Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H7: Non-Newtonian Flows I |
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Chair: Anette Hosoi, Massachusetts Institute of Technology Room: 24C |
Monday, November 19, 2012 10:30AM - 10:43AM |
H7.00001: Flow structure of polymer solutions close to walls, at less than one correlation length Patrick Tabeling, Zhenzhen Li, Marc Yonger, Fabrice Monti, Emmanuel Terriac, Choongyeop Lee We measure flow profiles in semi-dilute PEO solutions at distances less than a correlation length, close to smooth walls, using an evanescent wave technique we have been developing in the lab for a number of years. We observed unexpected flow structures in the first two correlation lengths from the wall, albeit consistent with recent numerical studies. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H7.00002: Interface Oscillation in the Side-by-Side (SBS) Tape Casting of Functionally Graded Ceramics (FGCs) Masoud Jabbari, Regina Bulatova, Jesper Hattel, Christian Bahl Room temperature magnetic refrigeration is a new highly efficient and environmentally protective technology. Although it has not been maturely developed, it shows great applicable prosperity and seems to be a potential substitute for the traditional vapor compression technology. Tape Casting is a common process in producing multilayer ceramics, which now is used for producing side-by-side (SBS) functionally graded ceramics (FGCs). These FGCs are mostly used in the magnetic refrigeration sectors due to the varying composition of the magnetocaloric materials so that the magnetic transition temperature of the magnetic regenerator varies along the paths. The main goal of this research is to study the multiple material flow in SBS tape casting and analyze its influence on the interface between the stripes. The materials used for the experimental part are $La_{0.85}Sr_{0.15}MnO_{3}$ and $Ce_{0.9}Gd_{0.1}O_{2}$ ceramic slurries. The rheological behavior of the slurries are extracted from experiments and used in the ANSYS FLUENT commercial code to develop a fluid flow model for the non-Newtonian ceramic slurries and evaluate the interface oscillation between the stripes in SBS tape casting. The Numerical results show reasonable agreement with corresponding experimental results. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H7.00003: Surface Characterization of pNIPAM Under Varying Absolute Humidity Arnav Chhabra, Ravitej Kanapuram, Harrison Leva, Juan Trejo, Tae Jin Kim, Carlos Hidrovo Poly(N-isopropylacrylamide) has become ubiquitously known as a ``smart'' polymer, showing many promising applications in tissue engineering and drug delivery systems. These applications are particularly reliant on its trenchant, thermally induced hydrophilic-hydrophobic transition that occurs at the lower critical solution temperature (LCST). This feature imparts the pNIPAM programmable adsorption and release capabilities, thus eliminating the need for additional enzymes when removing cells from pNIPAM coated surfaces and leaving the extracellular matrix proteins of the cells largely untouched. The dependence of the LCST on molecular weight, solvent systems, and various salts has been studied extensively. However, what has not been explored is the effect of humidity on the characteristic properties of the polymer, specifically the LCST and the magnitude of the hydrophilic-hydrophobic transition. We studied the surface energy variation of pNIPAM as a function of humidity by altering the absolute humidity and keeping the ambient temperature constant. Our experiments were conducted inside a cuboidal environmental chamber with control over the temperature and humidity inside the chamber. A controlled needle was employed to dispense size-regulated droplets. Throughout this process, a CCD camera was used to image the droplet and the static contact angle was determined using image processing techniques. The behavior of pNIPAM as a function of humidity is presented and discussed. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H7.00004: Particle Image Velocimetry During Injection Molding Thomas Bress, David Dowling Injection molding involves the unsteady non-isothermal flow of a non-Newtonian polymer melt. An optical-access mold has been used to perform particle image velocimetry (PIV) on molten polystyrene during injection molding. Velocimetry data of the mold-filling flow will be presented. Statistical assessments of the velocimetry data and scaled residuals of the continuity equation suggest that PIV can be conducted in molten plastics with an uncertainty of $\pm$2 percent. Simulations are often used to model polymer flow during injection molding to design molds and select processing parameters but it is difficult to determine the accuracy of these simulations due to a lack of in-mold velocimetry and melt-front progression data. Moldflow was used to simulate the filling of the optical-access mold, and these simulated results are compared to the appropriately-averaged time-varying velocity field measurements. Simulated results for melt-front progression are also compared with the experimentally observed flow fronts. The ratio of the experimentally measured average velocity magnitudes to the simulation magnitudes was found on average to be 0.99 with a standard deviation of 0.25, and the difference in velocity orientations was found to be 0.9 degree with a standard deviation of 3.2 degrees. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H7.00005: Quantitative analysis of the debonding structure of soft adhesives Matteo Nicoli, Francois Tanguy, Costantino Creton Pressure sensitive adhesives (PSAs) are viscoelastic or viscoplastic materials that adhere to a substrate upon the application of light pressure. The debonding mechanism is an interfacial process due to different phenomena, e.g. the creation of cavities and fibrils, the propagation of interfacial cracks and the lateral invasion of air fingers. The studies of adhesive performances of PSAs are carried out through the probe tack tester. We developed a boundary recognition algorithm to analyze the top-view images from probe tack experiments, allowing us to detect the nucleation of cavities, track their growth and measure various geometrical quantities. We tested three PSAs with different viscoelastic features, ranging from a more liquid to a more elastic behavior, at two debonding velocities ($1,10\, \mu m s^{-1}$). We measured the load bearing area and estimated the magnitude of the shear stress from the nominal force and the uniaxial tensile stress. From the characterization of the projected radius of each bubble, we tested the assumption of spherical growth of these cavities after the onset of their nucleation. The probe tack test combined with our methodology provides valuable data to understand the interfacial processes leading to the debonding of PSAs. