The scattered light is detected over a certain time period in order to monitor the movement of the particles. Small particles do not scatter much light, which leads to an insufficient measurement signal. In contrast, the lower size limit is defined by the signal-to-noise ratio. Therefore, the onset of sedimentation indicates the upper size limit for DLS measurements. If there is sedimentation, there is no random movement, which would lead to inaccurate results. A basic requirement for the Stokes-Einstein equation is that the movement of the particles needs to be solely based on Brownian motion. Further, the equation includes the viscosity of the dispersant and the temperature because both parameters directly influence particle movement. The speed of the particles is given by the translational diffusion coefficient D. The relation between the speed of the particles and the particle size is given by the Stokes-Einstein equation (Equation 1). If you know all other parameters which have an influence on particle movement, you can determine the hydrodynamic diameter by measuring the speed of the particles. As a result, smaller particles are moving at higher speeds than larger particles. The energy transfer is more or less constant and therefore has a greater effect on smaller particles. These collisions cause a certain amount of energy to be transferred, which induces particle movement. The principle of Brownian motion is that particles are constantly colliding with solvent molecules. When particles are dispersed in a liquid they move randomly in all directions. In this paper, the theory of dynamic light scattering technique and the mathematical principles of measurement are briefly presented.Dynamic light scattering (DLS) is based on the Brownian motion of dispersed particles. The application of this technique is expanding in the fields of nanomaterials, physics, chemistry, biology, medicine and fluid mechanics rapidly. Due to the reliable results providing by the technique, it is very popular in universities laboratories and industrial institutes. It is also a non-invasive, fast, easy, accurate, reliable and relatively inexpensive technology which requires very low amount of sample and low maintenance cost.Īlthough, this technique has got limitations such as: results fluctuations due to changes in temperature and viscosity sample preparation refractive index and a must for dispersed particles in Solution regarding to the physical and chemical interaction of the particles with the liquid, finding a right solution is a matter of difficulty. This spectroscopy method performs measurements at the nano scale and micro scale indirectly. ![]() When monochromatic light beam incidents to moving particles the intensity of the incoming light changes randomly, depending on the size of the particles. Brownian motion phenomena of the particles in solutions is used to determine the particle size distribution. This approach is one of the most general methods used to determine the particle size distribution. Dynamic light scattering is an optical measurement technique for describing nanoparticle systems. Particle size characterization affects many material properties and is a valuable factor of quality and performance.
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