Static Multiple Light Scattering (SMLS) is the most suited optical method to characterize concentrated liquid dispersions in their native state directly.
Many emulsions or suspensions are used in concentrated forms. Optical methods such as laser diffraction or dynamic light scattering offer limited possibilities in terms of the analysis of such formulations. Concentrated products have to be diluted and so mixed (mechanical stress) which alters the dispersion state. Static Multiple Light Scattering can be used to measure these formulations without dilution or perturbation of the initial form.
Therefore, SMLS offers a more accurate determination of physical phenomena occurring over time in complex formulations. It is also recommended (according to ISO TR 13097) that the determination of the shelf life of concentrated dispersions is done using direct optical methods that do not require sample preparations (such as dilution).
Monitor size change vs. time
Monitor local concentration change vs. time
Measure particles mean diameter or concentration
SMLS consists of sending photons (NIR light source, 880nm) into the sample. These photons, after being scattered many times by the particles (or droplets) in the dispersion emerge from the sample and are detected by 2 synchronous detectors:
Backscattering at 135° for opaque samples and Transmission at 0° from the light source for transparent samples.
Backscattering is directly related to the photon transport mean free path (l*). The l* (µm) is the distance above which the photon loses the initial direction of the incident beam. Transmission is directly related to the photon mean free path (l) which is the average distance between scatterers. Thus, Transmission and Backscattering light intensities both depend on particle size and concentration.
Turbiscan technology measures Transmission or Backscattering intensities versus over the sample height and aging time. It is possible to monitor particle diameter evolution and concentration change (sedimentation, creaming, …). Mean diameter can be calculated from Backscattering or Transmission intensities thanks to Mie theory using the above equations.