We demonstrate quantitative in situ viscosity measurements by tracking the thermal fluctuations of an optically trapped microsphere subjected to a small oscillatory flow. The measured power spectral density of the sphere's positions displays a characteristic peak at the driving frequency of the flow, which is simply proportional to the viscosity, when measured in units of the thermal power spectral density at the same frequency. Measurements are validated on different water-glycerol mixtures, as well as in a glycerol gradient, where no a priori knowledge of the solution is used to determine the glycerol concentration.
Bibliographical noteCopyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.