Lynn M. Walker

Rheology and structure of worm-like micelles

Although worm-like micelles have been studied for over 20 years, the diversity of macroscopic behavior and the potential analogy to polyelectrolytes has driven continued study. During the last year, development and application of more realistic scattering models has yielded a deeper understanding of micelle structure. Comprehensive studies on systematic systems are being reported that combine structural and macroscopic data. These studies provide the fundamental understanding necessary to quantify the coupling between micelle structure and rheology. Finally, with increased understanding of these systems, there is a growth in the number of novel applications of worm-like micelles.

Surface tension driven jet break up of strain-hardening polymer solutions

Experimental studies attempting to ascertain the influence of viscoelasticity on the atomization of polymer solution are often hindered by the inability to decouple the effect of shear thinning from the effect of extensional hardening. Here, the influence of viscoelasticity on the jet break up of a series of non-shear-thinning viscoelastic fluids is quantified. Previous characterization using an opposed-nozzle rheometer identified the critical extensional rates for strain hardening of these model fluids. The strain hardening fluids exhibit a beads-on-string structure with reduction or elimination of satellite drops. Capillary instabilities grow on the filaments connecting the spheres and eventually break the filaments up into a string of very small drops about one order of magnitude smaller than the satellite drops formed by a Newtonian fluid with the same shear viscosity, surface tension, and density. These results confirm that strain hardening is the key rheological property in jet break up and that the critical extensional rate of a fluid is pertinent in determining the final characteristics of break up. Results suggest that the opposed-nozzle rheometer does probe extensional behavior in the range of extensional rates that are relevant to jet break up, providing a tool to roughly predict jet break up.

Role of geometry and fluid properties in droplet and thread formation processes in planar flow focusing

Droplet formation processes in microfluidic flow focusing devices have been examined previously and some of the key physical mechanisms for droplet formation revealed. However, the underlying physical behavior is still too poorly understood to utilize it for generating droplets of precise size. In this work, we formulate scaling arguments to define dimensionless variables which capture all the parameters that control the droplet breakup process, including the flow rates and the viscosities of the two immiscible fluids, the interfacial tension between the fluids and the numerous dimensions in the flow focusing device. To test these arguments, we perform flow focusing experiments and systematically vary the dimensional parameters. Through these experiments, we confirm the validity of the scaling arguments and find a power law relationship between the normalized droplet size and the capillary number. We demonstrate that droplet formation can be separated into an upstream process for primary droplet formation...

Shake-gels: shear-induced gelation of laponite–PEO mixtures

Suspensions of clay particles (laponite), mixed with poly(ethylene oxide) (PEO) undergo a dramatic shear thickening when subjected to vigorous shaking, which transforms them from a low viscosity fluid into a ‘shake-gel’, a solid with elasticity sufficient enough to support its own weight. The shake-gel is reversible, relaxing back to a fluid with a relaxation time that is strongly dependent on PEO concentration. Shake-gels are observed for PEO concentrations slightly below the threshold for complete saturation of the laponite particles by the polymer. Light scattering measurements confirm that the PEO is adsorbed on the surface of the laponite particles, and suggests that shear induces a bridging between the colloidal particles, resulting in a gel network which spans the system. Desorption of the polymer reduces the bridging and thus relaxes the network. # 2002 Elsevier Science B.V. All rights reserved.

Effect of fluid relaxation time of dilute polymer solutions on jet breakup due to a forced disturbance

In inertia-dominated breakup of a low-viscosity liquid jet, complex disintegration mechanisms lead to a polydisperse distribution of the sizes of droplets formed. Macromolecules in solution increase the extensional viscosity and suppress the formation of satellite drops. Large extensional stresses lead to, and prevent, viscoelastic filaments from breaking up (beads-on-string structure). The drainage rate of fluid from the filaments into the beads is constant and can be used to estimate the relaxation time of the fluid. The nature of capillary breakup due to an imposed disturbance is investigated as a function of disturbance wavelength-to-diameter ratio and initial disturbance amplitude. We identify the key dynamics of the process and its relation to the fluid relaxation time; this allows us to control satellite drop formation. There is a minimum fluid relaxation time for suppression of satellite drops. Above this relaxation time, suppression of satellite drops is a function of the disturbance parameters. ...

