Volume 59, Issue 4, July 2015
Index of content:
Determination of characteristic lengths and times for wormlike micelle solutions from rheology using a mesoscopic simulation method59(2015); http://dx.doi.org/10.1122/1.4919403View Description Hide Description
We apply our recently developed mesoscopic simulation method for entangled wormlike micelle (WLM) solutions to extract multiple micellar characteristic lengths and time constants: i.e., average micelle length, breakage rate, and entanglement and persistence lengths, from linear rheological measurements on commercial surfactant solutions, one containing sodium lauryl one ether sulfate (SLE1S), and the other containing both SLE1S and cocamidopropyl betaine, as well as a perfume mixture, in both cases with a sample salt (NaCl) added. Measurements include both mechanical rheometry and diffusing wave spectroscopy, the latter providing the high-frequency data needed to determine micelle persistence length accurately. By fitting the experimental data ( and ) across the entire frequency range through our iteration procedure, the method is of practical use in predicting micellar parameters, which are difficult to obtain from other theoretical or experimental methods. The dependence of micellar parameters on added salt concentration, and the effect of micelle breakage mechanisms on viscoelasticity of WLM solutions, are also discussed.
59(2015); http://dx.doi.org/10.1122/1.4919405View Description Hide Description
We report in situ small-angle x-ray scattering studies of a prealigned cylindrically ordered styrene-ethylene butylene-styrene block copolymer melt subjected to extensional flow. Samples are prepared via lubricated planar extensional flow and tested using two initial conditions: “parallel” with cylindrical microdomains oriented along the extensional flow direction and “perpendicular” with domains aligned transverse to the flow direction. The experiments employ a counter-rotating drum extensional flow fixture housed in an oven designed for in situ synchrotron access. The impacts of initial condition, extension rate, and final Hencky strain on the melt structure are analyzed both during and following flow. Stress and flow kinematics are strongly influenced by the initial sample orientation. While parallel samples exhibit uniaxial deformation, perpendicular samples exhibit planar extensional kinematics, attributed to susceptibility to compression along the cylindrical microdomain axis due to a microscopic buckling instability. Scattering data reveal both anisotropic deformation of microdomain spacing and reorientation induced by the flow. Persistence of higher order reflections confirms hexagonal packing throughout the flow process, and strong alignment along the stretching direction is attained for both initial conditions. Flow-induced deformation of microdomain spacing and mechanical stress relax on similar time scales upon flow cessation, while negligible relaxation of orientation is observed.
59(2015); http://dx.doi.org/10.1122/1.4919970View Description Hide Description
The flow of dry and wet granular media is investigated in a Couette geometry using magnetic resonance imaging in order to test the applicability of the “fluidity model” for nonlocality in these materials. Local volume fraction measurements show that the systems become heterogeneous during flow. We find that the nonlocal rheology of suspensions can be correlated using the fluidity model, but the length scale that emerges is not a material property and the model cannot be used for predictive purposes. Rather, the suspension behavior is fully explained as a consequence of stress-driven particle migration and the resulting concentration gradient. The conclusion is less strong for the dry granular system, but it appears likely that the apparent nonlocal behavior is simply due to the formation of a shear band caused by granular dilatancy.