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1.
Phys Rev Lett ; 85(12): 2426-9, 2000 Sep 18.
Article in English | MEDLINE | ID: mdl-10978073

ABSTRACT

The outcome of the first stage of planetary formation, which is characterized by ballistic agglomeration of preplanetary dust grains due to Brownian motion in the free molecular flow regime of the solar nebula, is still somewhat speculative. We performed a microgravity experiment flown onboard the space shuttle in which we simulated, for the first time, the onset of free preplanetary dust accumulation and revealed the structures and growth rates of the first dust agglomerates in the young solar system. We find that a thermally aggregating swarm of dust particles evolves very rapidly and forms unexpected open-structured agglomerates.


Subject(s)
Models, Theoretical , Planets , Weightlessness
2.
Science ; 286(5448): 2319-22, 1999 Dec 17.
Article in English | MEDLINE | ID: mdl-10600737

ABSTRACT

Interstellar dust grains intercepted by the dust detectors on the Ulysses and Galileo spacecrafts at heliocentric distances from 2 to 4 astronomical units show a deficit of grains with masses from 1 x 10(-17) to 3 x 10(-16) kilograms relative to grains intercepted outside 4 astronomical units. To divert grains out of the 2- to 4-astronomical unit region, the solar radiation pressure must be 1.4 to 1.8 times the force of solar gravity. These figures are consistent with the optical properties of spherical or elongated grains that consist of astronomical silicates or organic refractory material. Pure graphite grains with diameters of 0.2 to 0.4 micrometer experience a solar radiation pressure force as much as twice the force of solar gravity.


Subject(s)
Cosmic Dust , Gravitation , Magnetics , Solar System
3.
Article in English | MEDLINE | ID: mdl-11970032

ABSTRACT

We present theoretical results for the source function in photophoresis of an arbitrary aggregate of spheres that are homogeneous and isotropic piecewise. This source function directly represents the distribution of electric fields inside the spheres. Our calculation is based on a rigorous analytic solution to the radiative multisphere-scattering problem developed recently [Y.-l. Xu, Appl. Opt. 34, 4573 (1995); 36, 9496 (1997); Phys. Lett. A 249, 30 (1998)]. When an aggregate degenerates to a single sphere, the results are exactly the same as those given by the Mie theory. We also discuss the numerical techniques necessary for obtaining an accurate numerical solution for the source function.

4.
Appl Opt ; 36(30): 8026-30, 1997 Oct 20.
Article in English | MEDLINE | ID: mdl-18264332

ABSTRACT

We present laboratory microwave scattering measurements for complex amplitude scattering matrices of three aggregates of 2, 8, and 27 identical spheres and compare them with theoretical predictions. Electromagnetic multiparticle-scattering calculations involve the determination of a large number of vector translation coefficients introduced by the addition theorems for vector spherical harmonics. For one of the two classes of vector translation coefficients there is an overall-sign discrepancy between two groups of formulations that exist in the literature. We compare our experimental data with the theoretical results from scattering calculations using the two different sets of formulas for computation of the translation coefficients. This comparison of experiment with theory reveals that Cruzan's original research on the vector addition theorems [Q. Appl. Math. 20, 33-40 (1962)] is correct, although many authors believe that Cruzan's formulation contains an overall-sign error.

5.
Appl Opt ; 32(21): 4088-100, 1993 Jul 20.
Article in English | MEDLINE | ID: mdl-20830050

ABSTRACT

We present angular scattering functions for loosely packed aggregates of 250 and 500 identical spheres near the Rayleigh size limit before and after the application of successive layers of an absorbing mantle. All measurements were obtained by using the microwave analog technique. Gross features of the scattering by aggregates without a mantle can be interpreted in terms of coherent scattering from the unit spheres acting independently of each other. The largest deviations from this approximation occur after the first minimum in forward scattering and extend to a scattering angle of 60° or 80° for our models. This intermediate range is also where the largest differences occur in the scattering from one aggregate to another. The angular extent of the range is largest for aggregates with the smallest dimensions. The scattering function is usually flat in the backscattering hemisphere and has little or no backscattering increase. The coherent scattering approximation breaks down when the aggregates are coated, and an equivalent spheres approximation becomes a better representation. The maximum degree of polarization near a scattering angle of 90° first decreases and then increases again as the mantle grows thicker.

6.
J Am Dent Assoc ; 120(4): 380, 1990 Apr.
Article in English | MEDLINE | ID: mdl-2319063
8.
Appl Opt ; 20(23): 4039-50, 1981 Dec 01.
Article in English | MEDLINE | ID: mdl-20372321

ABSTRACT

The light scattering properties of twenty-eight particles, spanning four sizes (near the resonance region) and seven related shapes (a 4:1 cylinder, 4:1 and 2:1 prolate spheroids, a sphere, 2:1 and 4:1 oblate spheroids, and a 4:1 disk), are presented for a common index of refraction, m = 1.61-i0.004, representing silicates. Microwave analog and theoretical methods were used to derive the scattered intensity and degree of polarization as a function of the scattering angle along with the extinction. All results refer to an ensemble or a cloud of identical particles because averages have been taken over random particle orientations. The degree of polarization, backscatter, and the radiation-pressure cross section are most sensitive to particle shape, implying that the use of Mie theory may be inappropriate for many applications.

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