Mechanics
and Materials Engineering
Academic Year 2005-2006
Monday, April
17, 2006
11:00 AM
479 EBU-II
Professor
Laser Thermal Laboratory
Department of Mechanical Engineering
“Micro/Nanoscale
Transport and Laser-Assisted Processing”
In the first part of the seminar,
research on micro/nanoscale fluidic transport will be
presented. Acoustically excited boundary layers in liquids are investigated by
micro-particle image velocimetry (-PIV). The
interplay between resonance standing modes and the highly confined near the
solid boundary viscous effects is analyzed experimentally and theoretically. Fluidic
transport in buried microchannels micromachined
in Si devices is probed by infrared thermal velocimetry.
Gas and water transport in sub-2 nm carbon nanotube
membranes is examined. The observed water flow rates are much too high to
reconcile with continuum mechanics, and the slip flow
corrections. On the other hand the gas transport exceeds the Knudsen
predictions. In the second part of the seminar, research on laser-assisted
micro and nanoprocessing, nanomachining,
nanolithography and nanodeposition is summarized.
Fundamental aspects of ultrafast laser coupling with
materials are probed by time-resolved diagnostics. A new paradigm is presented
for the maskless fabrication of passive and active
functional macroelectronic devices on flexible
substrates.
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Monday, March 20, 2006
11:00 a.m.
479 EBU-II
John Raymond Willis
Professor of Theoretical
“Scale Effects Induced by Strain-Gradient Plasticity and Interfacial
Resistance in Randomly-Inhomogeneous Media"
The theory and physical origin of
strain-gradient plasticity will be briefly outlined, and a "deformation
theory" (as opposed to "flow theory") version will be developed.
A distinctive aspect of the theory is that it requires an additional boundary condition, or condition across any interface. This may be
turned to advantage by introducing an "interfacial potential" that
penalizes the development of plastic flow at an interface, by impeding the
motion of dislocations. A scale-dependent hardening effect in any material such
as a composite or a polycrystal is generated thereby.
Treatment of such materials as having random microgeometry
renders exact solution intractable but approximations (which in some cases are
bounds) can be developed via a variational
formulation. This will be illustrated with simple examples.
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Monday, February 27, 2006
11:00 a.m.
479 EBU-II
Guglielmo Scovazzi, Ph.D.
1431 Computational Shock- & Multi-physics
Sandia National Laboratories
"SUPG-stabilized Lagrangian
shock hydrodynamics"
A new SUPG-stabilized formulation
for Lagrangian Hydrodynamics of materials satisfying
the Mie-Gr\"{u}neisen class of constitutive laws will be presented.
The proposed method can be used in conjunction with simplex-type
(triangular/tetrahedral) meshes, as well as the more commonly used brick-type
(quadrilateral/hexahedral) meshes. The motivation for the presented work is
that simplex-type meshes offer significant advantages in the automatic mesh
generation process, and they are usually preferred in multi-physics problems
involving radiation effects. The proposed method results in a globally
conservative formulation, in which equal-order interpolation (P1 or Q1 isoparametric finite elements) is applied to velocities,
displacements, and thermodynamic variables, namely pressure. As a direct consequence,
a natural representation of the pressure gradient on element interiors bypasses
all problematic issues related to pressure gradient reconstruction, typical of
standard, cell-centered, multidimensional hydrocode
implementations. SUPG stabilization in the Lagrangian
context involves specific design requirements such as Galilean invariance, an overlooked aspect in standard SUPG stabilized
methodologies. A discontinuity capturing operator in the form of a Noh-type
viscosity with artificial heat flux is used to preserve stability and
smoothness of the solution in the shock regions. Numerical results for the
Euler equations of gas dynamics will be presented.
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Monday, February 13, 2006
11:00 a.m.
479 EBU-II
Brad Lee Holian, Ph.D.
"Dynamics
at the Mesoscale"
Studying phenomena at the mesoscale -- between atomistic and macroscopic scales -- requires significant modifications of the atomistic-level molecular dynamics. In mesodynamics, the mass points, instead of being atoms, are mesoscopic, such as the centers of mass of polycrystalline grains or molecules. Mesodynamics requires the formulation of both an interaction potential between these mesoparticles and dissipative energy exchange that satisfies Galilean invariance. Mesodynamics simulations of shock waves in a crystalline polymer describe the thermal behavior nearly as well as all-atom molecular dynamics, at considerable computational savings.
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Monday,
February 6, 2006
11:00 a.m.
