MEAM Seminar Series Spring 2011

Seminars are held on Tuesday mornings, with coffee at 10:30 am in the Levine Hall Lobby and the seminar beginning at 10:45 am in Wu and Chen Auditorium (unless otherwise noted). Note: we will not hold a seminar on the dates of SEAS Faculty Meetings (February 8, March 15, April 12, May 10).

To be added to the MEAM Events mailing list (which sends notifications regarding all departmental seminars and events) please email us at meam-events@lists.seas.upenn.edu.

January 18

Chris Thorne, Ph.D. Candidate
Advisor: Mark Yim
"Toward the Design and Analysis of a Gyroscopically Controlled Micro Air Vehicle"

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Current research efforts in the field of Micro Air Vehicles (MAVs) focus on improving endurance and payload capacity by incrementally increasing subsystem performance. In this work, we adopt a system-level perspective for the development of a rotary-wing MAV and propose a new design that utilizes gyroscopic dynamics for attitude control. Unlike traditional MAVs where attitude control moments are generated by aerodynamic control surfaces, the proposed vehicle will leverage the existing angular momentum of its rotating components to generate gyroscopic moments for controlling attitude. This paradigm reduces mechanical complexity, precludes the use of complex mechanisms, such as the swashplate, for varying blade pitch, and has the potential to yield significant increases in agility, efficiency, and teleoperability when compared to state of the art micro Vertical-Take-Off-and-Landing (VTOL) vehicles. The evolution of the mechanical design, dynamic and aerodynamic models, control, and preliminary experimental results will be presented. Current challenges will also be presented and discussed.

Several other research efforts in the Modular Robotics Laboratory, including the design and implementation of the next generation of the ModLab modular robot, various latching mechanisms, and a brake mechanism, will also be highlighted in the presentation.

January 25

Daeyeon Lee, Assistant Professor of Chemical and Biomolecular Engineering, University of Pennsylvania
"Improving Mechanical Properties of Nanoparticle Thin Films and Bubble Shells"

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In this talk, I will describe our recent efforts to improve and understand the mechanical properties of nanoparticle thin films and bubble shells.  In the first part of the talk, a recently-developed method to mechanically reinforce nanoparticle thin films using atomic layer deposition will be discussed. Thin films composed of multiple nanomaterials have shown to exhibit emergent functionalities, making them useful for numerous advanced applications. These nanoparticle thin films, however, tend to fracture and abrade under small loads; this limitation presents a major bottleneck to their widespread use and commercialization. While high temperature sintering has been shown to improve the mechanical durability of these films on inorganic substrates such as glass, the process is not amenable to organic substrates such as polymers. We improve the mechanical durability of all-nanoparticle thin films at a low temperature using atomic layer deposition (ALD).  The effect of ALD on the mechanical and other physical properties of all-nanoparticle thin films will be discussed.  In the second part of the talk, our recent results on the stability of polymer-shelled bubbles with controlled dimensions will be presented. Monodisperse and stable bubbles have potential applications in the fabrication of functional lightweight materials and also in biomedical imaging as ultrasound contrast agents. I will introduce a new microfluidic approach to generate monodisperse and stable bubbles by employing an air-in-oil-in-water (A/O/W) compound bubble as a template.  It will be shown that the ratio of the shell thickness to bubble radius is critical in generating un-deformed polymer-shelled bubbles from A/O/W compound bubbles.  Multicomponent bubbles are also produced by incorporating a variety of materials into the nanoparticle shell.  We demonstrate the versatility of this approach in generating multicomponent bubbles by creating fluorescent and magnetic bubbles.



