Malvern Zetasizer Nano Series, Руководство пользователя

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What causes these bright and dark areas? The diagram below shows the
propagated waves from the light scattered by the particles. The bright areas of
light are where the light scattered by the particles arrive at the screen with the
same phase and interferes constructively to form a bright patch. The dark areas
are where the phase additions are mutually destructive and cancel each other out.
In the above example we said that the particles are not moving. In this situation
the speckle pattern will also be stationary - in terms of both speckle position and
speckle size.
In practice, particles suspended in a liquid are never stationary. The particles are
constantly moving due to Brownian motion. Brownian motion is the movement
of particles due to the random collision with the molecules of the liquid that
surrounds the particle. An important feature of Brownian motion for DLS is that
small particles move quickly and large particles move more slowly. The
relationship between the size of a particle and its speed due to Brownian motion
is defined in the
Stokes-Einstein equation.
As the particles are constantly in motion the speckle pattern will also appear to
move. As the particles move around, the constructive and destructive phase
addition of the scattered light will cause the bright and dark areas to grow and
diminish in intensity - or to put it another way, the intensity appears to fluctuate.
The Zetasizer Nano system measures the rate of the intensity fluctuation and
then uses this to calculate the size of the particles.
Interpreting scattering intensity fluctuation data
We know that the Zetasizer measures the fluctuation in scattering intensity and
uses this to calculate the size of particles within the sample - but how does it do
this?
C H A P T E R 1 3
Z e t a s i z e r N a n o S e r i e s
P a g e 1 3 . 2
M A N 0 3 1 7
Most light passes
through unscattered
Detector
The scattered light falling on the detector.
From Laser
Average
intensity
ILL
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