Photon Correlation Spectroscopy (PCS) is a technique used to measure the size and dynamics of small particles in a fluid. It works by shining a laser light on a sample, which causes the particles to scatter the light. The scattered light fluctuates over time due to the movement of the particles, and these fluctuations are analyzed to determine the particles' size and how quickly they are moving. PCS is particularly useful in fields like biotechnology and materials science, where understanding particle behavior is crucial. By providing insights into particle size distribution and diffusion rates, PCS helps researchers develop better products and processes, such as drug formulations and nanomaterials.