February 1

Mathew Mate, Hitachi San Jose Research Center, San Jose, CA
"How new disk drive technologies are pushing the nanoscale limits of materials and mechanics"

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Over the next five to ten years, the disk drive industry plans to incorporate two radically new technologies—thermally assisted recording (TAR) and bit patterned media (BPM)—that will greatly increase the storage capacity of a typical disk drive. Inherent with these new recording technologies, however, are high concentrations of mechanical, thermal, and tribological stresses, that are pushing the limits of our ability to design materials and nano-structures that tolerate these stresses. In this talk, I will describe the TAR and BPM technologies currently being developed by Hitachi GST and other companies, along with some of the technologies being developed to understand and ameliorate the high mechanical, thermal, and tribological stresses that occur within the nanostructures needed to implement TAR and BPM.

February 15

Xiulin Ruan, Assistant Professor of Mechanical Engineering, Purdue University
"Nanoscale Control of Photon and Phonon Transport for Enhanced Solar Energy Harvesting"

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Abstract:

Solar thermal and photovoltaics are effective processes of utilizing solar energy which may play critical roles in addressing the global energy challenge.  Photon and phonon transport are among the key issues that limit their efficiencies, and breakthroughs can arise via nanoscale engineering.  In this talk, I will first briefly introduce the progress and basic working principles of photovoltaic and solar thermal devices, and then discuss how photon, electron, and/or phonon transport and interaction can be manipulated using carefully-designed nanotube, nanowire, and quantum dot array structures, in order to enhance their efficiencies.  For carbon nanotube and silicon nanowire arrays, we use ab initio calculations, numerical electromagnetic simulations, and reflectivity measurements to show that the numerous nanocale cavities behave just like blackbodies that can trap and absorb solar irradiation extremely efficiently, while with only a fraction of materials needed compared to bulk devices.  The absorption of light can be further improved by introducing randomness into the nanostructures.  For quantum dot photovoltaic materials, we use non-adiabatic molecular dynamics simulations to show that the quantum confinement effects can significantly suppress electron-phonon coupling so as the heat generation loss. The findings are confirmed by our ultrafast laser measurements.  Future opportunities as well as challenges will also be discussed.

Biography:

Dr. Xiulin Ruan received his B.S. and M.S. from the Department of Engineering Mechanics at Tsinghua University. He then received an M.S. in electrical engineering (in 2006) and a Ph.D. in mechanical engineering (in 2007) from the University of Michigan at Ann Arbor.  After that he joined Purdue University as an assistant professor in the School of Mechanical Engineering.  Dr. Ruan's research is focused on nanoscale heat transfer and energy conversion.  He has published 20 journal papers and has given 8 invited talks at conferences and universities.  In 2010 he was selected as an Air Force Summer Faculty Fellow. His research is currently sponsored by National Science Foundation and the Air Force Office of Scientific Research.

 

February 22

**Tedori-Callinan Lecture**

Thursday, February 24 at 3:00 pm, Wu And Chen Auditorium
Lance R. Collins, the Joseph Silbert Dean of Engineering and Professor of Mechanical and Aerospace Engineering, Cornell University
"Role of Turbulence in the Atmospheric Processing of Clouds"
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March 1

Kyung-Suk Kim, Professor of Engineering, Brown University

"Unusual nanostructural instabilities caused by compressive stresses; graphene fracture operated in CNTs cut by sonication and dislocation motions during ion irradiation"

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Unusual mechanisms of nanostructual instabilities are found to be responsible for previously unexpalainable phenomena observed in nanofabrication processes. In the first example, our hybrid supercomputing simulations and experiments have enabled us to discover a new graphene fracture mechanism activated in carbon nanotube scission processes in water-sonication. The MD simulations reveal multi-scale compressive scission processes of the carbon nanotubes leading to the atomic-scale fracture. These processes are also evident in our sonication scission experiments with long single walled carbon nanotubes. These findings are in contrast to previous speculations that CNTs would be cut in tension or by high temperature reactions. In the second example, our molecular dynamics simulations reveal a unique saw-tooth pattern of sub-surface compressive stress accumulation and abrupt relaxation during the low energy ion bombardment of a large class of f.c.c. metals. The compressive stress build-up initiates with the formation of a (111) prismatic dislocation loop in the sub-surface region. The loop is highly mobile in the [110] in-plane burger’s vector orientation and grows by absorbing the surrounding interstitials. Once the prismatic loop reaches a critical size, the Burgers vector of the loop switches to the [101] direction through a mechanism of dislocation cross-slipping. In turn, the loop glides to the free surface and abruptly releases the compressive stress in the process. This dislocation assisted sub-surface atomic transport phenomenon couples stress relaxation to surface morphology evolution.

