https://hdl.handle.net/1721.1/7624 2026-04-17T08:18:05Z https://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 https://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 https://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 https://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 https://hdl.handle.net/1721.1/25319 Rheology and Dynamics of Associative Polymers in Shear and Extension: Theory and Experiment Tripathi, Anubhav; Tam, Kam C.; McKinley, Gareth H. We investigate the steady and transient shear and extensional rheological properties of a series of model hydrophobically modified ethoxylate-urethane (HEUR) polymers with varying degrees of hydrophobicity. A new nonlinear two-species network model for these telechelic polymers is described which incorporates appropriate molecular mechanisms for the creation and destruction of elastically-active chains. Like other recent models we incorporate the contributions of both the bridging chains (those between micelles) and the dangling chains to the final stress tensor. This gives rise to two distinct relaxation time scales; a short Rouse time for the relaxing chains and a longer network time–scale that depends on the aggregation number and strength of the micellar junctions. The evolution equations for the fraction of elastically-active chains and for the conformation tensors of each species are solved to obtain the total stress arising from imposed deformations. The model contains a single non-linear parameter and incorporates the non-linear chain extension, the shear-induced enhancement of associations and the stretch-induced dissociation of hydrophobic chains. In contrast to earlier closed-form models, we are able to obtain quantitative agreement between experimental measurements and the model predictions for three different series of telechelic polymers over a range of concentrations. The scaling of both the zero shear viscosity and the effective network relaxation time show good agreement with those measured in experiments. The model also quantitatively captures both the shear-thickening and subsequent shear-thinning observed in the rheology at high deformation rates and predicts transient extensional stress growth curves in close agreement with those measured using a filament stretching rheometer. Submitted to Macromolecules, July 2005 2005-08-22T19:34:56Z https://hdl.handle.net/1721.1/25318 Controlling the Location and Spatial Extent of Nanobubbles Using Hydrophobically Nanopatterned Surfaces Agrawal, Abhinandan; Park, Juhyun; Ryu, Du Y.; Hammond, Paula T.; Russell, Thomas P.; McKinley, Gareth H. The presence of nanobubbles—as imaged with tapping-mode atomic force microscopy— is controlled using nanopatterned surfaces possessing repeating patterns of polystyrene (hydrophobic domains) and polymethyl-methacrylate (hydrophilic domains). For nanobubbles to be present, we find that, in addition to controlling the degree of surface hydrophobicity, it is important for the spatial dimensions of the hydrophobic domains on the nanopatterned surface to be commensurate with the equilibrium topology of the nanobubbles. 2005-08-22T19:33:42Z https://hdl.handle.net/1721.1/18153 Two-Dimensional Self-Assembly in Diblock Copolymers Hosoi, A.E.; Kogan, Dmitriy; Devereaux, C.E.; Bernoff, Andrew J.; Baker, S.M. Diblock copolymers confined to a two-dimensional surface may produce uniform features of macromolecular dimensions (10 − 100 nm). We present a mathematical model for nanoscale pattern formation in such polymers which captures the dynamic evolution of a solution of poly(styrene)- b-poly(ethylene oxide), PS-b-PEO, in solvent at an air-water interface. The model has no fitting parameters and incorporates the effects of surface tension gradients, entanglement or vitrification, and diffusion. The resultant morphologies are quantitatively compared with experimental data. Submitted to Phys. Rev. Lett. 2005-06-20T20:01:31Z https://hdl.handle.net/1721.1/18087 Slippage and Migration in Taylor-Couette Flow of a Model for Dilute Wormlike Micellar Solutions Rossi, Louis F.; McKinley, Gareth H.; Cook, L. Pamela In this paper we explore a model, most appropriate for dilute or semi-dilute worm-like micellar solutions, in an axisymmetric circular Taylor-Couette geometry. This study is a natural continuation of earlier work on rectilinear shear flows. The model, based on a bead-spring microstructure with nonaffine motion, reproduces the pronounced plateau in the stress strain-rate flow curve as observed in laboratory measurements of steady shearing flows. We also carry out a linear stability analysis of the computed steady state solutions. The results show shear-banding in the form of sharp changes in velocity gradients, spatial variations in number density, and in alignment or stretching of the micelles. The velocity profiles obtained in numerical solutions show strong qualitative agreement with those of laboratory experiments. Submitted to J. Non-Newt Fluid Mechanics, June 2005 2005-06-09T19:49:57Z https://hdl.handle.net/1721.1/18086 Dimensionless Groups For Understanding Free Surface Flows of Complex Fluids McKinley, Gareth H. No abstract Submitted to Bulletin of the Society of Rheology, May 2005 2005-06-09T19:47:46Z https://hdl.handle.net/1721.1/18085 Visco-Elasto-Capillary Thinning and Break-Up of Complex