R and stellar wind drag. The inner planet has little effect on the distribution of dust, so that the planetary orbit could not be constrained. Reasonable agreement between the model and observations can only be achieved by relaxing the assumption of purely silicate dust and assuming a mixture of silicate and ice in comparable amounts.

Size Effects on the Scattering of Electron and Spherical Dust Grain in Dusty Plasmas

The finite size effects of the charged dust grain on the electron-dust grain collisions are investigated in complex dusty plasmas. The stationary phase analysis and the effective potential due to the renormalized dust charge are employed to obtain the phase shift for the scattering of the electron and the spherically charged dust grain as a function of the impact parameter, collision energy, Debye length, and dust radius. It is found that the size effect of the dust grain enhances the electron-dust grain scattering cross section in dusty plasmas. It is also found that the size effect on the scattering cross section increases with increasing plasma density. In addition, it is found that the size effect on the electron-dust scattering cross section decreases with an increase of the plasma temperature.

Computation of charge and ion drag force on multiple static spherical dust grains immersed in rf discharges

Charging of multiple spherical dust grains located in presheath and sheath regions of an rf discharge has been studied using a three-dimensional particle-particle-particle-mesh (P3M) code. First, dust charge, potential, and ion drag force on two dust particles for various interparticle separations are computed. It is found that for dust separations larger than the shielding length the dust parameters for the two dust particles match with the single particle values. As the dust separation is equal to or less than the shielding length, the transverse component of ion force increases which is due to dynamic shielding effect caused by neighboring dust particle. However, dust charge, potential, and ion drag are found not to be affected considerably. Further, dust charge and potential on an arrangement of nine dust particles are computed. The dust charge and potential do not differ much from the single particle values for the presheath. However the dust charges of multiple dust particles in the sheath are much less negative compared to the single dust case which is shown to be due to ion focusing.

10.1007/s11447-008-1014-3

The continuum approximation is used to analyze the effect of electron emission from the surface of a spherical dust grain immersed in a plasma on the grain charge by assuming negligible ionization and recombination in the disturbed plasma region around the grain. A parameter is introduced that quantifies the emission intensity regardless of the emission mechanism (secondary, photoelectric, or thermionic emission). An analytical expression for the grain charge Z d is derived, and a criterion for change in the charge sign is obtained. The case of thermionic emission is examined in some detail. It is shown that the long-distance asymptotic behavior of the grain potential follows the Coulomb law with a negative effective charge Z eff, regardless of the sign of Z d. Thus, the potential changes sign and has a minimum if Z d > 0, which implies that attraction is possible between positively charged dust grains.

Dust settling in magnetorotationally driven turbulent discs - I. Numerical methods and evidence for a vigorous streaming instability

In this paper, we have used the riemann code for computational astrophysics to study the interaction of a realistic distribution of dust grains with gas at specific radial locations in a vertically stratified protostellar accretion disc. The disc was modelled to have the density and temperature of a minimum mass solar nebula, and shearing box simulations at radii of 0.3 and 10 au are reported here. The disc was driven to a fully developed turbulence via the magnetorotational instability (MRI). The simulations span three gas scaleheights about the disc's midplane. We find that the inclusion of standard dust-to-gas ratios does not have any significant effect on the MRI even when the dust sediments to the midplane of the accretion disc. The density distribution of the dust of all sizes reached a Gaussian profile within two scaleheights of the disc's midplane. The vertical scaleheights of these Gaussian profiles are shown to be proportional to the reciprocal of the square root of the dust radius when large spherical dust grains are considered. This result is consistent with theoretical expectation. The largest two families of dust in one of our simulations show a strong tendency to settle to the midplane of the accretion disc. The large dust tends to organize itself into elongated clumps of high density. The dynamics of these clumps is shown to be consistent with a streaming instability. The streaming instability is seen to be very vigorous and persistent once it forms. Each stream of high-density dust displays a reduced rms velocity dispersion. The velocity directions within the streams are also aligned relative to the mean shear, providing further evidence that we are witnessing a streaming instability. The densest clumpings of large dust are shown to form where the streams intersect. We have also shown that the mean free path and collision time for dust that participates in the streaming instability are reduced by almost two orders of magnitude relative to the average mean free paths and collision times. The rms velocities between the grains also need to fall below a minimum threshold in order for the grains to stick and we show that a small amount of the large dust in our 10 au simulation should have a propensity for grain coalescence. The results of our simulations are likely to be useful for those who model spectral energy distributions of protostellar discs and also for those who model dust coagulation and growth.

