Light scattering

What is light scattering?

Particles scatter light, this is a fundamental fact and something we all encounter on a daily basis, the sky is blue. This is caused by stronger light scattering of blue light by atmospheric particles than red light. The surface finish be it glossy or matt is caused by the particles in the surface. 

The angle of light scatter, the frequency of the light scattering and the intensity of said scatter can be measured to determine the size, the charge and the molecular weight of materials. This is the core of many of our technologies.

Light Scattering Techniques

For laser diffraction and X-ray diffraction, small angle X-ray diffraction (SAXS), wide angle X-ray diffraction (WAXS), we harness the principle that particles of different sizes have a unique light scattering signature, so by accurately measuring the light scattering over a wide range of angles with high sensitivity and at extremely rapidly we can determine the particle / droplet size of powders, emulsions, sprays and suspensions. However, as particles get substantially into the nanometre range there is a big fall off in how particles scatter light. A 10 nm particle scatters 1 million times less than a 100nm particle, so there is a point at which even by reducing the wavelength of the light source (which increases the amount of scatter) that the light scattering is best analysed in alternate ways. There are multiple theories which can determine the light scattering from a particle size distribution (Mie scattering theory, Fraunhofer scattering theory, Rayleigh scattering theory), and an inversion algorithm can turn scattering into a size distribution.

We can look at the nano material at right angles to the laser and track how the particles diffuse (small particles move more rapidly than large particles) and from this determine the translational diffusion coefficient and hence the size (this is known as nanoparticle tracking analysis (NTA) or see how the scattered light changes over time as particles pass through it. If it changes quickly it can be determined that fine particles are present, slowly larger particles. This forms the basis of photon correlation spectroscopy / dynamic light scattering.

Electrophoretic light scattering involves passing an electric field through a liquid which makes particles move. The larger the charge on the particles, the faster they move. We pass a laser though the particles and then recombine the scattered light with another part of the same laser which hasn’t been scattered. The resulting interference pattern allows an incredibly accurate measure of the speed of the particles to be measured.

If we measure the light scattering as a function of concentration (of polymers or biopolymers) at a variety of angles we can determine information that allows us to determine the molecular weight of the material in question and information on its structure.