Rheology of region I flow in a lyotropic liquid‐crystal polymer: The effects of defect texture

A thorough rheological investigation into the region I flow of liquid‐crystalline hydroxypropylcellulose in water is reported. At low shear rates a region of shear thinning with a power‐law exponent 1/2 is observed over two decades in shear rate. Anomalous transient behavior is seen for applied stresses below the critical stress at the region I–II turnover such that the strain required to reach steady state increases as stress decreases. Recoil measurements show large recoverable strain and similar pseudostrain scaling in both regions I and II. The activation energy for the viscosity is also the same in regions I and II. This rheology combined with in situ flow‐small‐angle light scattering measurements of the defect texture size show the qualitative and quantitative accuracy of the defect texture stress balance previously proposed to explain region I flow.

Multisegmented Block Copolymers by 'Click' Coupling of Polymers Prepared by ATRP

Multisegmented block copolymers were prepared by the step-growth click coupling of well-defined block copolymers synthesized by atom transfer radical polymerization (ATRP). α,ω-Diazido-terminated polystyrene-block-poly(ethylene oxide)-block-polystyrene was coupled with propargyl ether in N,N-dimethylformamide in the presence of a CuBr/N,N,N´,N´´,N´´-pentamethyldiethylenetriamine catalyst. The preparation of multisegmented block copolymers was also demonstrated by the click coupling of propargyl ether with another diazido-terminated triblock copolymer, poly(n-butyl acrylate)-block-poly(methyl methacrylate)-block-poly(n-butyl acrylate), and a diazido-terminated pentablock copolymer, polystyrene-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-polystyrene. The formation of a product of higher molecular weight and broader molecular weight distribution was verified by triple-detection size exclusion chromatography, which revealed that typically five to seven block copolymers were linked together during the click reaction. Differential scanning calorimetry and dynamic mechanical analysis revealed that the amphiphilic block copolymer behaves as a viscoelastic fluid, while its corresponding multiblock copolymer is an elastic material. The multisegmented block copolymers with partially miscible segments exhibit higher glass transition temperatures than their precursors.

THE RHEOLOGY OF HIGHLY CONCENTRATED PBLG SOLUTIONS

We have characterized the rheology of two concentrated, liquid crystalline solutions of poly(γ benzyl–L–glutamate) of molecular weight 238 000 in m–cresol. Comparing these results to previous work on less concentrated, liquid crystalline solutions separates the effects of concentration from the direct influence of defect texture on liquid crystal polymer rheology. This work also defines the limitations of current polydomain models and suggests improvements. The solution at C=37wt% PBLG behaves similarly to moderately concentrated nematic solutions (12wt% <C< 30wt%) and much of the rheology can be described by a polydomain texture model. The relaxation behavior upon cessation of flow, including stress relaxation, moduli evolution and strain recoil, demonstrates that two different mechanisms with different dependence on concentration drive the relaxation. The higher concentration 40wt% PBLG solution exhibits Region I behavior and a smaller characteristic texture size. The material exhibits two steady state viscosity branches; the lower branch is probably the equilibrium branch while the upper is a metastable ‘‘glass–like’’ structure induced by shearing beyond a critical rate. The observed anomalous transient behavior for this sample concurs with Region I behavior observed for other lyotropic systems, suggesting universal behavior.

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces.

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.

Orthogonal versus parallel superposition measurements

Abstract The non-linear flow behaviour of viscoelastic fluids can be studied in detail by means of a perturbation analysis. For that purpose small amplitude oscillations can be superimposed on the steady-state shear flow. In this manner, detailed information about the spectral content of the material under the non-linear steady flow is obtained. The oscillatory flow can be either parallel or perpendicular to the main shear flow. Devices for both types are becoming readily available and apparently are being used without realizing the intricate nature of these flows. An analysis shows that linear superposition moduli do not obey the basic rules of linear viscoelasticity. This includes deviations from the Kramers–Kronig relation and from the usual relation between steady-state and dynamic viscosities. This is demonstrated on the basis of a Wagner I model for which analytical solutions of the superposition moduli can be derived. Other models give different results, consequently superposition flows could be used for the critical evaluation of rheological models. Preliminary data for both parallel and orthogonal superposition flows on a polyisobutene solution illustrate the potential of this technique. The relation between parallel and orthogonal superposition moduli derived by Bernstein for the K-BKZ model seems to be in agreement with the data. The results offer a potential for further theoretical work. The data also suggest that a physical interpretation of superposition moduli is not straightforward.