Malvern Zetasizer Nano Series, Руководство пользователя

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Within the instrument is a component called a digital correlator. A correlator
basically measures the degree of similarity between two signals over a period of
time.
If we compared the intensity signal of a particular part of the speckle pattern at
one point in time (say time = t) to the intensity signal a very short time later
(t+
d
t) we would see that the two signals are very similar - or strongly correlated.
If we then compared the original signal a little further ahead in time (t+2
d
t),
there would still be a relatively good comparison between the two signals, but it
will not be as good as at t+
d
t. The correlation is therefore reducing with time.
Now consider the intensity of the signal at ‘t’, with the intensity at a much later
time - the two signals will have no relation to each other as the particles are
moving in random directions (due to Brownian motion). In this situation it is said
that there is no correlation between the two signals.
With DLS we are dealing with very small time scales. In a typical speckle pattern
the length of time it takes for the correlation to reduce to zero is in the order of 1
to 10's of milliseconds. The "short time later" (
d
t) will be in the order of
nanoseconds or microseconds!
If we compare the signal intensity at (t) with itself then we would have perfect
correlation as the signals are identical. Perfect correlation is reported as 1 and no
correlation is reported as 0.
If we continue to measure the correlation at (t+3
d
t), (t+4
d
t), (t+5
d
t), (t+6
d
t),
etc, the correlation will eventually reach zero. A typical correlation function of
correlation against time is shown below.
Using the correlation function to get size information
How does the correlation function relate to the particle size? We mentioned
earlier that the speed of particles that are being moved by Brownian motion is
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 . 3
Correlation
1.00
0
t=0
Time
t=
¥
ILL
4022