Distinguished Visitors Lecture Series
Sponsored and funded annually by the Dean of the College of Sciences
Old Dominion University
Spring - Fall 2007
Public Talk, 7:30pm on March 29 at OCNPS 200
Recent Advances in Direct Numerical Simulations of Turbulent Viscoelastic Channel Flows: Towards a better Understanding of Polymer-Induced Drag Reduction
Prof. Antony N. Beris,
Department of Chemical Engineering, University of Delaware
Abstract: The phenomenon of polymer-induced drag reduction describes the effect that even minute quantities of a high molecular weight polymer can have, as small as of the order of ppm by weight, when added to a low molecular weight solvent, such as water or crude oil, in reducing considerably the turbulent drag. Despite a voluminous amount of research from its original discovery in the late 40s by Mysels and Tomms, unresolved issues still remain. Here I will describe the recent progress achieved thanks to Direct Numerical Simulations (DNS) of the turbulent channel flow of a dilute polymer solution modeled from first principles with a kinetic-theory-based (FENE-P) or network-based (Giesekus) constitutive equation. The main effect of viscoelasticity is shown to be the strengthening of the largest size turbulent structures which become much more coherent with a dynamics that changes at an appreciably lower rate than for the equivalent Newtonian structures. Our parametric study strongly suggests that this feature develops due to an enhanced resistance to extensional deformation induced due to viscoelasticity and it results to a lower energy transfer from the wall to the turbulent core, thus explaining the drag reduction. The numerical results provide thus evidence and in depth analysis to a mechanism proposed first in the 60s by Lumley and Metzner based on experimental observations: As the polymer elasticity increases, so does the resistance offered to extensional deformation. That, in turn, changes the structure of the most energy-containing turbulent eddies (they become wider, more well correlated, and weaker in intensity) so that they become less efficient in transferring momentum, thus leading to drag reduction. More recent analysis of the coherent structures using principal orhtogonal decomposition (POD) allows to further substantiate the evidence in support of the above-mentioned drag reduction mechanism. It also underscores the tremendous difficulty underling any effort towards a low-dimensional modeling of turbulent flows, gievn the large extent of scales of length and time characterizing turbulent dynamics. The POD analysis helps to elucidate the role of the larger scales and demonstrates the complicated interplay between the various coherent structures a key effect of intermittence and chaos characterising all turbulent flows.
For more information visit: http://sci.odu.edu/sci/news_events/Beris_1.pdf
Public Talk, 4:30pm on March 30 at OCNPS 200
Multiscale Analysis of Strong Extensional Flows of Polymer Solutions and Melts
Prof. Antony N. Beris,
Department of Chemical Engineering, University of Delaware
Abstract: The high resistance to extensional deformation exhibited by polymer solutions and melts is one of the most characteristic features that find numerous applications from drag reduction in turbulent flows to fiber spinning. Yet, it appears that we still do not understand very well how to describe it, especially for strong extensional flows under conditions under which the polymer chains are almost fully extended. Constitutive models for polymeric flows have been traditionally based on the Gaussian approximation assumption about the form of the distribution function dictating the conformation of polymer chains. However, although this is a fairly good approximation near equilibrium, it becomes consistently worst at high levels of extension when the chain distribution is fairly degenerate. The use of a maximum extensibility (in the form used in a traditional FENE-P like model) does not improve this situation---it actually makes it worst! Thus, as an artifact of this approximation, we have the paradox of an estimated extended free energy that becomes infinite in the limit of perfect chain extension. These observations are not limited to dilute polymer solutions but also hold for polymer melts where the role of individual chains is replaced by that of chain segments between entanglements. In fact, a new Non-Equilibrium Microscopic Lattice-based Monte Carlo technique that we have recently developed within our research group has been used in a non-equilibrium multiscale simulation to develop a new, thermodynamically consistent, constitutive model correcting the FENE-P expression for free energy (so called FENE-PB --- B standing for Bounded Free Energy) at high levels of extension. The model restores the consistency between the microscopic simulations and the macroscopic estimates for a dense amorphous phase (modeled by a Phan Thien and Tanner equation using the FENE-PB model to account for finite extensibility and a bounded free energy) quite a lot---up to the point where excluded volume effects are important. This new approximation can therefore be used under a variety of situations and it is expected to produce significant differences when there is a significant chain extension.
