(513h) Bi-Disperse Multiple Particle Tracking Microrheology for Characterizing Length Dependent Rheological Properties

Multiple particle tracking microrheology (MPT) is a powerful tool for quantitatively characterizing rheological properties of soft matter. Traditionally, MPT uses a single particle size to characterize rheological properties. But in complex systems, MPT measurements with a single size particle can characterize distinct properties that are linked to the materials’ length scale dependent structure. By varying the size of probes, MPT can measure the properties associated with different length scales within a material. We develop a technique to simultaneously track a bi-disperse population of probe particles. 0.5 and 2 μm particles are embedded in the same sample and these particle populations are tracked separately using a brightness-based squared radius of gyration, R_g². Bi-disperse MPT is validated by measuring the viscosity of glycerol samples as a function of concentration. Bi-disperse MPT measurements agree well with literature values. We then characterize a homogeneous poly(ethylene glycol)-acrylate: poly(ethylene glycol)-dithiol gelation reaction. The critical relaxation exponent and critical gelation time are consistent between the two particle sizes and agree with previous measurements using a single particle. Degradation of a heterogeneous hydrogenated castor oil colloidal gel is characterized. The two particle sizes measure different values of the critical relaxation exponent, indicating that they are probing different structures within the evolving material. Analysis of material heterogeneity shows measured heterogeneity is dependent on probe size indicating that each particle is measuring rheological evolution of a length scale dependent structure. Finally, we measure the length scale dependent rheological properties around an encapsulated human mesenchymal stem cell in a synthetic cell-degradation polymer-peptide hydrogel scaffold. We measure statistically different particle movement, with the larger particles moving more than the smaller particles. The larger particles are moved by cytoskeletal tension on the larger network while the smaller particles measured cell-mediated scaffold degradation. Overall, bi-disperse MPT increases the amount of information gained in a single measurement, enabling more complete characterization of complex systems that range from consumer care products to biological materials.