Volume 50, Issue 1, January 2006
Index of content:
Transient rheology of solvent-responsive complex fluids by integrating microrheology and microfluidics50(2006); http://dx.doi.org/10.1122/1.2135329View Description Hide Description
A new microrheology set-up which allows us to quantitatively measure the transient rheological properties of solvent-responsive complex fluids was constructed by integrating particle tracking microrheology and microfluidics. The dialysis cell consists of a reservoir, porous dialysis membrane, and sample chamber. Solvent molecules can freely diffuse between the reservoir and the sample chamber while macromolecular sample components are trapped in the sample chamber with a rigid semipermeable dialysis membrane. The design enables manipulation of the solvent composition in the sample chamber by simply switching the fluid composition in the reservoir. Validation experiments for solvent diffusion in the dialysis cell showed good agreement with numerical solutions of the transport equations and confirmed that the solvent composition in a sample can be changed in a controlled and predictable fashion within a few minutes due to the small device dimensions. For aqueous solutions of sodium alginate and sodium polystyrene sulfonate, transient rheological properties were measured in response to changes in salt concentration. The results showed that the dialysis cell is a useful and versatile tool to study the response of complex fluids to reversible changes in solvent composition.
50(2006); http://dx.doi.org/10.1122/1.2127907View Description Hide Description
The linear viscoelastic properties of binary blends of monodisperse linear polymers of various polymers, including polyisoprene, polybutadiene, and polystyrene, are compared with the predictions of the model of Park and Larson (2004), which is an extension of the Milner-McLeish model [Milner and McLeish (1998)]. For each polymer system the model parameters needed in the predictions of the binary blend are obtained from experimental data for monodisperse polymers. For well entangled blends of nearly monodisperse polymers, the model predictions are in good agreement with experimental data for a wide range of values of the Graessley parameter Gr, which is proportional to the ratio of the reptation time of the long chain in the undilated tube to the constraint-release Rouse time. Our results show that for polymers of multiple chemical types, the terminal behavior of the long chain is controlled by reptation in an undilated tube for small , and by reptation in a dilated tube for , where is around 0.06. For values of Gr near , behavior intermediate between reptation in a dilated and an undilated tube is found.
50(2006); http://dx.doi.org/10.1122/1.2135330View Description Hide Description
Slip occurs at the interfaces between immiscible polymer melts at high shear stress. We demonstrate that this reduces adhesion during coextrusion. A 20-layer polystyrene (PS)/poly(methyl methacrylate) (PMMA) alternating layer sample was coextruded and the adhesion at each internal interface was measured with the asymmetric dual cantilever beam crack propagation test. When the shear stress experienced by an interface is low, interfacial slip is negligible and interfacial adhesion is high, comparable to a laminated interface. When the shear stress exceeds a critical value, interfacial slip begins to develop and interfacial adhesion begins to decrease with shear stress. Above another critical stress, full slip has been developed at the interface and adhesion reaches a plateau value, which is about of the equilibrium value. The changes in adhesion versus shear stress follow a master curve for different flow rates. This supports the hypothesis that polymer chains at the interface are disentangled by the shear stress during coextrusion. It was also found that annealing restored adhesion on the reptation time scale indicating that entanglements were reestablished at the interface. Creating block copolymer by a coupling reaction at the interface during coextrusion increased adhesion to level even higher than the laminated interface.
50(2006); http://dx.doi.org/10.1122/1.2135331View Description Hide Description
The stress relaxation behavior of a well-entangled polystyrene following step shear deformations is investigated. New experimental data are reported for the relaxation of shear stress and first normal stress difference that are not affected by imperfect strain history or transducer compliance. Anomalous (type C) shear stress relaxation behavior is observed indicating significant discrepancies from the tube model prediction for the damping function. First normal stress difference data are used to evaluate the Lodge-Meissner relation and small deviations are observed. We find that slip between the test fluid and cone-and-plate fixtures is responsible for the observed type C behavior, but only leads to small departures from the Lodge-Meissner relation. Simulations of step strain flows with wall slip show qualitative agreement with experimental results. In addition, we examine the issue of polydispersity in the polystyrene standards used to prepare well-entangled solutions for this and similar studies.
50(2006); http://dx.doi.org/10.1122/1.2139098View Description Hide Description
The microrheology of a colloidalsuspension is measured using laser tweezers. Suspensions of refractive index-matched fluorinated ethylene propylene (FEP) particles are seeded with index-mismatched polystyrene or silica probe particles. Laser trapped probes are then subjected to steady uniform flows, enabling measurements of the suspension microviscosity as a function of FEP volume fraction and flow velocity. The microrheology results agree with bulk rheology, and both exhibit the same volume fraction dependence of the Krieger-Dougherty relationship for hard spheres. As volume fraction increases, the microrheology more closely agrees with the infinite shear bulk viscosity. In this regime, measurements using small probes exhibit additional shear thinning. Using confocal microscopy and fluorescent poly(methylmethacrylate) dispersions, we demonstrate that the nonlinear microrheology is consistent with the development of an anisotropic nonequilibrium pair distribution function between the probe and bath particles, with a denser region at the leading surface of the probe and a wake trailing it. The nonlinear response and underlying microstructure are in qualitative agreement with recent theory [T. M. Squires and J. F. Brady, Phys. Fluids17, 073101 (2005)].