March 22

Timothy J. Healey, Professor, Departments of Mechanical & Aerospace Engineering and Mathematics, Cornell University

"Stable equilibria of some 2-phase problems of nonlinear elasticity via global bifurcation"

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We consider 3 problems from nonlinear elasticity modeling various 2-phase phenomena:  wrinkling of thin elastic sheets, shear-induced phase transition in shape-memory alloys, and pressurized Giant Unilamellar Vesicles (GUV's).  We identify a common mathematical structure in the elastic potential energy density for each of these: a convex term in the second-gradient of the deformation - characterized by a multiplicative small parameter - plus a non-convex term in the first gradient.  E.g., the small parameter is directly related to the thickness of the structure in the first and third problems mentioned above.  Using global bifurcation theory combined with a-priori bounds, we obtain the existence of solutions corresponding to arbitrarily small, non-zero values of the parameter.  With this in hand, we are able to efficiently compute branches of such solutions, identifying those that render the total energy a local minimum (stable).  We give applications to the above mentioned problems.

Monday, March 28

**Doctoral Defense** 3:00PM in Towne 337
Jason A. Thompson, Ph.D. Candidate
Advisor: Haim H. Bau
"Microbead-Based Biosensing in Microfluidic Devices"

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Microbeads are frequently used as a solid support to capture target analytes of interest, such as proteins and nucleic acids, from a biological sample. The integration of microbeads into microfluidic systems for biological testing is an area of growing interest. Such "lab-on-chip" systems are designed to integrate several functions of a conventional laboratory onto a single chip. As a platform to capture targets, beads offer several advantages over planar surfaces such as large surface areas to support biological interactions (increasing sensitivity), the availability of libraries of beads of various types from many vendors, and array-based formats capable of detecting multiple targets simultaneously (multiplexing). I describe the development and characterization of microbead-based biosensing devices. A customized hot embossing technique was used to stamp an array of microwells in a thin plastic substrate where appropriately functionalized agarose microbeads were selectively placed within a conduit. Functionalized quantum dot nanoparticles were pumped through the conduit and used as a fluorescent label to monitor binding to the bead. Three-dimensional finite element simulations were carried out to model the mass transfer and binding kinetics on the beads’ surfaces and within the porous beads. The theoretical predictions were critically compared and favorably agreed with experimental observations. A novel method of bead pulsation was shown to improve binding kinetics in porous beads. In addition, the thesis discusses other types of bead arrays and demonstrates alternative bead-based target capture and detection strategies. This work enhances our understanding of bead-based microfluidic systems and provides a design and optimization tool for developers of point-of-care, lab-on-chip devices for medical diagnosis, food and water quality inspection, and environmental monitoring. 

March 29

Pedro Reis, Esther and Harold E. Edgerton Assistant Professor of Mechanical Engineering and Civil and Environmental Engineering, Massachusetts Institute of Technology

"The wonders of thin objects: From torn tape and sinking flowers to graphene ribbons and grabbing water"

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The reviving study of thin elasticity objects is a rapidly burgeoning field that is bringing together seemingly separate communities. Recent work has shown that the strong non-linearities arising primarily from geometry are responsible for universal modes of deformation. Moreover, coupling the elasticity of thin objects with phenomena such as fracture, interfacial forces and flow opens new fundamental problems arising frequently in technology and nature.

