MEAM Seminar Series Archive: Fall 2008

September 11
Safety Seminar

Speakers: Joe Passante, CIH
Sr. Industrial Hygienist Environmental Health & Radiation Safety
Karen Kelley, CIH
Industrial Hygienist, Laser Safety Officer
Terry Kientz
Electronic Engineer
Melissa Muth
Sr. Information Security Specialist

Topics Covered: Computer Security and Copyrights, General Lab Safety, Laser Safety, and Additional Lab Issues

*All first year MEAM graduate students must attend.

September 18
Howard Brenner
"Bi-velocity hydrodynamics"

Speaker: Howard Brenner
Willard H. Dow Professor, Department of Chemical Engineering
Massachusetts Institute of Technology

Based upon linear irreversible thermodynamics jointly with Boltzmann's molecular-kinetic model of a dilute gas, it is argued that single-component continuum hydrodynamics cannot be correctly characterized by means of but a single velocity field. Rather, contrary to accepted fluid-mechanical principles, two independent velocities are theoretically shown to be required - one quantifying the movement of mass, the other that of volume. Experimentally, two distinct velocities are invariably observed when monitoring the (gravity-free) movement of a fluid undergoing steady-state heat conduction by the respective additions thereto of: (1) a dye; and (ii) a small, inert, rigid, macroscopic, nonBrownian, tracer-particle. Whereas the dye remains macroscopically at rest, the tracer-particle moves from hot to cold (thermophoresis). These two different experimentally-observed velocities constitute the physical realization of the theoretical bi-velocity prediction.

September 24
Santi Swaroop Adavani, PhD Candidate
"Numerical methods for inverse problems constrained by elliptic and parabolic partial differential equations on regular domains"

Speaker: Santi Swaroop Adavani
PhD Candidate
Advisor: George Biros
Mechanical Engineering and Applied Mechanics, University of Pennsylvania

The main goal of this project is to design and implement fast algorithms for the solution of inverse problems constrained by elliptic and parabolic partial differential equations (PDEs) on regular domains. Our main motivation is the application of these algorithms for inverse problems in electrocardiography. We considered the following two formulations: 1) an elliptic source inversion problem, and 2) a parabolic source inversion problem. We formulated these problems as PDE-constrained optimization problems and used Tikhonov regularization in both the problems for stability. We formed the Lagrangian functional and used a reduced space approach in which we eliminate the state and the Lagrange variables and we iterate in the inversion parameter space using Conjugate Gradients (CG). We used the Fourier-Galerkin method for the spatial discretization of the PDEs in 2D and 3D. We exploited the structure of the reduced Hessian in both the cases, which was a result of using spectral methods for the discretization of the PDEs. We derived the Singular Value Decomposition (SVD) of the Hessian analytically and designed preconditioners to accelerate the convergence of CG when applied to the elliptic source inversion problem. The overall complexity of reconstructing the source is O(N log N), where N is the number of parameters used to represent the source. We derived analytical expressions for the eigenvalues of the Hessian and proposed multigrid based preconditioners to accelerate the convergence of CG to solve the parabolic source inversion problem with full and partial domain observations. We derived analytical expressions for the entries of the Hessian and proposed low rank Hessian approximation based preconditioners for the parabolic source inversion problem with boundary observations. The overall complexity of solving the parabolic source inversion problem in the case of domain observations and boundary observations is O(Nt N + N log2 N) and O(Nt N log N) respectively, where N is the number of grid points and Nt is the number of time steps. We showed that these methods are algorithmically robust to the value of the regularization parameter using numerical experiments, and thus, useful for the cases in which we seek high-fidelity reconstructions. We have observed that we cannot reconstruct the sources completely by solving the elliptic source inversion problem using the boundary data. On the other hand, by solving the parabolic source inversion problem using the boundary data we have observed an accurate reconstruction of source. We have solved a 3D parabolic source inversion problem with 2 million inversion variables and 0.2 billion state variables on 32 processors in 6 minutes including the setup of the preconditioner.