For more information visit: http://sci.odu.edu/sci/news_events/Beris_2.pdf
Public talk, 3:00pm on March
20 at OCNPS 200
New Insights into Hard Problems with Soft Materials
Prof. David A. Weitz
Department of Physics,
Harvard
University
Abstract: This talk will present results
of studies of both crystallization and the glass
transition performed by real-space imaging of individual particles in colloidal
suspensions, providing rich new insight into the behavior of these most fundamental phase transitions.
Public talk, 7:30pm on March
21 at OCNPS 200
Dripping, Jetting, Drops and Wetting: the Magic of Microfluidics
Prof. David A. Weitz
Department of Physics,
Harvard University
Abstract:
This talk will discuss some
of the new opportunities that
arise by precisely controlling fluid flow and mixing using
microfluidic devices. These devices take advantage of the advances
in computer technology, but apply them to the control of fluid flow The talk will describe the remarkable instabilities that can
be used to create very small droplets and will show how these can be
used to fabricate new classes of materials. The talk will also
describe how the exquisite control afforded by the microfluidic
devices provides the enabling technology to use droplets as
nanoreactors for very high throughput combinatorial screening of
chemical reactions for drug discovery and validation.
Public Talk, 7:30 PM on February 19 at OCNPS 200 (Oceanography/Physical Sciences Building)
Modeling liquid crystal materials and processes in biological systems
Prof. Alejandro D. Rey
,
Department of Chemical Engineering,
McGill University
Abstract: Liquid crystal phases are found in DNA, proteins, lipids and polysaccharides. Frozen-in, chiral liquid crystal ordering also occurs in solid biocomposites such as insect cuticle, muscle, plant cell walls and collagen, where the helicoid structure is believed to arise by self-assembly processes. Spinning of silk fibers by spiders is another biological polymer process that relies on liquid crystal self-assembly. I will discuss the progress and challenges of modeling in three such applications: (1) Biological helicoids form by directed self-assembly. Theory and computer simulation of chiral phase ordering show that the directed self-assembly process reproduces the natural structures. The computational results shed light on the role of chiral ordering on the formation of helicoidal monodomains. (2) Spinning of spider silk involves a complex sequence of phase transitions that includes nematic phase ordering in the duct section of the spinning apparatus. Simulation of phase ordering under capillary confinement replicates the observed structures found in Nephila clavipes and other orb-weavers. The computational results shed light on the role of defect textures in the fiber spinning process. (3) Biological membranes are smectic liquid crystals that display flexoelectricity, or coupling between electric fields and curvature. Models based on smectic elasticity and polarization thermodynamics are used to derive the electroelastic shape equation, whose solution gives the membrane shape under external fields. The theoretical results shed light on the various ways electric fields affect membrane shape and functioning.
