  Important aspects in the field of microrheology are the studies of the
viscosity of fluids within structures with micron dimensions and fluid samples
where only microlitre volumes are available. We have quantitatively
investigated the performance and accuracy of a microviscometer based on
rotating optical tweezers, that requires as little as one microlitre of sample.
We have characterised 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
un-stimulated eye fluid.
