Fast optical sizing without dilution

Particle size is a key characteristic of colloidal dispersions. Small-angle light scattering, the standard measurement method, requires extensive sample dilution before size measurement. This process may change the sample. It is also time-consuming and makes on-line process control difficult. We present a new method of particle sizing that does not require dilution. It is based on the static (steady) scattering of incoherent light in the multiple scattering limit. We call it steady light transport (SLT). We focus a laser onto the sample and measure the halo of backscattered light. The technique is fast, robust, inexpensive and non-invasive. Diameters between tens of nanometers and several micrometers can be measured reliably. We first describe a simple technique that does not use the polarization of light. It gives the average size when the particle volume fraction and optical contrast are known. A second, more sophisticated version measures the full light-polarizing properties of the sample. This method can give both average particle size and volume fraction. Only the optical contrast needs to be measured independently. We demonstrate the use of SLT to size emulsions and to measure emulsification in real time.