September 25
Michael D. Graham
"Transport and collective dynamics in suspensions of swimming microorganisms"

Speaker: Michael D. Graham
Department of Chemical and Biological Engineering
University of Wisconsin-Madison

A suspension of swimming organisms is an example of an “active” complex fluid. At the global scale, it has been suggested that swimming organisms such as krill can alter mixing in the oceans. At the laboratory scale, experiments with suspensions of swimming cells have revealed characteristic swirls and jets much larger than a single cell, as well as increased effective diffusivity of tracer particles. This enhanced diffusivity may have important consequences for how cells reach nutrients, as it indicates that the very act of swimming toward nutrients alters their distribution. The enhanced diffusivity has also been proposed as a scheme to improve transport in microfluidic devices and might be exploited in micro?uidic cell culture of motile organisms or cells.

The feedback between the motion of swimming particles and the fluid flow generated by that motion is thus very important, but is as yet poorly understood. In this presentation we describe theory and simulations of hydrodynamically interacting microorganisms that shed some light on the observations. In the dilute limit, simple arguments reveal the dependence of swimmer and tracer velocities and diffusivities on concentration. As concentration increases, we show that cases exist in which the swimming motion generates dramatically enhanced transport in the fluid. This transport is coupled to the existence of long-range correlations of the fluid motion. Furthermore, the mode of swimming matters in a qualitative way: microorganisms pushed from behind by their flagella are predicted to exhibit enhanced transport and long-range correlations, while those pulled from the front are not. A physical argument supported by a mean held theory sheds light on the origin of these effects. These results imply that different types of swimmers have very different effects on the transport of nutrients or chemoattractants in their environment; this observation may be related to the evolution of different modes of swimming.

October 2
Hui Zhao, PhD Candidate
"Modeling electrokinetics with applications to micro and nano fluidic systems"

Speaker: Hui Zhao
PhD Candidate
Advisor: Haim Bau
Mechanical Engineering and Applied Mechanics, University of Pennsylvania

With the rapid growth in lab-on-a-chip technology, the prospects of detecting viruses in a microfluidic diagnostic device or immobilizing single bio molecules have become more and more promising. This poses a challenge for particle manipulation such as nano particle or bio molecule assembly and separation, which are far from trivial due to the low Reynolds number. Electrokinetics provides an opportunity to fulfill this task. After briefly discussing the fundamentals of electrokinetics, the talk focuses on the polarization of the electric double layer of charged dielectric particles, which plays a critical role in particles' polarization under the action of an electric field. The effect of the electric field on three types of particles including a cylindrical particle, a spherical particle and an elongated cylindrical particle are studied by the standard model consisting of the Poisson-Nernst-Planck equations, in particular, when the double layer thickness is of a large portion of the particle's radius or even larger. In contrast to the case of the thin double layer, the particle's polarization exhibits interesting and different behavior. The results explain how a nano particle or a bio molecule migrates in response to an electric field, which can guide the design of biophysical and biomedical experiments such as DNA manipulation, cell separation and cell sorting.

October 9
Andrea Prosperetti
"Some flows with bubbles and particles"

Speaker: Andrea Prosperetti
Charles A. Miller, Jr. Distinguished Professor of Mechanical Engineering
The Johns Hopkins University
The Berkhoff Professor of Fluid Dynamics
Department of Applied Sciences and Burgerscentrum
University of Twente, The Netherlands

Some recent work on a few flow problems involving bubbles and particles will be described. (1) Long gas bubbles rising in a tube (the so-called Taylor bubbles) are found to lose axial symmetry when the liquid flows against gravity; a description of the underlying physical mechanism will be given. (2) A buoyant particle in a horizontally rotating drum finds an equilibrium position where it rotates with an angular velocity which may exceed that of the drum; the results of experiments and simulations will be illustrated. (3) The gravitational settling of a large number of particles is a challenging computational problem; a new, physics-based, efficient algorithm to simulate the process will be described. (4) The talk concludes with a micro-fluidic "acoustic fish".