Computation of ion drag force on a static spherical dust grain immersed in rf discharges

The ion drag force on static spherical dust grains located in an argon rf discharge under typical laboratory experiment conditions has been computed using a three-dimensional particle-particle-particle-mesh code. Elastic and inelastic collisions have been included in the current model to obtain realistic rf discharge plasma conditions. The ion drag has been computed for various sizes of dust placed at different locations in the rf discharge under different gas pressures. The orbital drag force is typically found larger than the collection drag force. Ion-neutral collisions increase flux to the dust and hence the total drag force for collisional case is found larger than the collisionless case. Within the pressure range investigated, the drag forces do not vary much with pressure. The size dependence of the drag force is nonlinear and agrees well with the forces computed from the analytical models.

Ion drag force on dust grains in the magnetized edge plasma

A 3-dimensional Particle–Particle Particle–Mesh (P3M) code [K. Matyash, R. Schneider, F. Taccogna, D. Tskhakaya, J. Nucl. Mater. 363–365 (2007) 458] is applied to simulate a small-size (smaller than a Debye length) spherical dust grain confined in the magnetized plasma near the material wall of a tokamak. Plasma particles (electrons and ions) are treated kinetically (Particle-in-Cell with Monte Carlo Collisions (PIC MCC)), which allows to resolve self-consistently the electrostatic sheath in front of the wall. In order to describe accurately the plasma particles’ motion close to the dust grain, the PIC technique is supplemented with Molecular Dynamics (MD), employing an analytic electrostatic potential for the interaction with the dust grain. The charging of a spherical, conducting dust grain confined in the sheath potential close to the wall of a tokamak is simulated. A magnetic field normal to the wall was investigated. The ion drag force resulting from dust grain collisions with the streaming ions is calculated. This force is critical for a realistic description of the dust particle dynamics and transport in fusion plasmas.

Obliquely propagating Alfvén waves in a Maxwellian dusty plasma

A kinetic formulation developed to analyze wave propagation in dusty plasmas, which takes into account the charge variation of the dust particles, is utilized to study the propagation and damping of Alfvén waves propagating in oblique directions relative to the ambient magnetic field. A dusty plasma containing spherical and immobile dust grains in a homogeneous ambient magnetic field is considered. The charging process of the dust grains is assumed to be associated with the capture of electrons and ions by the dust particles during inelastic collisions between them and plasma particles. The dispersion relation and the damping rates of obliquely propagating Alfvén waves are obtained assuming Maxwellian distributions for electrons and ions in equilibrium. For the numerical analysis of the dispersion relation we use the average values of the inelastic collision frequency as an approximation, instead of the momentum dependent expressions originally derived in the kinetic formulation, and study the modifications which the presence of the dust particles causes in both the propagation and the damping of the Alfvén waves. In particular is discussed the competition between the different damping mechanisms, namely, the Landau damping and the damping associated with the dust charge variation, and it is shown that the inelastic collision frequency plays a pivotal role in the magnitude of the damping rates.

Computation of dust charge and potential on a static spherical dust grain immersed in rf discharges

Dust charge and potential on static spherical dust grains located in an argon rf discharge under typical laboratory experiment conditions have been computed using a three-dimensional particle-particle-particle-mesh code. Elastic and inelastic collisions have been included in the current model to obtain realistic rf discharge plasma conditions. Dust charge, potential, and potential distribution around the dust have been computed for various sizes of dust placed at different locations in the rf discharge. The dust charge is found to be smaller than the values from the simple orbit motion limited model due to ion-neutral collisions. Further, the dust potential has been found to be increasing with dust size. Moreover, the shielding length of the dust has been found between electron and ion Debye lengths.

