Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers

We briefly review the recent advances in the rheology of entangled polymers and identify emerging research trends and outstanding challenges, especially with respect to branched polymers. Emphasis is placed on the role of well-characterized model systems, as well as the synergy of synthesis-characterization, rheometry and modeling/simulations. The theoretical framework for understanding the observed linear and nonlinear rheological phenomena is the tube model which is critically assessed in view of its successes and shortcomings, whereas alternative approaches are briefly discussed. Finally, intriguing experimental findings and controversial issues that merit consistent explanation, such as shear banding instabilities, multiple stress overshoots in transient simple shear and enhanced steady-state elongational viscosity in polymer solutions, are discussed, whereas future directions such as branch point dynamics and anisotropic monomeric friction are outlined.Comment: 25 pages, accepted for publication in Journal of Physics Condensed Matter (August 2015

Three Experiments on Complex Fluids

The behaviour of complex fluids is fundamentally interesting and important in many applications. This thesis reports on three experiments on the thermal and rheological behaviour of complex fluids. The first is a study of the rheological properties of and heat transport in a saline solution of hydroxyethyl cellulose. This material has been used as a tissue phantom in testing the behavior of medical devices in MRI scanners. We find it behaves as a typical entangled polymer, and flows in response to local heating, such as could occur due to eddy-current heating of metallic devices in an MR scanner. We use laboratory experiments and numerical simulations to determine the convective and conductive contributions to the heat transport in a simple model of this system. Our results indicate that convective heat transport is of the same order of magnitude as conductive transport under conditions typical of MRI device tests. The second project is an investigation of the start-up flow and yielding of a simple yield-stress fluid (Carbopol 940) in a vertical pipe. The Carbopol was displaced from below by an immiscible Newtonian liquid (Fluorinert FC-40) injected at a constant, controlled rate. Rough and smooth-walled pipes were used to study the effects of wall boundary conditions. In the rough-walled pipe, the yielding involved a long transient with several steps: elastic deformation, the onset of wall slip, yielding at the wall, and finally a steady-state plug flow that is well-described by the predictions of the Herschel-Bulkley model. In contrast, in the smooth-walled pipe, the wall shear stress never exceeded the yield stress. In the third project, we study the flow of Carbopol solutions confined to square microchannels with sides ranging from 500 down to 50 um. In the larger channels, the measured velocity profiles agreed well with simulations based on the bulks-scale rheology of the Carbopol and the Herschel-Bulkley model. In contrast, in microchannels with sides less than 150 um the velocity profiles could not be fitted by a model with a finite yield stress, but instead were described by a power-law model with zero yield stress. We explain the vanishing of the yield stress in terms of the confinement of the Carbopol’s microstructure by the microchannels

Shape evolution of 3D periodic structure fabricated by direct-write assembly of concentrated colloidal gels

Scope and Method of Study: 3D periodic structures were fabricated by direct-write assembly of concentrated colloidal gels with self-supporting features. The rheological behavior of the gel was characterized in linear viscoelastic regions. The flow behavior of the gel was modeled by using structural kinetics theory. Based on this model, the dynamic extrusion process of the gel was simulated by incorporating slip wall boundary conditions. A viscoelastic catenary model was developed to describe span shape and compare the results to previous results that used a simple elastic beam theory. The shape evolution (i.e., spanning behavior) of spanning filaments observed was related to shear stress conditions and a limited set of rheological parameters.Findings and Conclusions: The rate and magnitude of microstructure change within a colloidal gel ink are crucial factors for shape evolution of 3D structures assembled by direct write techniques. The events that set the equilibrium shape of 3D structure occur within the initial few seconds after deposition and gels microstructure recovery within this period is critical to geometric fidelity. The shape evolution of 3D structures may be predicted by knowledge of the rheological behavior of the colloidal gel in shear loading. Rheological behavior can be related to the structural recovery time of the colloidal gel and this may be measured with a series of equilibrium flow measurements. Successful completion of this research advances science-based ink design methods and optimization of deposition variables. Better control of shape evolution will lead to improvements in advanced applications such as photonic band gap structures, artificial bone structures, and metal-ceramic composites. The improved connections between time-dependent shear behavior and shape evolution in an extrusion process will also impact other industries (e.g., clay extrusion for catalytic converter substrates) and improve industrial productivity through better paste design. Although the current work is limited to colloidal gels, the knowledge gained here may be easily extended to other complex ink systems such as partially melted thermoplastic polymers and metals

Non-linear oscillatory rheological properties of a generic continuum foam model: comparison with experiments and shear-banding predictions