October 27
Nathan Michael, PhD Candidate
"Planning and control for teams of robots in complex environments"

Speaker: Nathan Michael
PhD Candidate
Advisor: Vijay Kumar
Mechanical Engineering and Applied Mechanics, University of Pennsylvania

We address the challenge of controlling a team of robots through a complex environment, a capability relevant to applications such as environmental monitoring or surveillance. The difficulty of the problem lies in the design of individual control laws for each of the agents that account for inter-agent interactions while preserving the convergence properties of the control law and drive the system to a desired state. We present an abstract statistical representation that reduces the complexity of controller design and is invariant to the number of robots in two and three dimensions. We develop control laws that require limited global state information, resulting in decentralized control laws at the agent level with collision avoidance and show convergence and stability properties. We propose estimation of the representation through the development of a distributed consensus algorithm and the inclusion of a supervisory aerial robot acting as a global observer. We define energy metrics based on the abstract representation that reduce the complexity of the planning problem to designing trajectories for the abstract representation of the system and discuss strategies for splitting and merging the robot formation. We review an experimental infrastructure developed to validate these multi-robot planning and control strategies and provide experimental verification on teams of nonholonomic robots. We conclude by proposing future research opportunities.

October 30
Howard H. Hu
"Simulations of particulate flows"

Speaker: Howard H. Hu
Professor, Department of Mechanical Engineering and Applied Mechanics
University of Pennsylvania

In this presentation, three topics will be discussed. The first is on direct numerical simulation of rigid particulates in viscous fluid flows using an explicit finite-difference scheme. The scheme enforces the no-slip condition on the particle surface based on matching the Stokes flow solutions next to the particle surface with a numerical solution slightly away from it. The second topic is on a monolithic solver for fluid-structure interactions, particularly to simulate dynamics of elastic objects in viscous fluids. The numerical scheme solves the deformation of incompressible elastic solids with a velocity-stress formulation, which models the elastic solid as a “neo-Hookean” material where the displacement field is eliminated. The third topic is on numerical models that simulate motion of dielectric particles in an electrolyte. Most suspensions in biological applications involve electrolyte where an ionic double layer may be formed next to particle surface due to the induced-charge on particle's surface. This double layer affects the dielectrophoretic motion of the particle. The effects of the double layer on the dielectrophoretic motion of particles will be discussed.

November 13
Roderic Lakes
"Extreme materials: creation of materials with negative or very large values of material properties"

Speaker: Roderic Lakes
Department  of Engineering Physics, Engineering Mechanics Program
University of Wisconsin-Madison


Material properties which we choose to analyze or to measure are in part conditioned by our perception of what is reasonable or possible. During the formative period of the theory of elasticity, there was disagreement regarding how much freedom should be incorporated in the mathematical description of the properties. Scholars such as Navier, Cauchy, Poisson, and Lam favored a model based on analysis of interatomic forces while Green favored a tensorial approach. The former approach predicted a Poisson's ratio of 1/4 for all materials while the latter approach, now formalized as the classical theory of elasticity predicts allowable Poisson's ratios between -1 and 1/2. Experimental measurements (about a century ago) of Poisson's ratio of about 1/3 in most common materials led to the replacement of uniconstant elasticity by the more general classical elasticity. Known isotropic materials have Poisson's ratio in the range 0 to 1/2, and some recent text books even claim that is the allowable range.

In our laboratory we have made materials with Poisson's ratio as low as -0.8. Cellular solids with a negative Poisson's ratio exhibit superior resilience and toughness as a result of the unfolding of the cells. Since that development, other groups have found a variety of systems which exhibit such behavior. Structural heterogeneity, leading to non-affine deformation or non-central forces, is responsible for such effects. Guided structural heterogeneity has enabled the synthesis of materials with high positive or negative values of thermal expansion, and of cellular solids with ratios of strength to weight orders of magnitude higher than those of conventional foam or honeycomb.

We consider composite material micro-structures, which give rise to high stiffness combined with high viscoelastic loss, a combination beneficial in the context of vibration - absorbing damping layers. We characterize several candidate materials isothermally over 11 decades of time and frequency with a novel instrument. In this context we have developed composites in which one phase has a negative stiffness. Such composites can exhibit extreme mechanical damping and stiffness, even a Young's modulus approaching ten times that of diamond. The damping, tan delta, can approach a singularity in such heterogeneous systems. Negative stiffness is by itself unstable, but regions of negative stiffness can be stabilized in composite micro- structures.