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H7.00006: Shear Banding in Polymer Solutions with a Monotonic Constitutive Curve Michael Cromer, Michael Villet, Glenn Fredrickson, Gary Leal Shear banding is a well-documented phenomenon occurring in the flow of various complex fluids, e.g. wormlike micellar solutions. In recent years, experiments have revealed that shear banding can occur in entangled polymer solutions. To model this behavior it has been assumed, for over 40 years, that an underlying non-monotonic constitutive curve is required to theoretically generate steady shear banding. It is widely believed, however, that the underlying constitutive curve for polymer solutions is strictly increasing. In this we talk we show that the linear instability of a monotonic curve driven by the Helfand-Fredrickson mechanism of polymer moving up stress gradients results in a shear-banded profile with a non-uniform concentration distribution. In addition, a subcritical, nonlinear instability exists resulting in multiple steady states, which agrees with a recent experimental observation. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H7.00007: Rheological hysteresis in soft glassy materials Thibaut Divoux, Vincent Grenard, Sebastien Manneville The nonlinear rheology of a soft glassy material is captured by its constitutive relation, shear stress vs shear rate, which is most generally obtained by sweeping up or down the shear rate over a finite temporal window. For a huge amount of complex fluids, the up and down sweeps do not superimpose and define a rheological hysteresis loop. By means of extensive rheometry coupled to time-resolved velocimetry, we unravel the local scenario involved in rheological hysteresis for various types of well-studied soft materials. Building upon a systematic experimental protocol, we introduce two observables that quantify the hysteresis in macroscopic rheology and local velocimetry respectively, as a function of the sweep rate $\delta t^{-1}$. Strikingly, both observables present a robust maximum with $\delta t$, which defines a single material-dependent timescale that grows continuously from vanishingly small values in simple yield stress fluids to large values for strongly time-dependent materials. In line with recent theoretical arguments, these experimental results hint at a universal timescale-based framework for soft glassy materials, where inhomogeneous flows characterized by shear bands and/or wall slip play a central role. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H7.00008: Structure evolution in electrorheological fluids Bian Qian, Ahmed Helal, Maria Telleria, Mike Murphy, Marc Strauss, Gareth McKinley, Anette Hosoi Enhanced knowledge of the transient behavior and characteristics of electrorheological (ER) fluids subject to time dependent electric fields carries the potential to advance the design of fast actuated hydraulic devices. In this study, the dynamic response of electrorheological fluid flows in rectilinear microchannels was investigated experimentally. Using high-speed microscopic imaging, the evolution of particle aggregates in ER fluids subjected to temporally stepwise electric fields was visualized. Nonuniform growth of the particle structures in the channel was observed and correlated to field strength and flow rate. Two competing time scales for structure growth were identified. Guided by experimental observations, we develop a phenomenological model to quantitatively describe and predict the evolution of microscale structures and the concomitant induced pressure gradient. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H7.00009: Experimental Study of Settling of Spherical Particles in Unbounded and Confined Shear Thinning Viscoelastic Fluids Mukul M. Sharma, Sahil Malhotra An experimental study is performed to understand and quantify settling velocity of spherical particles in unbounded and confined surfactant-based shear thinning viscoelastic fluids. Experimental data is presented to show that elastic effects can increase or decrease the settling velocity of particles, even in the creeping flow regime. A significant drag reduction occurs with increase in Weissenberg number. This is followed by a transition to increasing drag at higher Weissenberg numbers. A new correlation is presented for the sphere settling velocity in unbounded viscoelastic fluids as a function of the fluid rheology and the proppant properties. The wall factors for sphere settling velocities in viscoelastic fluids confined between solid parallel plates are calculated from experimental measurements made over a range of Weissenberg numbers. Results indicate that elasticity reduces the effect of the confining walls and this reduction is more pronounced at higher ratios of the particle diameter to spacing between the walls. Shear thinning behavior of fluids is observed to reduce the retardation effect of the confining walls. A new empirical correlation for wall factors for spheres settling in a viscoelastic fluid confined between two parallel walls is presented. [Preview Abstract] |
Monday, November 19, 2012 12:27PM - 12:40PM |
H7.00010: Micro-Macro Simulation of Viscoelastic Fluids in Three Dimensions Alexander R\"uttgers, Michael Griebel The development of the chemical industry resulted in various complex fluids that cannot be correctly described by classical fluid mechanics. For instance, this includes paint, engine oils with polymeric additives and toothpaste. We currently perform multiscale viscoelastic flow simulations for which we have coupled our three-dimensional Navier-Stokes solver NaSt3dGPF with the stochastic Brownian configuration field method on the micro-scale. In this method, we represent a viscoelastic fluid as a dumbbell system immersed in a three-dimensional Newtonian liquid which leads to a six-dimensional problem in space. The approach requires large computational resources and therefore depends on an efficient parallelisation strategy. Our flow solver is parallelised with a domain decomposition approach using MPI. It shows excellent scale-up results for up to 128 processors. In this talk, we present simulation results for viscoelastic fluids in square-square contractions due to their relevance for many engineering applications such as extrusion. Another aspect of the talk is the parallel implementation in NaSt3dGPF and the parallel scale-up and speed-up behaviour. [Preview Abstract] |
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