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Nanomechanical properties of biological fibers are governed by the morphological features and chemically heterogeneous constituent subunits. However, very little experimental data exist for nanoscale correlation between heterogeneous subunits and their mechanical properties. We have used keratin-rich wool fibers as a model of composite biological fibers; a wool fiber is a simple two component cylindrical system consisting of a core cellular component surrounded by an outer cell layer and their ultrastructure and chemical composition are well-characterized. The core is 16-40 micrometer in diameter and rich in axially aligned keratin microfibrils. Outer cells have multiple laminar layers, 60-600 nm thick and distinctly rich in disulfide bonds. We used an atomic force microscope (AFM) to examine the nanomechanical properties of various structural components using complementary techniques of force-volume imaging and nano-indentation. AFM images of transverse sections of fibers were obtained in ambient environment, and the mechanical properties of several identified regions were examined. The outer cell layer showed a significantly higher mechanical stiffness than the internal cellular core region. Chemical reduction of disulfide bonds eliminated such dichotomy of mechanical strengths, indicating that the higher rigidity of the outer layer is attributed primarily to the presence of extensive disulfide bonding in the exo-cuticle. This is the first detailed correlative study of nano-indentation and regional elasticity measurements in composite biological systems, including mammalian biological fibers.

Effect of the attractive interactions on the phase behavior of the Gay-Berne liquid crystal model Rotational diffusion of asymmetric molecules in liquid crystals: A global analysis of deuteron relaxation dataThe liquid crystalline phase diagrams for aqueous solutions of the homologous series of surfactants CF 3 ͑CF 2 ͒ n CO 2 Ϫ Cs ϩ (nϭ5 -8) have been determined. They all exhibit the classical isotropic (I)-to-discotic nematic (N D ) and N D -to-smectic lamellar (L) sequences of transitions with increasing concentration, as previously established for the nϭ6 system ͓N. Boden, S. A. Corne, and K. W. Jolley, J. Phys. Chem. 91, 4092 ͑1987͔͒. The effect of increasing n is to displace the transitions to higher temperatures. The behavior of all of the surfactant systems can be represented on a universal phase diagram. Both the I-to-N D and the N D -to-L transitions at corresponding concentrations are found to occur when the axial ratio of the disklike micelles attains a singular value in each case, irrespective of the value of n. The form of the experimental phase diagrams can be qualitatively understood in terms of a simple ''zeroth'' order theory which uses the results of Onsager's theory ͓L. Onsager, Ann. NY Acad. Sci. 51, 627 ͑1949͔͒ applied to disks to find the critical axial ratios in the coexisting isotropic and nematic phases and, separately, a dilute solution, self-assembly theory of disklike micelles ͓W. E. McMullen, A. Ben-Shaul, and W. M. Gelbart, J. Colloid Interface Sci. 98, 523 ͑1984͔͒ to determine the temperatures at which these axial ratios are achieved at each concentration. The same treatment is also shown to account for the experimental phase behavior of mixed-chain-length systems.

Magnetic force microscopy has been used to characterize the paramagnetic to ferromagnetic transition caused by carbide precipitation in a HPM alloy used in ethylene production. The MFM images are compared to conventional atomic force microscope micrographs. The element segregation in the alloy is quantified using scanning electron microscope energy dispersive x-ray analysis linescans, and compared to Curie temperature data for iron, nickel and chromium alloys.