However, a gas laser usually has a relatively large volume that makes it very bulky. Gas laser has the advantages of stable wavelength emission with relatively low cost. The most commonly used solid-state laser is neodymium-doped yttrium aluminum garnet, abbreviated as Nd: YAG. These dopants are often rare-earth minerals such as neodymium, chromium, and ytterbium. In a solid-state laser, small amounts of solid impurities called “dopant" are added to the gain medium to change its optical properties. A solid-state laser refers to a laser device where a solid act as the gain medium. Typical example of gas laser in DLS setup is helium-neon laser which emits laser with a wavelength of 632.8 nm. The majority of the laser devices in DLS instruments are gas lasers and solid-state lasers. These industries include but are not limited to semi-conductors, renewable energy, pharmaceuticals, inks, pigments, batteries, et cetera. DLS provides various industries the opportunity to control their products' quality and therefore maximizing product performance by controlling particle size. With DLS, the particle size distribution can be measured at different temperatures, and thus a thermal analysis can be conducted on the test sample. The testing process is nearly all automatic, minimizing operation errors from different operators. With the DLS method, results with great repeatability and accuracy can be obtained within a few minutes. A small amount of sample is sufficient for a diluted solution preparation. First, DLS is non-invasive to the samples, meaning that the structure of the molecules would not be destroyed during sizing. There are several advantages of using the DLS when sizing particles. Typically, the measuring size limit of DLS falls in the nano and sub-micro range, with the magnitude of 1 nanometer to 10 micrometers. The Stokes-Einstein equation is expressed as follows:īy analyzing the fluctuating scattered light intensity due to Brownian motion, DLS can obtain the particle size distribution of small particles suspended in a diluted solution. This technique is called the dynamic light scattering, abbreviated as DLS. Then using the Stokes – Einstein equation, the particle size distribution can be calculated from the diffusion coefficient. The translational diffusion coefficient has the unit of area per unit time, where the area is introduced to prevent the sign change convention when the particle is moving away from its origin. Notice here the diffusion coefficient is specified by the word “translational”, indicating that only the translational, but not the rotational movement of the particle is taken into account. To quantify the speed of Brownian motion, the translational diffusion coefficient is modelled by the Stokes-Einstein equation. The fluctuations in scattered light intensity with time allows us to calculate the diffusion coefficient through the auto-correlation function analysis. The scattered light intensity is not a constant value it fluctuates over time due to the random walk of particles that are undergoing Brownian motion which refers to the particle's continuous and spontaneous random walk when placed in the medium resulting from the collisions between the particles and the medium molecules. The scattered light intensity depends on the particle's intrinsic physical properties such as size and molecular weight. During quasi-elastic light scattering, the frequency changes between scattered light and incident light are small, and the light scattered by the oscillating dipole has a spectrum that broadens around the incident light frequency. Light scattering refers to the emission of light in all directions from oscillating dipole. What is light scattering? When a monochromatic and coherent light source irradiates onto the particle, the electromagnetic wave will interact with the charges in atoms that compose the particle, and thus induce the formation of an oscillating dipole in the particle.
0 Comments
Leave a Reply. |