Measurement of viscosity of liquids using optical tweezers

We propose a method for measuring viscosities of unknown liquids by using optical tweezers combined with optical microscopy. We trapped 1-μm particles in water–glycerin mixtures and analyzed the dependence of the motion on viscosity. Based on our calibration with various water–glycerin mixtures, we propose a method for determination of viscosities of unknown liquids with high accuracy. We discuss how the method can be applied to measure the viscosity of liquids that are available only in small quantities. This non-invasive method of studying viscosities could be especially applicable in investigations of biological samples.