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The scattered light intensity signal changes over time.
The ACF aims to identify patterns from this signal to
give us the information we need to assign a size to
particles with DLS. It does this by creating a delayed
copy of the signal, then correlating the signal from the
copy to the original. In the figure on Page 25 we see
this delay shi as tau. This process is repeated with a
series of delays, and we examine how similar the signal
to the original is over time.
This is where the idea of the ACF comes from – the function that
relates the similarity of the signal to itself with a given delay. In DLS
measurements, the particles are changing location via Brownian
motion, i.e. random motion, so their movement is not a perfectly
repeating pattern. They will always look dissimilar to the delay
copy compared to the original signal. And as the delay becomes
longer, they will look less and less similar to the original. Thus,
the ACF for DLS measurements is almost always a decay function.
However, the rate of decay will be dependent on particle size
because size affects the rate of motion.
The decay function from this process of continually shi ing the
signal is extracted using the appropriate mathematical models.
From this, D can be calculated, which is then used to calculate the
size of the particles in solution.
The ACF is indicative of how likely
a particle will be in the same place
over time. The larger a particle is, the
slower it moves, and the more likely it
is to be found in the same place.
