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Passive microrheology exploits the Brownian motion of colloidal tracerparticles. From the mean-squared displacement (MSD) of the tracers, the bulkrheological 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 butprovides parameters commonly acquired in light scattering experiments. Based onthe spatial Fourier transform of image differences, the intermediate scatteringfunction and subsequently the MSD is calculated. Then the usual microrheologyprocedure and the empirical Cox-Merz rule yield the steady-shear viscosity.This method, $\eta$-DDM, is tested and illustrated using three differentsystems: Newtonian fluids (glycerol-water mixtures), colloidal suspensions(protein samples) and a viscoelastic polymer solution (aqueous poly(ethyleneoxide) solution). These tests show that common lab equipment, namely abright-field optical microscope, can be used as a convenient and reliablemicroliter viscometer. Because $\eta$-DDM requires much smaller sample volumesthan classical rheometry, only about a microliter, it is particularly usefulfor biological and soft matter systems.

Microliter viscometry using a bright-field microscope: $\eta$-DDM