Effect of Opaque Materials on Particle Size Determination

The ability of matter to transmit light waves is defined as transparency. If “something positioned behind a material” is relatively clearly recognizable, the material is considered transparent or lucent, like in the case of window glass. This is the case for most gaseous, as well as many liquid substances and some solid materials.

Transparent and Opaque Materials

If the degree of transparency is based on the wavelength of the light, then the transparent substance is tinted through absorptive acting particles like some metal oxide molecules in glass or colored smoke. The higher the concentration of these particles, the less transparent the material becomes.

Impact of Transparent and Opaque Materials on Particle Size Distribution

Some materials, such as frosted glass, transmit light in a way that no objects can be recognized behind them. They are considered opaque as oppose to materials such as wood and metal, which are non-translucent. Opacity measures the obfuscation of a material and is defined as the reciprocal of translucidity. Based on the size of the scattering particles, another characteristic is generally defined: opalescence. Opalescence is the colorfulness of certain materials that occurs through scattering and possibly interference of light on small structures.

When the scattering particles are smaller than the wavelength, a strongly wave-dependent Rayleigh scattering takes place. The scattering intensity substantially increases with a decrease in the wavelength. Therefore, a blue light is more strongly scattered than a red one. This is, for example, what causes the blue color of the sky.

When particles become larger than the wavelength of the light, the Mie-scattering occurs. However, in this case the wavelength dependency of the scattering light intensity is less visible. The colorfulness of the scattered white light is lost and a transition from opalescence to opacity is observed.

The following graph shows the measured particle size distribution of ground cryolite, a substance used for tinting frosted glass.

Different visual impressions occur depending on the particle size. The wet measuring was performed with a saturated cryolite solution, in which a background measurement with the ANALYSETTE 22 was carried out before adding the sample. This prevented the de-stabilization of the solution, which would have been observed with the same sample in pure water. Unstable solutions lead to a shifting of the measuring curve during successive measurements and to smaller measured particle diameters. As an alternative, the sample can be measured in alcohol by using a small volume dispersion unit.

This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.

For more information on this source, please visit FRITSCH GMBH - Milling and Sizing.

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