In this talk I will present three instances of the coupling of thin elastic sheets with adhesive forces at solid and liquid interfaces. First, I will discuss the annoying quotidian problem of torn adhesive tape which produces triangular flaps. Secondly, understanding this mechanism enables us to fabricate tapered graphene ribbons from an adhesive substrate, albeit at the nanoscale. The final example is inspired by sinking aquatic flowers. Reversing the process, as a flexible petal-shaped object is withdrawn from the interface of a liquid bath, it can close, pinch-off and thereby grab a droplet of liquid. We propose this as a novel passive pipetting mechanism that relies purely on the coupling of the elasticity of thin plates and the hydrodynamic forces at the liquid interface.


April 5

Silvia Salinas Blemker, Assistant Professor, Departments of Mechanical & Aerospace Engineering, Biomedical Engineering, and Orthopaedic Surgery, University of Virginia

"Meso-scale computational models of skeletal muscle provide insights into muscle injuries"

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Muscles are most commonly injured when they undergo lengthening contractions.  For example, the hamstrings muscles are thought to experience strain injuries during the terminal swing phase of sprinting.  At the micro-scale, strain injuries have been shown to result from over-stretched sarcomeres, and weak sarcomeres are commonly implicated as the source of strain injury.  At the macro-scale, global assessments – such as overall strength, flexibility, and coordination – are commonly explored as possible indicators of injury susceptibility.  However, skeletal muscle has a hierarchical structure, and the effects of features at intermediate levels of the hierarchy (i.e., at the tissue level, fascicle level, and fiber level) are not traditionally explored as potentially affecting injury susceptibility.  We have built computational models that predict the behavior of muscle at multiple scales, including the tissue, fascicle, and fiber levels.  These models demonstrate how structural features of muscle at these meso-scales greatly influence on strain distributions and injury susceptibility. Based on these insights, new ideas regarding injury prevention and rehabilitation have emerged.  The focus of this talk will be to describe our computational approach as well as the insights into muscle injury gained from these models.

April 19

Jun Zhang, Associate Professor, Department of Physics and the Courant Institute of Mathematical Sciences, New York University

"Reversed flapping flight and inverted hydrodynamical drafting"

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Flying birds and swimming fish are familiar sights to everyone, but their remarkable locomoting abilities are often poorly understood. Inspired by these examples found in nature, we discuss two recent laboratory experiments on interactions between unsteady flows and dynamic boundaries (here flapping wings or fins). In the first experiment, we study the physical origin of flapping flight and investigate how wing flexibility
determines the flight speed and even the flight direction. There, we found that the finite wing rigidity (which allows passive pitching when the wing is flapped) could lead to reversed flapping flight. In the second experiment, we investigate the group dynamics of multiple locomotors as they interact with each other through the passing flows. Unlike the well-known hydrodynamic drafting of rigid objects placed in tandem, flexible structures like flags show inverted drafting where the leading body enjoys a reduced drag while the follower suffers a drag increase.

April 26

Shizhi Qian, Assistant Professor, Department of Mechanical and Aerospace Engineering, Old Dominion University

"Electrokinetic Particle Transport in Micro/Nano-fluidics"

Read the Abstract and Biography

Abstract:

Electrokinetics has emerged as one of the most promising techniques to manipulate fluid and particles in micro/nano-fluidic devices, which have the potential of revolutionizing various bioanalytical applications.

The first half of this talk will be on experimental and numerical studies of electrokinetic particle transport in several types of microfluidic channels, with emphasis on dielectrophoretic (DEP) effect. The numerical predictions are in quantitative agreement with our own and other researchers’ experimental results. It has been found that the DEP effect could be utilized to separate, focus and trap particles in microfluidic devices. DEP particle-particle interactions could assemble and form particle chains parallel to the applied electric field, based on which a microfluidic device has been developed to achieve a high throughput cell electrofusion (Fig.1).

