This thesis reports experimental studies of the rheology of biological materials, with an emphasis on interfacial layers, including protein layers and bacterial biofilms. Results are presented on layers formed by the proteins lysozyme and Staphylococcal nuclease at the air--water interface, on biofilms formed by Pseudomonas bacteria at the oil--water interface, and on the bulk rheological properties of fibrin and cystic fibrosis mucus with an eye toward its role as an interfacial barrier in the lung. The evolution of interfacial mechanical response through time is interpreted in terms of the changing microscopic structure of the layer.The studies employ interfacial microrheology, which uses the motion of micrometer-scale particles embedded in the interface to probe the mechanical response of the surrounding material and infer its rheology. Passive measurements, which rely on thermal forces to drive the particles, are complemented by active measurements, in which ferromagnetic nanowires were rotated using magnetic fields. Additionally, the study of fibrin and cystic fibrosis mucus employs a novel technique using custom fluorescent particles that can be selectively ``switched on;;;; and used to characterize rheology and particle mobility over physiologically relevant time and distance scales.This thesis also presents a software toolkit, developed as part of the thesis work to meet the demands of this research, that has found applications by other researchers in other areas.This work was conducted under the supervision of Professor Robert L. Leheny and Professor Daniel H. Reich.
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