http://hdl.handle.net/1721.1/7624 2013-05-18T08:24:12Z http://hdl.handle.net/1721.1/33792 Ex vivo Rheology of Spider Silk Kojic, Nikola; Bico, Jose; Clasen, Christian; McKinley, Gareth H. We investigate the rheological properties of microliter quantities of the spinning material extracted ex vivo from the major ampullate gland of a Nephila clavipes spider using two new micro-rheometric devices. A sliding plate micro-rheometer is employed to measure the steady-state shear viscosity of ~1µL samples of silk dope from individual biological specimens. The steady shear viscosity of the spinning solution is found to be highly shear-thinning with a power-law index consistent with values expected for liquid crystalline solutions. Calculations show that the viscosity of the fluid decreases ten-fold as it flows through the narrow spinning canals of the spider. By contrast, measurements in a microcapillary extensional rheometer show that the transient extensional viscosity (i.e. the viscoelastic resistance to stretching) of the spinning fluid increases more than one hundred-fold during the spinning process. Quantifying the properties of native spinning solutions provides new guidance for adjusting the spinning processes of synthetic or genetically-engineered silks to match those of the spider. Submitted to Proceedings of the Royal Society B 2005-08-05T15:01:33Z http://hdl.handle.net/1721.1/31211 Thermorheological Properties Near the Glass Transition of Oligomeric Poly(methyl methacrylate) Blended with Acrylic Polyhedral Oligomeric Silsesquioxane Nanocages Kopesky, Edward T.; Boyes, Stephen G.; Treat, Neil; Cohen, Robert E.; McKinley, Gareth H. Two distinct oligomeric species of similar mass and chemical functionality (Mw ≈ 2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration Rg ≈ 1.1 nm) and the other a hybrid organic-inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r ≈ 1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (Tg = −47.3°C) and the pure POSS (Tg = −61.0°C). Blends containing high POSS contents (with volume fraction φ_POSS ≥ 0.90) exhibited enhanced enthalpy relaxation in DSC measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m = 59) and the POSS (m = 74). The temperature dependences of the viscosities were fitted by the free volume-based WLF-VFT framework and a dynamic scaling relation. The calculated values of the fragility from the WLF-VFT fits were similar for the POSS (m = 82) and for the oligomer (m = 76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids, the temperature dependence of the viscosity was found to be similarly fragile for the two species. The difference in shape of the nanocages and oligomer chains is unimportant in controlling the glass-forming properties of the blends at low volume fractions ( φPOSS < 0.20); however, at higher volume fractions, adjacent POSS cages begin to crowd each other, leading to an increase in the fractional free volume at the glass transition temperature and the observed enhanced enthalpy relaxation in DSC. Submitted to Rheologica Acta 2005-08-26T16:50:23Z http://hdl.handle.net/1721.1/31210 Iterated Stretching, Extensional Rheology and Formation of Beads-on-a-String Structures in Polymer Solutions Oliveira, Monica S. N.; Yeh, Roger; McKinley, Gareth H. The transient extensional rheology and the dynamics of elastocapillary thinning in aqueous solutions of polyethylene oxide (PEO) are studied with high-speed digital video microscopy. At long times, the evolution of the thread radius deviates from self-similar exponential decay and competition between elastic, capillary and inertial forces leads to the formation of a periodic array of beads connected by axially-uniform ligaments. This configuration is unstable and successive instabilities propagate from the necks connecting the beads and ligaments. This iterated process results in multiple generations of beads developing along the string in general agreement with predictions of Chang et al. [Phys Fluids, 11, 1717 (1999)] although the experiments yield a different recursion relation between the successive generations of beads. At long times, finite extensibility truncates the iterated instability, and slow axial translation of the bead arrays along the interconnecting threads leads to progressive coalescence before the ultimate rupture of the fluid column. Despite these dynamical complexities it is still possible to measure the steady growth in the transient extensional viscosity by monitoring the slow capillarydriven thinning in the cylindrical ligaments between beads. Accepted for publication in JNNFM, December 2005. 2005-12-01T16:48:17Z http://hdl.handle.net/1721.1/31209 Nonlinear Shear and Extensional Flow Dynamics of Wormlike Surfactant Solutions Yesilata, B.; Clasen, Christian; McKinley, Gareth H. Nonlinear shear and extensional flow dynamics of rheological properties of a wormlike micellar solution based on erucyl bis (2-hydroxyethyl) methyl ammonium chloride, EHAC, are reported here. The influences of surfactant (EHAC) and salt (NH4Cl) concentrations on the linear viscoelastic parameters are determined using small amplitude oscillatory shear experiments. The steady and time-dependent shear rheology is determined in a double gap Couette cell, and transient extensional flow measurements are performed in a Capillary Breakup Extensional Rheometer (CABER). In the nonlinear shear flow experiments, the micellar fluid samples show strong hysteretic behavior upon increasing and decreasing the imposed shear stress due to the development of shear-banding instabilities. The non-monotone flow curves of stress vs. shear rate can be successfully modeled in a macroscopic sense by using the single-mode Giesekus constitutive equation. The temporal evolution of the flow structure of the surfactant solutions in the Couette flow geometry is analyzed by instantaneous shear-rate measurements for various values of controlled shear-stress, along with FFT analysis. The results indicate that the steady flow bifurcates to a global time-dependent state as soon as the shear banding/hysteresis regime is reached. Increasing the salt/surfactant ratio or the temperature is found to stabilize the flow, as also confirmed by the decreasing values of anisotropy factor in the Giesekus model. Finally we have investigated the dynamics of capillary breakup of the micellar fluid samples in uniaxial extensional flow. The filament thinning behavior of the micellar fluid samples is also accurately predicted by the Giesekus constitutive equation. Indeed quantitative agreement between the experimental and numerical results can be obtained providing that the relaxation time of the wormlike micellar solutions in extensional flows is a factor of three lower than in shear flows. Accepted for publication in JNNFM. 2005-10-04T16:46:48Z