Photon correlation spectroscopy (PCS) has become a mature and popular technology for probing the diffusion of particulate materials either in solution or in suspension. By determining the rate of diffusion (the diffusion coefficient), information regarding the size of particles, the conformation of macromolecular chains, various interactions among the constituents in the solution or suspension, and even the kinetics of the scatterers can be obtained without the need for calibration. The advantages of PCS, which include the fact that it is a non-invasive absolute technique requiring only a small amount of sample, and does not require extensive sample preparation, have made this technology the method of choice for sizing submicron particles. An international standard covering the use of PCS to obtain mean particle size in a dilute suspension has been established.

In a PCS experiment the fluctuations (temporal variation, typically in a µs to ms time scale) of the scattered light from scatterers in a medium are recorded and analyzed in correlation delay time domain. The scatterers can be anything that has a different refractive index than that of the medium and are stable throughout the duration of the measurement. In the typical situation the scatterers are solid particles (such as metal oxides, mineral debris, and latex particles) or soft particles (such as vesicles and micelles) in suspension, or macromolecular chains (such as synthetic polymers and biomaterials) in solution. The common property of these particles probed in a PCS measurement is their movement. This movement arises from the random thermal motion of the medium’s molecules (this motion, first observed by the English botanist Robert Brown while using an optical microscope to observe flower pollens in water, is known as Brownian motion), and is free from external forces such as turbulence or the gravitational force. The detected scattering may be from individual particles (single scattering), or from the multiple scattering in a concentrated solution or suspension. Fluctuations in the scattering intensity at a given scattering angle arise because the phase and polarization of the light scattered by each particle changes with time and the particles continuously rearrange their configuration in the scattering volume due to Brownian motion. The relative positions of the particles in the scattering volume at any instant determine the magnitude of constructive or destructive interference of the scattered light at some fixed point in space. In practice, that point is where the detector is located. Since the diffusion rate of particles is determined by their sizes in a given environment, information about their size is contained in the rate of fluctuation of the scattered light.

Generally speaking, the lower size limit of this type of measurement is determined by the detectable scattering fluctuations of particles versus the experimental noise. The measured scattering fluctuations must be greater than the experimental noise created by various sources, including environment disturbances, temperature fluctuations, and the inherent electronic noise, in order to obtain an unbiased result. The upper size limit of these measurements is determined primarily by the sedimentation limit. Particles that are being analyzed must be stably suspended. Practically, the upper size limit in a PCS experiment is about a few microns depending on material density and medium viscosity, and the lower size limit is about a few nanometers depending on the refractive index difference between the particles and the medium.