479 EBU-II
Professor Federico Rosei
INRS Energie, Materiaux et Telecommunications
"Strategies for controlled assembly at the nanoscale"
The bottom–up approach is emerging as a viable alternative for low cost manufacturing of nanostructured materials [1]. It is based on the concept of self–assembly of suitable nanostructures on a substrate. We propose various strategies to control the assembly of nanostructures (both organic and inorganic) at the nanoscale [1, 2]. Our approaches include surface patterning through a nanostencil [3, 4] (i.e. a shadow mask with nanoscale features) and deposition on naturally patterned substrates, which take advantage of long–range reconstructions [5, 6]. New experimental tools are presented to gain atomic scale insight into the surface processes that govern nucleation, growth and assembly [7–9]. The controlled assembly of building blocks at the nanoscale will be effective for a variety of applications, ranging from nanoelectronics to chemical and biosensors.
[1] F. Rosei, J. Phys. Cond. Matt. 16,
S1373 (2004).
[2] F. Rosei et al., Prog. Surf. Sci. 71, 95 (2003).
[3] C.V. Cojocaru, C. Harnagea, F. Rosei et al., Appl. Phys. Lett 86, 183107
(2005).
[4] C.V. Cojocaru, et al., Microel.
[5] A. Sgarlata, et al., Appl.
Phys. Lett. 83, 4002 (2003).
[6] R. Otero, Y. Naitoh, F. Rosei
et al., Angew. Chem. 43, 4092
(2004).
[7] F. Ratto, F. Rosei et
al., Appl. Phys. Lett. 84, 4526 (2004).
[8] F. Ratto, F. Rosei et al., J. Appl. Phys. 97,
043516 (2005).
[9] F. Ratto, F. Rosei et
al., Small in press (2006).
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Friday,
February 3, 2006
3:00 p.m.
CMRR Auditorium
Wing
Kam Liu, Walter P. Murphy Professor
Director of NSF Summer Institute on Nano Mechanics and Materials
Department of Mechanical Engineering
NORTHWESTERN UNIVERSITY
"Modeling
Electric Field Guided Assembly of Nano/Bio Filaments"
The characterization
and manipulation of complex biological systems has reached a stage to resolve
various levels of details. We briefly outline the immersed electrohydrodynamics
finite element method (IEFEM) coupled with multiphysics
features such as protein molecular dynamics and adhesion mechanics for solving
a class of bio-nano-fluidics problems. We then apply the multiphysics
of the composite electric field for the guided alignment of the carbon nanotube (CNT) and DNA. Preliminary multi-scale and
multi-physics examples demonstrate that the proposed IEFEM provides an ideal
modeling platform for the modeling of multi-physics biological systems,
including heart, arteries and veins, microcirculation blood flow, cell-extra
cellular matrix interaction, and electric field guided assembly of nanowires. In particular, the IEFEM code is being used in
the modeling of nano-electromechanical (NEM) sensor fabrications. The dynamic
process of the attraction, alignment, and deposition of nano/bio filaments
between micro-electrodes is modeled by integrating electrophoretic
and dielectrophoretic forces in addition to a drag
force caused by electroosmosis. The various dynamic
processes and assembled patterns are explored by comparing our simulation
results with experimental observations. The NEM sensors will be used for the
measurement of cell traction forces for the understanding of the focal adhesion
complex and cell motility.
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Wednesday,
January 25, 2005
2:00 p.m.
479 EBU-II
Professor John W. Hutchinson
Division of Engineering and Applied
"On
The Design Of Metallic Plate Structures Under Intense
Dynamic Loads"
Can metal sandwich plate
construction be more effective in resisting intense dynamic loads than solid
plate structure of the same mass and material?
The answer is yes, but establishing this relative advantage and
designing the most effective structure requires an understanding of basic
aspects of fluid-structure interaction and plastic deformation of the components
(faces and core) of the sandwich plate.
Core topology (e.g., honeycomb, corrugated plate or truss elements) is
an important consideration in the development of effective sandwich plates, as
is the relative allocation of material to the faces and core. Dissipation of energy occurs in compaction of
the core and in bending and stretching of the faces. The seminar will present an overview of the
mechanics underlying these issues and will review simulations of plates subject
to impulsive loads. Results of sandwich
plates optimized against air and water shocks are discussed.
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Monday, January 9, 2006
11:00 a.m.
479 EBU-II
E.M. Bringa
Chemistry and Material Sciences, Materials Science and Technology
Division
“Atomistic simulations of high
strain rate deformation of materials: from nanocrystals to astrophysics”
High strain rate deformation of condensed materials offers a window to new phases, novel material properties, and ultra-fast microstructure evolution. Recent experiments using laser-induced shocks have unveiled a rich deformation scenario depending on shock pressure and strain rate. The new National Ignition Facility (NIF), which just started reduced operation, will give a new window to study materials at extreme conditions.
