| Typically, rheology is measured using bulk rheological techniques, which give information about the macroscopic properties of the fluid, such as viscosity, and elastic modulus. However, the recent advances in sensitive microscopic and optical techniques have led to the emergence of a new field, known as microrheology. Microrheological measurements are capable of resolving forces on the order of piconewtons, and can obtain very localized structural information in the test fluid. All microrheological techniques utilize embedded probe particles to determine the surrounding fluid properties. The most common form of microrheology is "passive" microrheology, wherein the probe particles execute purely Brownian or thermal motion. Alternatively, in "active" microrheology, probe particles are manipulated using an external source of force, such as laser tweezers or magnetic tweezers. Both techniques present specific advantages. Passive microrheology has been widely adopted because the design and implementation of experiments is relatively straightforward. However, since passive microrheology is constrained to rely on purely thermal motion, the recovered fluid behavior is always in a linear response regime. The goal of the present work is to adapt active microrheology to measure non-linear fluid properties such as shear thinning, and to determine the conditions where such measurements agree with bulk rheology.;We begin by developing a small amplitude, oscillatory active microrheological technique, where a single probe particle is trapped and oscillated in a suspension of small bath particles, whose diameter that is approximately twenty times smaller than that of the probe diameter. The suspension microviscosity is recovered across a frequency range of 5-1000 Hz, and applied amplitudes which are approximately 10% of the probe diameter. We find that the suspension exhibits thinning behavior with increasing frequency. In addition, we obtain quantitative agreement between our microrheological measurements, drag microrheology measurements, and bulk rheology, suggesting that this technique is capable of recovering bulk viscosity values. Finally, we compare our results to a new theoretical model that accounts for the three sources of stress in active microrheological measurements of colloidal suspensions: (i) direct interactions between the probe and bath particles (ii) indirect interactions between bath particles, and (iii) Einstein stresses that arise from the particles' inability to shear in the same manner as the surrounding fluid. Notably, while indirect interactions and Einstein stresses are present in macroscopic techniques, direct interactions have no macroscopic analog. Therefore, if indirect interactions dominate in the measurements, the bulk suspension viscosity can be recovered. We find that indirect interactions dominate in the limit that the probe particle is far larger than the surrounding bath particles, and therefore determine both experimentally and analytically that our measurements recover the bulk suspension viscosity.;Next, we analyze single probe drag measurements in a suspension of fluorescently labeled bath particles. We recover the suspension microviscosity, and directly correlate this to the microstructural deformation in the bath suspension. We find that the suspension exhibits thinning behavior as the velocity of the probe increases. In addition, the measured microviscosity is found to be in good agreement with recent computational studies in the "direct" collision limit, which is a measurement artifact when considering the bulk, non-linear response in a suspension. In addition, we find some departure between our experimental results, and recently performed theoretical studies, which we attribute to the presence of hydrodynamic interactions in our system.;We further characterize the anisotropic, non-linear structures formed in the direct probe limit by performing two probe experiments in a colloidal suspension. We hypothesize that these structures could potentially lead to interprobe interactions. First, we hold two particles such that the line joining their centers is normal to the flow direction, and then conduct measurements as a function of probe velocity and interparticle separation. Both the drag force and microstructural deformation surrounding the particles are recovered. Intriguingly, we find that the microstructure induces a slight attraction between the probe particles, particularly at close separations, constituting a "non-equilibrium" depletion interaction. We then conduct two point measurements with the line joining the particles aligned parallel to the flow direction, and reexamine the forces on the probes, and the microstructural effects in the bath suspension. We find that the drag force experienced by both particles is the same, despite their orientation to the flow direction. In addition, we find several microstructural effects that differ from both the single probe and perpendicular case. |
