Dynamic Light Scattering (DLS)
DLS,
also known as Photon Correlation Spectroscopy (PCS) uses the time dependence
of the intensity of the scattered light to determine the translational
diffusion coefficient of small particles.
If a laser beam illuminates a suspension, and the scattered light is
collected over a small solid angle, then the light will exhibit intensity
fluctuations on the order of microseconds to milliseconds time scale. This
is because the Brownian motion of the scatterers causes the phase of the
light scattered from each particle to change with time, hence the total
scattered intensity also fluctuates with time.
From the intensity fluctuations it is possible to determine the electric field auto-correlation function:
Where Γ=K2DT is the decay constant, DT is the translational diffusion coefficient and K is the magnitude of the scattering vector. For spheres, the Stokes-Einstein relationship applies and the diffusion coefficient is related to the hydrodynamic radius r (a measure of particle size) by DT=kBT/6πηr. Here kB is Boltzmann's constant, T the absolute temperature, and η is the viscosity of the suspending liquid.
- DLS can generally measure sizes in the range 1 to 1000 nm.
- Our group has a Brookhaven Instruments light scattering instrument which contains a goniometer (BI-200SM) and autocorrelator (BI-2030AT).
- The light source is a water-cooled Lexel Argon ion laser.
- We use control software for both DOS and Windows platforms.
One of our interests has been improving the accuracy and reliability of particle size distribution by using multi-angle Dynamic Light Scattering. This data is then combined with static light scattering measurements at the same angles. We have written a program that can incorporate data from these types of measurements and hence, provide a more precise description of the particle size distribution.