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Photon correlation spectroscopy on tears.
Photon Correlation Spectroscopy (PCS) is a technique for measuring the size of particles in suspension by analyzing the intensity fluctuations of scattered light. The speed of movement is used to determine particle size distribution. Human tears were analyzed by PCS. Statistical difference was found between contact-lens wearers and non-wearers, the average hydrodynamic diameter being $\sim 60$ nm higher for wearers. The difference is attributed to a change of the state of aggregation/interaction of tear content due to contact-lens wear. PCS is very sensitive to aggregations/interactions between particles, which modify their motion.