  Passive microrheology exploits the Brownian motion of colloidal tracer
particles. From the mean-squared displacement (MSD) of the tracers, the bulk
rheological and viscometric properties of the host medium can be inferred.
Here, the MSD is determined by applying Differential Dynamic Microscopy (DDM).
Compared to other microscopy techniques, DDM avoids particle tracking but
provides parameters commonly acquired in light scattering experiments. Based on
the spatial Fourier transform of image differences, the intermediate scattering
function and subsequently the MSD is calculated. Then the usual microrheology
procedure and the empirical Cox-Merz rule yield the steady-shear viscosity.
This method, $\eta$-DDM, is tested and illustrated using three different
systems: Newtonian fluids (glycerol-water mixtures), colloidal suspensions
(protein samples) and a viscoelastic polymer solution (aqueous poly(ethylene
oxide) solution). These tests show that common lab equipment, namely a
bright-field optical microscope, can be used as a convenient and reliable
microliter viscometer. Because $\eta$-DDM requires much smaller sample volumes
than classical rheometry, only about a microliter, it is particularly useful
for biological and soft matter systems.
