Picoliter viscometry using optically rotated particles

Important aspects in the field of microrheology are studies of the viscosity of fluids within structures with micrometer dimensions and fluid samples where only microliter volumes are available. We have quantitatively investigated the performance and accuracy of a microviscometer based on rotating optical tweezers, which requires as little as one microliter of sample. We have characterized our microviscometer, including effects due to heating, and demonstrated its ability to perform measurements over a large dynamic range of viscosities (at least two orders of magnitude). We have also inserted a probe particle through the membrane of a cell and measured the viscosity of the intramembranous contents. Viscosity measurements of tears have also been made with our microviscometer, which demonstrate its potential use to study unstimulated eye fluid.