DYNAMICS LIGHT SCATTERING IN PHYCICS .pptx

this is ppt for dynamics light scattering which related to mesure the size of particles in suspansion

1. Dynamics light scattering (DLS) Key points: Introduction Brownian motion Sample preparation Working mechanism Auto correlation function Stock Einstein equation Calculations Advantage and disadvantage

2. INTRODUCTION: Dynamics light scattering is a technique in physics that can be used to determine the size of distribution profile of small particle in suspension or polymers in solution. This technique develop in the 1960s for measuring range , in a suspension or solution The first theoretical basis for DLS was established by Berne and Pecora in 1970s. The method is based on the analysis of the fluctuations of intensity of scattered light, caused by the brownian motion of the particle .

3. Brownian motion: Brownian motion refers to the random movement of particles suspended in a fluid(liquid or gas) resulting from collisions with the molecules of the fluid. In the context of DLS Brownian motion causes the particles in a suspension to move randomly,leading to fluctuations in intensity of scattered light. The fluctuations In intensity is more wide for smaller particles as compare to large size particle,because the thermal energy of small particles is greater so its move faster then large particles. Fig. 2 intensity pattern with particle movements Fig. 1 brownian motion of the particles

4. SAMPLE PREPRATION

5. WORKING MECHENISM In this technique we use monochromatic laser light, when laser light intract with particle then it goes to scattered . particle performs brownian motion so that it change own distance continuously then we got a fluctuation intensity pattern in motion. The intensity pattern change with time. Diagram for DLS working DLS Appratus .

6. Auto correlation function In DLS we use auto correlation function,the autocorrelation is a crucial tool for analyzing the motion of particle in a fluid. It provide information about size dynamics by examining , how the intensity of scattered light fluctuated over time. The autocorrelation function G( τ ) measure the intensity of the scattered light with a delayed version of itself as a fluction of time τ . G( τ ) = < I(t) . I(t+ τ )> Here: <I(t)> = intensity at time t.

7. for monodisprsion spherical particle , the autocorrelation function typically decays exponentially.   G ( q;τ )= exp ⁡(− Γτ )