Particle size and size distribution from tens of nanometers to millimeters can be rapidly and reliably determined by laser diffraction. In Laser diffraction, light scattering from the particles as a function of angle is measured and this data is used along with the refractive index of the particle and dispersion medium to find the particle size distribution. The value of refractive index can be important for determining particle size. That is, if the correct value of refractive index is used for the calculations, the correct particle size distribution is obtained. The sensitivity of the sizing result to the refractive index choice varies with particle size. If an incorrect value of refractive index is chosen, the obtained particle size distribution will be incorrect and that error is a function of the error in refractive index choice for some particle size ranges. Unfortunately, refractive index values are not always readily available and the effect of refractive index errors on particle size results is complex. For example, the effect of refractive index on determined particle size is widely considered to be small for large particles. Here, the effect of refractive index choice on determined particle size is quantitatively analyzed for a number of realistic particle samples. It was found that the effect of refractive index choice becomes weaker for larger particles, as expected. Interestingly, the effect of refractive index choice on determined size becomes weak even for particles that are still small enough that a refractive index free approximation (e.g., Fraunhofer) is invalid. This work will provide some practical idea of the relative importance of refractive index choice in laser diffraction work.

Particle sizing and laser diffraction: Refractive index effects

248th National Meeting (2014)

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