For more information visit: http://sci.odu.edu/sci/news_events/Rey_1st.doc
Public Talk, 3:30 PM on February 20 at Hampton/Newport News room
(Webb Center)
Computational materials science of liquid crystals
Prof. Alejandro D. Rey
,
Department of Chemical Engineering,
McGill University
Abstract: Liquid crystals are orientationally ordered soft phase materials with many functional and structural applications. Understanding and optimizing the functionality and processability of these anisotropic, textured, viscoelastic materials requires the integrated use of optics, rheology interfacial science, phase transitions, defect physics and topology. Computational materials science is a methodology based on the integration of all these disciplines. I will discuss several challenges in liquid crystal research in which computational materials science has helped to elucidate structure and dynamics relevant to functionality and processability. Processing of liquid crystalline polymers to produce fibers involves flow-induced texturing, pattern formation, and texture transformation through defect nucleation and annihilation. I will discuss how computational rheooptics together with defect physics and bifurcation theory yield quantitative information on orientation field symmetry, defect density, and rheological functions, as well as inverse problems, such as extracting heterogeneous orientation information from optical signals. Carbonaceous mesophases are discotic liquid crystals used as precursors for carbon super-fibers, composites, foams, and nanofibers. Mesophase orientation and self-assembly under non-planar confinement invariably leads to characteristic heterogeneous textures and defect lattice structures. I will discuss the use of phase ordering models, computational reflection polarized microscopy, and differential geometry to develop an understanding of how defect lattices and orientation heterogeneities arise under confinement. Texturing by chemical, electrical, and geometrical heterogeneities of substrates in contact with liquid crystals can be detected by transmitted optical microscopy, an optoelastic process known as liquid crystal vision. I will present progress and challenges in computational polarized optical microscopy of textured liquid crystals based on Maxwell equations and device models based on liquid crystal vision.
For more information visit: http://sci.odu.edu/sci/news_events/Rey_2nd.doc
Public talk, 7:30pm on February 7 at OCNPS 200
Nano-materials: what's really different and what's math got to do with it?
M. Gregory Forest,
Grant Dahlstrom Distinguished Professor
Departments of Mathematics & Biomedical Engineering
Co-Director, Institute for Advanced Materials, Nanoscience & Technology
University of North Carolina at Chapel Hill
Abstract: By now everyone has heard about "nano", if by no other means than from the IPod nano. In this lecture I will address some of the reasons why the design of materials at the nanometer scale really is different from traditional materials. I will have very little to say about your IPod, I confess. I will survey some of the promise that nano-materials hold for applications in technology and medicine. Yet, there are challenges that come with great promise, and I will provide some perspectives on why Nature presents unique obstacles in nano-science and nano-technology. Finally, I will respond to a slight variation on Tina Turner's rhetorical question: what's math got to do with it?
For more information visit:
http://sci.odu.edu/sci/news_events/MGregoryForest.pdf
Colloqium, 4:30pm on February 8 at OCNPS 100
Modeling the nano-materials pipeline: some successes and the challenges ahead
M. Gregory Forest,
Grant Dahlstrom Distinguished Professor
Departments of Mathematics & Biomedical Engineering
Co-Director, Institute for Advanced Materials, Nanoscience & Technology
University of North Carolina at Chapel Hill
Abstract: There is a preponderance of evidence to suggest that property performance capabilities are greatly enhanced with designer nano-scale particles. These particles could be synthesized or naturally occurring, they could be macromolecules or larger particles, but with at least one dimension on the order of nanometers to "qualify" for the nano label. I will talk about a special class of nano-materials that involve macromolecules which are highly anisotropic: either thin rods or thin platelets. There are many examples of such nano-materials with a variety of remarkable performance properties that have been illustrated. This lecture will focus on a strategy for modeling the pipeline, from the raw materials throught processing to performance evaluation, and an update on progress thus far. One of the take-home messages is that there are fundamental scientific advances necessary to control and optimize nano-materials, with a wealth of challenges ahead.
Past Lecture Series:
- 1995 - Chaos and the Environment
- 1996 - Global change and "Geophysiology": Man's Impact on Biogeochemical Cycles
- 1997 - The Frigid World of Ultracold Matter: From Bose-Einstein Condensation to Optical Tweezers
- 1998 - Naturally Occurring Rapid Change in Global Climate
- 1999 - Emerging Infectious Diseases
- 2000 - Investigating Modern Pseudoscience
- 2001 - Bioinformatics
- 2002 - Emerging Scientists
- 2003 - Science and Health Promotion in the 21st Century
- 2004 - High Performance Computing and its Application to Science
- 2005 - Digital Preservation
- 2006 - PORTS - Portal to the World Distinguished Lecture Series
