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  1. We show that for spherical particles greater than ca. 5 µm, the differential scattering cross section is only weakly dependent on the real and imaginary parts of the refractive index (m=n+iκ<#comment/>) when integrated over angle ranges near37±<#comment/>5∘<#comment/>and115±<#comment/>5∘<#comment/>, respectively. With this knowledge, we set up an arrangement that collects scattered light in the ranges37±<#comment/>5∘<#comment/>,115±<#comment/>5∘<#comment/>, and80±<#comment/>5∘<#comment/>. The weak functionality on refractive index for the first two angle ranges simplifies the inversion of scattering to the particle properties of diameter and the real and imaginary refractive indices. Our setup also uses a diamond-shaped incident beam profile that allows us to determine when a particle went through the exact center of the beam. Application of our setup to droplets of an absorbing liquid successfully determined the diameter and complex refractive index to accuracies ranging from a few to ten percent. Comparisons to simulated data derived from the Mie equations yielded similar results.

  2. The scattered intensity from large spheres with a real part of the refractive index ofn=1.33,1.5,2.0is investigated as the radiusRand an imaginary part of the refractive indexκ<#comment/>are varied. It is shown that the product ofκ<#comment/>and the size parameterkR,κ<#comment/>kR, is a universal parameter describing the quenching of the refraction phenomenon of the scattered light: the refraction hump, the generalized rainbows, and the glory. The physical reason for this is thatκ<#comment/>kRis the inverse of the relative skin depth of light penetration into the sphere, which is demonstrated by calculations of the internal fields that darken universally asκ<#comment/>kRincreases.

  3. We present measurements of light scattering intensity from aerosolized, micron sized, irregularly shaped, molybdenum disulfide (MoS 2 ) particles in order to study the effects of a refractive index, m = n + i κ, with large real and imaginary parts. Light scattering was measured over a range of angles from 0.32 °to 157 °. Calibration was achieved by scattering with micron sized, spherical silica particles. Light scattering for both particle types was compared to theoretical Mie scattering calculations using size distributions deter- mined by an aerodynamic particle sizer. Effects of the intensity weighted size distribution are discussed. We find that scattering by these irregularly shaped, highly refractive particles is well described by Mie scattering. We also find that when the quantity κkR, where kR = 2 πR/ λis the size parameter, is greater than one, there is no enhancement in the backscattering. Finally, we show that Guinier analysis of light scattering by highly refractive particles yields intensity weighted mean sizes of reasonable accuracy for any shape.
  4. We present measurements of the scattered light intensity by aerosolized hematite aggregate particles. The measurements were made at a wavelength of 532 nm in the scattering angle range from 0.32 °to 157 °. Hematite has high values of the real and imaginary parts of the refractive index m = n + i κ= 3 + i0.5 at the studied wavelength. Scanning electron micrographs (SEM) indicated that the particles were aggregates whereas the optical microscope pictures showed that the aerosol had a bimodal distribution with effective mean diameters of roughly 1 and 10 μm. This is consistent with the light scattering results which displayed two Guinier regimes. The aggregates were composed of smaller grains with an approximate size of 200 nm. Ultra Small-Angle X-ray Scattering (USAXS) indicate that the aggregates were uniform and non-fractal. Mie calculations for a sphere equivalent to the aggregate size were compared to the experimentally observed results. The observed results showed an enhanced backscattering, whereas the Mie calculations did not due to the large imaginary part of the refractive index. Hematite aggregates were simulated by assuming they were composed of spherical monomers inside a spherical volume. Then the light scattering was calculated using the T-matrix method formore »these simulated aggregates. The calculated results show an enhanced backscattering. We present a dimensional analysis to estimate the extent of multiple scattering within the aggregate and find a correlation between the average number of scattering events within the aggregate and the enhancement in the backscattering.« less
  5. We present investigations of the kinetics of the colloidal sol-to-gel transition by combining small angle static light scattering (SASLS) and dynamic light scattering (DLS) techniques. Dilute monomer volume fractions were used to allow for a full investigation of the gelation to obtain all possible kinetic regimes. Our data verify the predictions of a kinetic theory, the ideal gel point (IGP) theory, where three regimes of kinetics are expected. We observe the first regime, the well-known cluster-dilute regime, with a kinetic exponent of z = 1. Followed by a cluster-dense regime with an enhanced kinetics and z ’ 2. Finally, a gelation regime is observed where the aggregate growth slows and ceases to grow at the IGP predicted size, Rg,G. These results quantitatively verify the IGP theory. We conclude that kinetic description provides a complete theory of the gelation process from sol to gel.
  6. A comprehensive theory encompassing the kinetics of the sol-to-gel transition is yet to be formulated due to break-down of the mean-field Smoluchowski Equation. Using high temporal-resolution Monte Carlo simulation of irreversible aggregation systems, we show that this transition has three distinct regimes with kinetic exponent z 2 1 ½ ; 2Þ corresponding to aggregation of sol clusters proceeding to the ideal gel point (IGP); z 2 ½2; 5:7Þ for gelation of sol clusters beyond IGP; and z 2 ½2; 3:5Þ for a hitherto unidentified regime involving aggregation of gels when monomer-dense. We further establish universal power-law scaling relationships that connect the kinetics of these three regimes. Improved parameterizations are performed on the characteristic timescale parameters that define each regime.