The non-radial component of the solar wind and motion of dust near mean motion resonances with planets

We investigate the effect of solar wind and solar electromagnetic radiation on the dynamics of spherical cosmic dust particles. We also consider the non-radial component of the solar wind velocity, in the reference frame of the Sun. We apply the equation of motion to the motion of dust grains near commensurability resonances with a planet – mean motion orbital resonance (MMR; a particle is in resonance with a planet when the ratio of their mean motions is approximately the ratio of two small integers) – and possible capture of the grains in the resonances. Up to now, only nonspherical grains, under action of the electromagnetic radiation of the central star, were known to exhibit an increase of semimajor axis before capture into the MMR. This paper shows that the same result can be generated by the non-radial component of the solar wind even for spherical dust particles. Spherical dust grains enable the treatment of the problem in an analytic way (at least partially), which is not the case for the effect of electromagnetic radiation on nonspherical dust grains. The situation treated in the paper presents the second known case when resonant trapping of a cosmic body occurs for diverging orbits. The paper presents the first case of secular evolution of the eccentricity of a body captured in the resonance derived in an analytic way for a body characterized by a diverging orbit.

The Physics of Protoplanetesimal Dust Agglomerates. II. Low‐Velocity Collision Properties

For the investigation of collisions among protoplanetesimal dust aggregates, we performed microgravity experiments in which the impacts of high-porosity millimeter-sized dust aggregates into 2.5 cm high-porosity dust aggregates can be studied. The dust aggregates consisted either of monodisperse spherical, quasi-monodisperse irregular, or polydisperse irregular micrometer-sized dust grains and were produced by random ballistic deposition with porosities between 85% and 93%. Impact velocities ranged from ~0.1 to ~3 m s−1, and impact angles were almost randomly distributed. In addition to the smooth surfaces of the target aggregates formed in our experiments, we "molded" target aggregates such that the radii of the local surface curvatures corresponded to the projectile radii, decreasing the targets' porosities to 80%-85%. The experiments showed that impacts into the highest porosity targets almost always led to sticking, whereas for the less porous dust aggregates, consisting of monodisperse spherical dust grains, the collisions with intermediate velocities and high impact angles resulted in the bouncing of the projectile with a mass transfer from the target to the projectile aggregate. Sticking probabilities for the impacts into the "molded" target aggregates were considerably decreased. For the impacts into smooth targets, we measured the depth of intrusion and the crater volume, and were able to derive some interesting dynamical properties which can help to derive a collision model for protoplanetesimal dust aggregates. Future models of the aggregate growth in protoplanetary disks should take into account noncentral impacts, impact compression, the influence of the local radius of curvature on the collisional outcome, and the possible mass transfer between the target and projectile agglomerates in nonsticking collisions.

Stratified dust grains in the interstellar medium - I. An accurate computational method for calculating their optical properties

We present an accurate method of computing the optical properties of layered spherical grains in which the grain core and the layers may each have different refractive indices. We show how the results of this method differ from effective medium approximations commonly in use. We use the method to compute the optical properties of spherical dust grains composed of silicate cores coated with two carbonaceous layers, the inner layer of sp 2 carbon and the outer layer of sp 3 carbon, and explore the consequences of varying the core-grain size distribution and the thicknesses of the carbonaceous layers. Guided by these results, we compute a family of interstellar extinction curves in which the underlying extinction is caused by a size distribution of silicate cores coated with two carbonaceous layers; the well-known extinction bump at 217.5 nm is attributed to a large population of polycyclic aromatic hydrocarbon (PAH) molecules for which a data base of computed optical properties exists. The family of curves is created by varying the extents of the sp 2 and the sp 3 layers. The so-called ‘standard’ observed interstellar extinction curve is well matched by one of this family of computed curves. The total amount of carbon required in this model is within the carbon budget for the interstellar medium. We also show that thin carbon mantles tend to suppress 10 and 20 µm emission from very small silicate cores that are heated by the absorption of a single photon.

Effect of electron emission on the charge and shielding of a dust grain in a plasma: A continuum theory

The continuum approximation is used to analyze the effect of electron emission from the surface of a spherical dust grain immersed in a plasma on the grain charge by assuming negligible ionization and recombination in the disturbed plasma region around the grain. A parameter is introduced that quantifies the emission intensity regardless of the emission mechanism (secondary, photoelectric, or thermionic emission). An analytical expression for the grain charge Z{sub d} is derived, and a criterion for change in the charge sign is obtained. The case of thermionic emission is examined in some detail. It is shown that the long-distance asymptotic behavior of the grain potential follows the Coulomb law with a negative effective charge Z{sub eff}, regardless of the sign of Z{sub d}. Thus, the potential changes sign and has a minimum if Z{sub d} > 0, which implies that attraction is possible between positively charged dust grains.