Fluids McKinley, Gareth H. The progressive break-up of an initially stable fluid column or thread into a number of smaller droplets is an important dynamical process that impacts many commercial operations from spraying and atomization of fertilizers and pesticides, to paint application, roll-coating of adhesives and food processing operations such as container- and bottle-filling. The progressive thinning of a fluid filament is driven by capillarity and resisted by inertia, viscosity and additional stresses resulting from the extensional deformation of the fluid microstructure within the thread. In many processes of interest the fluid undergoing break-up is non-Newtonian and may contain dissolved polymer, suspended particles, surfactants or other microstructural constituents. In such cases the transient extensional viscosity of the fluid plays an important role in controlling the dynamics of break-up. The intimate connection between the degree of strain-hardening that develops during free extensional flow and the dynamical evolution in the profile of a thin fluid thread is also manifested in heuristic concepts such as ‘spinnability’, ‘tackiness’ and ‘stringiness’. In this review we survey recent experimental and theoretical developments in the field of capillarydriven thinning and break-up with a special focus on how quantitative measurements of the thinning and rupture processes can be used to quantify the material properties of the fluid. As a result of the absence of external forcing the dynamics of the necking process are often self-similar and observations of this ‘self-thinning’ can be used to extract qualitative, and even quantitative, measures of the transient extensional viscosity of a complex fluid. Submitted to Annual Rheology Reviews, 2005. 2005-06-09T19:46:59Z https://hdl.handle.net/1721.1/16543 Shock Dynamics in Particle-Laden Thin Films Dupuy, B.; Bertozzi, A.L.; Hosoi, A.E. We present theory and experiments for thin film particle-laden flow on an incline. At higher particle concentration and inclination angle, a new phenomenon is observed in which a large particle-rich ridge forms at the contact line. We derive a lubrication theory for this system which is qualitatively compared to preliminary experimental data. The ridge formation arises from the creation of two shocks due to the differential transport rates of fluid and particles. This parallels recent findings of double shocks in thermal-gravity driven flow [A. L. Bertozzi et. al., PRL, 81, 5169 (1998), J. Sur et. al., PRL 90, 126105 (2003), A. M¨unch, PRL 91, 016105 (2003)]. However, here the emergence of the shocks arises from a new mechanism involving the settling rates of the species. PRL 94(11) March 25, 2005 117803 2005-04-22T15:46:39Z https://hdl.handle.net/1721.1/16534 Miscibility and Viscoelastic Properties of Acrylic Polyhedral Oligomeric Silsesquioxane-Poly(methyl methacrylate) Blends Kopesky, Edward T.; Haddad, Timothy S.; McKinley, Gareth H.; Cohen, Robert E. We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d ≈ 2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to φ = 0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature Tg and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings φ < 0.10. At a loading φ = 0.05, the unmodified acrylic POSS induces a 4.9°C decrease in the <i>T</i>_g of PMMA, far less than the 17.4°C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS-PMMA blend compared with the DOP-PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume <i>f</i>₀ = 0.046 for PMMA at <i>T</i>₀ = 170°C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA <i>f</i>₀ = 0.057, which corresponds to an addition of 0.47 nm³ per added POSS molecule at φ = 0.05. The degree of dispersion was characterized using both wide-angle x-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at φ = 0.20 and higher, but no significant phase separation is evident at φ = 0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings φ = 0.10, while no evidence of phase separation is present in the φ = 0.05 blend in DMA. Submitted to POLYMER, January 2005 2005-04-19T11:13:52Z https://hdl.handle.net/1721.1/16532 Iterated Stretching and Multiple Beads-on-a-String Phenomena in Dilute Solutions of Highly-Extensible Flexible Polymers Oliveira, Monica S. N.; McKinley, Gareth H. The dynamics of elastocapillary thinning in high molecular weight polymer solutions are re-examined using high-speed digital video microscopy. At long times, the evolution of the viscoelastic thread deviates from self-similar exponential decay and competition of elastic, capillary and inertial forces leads to the formation of a periodic array of beads connected by axially-uniform ligaments. This configuration is itself unstable and successive instabilities propagate from the necks connecting the beads and ligaments. This iterated process results in the development of multiple generations of beads in agreement with predictions of Chang et al.1, although experiments yield a different recursion relation between successive generations. At long times, finite extensibility truncates the iterated instability and axial translation of the bead arrays along the interconnecting threads leads to progressive coalescence before rupture of the fluid column. Preprint submitted to Phys. Fluids (Letters) 2005-04-13T02:27:24Z