Large-scale non-equilibrium atomistic molecular dynamics simulations of shocks in embedded atom method (EAM) metals, using up to 4 108 atoms during 0.2 ns will be presented. A new method, informed by molecular dynamics results, and mixing on the fly dislocation dynamics and finite element methods coupling, will be presented to describe microstructure produced by isentropic (ICE) loading. These simulations help interpreting experimental data and suggest new experiments to elucidate deformation pathways at high strain rate. The onset of dislocation slip and twinning, together with the “long”-time plastic evolution and resulting X-ray diffraction evolution will be discussed, explaining recent experimental X-ray diffraction results. Simulations of shocks in samples with dislocation loops, voids (1) and grain boundaries have been carried out to study the effect of pre-existing defects on the final shock-induced microstructure. In particular, there is an experimental and modeling effort at LLNL to understand shocks in nanocrystals (2). New experimental results on shocks in nanocrystalline Ni will be presented, which are consistent with atomistic simulation results. Finally, experiments and simulations on high-strain rate deformation of sub-micron clusters/grains will be discussed, in the context of damage due to micrometeoroid impacts relevant to space missions.
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Monday, December 5, 2005
11:00 a.m.
479 EBU-II
Katja Lindenberg
Distinguished
Professor of Chemistry
Department of
Chemistry and Biochemistry
“Energy Localization and Dynamics in Nonlinear Molecular and Granular
Media”
The localization of
mechanical energy in discrete nonlinear molecular arrays has attracted a huge amount of interest in the past two
decades as a possible mechanism for the efficient storage and transport of
energy. The reasons for the broad interest in these phenomena are at least
two-fold: on the one hand, they embody many of the interesting effects of the
interplay of nonlinearity, discretization, and stochasticity, and on the other they may be of use in explaining a variety of physical and
biophysical phenomena. The examination of such arrays as energy storage
and transfer assemblies for chemical or photochemical processes goes back to
the early days of this problem. More recently, the localization and transport
of vibrational energy has been invoked in a number of
physical settings including DNA molecules, hydrocarbon structures, the creation
of vibrational intrinsic localized modes in anharmonic crystals, photonic crystal waveguides, and
targeted energy transfer between donors and
acceptors in biomolecules.
A parallel and equally important context for energy localization occurs at a more macroscopic level in the propagation of pulses in dense strongly nonlinear granular materials. A recent revival of interest in the subject has been triggered in part by a concern with important technological applications such as the design of efficient shock absorbers, the detection of buried objects, and the fragmentation of granular chains.
While
these interesting applications fuel the continued intense study of these
phenomena, so does the desire
to understand these behaviors at a fundamental level. In this talk we will
present highlights of the analytical and numerical analysis of energy
localization and dynamics in model nonlinear discrete media. We focus on the
roles and interplays of linear and nonlinear interactions, thermal fluctuations
where appropriate, and frictional energy dissipation on these behaviors.
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Monday,
November 14, 2005
11:00 a.m.
479 EBU-II
Professor Bernard D. Coleman
J. Willard Gibbs Professor of
"On the elastic behavior of
DNA, with intramolecular electrostatic effects and
intrinsic curvature"
A molecule of DNA can be treated as though it is a rod-like structure that in a nanoscale drawing looks roughly like a stack of dominos (called base pairs) with each rotated approximately 36 degrees relative to its predecessor in the stack. The step from one base pair to the next can be one of ten distinct types each having its own mechanical properties (that are determined by the nucleotide composition of the step). The complexities of the genetic code are such that a DNA molecule found in nature is not mechanically homogeneous.
Of particular importance is the realization of the fact that, because each base
pair carries two (negative) charges, under physiological conditions a DNA
molecule is subject to strong long range intramolecular
electrostatic forces that, because they are partially screened out by
positively charged counter ions, can render the equilibrium configurations of
the DNA sensitive to changes in the concentration of salt in the medium.
Thus DNA is not what we call "a simple material", "a higher
gradient material", or even a material with mechanical behavior that can
be well approximated by the behavior of such materials.
This talk will be about the mathematical theory of a naturally discrete (i.e.,
base-pair level) model of the mechanics of DNA molecules that takes account of
both the long-range intramolecular electrostatic
forces and the dependence of the elastic properties of DNA on the base-pair
sequence. The theory was developed by Yoav Biton, David Swigon, and the
speaker. Recent calculations of Yoav Biton will be presented for cases in which the theory
predicts that the dependence of DNA configurations on salt concentration is
much stronger than might have been conjectured previously.
