<p>Friction is the resistive force that arises when two contacting surfaces move relative to each other. Frictional interactions are important from both engineering and biological perspectives. In this research I focus on the fundamental understanding of friction on polymeric and biological surfaces in aqueous environments. First, I examine the frictional properties of a stimulus-responsive hydrogel, poly-N-isopropylacrylamide (pNIPAAm), to understand how different phase states affect its tribological properties. My measurements indicate that gels in a collapsed conformation at low shear rates, exhibit significantly larger friction than swollen gels. These differences arise from changes in surface roughness, adhesive interactions, and chain entanglements of the gel surfaces associated with the phase transition. Importantly, I show that the changes in friction, triggered by an external stimulus, are reversible. </p><p>Second, I examine details of the boundary lubrication mechanism involved in mediating friction and wear in diarthrodial joints. Specifically, I looked at the constituents of the synovial fluid, lubricin and hyaluronic acid (HA) and examined their interactions on model substrates, (1) to determine the effect of surface chemistry on adsorption using surface plasmon resonance (SPR), and (2) to study normal force interactions between these surfaces using colloidal probe microscopy (CPM). I found that lubricin is highly surface-active, adsorbed strongly onto hydrophobic, hydrophilic and also collagen surfaces. Overall, lubricin develops strong repulsive interactions. This behavior is in contrast to that of HA, which does not adsorb appreciably, nor does it develop significant repulsive interactions. I speculate that in mediating interactions at the cartilage surface, an important role of lubricin is one of providing a protective coating on cartilage surfaces that maintains the contacting surfaces in a sterically repulsive state.</p> / Dissertation

Organic semiconductors show promise to yield a novel class of bendable electronic devices, and much research efforts have focused on the optimization of these films for device performance. It is well known that the structure of organic films has a large influence over the electronic properties. In particular, the carrier mobility is often highly anisotropic, and domain boundaries have a detrimental effect on charge transport. Therefore the domain structure and lattice orientation are of particular interest. However, little is known about the domain structure of organic films, and techniques to study these properties have only begun to emerge in recent years. In this thesis, we apply two experimental techniques, Grazing-Incidence X-ray Diffraction (GIXD) and Lateral Force Microscopy (LFM), toward studying the lattice and domain structure of tetracene films grown on the silicon(001)-monohydride surface. We describe the necessary steps toward optimizing the sensitivity of these techniques to the domain structure. Results show that the crystalline tetracene films form a layered morphology in which the a-b plane lies parallel to the substrate surface. The film lattice structure is similar to bulk tetracene, and the lattice is confined to two orthogonal orientations, forming a partially-commensurate relationship with the substrate surface lattice along the film 'a' axis. LFM images reveal two types of polycrystalline domains. The first type ("major domains") are tens of microns in size, and are classified by their lattice orientation. They are subdivided into the second type ("sub-domains"), which range from 0.1 to 5um in size, and are argued to represent regions of uniform molecular tilt direction. The GIXD data show that the single-crystal domains which comprise these two larger domain types are anisotropic in size, being up to two times longer along the film 'b' axis than along 'a'. The single-crystal domains range from 0.05 to 0.2um in size, depending on lattice orientation and film thickness. The mathematical basis for these single-crystal domain size calculations is presented. The single-crystal domain sizes are thickness-dependent, and are two orders of magnitude smaller than a typical surface island observed in atomic-force microscopy (AFM) topographs. Substrate steps can also significantly influence the film structure by inducing boundaries in the single-crystal domains and sub-domains, but not in the major domains. This detailed knowledge of the domain structure of organic thin-films may assist in our understanding of the factors which affect charge transport in thin films, and may help to direct research efforts in optimizing the film structure for device performance. / Natural Sciences and Engineering Research Council (NSERC), Canadian Foundation for Innovation (CFI), Ontario Innovation Trust (OIT).