Emma Hunt Final Defense

Adhesion to planar surfaces is relevant to numerous natural, lab, and industrial settings. Pull-off force and contact angle or contact area measurements are frequently used to study the strength of microscale adhesions, but these methods may be low throughput, rupture the contact, require appropriate models, or be highly resolution limited. We employ activity microscopy to determine the contact stiffnesses and the diffusion and drag coefficients of hundreds of silica and polystyrene microspheres immersed in a PBS medium by measuring their thermal fluctuations. The results show adhered particles act as harmonic oscillators, except in extremely high salt conditions. Silica particles were found to adhere more strongly than polystyrene particles as ionic strength increases and adhesive contact stiffnesses were shown to increase with ionic strength as diffusion coefficients decrease. Further, adhesive contacts were also shown to have stiff and soft fluctuation axes, presumably from deformation along one axis damping deformation along the perpendicular axis. Although drag was found to be independent of adhesion strength, there is some evidence that the reduction in the diffusion coefficient at higher ionic strengths is the result of additional drag on the particle that's imparted by the contact itself.

The notion that thermal fluctuations can be used to study friction was also applied to individual collagen fibrils, the primary component of the extracellular matrix (ECM) that gives tissues structure and helps mediate mechanical communication between cells. A fibril's thermally driven shape fluctuations offer a valuable probe of a fibril's bending stiffness, internal friction, and possible tension. In this study, mean-squared displacements of the fibril's thermal fluctuations are used to measure the characteristic relaxation times along the fibril's contour. Single collagen fibrils show two relaxation timescales, whether they were isolated and attached to carbon grids or embedded in a network. The source of the two time scales was determined to be the result of internal bending and shearing friction within the fibril. These were each an order of magnitude higher in glutaraldehyde treated networks. Notably, the ratio between the internal shearing and bendingfriction coefficients also increased after glutaraldehyde treatment, implyingminhomogeneity in the bending and shearing e ects of glutaraldehyde treatment.