Secondary Emission From Glass Grains: Comparison of the Model and Experiment

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The surface potential of dust grains immersed in surrounding plasma results from the balance of many charging processes such as photoemission, electron/ion attachments, and secondary and field emissions. Since hot electrons are often present in space as well as laboratory plasmas, the understanding of the secondary electron (SE) emission process for small dust grains is of great interest because their size effects modify well-known characteristics of large samples. This paper compares the measured surface potential of <formula formulatype="inline"><tex>$ \hbox{SiO}_{2}$</tex></formula> spherical dust grains with the results of the Monte Carlo model of secondary emission developed for metallic samples. It was found that 1) the model can be used for description of the secondary emission process from cosmic dust, 2) the backscattering of primary beam electrons is the most important factor for the charging of small grains, and 3) the actual value of the surface grain potential is given by the energy spectrum of SEs. </para>

Model of secondary emission and its application on the charging of gold dust grains

Dust grains of various shapes and materials can be found anywhere in the space where the temperature decreases under a melting point. Many plasma processes are driven by the grain charge but, on the other hand, dust grain charging can be influenced by several different processes, too. Among others, secondary emission can play a dominant role wherever the plasma is hot. In the present paper, a further development of our previous simple numerical secondary emission model, now using techniques for a beam interaction modelling applied in electron microscopies, is sketched and its application onto charging of spherical dust grains by a narrow electron beam is discussed. The model fits experimental data rather well and, thus, our conclusion suggests that the model is a very useful tool for considerations on the dust charging under different conditions in the space.

Simulation study of spheroidal dust gains charging: Applicable to dust grain alignment

The charging process of nonspherical dust grains in an unmagnetized plasma as well as in the presence of a magnetic field is studied. It is shown that unlike the spherical dust grain, due to nonhomogeneity of charge distribution on the spheroidal dust surface, the resultant electric forces on electrons and ions are different. This process produces some surface charge density gradient on the nonspherical grain surface. Effects of a magnetic field and other plasma parameters on the properties of the dust particulate are studied. It has been shown that the alignment direction could be changed or even reversed with the magnetic field and plasma parameters. Finally, the charge distribution on the spheroidal grain surface is studied for different ambient parameters including plasma temperature, neutral collision frequency, and the magnitude of the magnetic field.

Interstellar extinction by spheroidal dust grains

Observations of interstellar extinction and polarization indicate that the interstellar medium consists of aligned non- spherical dust grains which show variation in the interstellar extinction curve for wavelengths ranging from NIR to UV. To model the extinction and polarization, one cannot use the conventional Mie theory which assumes the grains as solid spheres. We have used a T-matrix based method for computing the extinction efficiencies of spheroidal silicate and graphite grains of different shapes (axial ratios) and sizes and used these efficiencies to evaluate the interstellar extinction curve in the wavelength range 3.4−0.1 µm. A best fit linear combination of silicate and graphite grains of not very large axial ratio, fits the observed extinction curve reasonably well. We calculate the volume extinction factor Vc, which is an important parameter from the point of view of the cosmic abundance, for the spheroidal grain models that reproduce the interstellar extinction curve. We find that the shape of the grains does not affect the volume extinction factor. Finally we have also studied the extinction and linear polarization efficiencies for aligned spheroids. The results show that the shape of grains affects the linear polarization efficiencies considerably for various orientation angles of the spheroids.

Dust-lower-hybrid instability in the presence of dust charge fluctuations in a magnetized dusty plasma

The growth and damping of the low-frequency electrostatic dust-lower-hybrid (DLH) wave have been examined in the presence of dust charge fluctuations in a uniform magnetized dusty plasma. To obtain the dust charge fluctuation effects, the charging currents of electrons and ions to a spherical dust grain in a uniform magnetized dusty plasma have been calculated under the orbit-limited motion approximation. It is found that the external magnetic field reduces the charging currents, thereby reducing the dust charge fluctuation damping of a low-frequency electrostatic wave in a dusty plasma. However, for sufficiently low-frequency DLH wave having a finite parallel wave number, the thermal electrons may give rise to an instability of the DLH wave.