November 14
David Saintillan
"Dynamics, pattern formation and mixing in active particle suspensions"

Speaker: David Saintillan
Assistant Professor, Mechanical Science and Engineering
University of Illinois at Urbana-Champaign

Suspensions of swimming microorganisms are characterized by complex dynamics involving strong fluctuations and large-scale correlated motions. These motions, which result from the many-body interactions between particles, are biologically relevant as they impact mean particle transport, mixing and diffusion, with possible consequences for nutrient uptake. Using direct numerical simulations, I first investigate aspects of the dynamics and microstructure in suspensions of interacting self-propelled rods at low Reynolds number. A detailed model is developed that accounts for hydrodynamic interactions based on slender-body theory. It is first shown that aligned suspensions of swimming particles are unstable as a result of hydrodynamic fluctuations. In spite of this instability, a local nematic order persists in the suspensions over short length scales and has a significant impact on the mean swimming speed. Consequences of the large-scale orientational disorder for particle dispersion are discussed and explained in the context of generalized Taylor dispersion theory. Dynamics in thin liquid films are also presented, and are characterized by a strong particle migration towards the interfaces. The results from direct numerical simulations are then complemented by a kinetic model, in which the dynamics are captured using a conservation equation for the particle configurations, coupled to a mean-field description of the flow arising from the active stress exerted by the particles on the fluid. Based on this model, the stability of isotropic suspensions of particles is investigated. I demonstrate the existence of an instability in which shear stresses are eigenmodes and grow exponentially at long scales, and propose an interpretation in terms of the system entropy. Non-linear effects are also studied using numerical simulations of the kinetic equations in two dimensions. These simulations confirm the results of the stability analysis, and the long-time non-linear behavior is shown to be characterized by the formation of strong density fluctuations, which merge and break up in time in a quasi-periodic fashion. These complex motions result in very efficient fluid mixing, which is quantified by means of a multiscale mixing norm.

November 20
Ting Zhu
"Nanomechanics of plasticity"

Speaker: Ting Zhu
Woodruff School of Mechanical Engineering
Georgia Institute of Technology

Recent nanoscale experiments have revealed a host of plastic flow phenomena controlled by the nucleation and reaction of dislocations. We have developed the multiscale and atomistic modeling methods to quantitatively understand the dislocation processes in these experiments. We first develop a multiscale modeling approach of interatomic potential finite element method for simulating nanoindentation. This facilitates the modeling at the length scales of laboratory experiments, while remaining faithful to the nonlinear interatomic interactions. Our results demonstrate that the hyperelasticity and crystallography control critically the onset of plasticity during the nanoscale contact. We further bridge the timescale gap between atomistic simulations and laboratory experiments by integrating the transition state theory and the exploration of atomistic energy landscape. We show that the interfacial dislocation reaction is the rate-controlling mechanism in nano-twinned copper, giving rise to an unusual combination of ultrahigh strength and high ductility. Our results also reveal a small activation volume associated with surface dislocation nucleation. This leads to the sensitive temperature and strain-rate dependence of the nucleation stress, providing an upper bound to the size-strength relation in nanopillar compression experiments.

December 4
MinJun Kim
"Finding nano: Cellular mechanics & molecular dynamics"

Speaker: MinJun Kim
Department of Mechanical Engineering & Mechanics
School of Biomedical Engineering, Science and Health Systems
Drexel University

The use of biological nanostructures in engineered systems represents a critical step toward understanding both how the biological world has evolved at the nanoscale as well as how scientists and engineers can mimic and improve on nature using modern fabrication and assembly. Two topics are treated within this seminar. First, we will discuss the practical integration of biomolecular motors for biologically powered robotic nano/microswimmers as well as the development of polymeric protein nanostructures such as bacterial flagellar filaments for use in micro and nanoscale engineered devices. The ability to integrate multiple levels of functionality with a control hierarchy will be highlighted to show the realization of miniaturized robots with applications to machines for micro-transport and assembly and drug delivery systems. Second, this talk will be focused on current nanopore technology for single molecule analysis. Nanometer-sized pores can be used to detect and characterize biopolymers, such as DNA, RNA, and polypeptides, with single-molecule resolution. In addition to the single molecule analysis, the possibility of using nanopore sensors and ionic current blockade techniques will be introduced to detect and configure candida/bacteria/viruses in solutions for the identification of certain types of pathogens.

