Experimental investigation of pressure responsive 'smart' materials found in the natural world

The unique ability to sense environmental stimuli and respond appropriately in a controlled and reversible fashion is a distinguishing characteristic of a novel class of materials collectively known as 'smart,' 'adaptive,' or 'responsive' materials. This seemingly 'smart' behavior is, thus, controlled by internal 'feedback loops' generated by the interconnectivity of various system elements which enables them to operate cooperatively and exhibit more complex behaviors as a collective. This thesis investigates, experimentally, three natural systems found in both the biological and non-biological domains which exhibit responsive or adaptive behaviors in the presence of external mechanical pressures by exploiting uniquely different chemical and physical principles.The first system focuses on articular cartilage, an example of an adaptive material which utilizes its distinctive hierachical, multi-scale structure in order to adjust and change its physical and lubricating properties to suit the specific needs under a broad range of normal and shear stresses. A compression cell designed to fit inside an NMR spectrometer was used to investigate the in situ mechanical strain response, structural changes to the internal pore structure, and the diffusion and flow of interstitial water in full thickness cartilage samples as it was deforming dynamically under a constant compressive load (pressure). We distinguish between the hydrostatic pressure acting on the interstitial fluid and the pore pressure acting on the cartilage fibril network. Our results show that properties related to the pore matrix microstructure such as diffusion and hydraulic conductivity are strongly influenced by the hydrostatic pressure in the interstitial fluid of the dynamically deforming cartilage which differ significantly from the properties measured under static i.e. equilibrium loading conditions (when the hydrostatic pressure has relaxed back to zero). The magnitude of the hydrostatic fluid pressure also appears to affect the way cartilage's pore matrix changes during deformation with implications for the diffusion and flow-driven fluid transport through the deforming pore matrix. We also show strong evidence for a highly anisotropic pore structure and deformational dynamics that allows cartilage to deform without significantly altering the axial porosity of the matrix even at very large strains.The second system looks at the geological phenomena of 'pressure solution' where the solubility of quartz is either enhanced or retarded by being under pressure and in close proximity to certain mineral surfaces. We have measured changes in the electrical potential difference between quartz and mica surfaces that correlate with the changing quartz dissolution rate when surfaces are pressed together at relatively low pressures (2-3 atm) in aqueous electrolyte solutions of 30 mM CaCl2 at 25°C using a Surface Force Apparatus. No detectable dissolution or voltage potential is measured in symmetrical systems (e.g. mica-mica or quartz-quartz) or between dry surfaces under similar pressures, indicating that the dissolution can not be attributed to a simple pressure effect, slow aging (creep), or plastic deformation of the quartz surface. In quartz-mica systems brought under pressure or close proximity in electrolyte solution, the onset of quartz dissolution is marked by a sudden, rapid decrease in the quartz thickness at initial rates in the range from 1 to 4 nm/min which gradually settle after several hours into a constant rate of approximately 0.01 nm/min (&sim5 mum/yr). The dissolution is 'non-uniform': the surfaces become rough as dissolution proceeds, with the appearance of pits in a manner analogous to corrosion. The decrease in the decay rate is interpreted as the gradual transformation of the quartz surface from an initially smooth crystalline lattice into a rougher, more amorphous and porous structure as material is removed.Finially, the third system investiagates actin protein, an example of a biological 'molecular machine' that belongs to a unique and highly versatile class of 'smart' materials known as multistable systems. Here we describe a new technique for studying actin-surface interactions using a Surface Forces Apparatus which is able to directly visualize and quantify the collective forces generated by adsorbed layers of non-interconnected, end-tethered actin filaments confined between two (mica) surfaces. We also identify a new force-response mechanism in which filaments stiffen under compression to produce an amplification in the elastic opposing forces. (Abstract shortened by UMI.)...