The occurence of shear bands in a complex fluid is generally understood as resulting from a structural evolution of the material under shear, which leads (from a theoretical perspective) to a non-monotonic stationnary flow curve related to the coexistence of different states of the material under shear. In this paper we present a scenario for shear-banding in a particular class of complex fluids, namely foams and concentrated emulsions, which differs from other scenarii in two important ways. First, the appearance of shear bands is shown to be possible both without any intrinsic physical evolution of the material (e.g. via a parameter coupled to the flow such as concentration or entanglements) and without any finite critical shear rate below which the flow does not remain stationary and homogeneous. Secondly, the appearance of shear bands depends on the initial conditions, i.e., the preparation of the material. In other words, it is history dependent. This behaviour relies on the tensorial character of the underlying model (2D or 3D) and is triggered by an initially inhomogeneous strain distribution in the material. The shear rate displays a discontinuity at the band boundary, whose amplitude is history dependent and thus depends on the sample preparation.Comment: 18 pages - 17 figure

不均一な絡み合い高分子系のレオロジー

Tohoku University川勝年洋課

Thermal fluid models of a hydrogel delivery system for pancreatic cancer treatment

Pancreatic cancer is one of the most devastating cancers with low survival rates. This disease is difficult to detect due to the pancreas\u27s location deep within the body. Therefore, diagnoses are often made in the later stages, making treatment options more limited and difficult. It has been hypothesized that direct injection into the tumor would enhance drug effectivenes. Therefore, we examined the use of endoscopic ultrasound (EUS) combined with a fine needle injection to deliver a drug-eluding thermosensitive hydrogel directly into the tumor. Unfortunately normal body temperatures surrounding the EUS can warm the hydrogel drug combination beyond its phase transition temperature or lower critical solution temperature (LCST) before its final destination inside the tumor. A modified version of FocalCool\u27s technology CoolGuide(TM) catheter, now called the CoolGuide(TM) sheath, will be used to provide temperature control along the injection pathway, ensuring that the hydrogel remains below its phase transition temperature LCST. The objective of this work is to build and explore thermal fluid models of a temperature controlled device using a finite volume conjugate heat transfer approach. Using experimental results for validation we intend to demonstrate that the sheath has the ability to control and deliver 30% hydrogel (Pluronic F127) below its LCST under body temperature conditions. While these experiments are instrumental in the development of successful in vitro testing to help patients with pancreatic cancer, modeling will allow a broader range of possible designs for the CoolGuide(TM) sheath to deliver hydrogel deeper inside the body. This new drug delivery system will provide the necessary data to achieve a successful in vivo safe testing

A mesoscopic model for the rheology of soft amorphous solids, with application to mi- crochannel flows

We study a mesoscopic model for the flow of amorphous solids. The model is based on the key features identified at the microscopic level, namely peri- ods of elastic deformation interspersed with localised rearrangements of parti- cles that induce long-range elastic deformation. These long-range deformations are derived following a continuum mechanics approach, in the presence of solid boundaries, and are included in full in the model. Indeed, they mediate spatial cooperativity in the flow, whereby a localised rearrangement may lead a distant region to yield. In particular, we simulate a channel flow and find manifestations of spatial cooperativity that are consistent with published experimental obser- vations for concentrated emulsions in microchannels. Two categories of effects are distinguished. On the one hand, the coupling of regions subject to different shear rates, for instance,leads to finite shear rate fluctuations in the seemingly un- sheared "plug" in the centre of the channel. On the other hand, there is convinc- ing experimental evidence of a specific rheology near rough walls. We discuss diverse possible physical origins for this effect, and we suggest that it may be associated with the bumps of particles into surface asperities as they slide along the wall

Modeling and Simulation of Mucus Flow in Human Bronchial Epithelial Cell Cultures – Part I: Idealized Axisymmetric Swirling Flow

A multi-mode nonlinear constitutive model for mucus is constructed directly from micro- and macro-rheology experimental data on cell culture mucus, and a numerical algorithm is developed for the culture geometry and idealized cilia driving conditions. This study investigates the roles that mucus rheology, wall effects, and HBE culture geometry play in the development of flow profiles and the shape of the air-mucus interface. Simulations show that viscoelasticity captures normal stress generation in shear leading to a peak in the air-mucus interface at the middle of the culture and a depression at the walls. Linear and nonlinear viscoelastic regimes can be observed in cultures by varying the hurricane radius and mean rotational velocity. The advection-diffusion of a drug concentration dropped at the surface of the mucus flow is simulated as a function of Peclet number