December 5
J. Christian Gerdes
"Lanekeeping assistance at the vehicle handling limits"

Speaker: J. Christian Gerdes
Design Group, Department of Mechanical Engineering
Stanford University

Each year there are approximately 40,000 fatalities on US roadways, 40% of which result from a collision with a fixed obstacle in the environment. Thousands of lives, therefore, could be saved by simply helping the driver keep the vehicle in the lane. This talk describes an approach to driver assistance based on artificial potential fields that define the lane boundaries as hazards with the minimum hazard in the center of the lane. Analogous to a marble rolling in a valley, the lanekeeping assistance system attempts to nudge the vehicle back to the minimum hazard. When the driver is tracking the lane, the car feels exactly how it would without any assistance; as the driver deviates from the center, the car gently adds an additional steering command, producing an effect much like being attached to the road with a light spring.

Mathematically, the system can be formulated in terms of Lagrangian dynamics with the lankeeping system adding a force on top of the existing vehicle dynamics. Forming a Lyapunov function for the system, lanekeeping performance can be guaranteed even in the presence of curves or other disturbances. This formulation also gives insight into how the lanekeeping system performs at the handling limits, enabling stability and performance guarantees even when the tire friction limits are exceeded.

The mathematical results suggest that very simple algorithms can lead to highly robust vehicle control. To further support this claim, the talk presents the results of experiments involving a 1997 Chevrolet Corvette and an all-electric student-built steer-by-wire vehicle, P1. Data and video from these experiments show that the theoretical performance guarantees hold even in real-world situations when the car is pushed beyond the friction limits.

December 11
Mark R. VanLandingham
"Experimental nanomechanics and micromechanics: Opportunities in polymer science"

Speaker: Mark R. VanLandingham
U. S. Army Research Laboratory
Weapons & Materials Research Directorate - Materials Division
Aberdeen Proving Ground, Maryland

Homogeneous polymer networks have a limited capacity to dissipate energy, because the chemical crosslinks restrict polymer chain motion, such that dissipation mechanisms are typically limited to viscoelastic deformation of the network and chain rupture. High glass transition temperature (Tg) or glassy systems typically suffer from low ductility and toughness, such that the amount of viscoelastic deformation prior to fracture is small. Elastomeric (i.e., low Tg) systems are extremely compliant, such that, although highly ductile, limited energy is expended deforming and fracturing these materials. To increase local volumetric deformation, the addition of heterogeneity to create multiscale structure is often utilized for both glassy and elastomeric polymers. Efforts to toughen high Tg systems, such as the incorporation of elastomers and thermoplastic polymers that phase separate during cure or small molecule plasticizers / fortifiers that interact with the network polymer at a molecular level, often lower stiffness and/or Tg. In any case, these types of modifications often result only in modest improvements in toughness. Efforts to stiffen and strengthen elastomers and gel networks typically involve the addition of fiber and/or particle reinforcement and can also include the creation of multiple or interpenetrating network structures. The mechanical behavior of these materials is highly dependent on the reinforcing constituents and the ability of the network to transfer stress at polymer-filler interfaces.

Further inspiration for energy dissipating network structures can be found in the hierarchical structures of many biological materials, in which the structure undergoes a wide variety of deformation mechanisms at many length scales to produce mechanical property amplification along with other types of functionality. While producing such highly complex, multiscale architectures through bottom-up techniques remains as a significant technical challenge, many possibilities exist for creating multiscale structure within crosslinked polymer networks to enhance mechanical properties and multifunctionality. In this presentation, new research initiatives will be discussed, in which the fundamental physics of polymer networks is being explored, with emphasis on the development and application of nanomechanical and micromechanical techniques to study energy dissipation in polymer-based materials.