The light scattering pattern is rather diffuse, but more light is scattered in forward direction than to the side or back. Furthermore, the intensity of the diffraction maxima would increase.įor the 1 µm particles these diffraction rings are no longer observed. For larger particles, the diffraction angles would be smaller, and the rings would be closer to the center. The figure shows the scattered light patterns of suspensions with 1 µm and 10 µm particle size, respectively.įor the 10 µm particles, the scattered light pattern shows a characteristic ring structure, which can be explained mainly by diffraction. Static light scattering is often referred to as laser diffraction or laser diffractometry, even independent of the size of the particles considered and the phenomena that occur. Literature values are available for the refractive indices of almost all solids, so Mie theory can be applied very reliably for static light scattering. This is described by the Mie theory, which, however, also includes diffraction and therefore allows a comprehensive evaluation of light scattering phenomena. "Large particles" in this context means "significantly larger than the wavelength of light".įor the description and evaluation of scattered light patterns of smaller particles, the optical properties, essentially the refractive index, must be considered. This is sufficiently described by the so-called Fraunhofer theory. The angular intensity distribution is used to determine the particle size by the Stokes-Einstein equation.The characteristic light scattering patterns which are formed when a laser interacts with particles are caused by diffraction, refraction, reflection, and absorption (as shown on the figure).įor large particles, diffraction, which occurs at the contour of particles, is the dominant mechanism. Signal variation is due to the random Brownian motion of the particles. The light scattering is observed at a certain angle over time. In DLS analysis the sample suspension is illuminated by a laser beam, after which the laser light scatters in all directions. By measuring the random motion changes in the intensity of light scattering, DLS allows for the particle size and particle size distribution to be determined. Therefore, the distance between the scatterers in the solution is constantly changing with time. It is due to the collisions between the particles and the medium molecules. The scattered light intensity is not a constant value because the scattering intensity fluctuates over time as the result of the random movement of particles called the Brownian motion. The scattered light intensity is affected by the physical properties of the particle, such as size and molecular weight. The theory of Brownian motion states that small particles move faster than larger particles in a liquid or gas medium. Size distribution information is obtained from the Brownian motion of particles. DLS is a spectroscopy method that is used to determine the size distribution of particles in solution or suspension. What is DLS analysis and how does it work?ĭynamic light scattering (DLS) is also called photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS).
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