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Friday,
October 28, 2005
1:30 p.m.
479 EBU-II
Chantal Darquenne, Ph.D.
"The
use of experimental and computational techniques to study aerosol transport and
deposition in the lung"
Because of the difficulty of obtaining in-vivo data of
particle transport in the alveolar region of the lung, most studies have been
based on non-invasive measurements and computational modeling. Our laboratory
has undertaken an extensive experimental and computational research program on
the effect of gravity on aerosol transport in the human lung. The results from
these studies will be presented in the seminar.
Experimental studies. First,
an overview of the anatomy of the lung and of the mechanisms affecting aerosol
transport in the lung will be discussed. Then the experimental studies carried
aboard the NASA Microgravity Research Aircraft in microgravity (µG), normal
gravity (1G) and hypergravity (~1.6G) will be
presented. In the first set of studies, human volunteers continuously breathed
aerosol with a size ranging between 0.5 and 3 µm in diameter. Data showed that
deposition (DE) increased with increasing G. However, in µG, DE of the small
particles was higher than predicted by the numerical models. As inertia is
negligible for these small particles and sedimentation is absent in µG, the
higher DE was explained by a larger DE by enhanced diffusion resulting from unaccounted
mixing effects.
A series of bolus deposition (DE) and dispersion (H) studies in altered G levels was then performed. A bolus containing 0.5, 1.0 or 2.0 mm aerosol particles was introduced at predetermined points in an inspiration immediately followed by an expiration. Penetration volumes (Vp) of the bolus ranged from 150 to 1500 ml. For each particle size (dp), the data show that, at shallow Vp, DE and H were not different between gravity levels. In contrast, at larger Vp, when the aerosol bolus reached the alveolar regions of the lung, DE and H were strongly dependent on the G level. The steady increase in H with increasing Vp suggests a continued presence of mixing processes in the early generations of the acinar region. This mixing may facilitate particles entering the alveolar cavities and eventually depositing. In the studies performed in μG, the lung is in the most uniform state possible, and there are no losses due to sedimentation. Yet there is clearly a consistent increase in bolus dispersion as Vp increases. This is the first clear experimental evidence that airway geometries, lung expansion, and the flow patterns that they generate directly result in convective mixing in the human lung.
Computational studies. For particle sizes that are most affected by gravitational
sedimentation (0.5-5µm), a comprehensive study of aerosol deposition was
performed in a two-dimensional (2D) model of a symmetric 6-generation structure
of identical alveolated ducts (AD) and in a
three-dimensional (3D) model of a single bifurcation of AD. Simulations were
performed for a typical breath pattern consisting of a 2-sec inspiration
immediately followed by a 2-sec expiration. Up to five
breath cycles were simulated. Simulations were performed for different orientations
of the structure with respect to the gravity vector. The data clearly showed
that, for each dp
and structure orientation, there was a larger heterogeneity in DE among ducts
of the structure. Local concentrations of deposited particles could be at least
one order of magnitude larger than mean alveolar DE. A large number of the
small particles (0.5-2 mm) failed to exit the structure at the end of expiration and remained
in suspension in the distal part of the structure. During subsequent breaths,
these particles penetrated beyond the inspired volume of air where they
eventually deposited. These are important observations when one has to
determine the potential effects of airborne pollutants to human health or the
effectiveness of drugs administered by inhalation therapy.
More recently, a 3D model of an alveolated
duct with expanding and contracting walls has been developed in which similar
DE predictions were made for 1 to 5 µm particles. Data showed that for each dp, DE was
significantly higher in the moving-walled model than in a comparable
rigid-walled model. These results
strongly demonstrate the importance of modeling the expansion and contractions
of the alveolar structure in future modeling studies.
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Monday,
October 24, 2005
11:00 a.m.
479 EBU-II
Professor Subhash Mahajan
Department of Chemical and
"Origins
and Reduction of Threading Dislocation in GaN layers
Deposited on (0001) Sapphire"
We have investigated origins of threading dislocations (TDs) in GaN Layers grown on (0001) sapphire substrates by metalorganic chemical vapor deposition using two-step epitaxy. This growth protocol involved the deposition of GaN nucleation layers (NLs) at low temperature, followed by the growth of GaN layers at high temperatures (HT). Two sources of TDs have been identified: highly defective NLs and points defects in HT GaN layers. TDs evolve by glide and climb of dislocations from NLs into HT overgrowths and condensation of point defects to form dislocation loops. In no case, TDs are observed at the coalescence of growth islands.
