Viscosity measurements on micron-size scale using optical tweezers

We describe how a single optical tweezer can be used as a quantitative tool to perform absolute and accurate viscosity measurements on a micrometer-size scale. Our method combines an unbiased position detector calibration procedure and frequency analysis of the Brownian motion of optically confined polystyrene microspheres. The position detector used to monitor the displacement of the particle is properly calibrated using the same trapped particle. With respect to the standard calibration method based on beads attached over the cover slip surface, our approach eliminates bias caused by surface effects and spherical aberrations. We apply our method to pure water samples determining their viscosity with a precision of 12%.