Ethan Van Goor and Kevin Booth
Spring 2014
When laser strikes a particle which is smaller than the laser’s wavelength, the laser is scattered by some angle. The intensity of the scattered light fluctuates due to the Brownian motion of the particle while it is suspended in a fluid. If the viscosity of the fluid is known, the size of the particle can be measured by this process, which is known as Dynamic Light Scattering(DLS.)
In this experiment, the size of the scattering particles was known and the viscosity of the suspending fluid was calculated. The scattering particles were three different sizes of mono-disperse polystyrene spheres whose diameters were:(340±10)nm, (489±13)nm, and (1053±10)nm.
It was our goal to determine how well the DLS technique could be used to measure different viscosities. We used all three sphere sizes to measure the viscosity of five different fluids:. Water, Methanol, 2-Propanol, a 10% by volume Glycerin Water solution, and a 40% by volume Glycerin Water solution
A specific number of coherence areas of the scattered light were measured with a photodiode. A coherence area is an area of light where all the interference is either constructive or destructive.
The intensity fluctuations in the frequency domain described how the spheres diffused in the fluid. From this, the diffusion coefficient of the suspended spheres was calculated. Subsequently, the viscosities of the suspending fluids were calculated using the Stokes Einstein equation and compared to the literature values.
The concentration by volume of spheres in the fluid was kept at 1:1000 in order to maintain Brownian motion.