We have developed a simple growth scheme to reduce the density of TDs in GaN overgrowths. This process entails in situ deposition of very thin layers (2nm or less) of silicon nitride on NLs, followed by thermal annealing. This step creates GaN seeds that are separated from each other, by silicon nitride. The subsequent growth of HT GaN appears to initiate from these seeds. Initially, GaN growth occurs vertically, and later on lateral growth takes over, resulting in a continuous film. Using this approach, the density of TDs is reduced from ~1010 cm-2 to 3X108 cm-2. Observations will be presented to understand the mechanism of reduction.
The support of the above work by AFOSR, NSF and ONR is gratefully acknowledged.
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Monday,
October 17, 2005
11:00 a.m.
479 EBU-II
Elena Grekova
Institute for Problems in Mechanical Engineering of
St.
“Wave propagation in rocks and
soils modelled as reduced Cosserat
continuum”
We consider rocks and compressed soils as elastic reduced Cosserat continuum, i.e. an elastic medium whose point-bodies are able to rotate,which induces asymmetric stresses, but according to our constitutive hypothesis does not induce couple stresses. The basic idea for such a modelling is that in a real rock or a compressed soil there are many heterogeneities, posessing rotational degrees of freedom, which influence wave propagation. Instead of considering in a detailed way a complex scattering problem we take as an effective model the reduced Cosserat continuum. We consider wave propagation for an isotropic case, and for the case of general weak anisotropy in coupling of rotational and translational degrees of freedom. In both cases the microstructure of the kind under consideration provides a strong dispersive behaviour, and the "attenuation" caused by the trapping of kinetic energy by rotation, for certain frequencies. The Green functions are constructed for the isotropic medium. The influence of inhomogeneities in the inertial properties on the wave propagation is considered for some cases.
This work has been done in
collaboration with Gerard C. Herman (Shell E. & P.,
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Friday,
October 14, 2005
11:00 a.m.
584 EBU-II
Professor Antonio Castellanos
Physics Faculty, Avda. Reina
Seville
"Flow
regime boundaries in cohesive powders"
Granular materials exhibit several regimes of behavior: plastic, inertial, fluidized, and entrained flow. Particle size, particle density, cohesiveness and gas flow govern which of these types of behavior occur, but not all materials can pass through all of these states. For each regime of granular behavior there is a dominant mechanism that determines the order of magnitude of the stresses in the bulk, and the transitions between the various flow regimes can then be obtained by comparing the magnitude of these stresses. In general the motion of coarse granular material is characterized by transition from plastic to inertial flow, whereas a fine powder at atmospheric pressure is characterized by the transition from plastic to fluidized flow. This transition makes powders qualitatively different from coarse granular materials, and it is the most important single fact to be taken in account when designing industrial devices for handling, transport and mixing of fine powders.
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Monday, October 3, 2005
11:00 a.m.
479 EBU-II
M. I. Baskes
“Atomistic calculations
of shock induced phase transformations and spall”
Shock waves in materials produce a number of interesting
phenomena including phase transformations and spall.
This presentation will discuss recent calculations of these phenomena at the
atomic level using an embedded atom method (EAM) potential. Since the
phenomena of interest involve subtle energy differences in the phase
transformations and generation of free surface in spall,
a realistic many-body EAM potential that represents the properties of nickel, a
typical fcc material is
used.
Using a standard flyer-plate geometry, many molecular
dynamics calculations at different sample sizes, orientations, grain structure,
and initial velocity were performed. In general the following observations
about shock induced phase transformations may be made: (1) the fcc crystal transforms to a bcc-like crystal structure just
behind the shock wave; (2) after a period of time, the bcc structure transforms
to a layered hcp/fcc material; and (3) when the reflected
shock wave traverses this material, the system mostly transforms back to fcc. These phase transformations produce a multi-wave
velocity profile at the free surfaces of the sample in agreement with many
experimental observations.
Due to the tension produced by the reflected shock waves, spall
then occurs in this almost pristine material by void nucleation at a grain
boundary facet and growth of this void. The spall
strength is found to be a strong function of the large strain rates in these
computer experiments. Extrapolation of the spall
strength to true experimental strain rates yields excellent agreement with
experiment.
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Monday,
September 26, 2005
11:00 a.m.
479 EBU-II
Professor Joseph ZARKA
Scientific Director CADLM
“Some Questions in Materials and Structures”
During the seminar this fundamental question will be underlined:
Nobody in mechanical, civil or transportation engineerings can escape from nano modelling/nano technologies, is it possible to keep our main hypothesis of continuous media ?
What are the possible directions to reach solutions ?
Using some examples, it will be shown the importance of the Advanced Intelligent Design of Systems to such general problems when we know that we don’t know and when we need to reach real practical and optimal solutions.
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Information: (858) 534-3980