Laser Doppler Velocimetry

• Laser beams are aligned at the stationary layer in the cell
• At the crossing point of the beams, Young's interference fringes of known spacing are formed
• Particles moving through the fringes under the influence of the applied electric field scatter light whose intensity fluctuates with a frequency that is related to the particles velocity
• Photons detected are input to a digital correlator
• A frequency spectrum is produced from which the mobility and hence zeta potential are calculated

What about particles with no charge?

• Particles with no charge will not move in an applied electric field
• They will not be able to be detected
• This problem is overcome by making the interference fringes move
• This is done by using a device called a modulator

What happens if the fringes move but the particles do not?

What happens if the fringes and the particles are moving?

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