The second half of this talk will present numerical study of electrokinetic particle transport in nanofluidics. A continuum-based numerical model, which is capable of dynamically tracking the particle translocation through a nanopore with the consideration of the finite electrical double layers adjacent to charged surfaces, has been developed. The predictions on the ionic current change due to the presence of particles inside the nanopore are in qualitative agreement with existing experimental results. Furthermore, field effect control of nanoparticle translocation through a nanopore has been numerically demonstrated.

Biography:

Shizhi Qian received his B.S. and first Ph.D. from Huazhong University of Science and Technology, China in 1994 and 1998, respectively. He worked afterwards as a senior engineer in industry for two years. From February 2001 to August 2002 he worked as a Postdoctoral Researcher in Mechanical Engineering and Applied Mechanics (MEAM), University of Pennsylvania (PENN) under the supervision of Professor Haim H. Bau. From September 2002 to December 2004, he pursued another Ph.D with Professor Bau. He was then promoted to Research Associate, and worked at PENN until he became Assistant Professor in the Department of Mechanical Engineering at the University of Nevada Las Vegas (UNLV) in August 2005. He joined the Department of Aerospace Engineering at the Old Dominion University as an Assistant Professor in July 2008. He is a WCU (World Class University) Fellow in Korea. Since 2002, he has published 70 peer-reviewed journal articles, 4 book chapters, and 55 works in conference proceedings. As of March 2011, ISI Citation Index has shown over 670 citations of his work with h-index of 16 (16 articles have been cited at least 16 times).

May 3

**3:00 pm in Wu and Chen Auditorium, Levine Hall**
Manu Prakash, Harvard University
"Hydraulic constraints on feeding and metamorphosis: the case of a hungry fly"

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Abstract: Physical laws combined with plethora of biological design constraints ultimately determine the form of living systems. Although sometimes clear in hindsight, quantitative means to tease out these threads that explain shape, form and dynamics of biological entities has often remained elusive. By developing novel experimental tools to probe and perturb biodynamics in a broad range of systems - our group discovers novel design principles of biological machines at molecular, cellular and organismic scales. In this talk I will focus on two examples from organismic biology highlighting the role of fluid dynamics in feeding and metamorphosis in flies. By developing a hybrid X-ray and fluorescence microscopy setup with electrophysiology capabilities, we have been able to generate the first video rate images of inner mechanisms of insect feeding behavior in freely moving flies, bumble-bees and moths. Furthermore, we have discovered viscosity dependent feeding modes in house flies. Implications of this discovery to the energy budget of a fly and neuroethology of feeding behavior will be discussed. In the second part of the talk, I will briefly describe the role of physical fluid stresses during a developing pupae of a fly. Via live imaging techniques, we have uncovered a bubble nucleation event that occurs during early stages of pupal development. Although known for the last 50 years, the origin and control of this nucleation event has remained completely mysterious. How is the location of the bubble nucleation site encoded in the pupae? Where does the driving pressure for bubble nucleation come from? How do molecular processes control such a physical event? What is the role of this bubble during fly development? These questions will be answered and implications to mechanics of pupal architectures will be discussed.

Short Bio: Manu is a Junior Fellow (Biophysics) at the Harvard Society of Fellows for the term 2008-2011 and an Assistant Professor (on leave) in the Department of Bioengineering at Stanford University. His inventions range from microfluidic computers for ultra-high throughput biology in nanoliter droplets to glass windows that double as wireless antennas. Dr. Prakash’s new interdisciplinary lab explores mechanistic understanding of living matter ­ from molecular to organismic scales ­ often uncovering hidden physical constraints shaping life forms. An MIT alumnus, for his ideas and inventions, Dr. Prakash was awarded the 2008 Lemelson Student Finalist Award, Milton Fund New Innovator award, MIT 100K development prize and MIT IDEAS award. To pursue his interests in inventive solutions for global health, Dr. Prakash recently cofounded Appropria Medical Devices.

May 6

**Doctoral Defense** 9:30 am in Room 307 Levine Hall
Nora Ayanian, PhD Candidate
"Coordination of Multirobot Teams and Groups in COntrained Environments: Modeling, Abstractions and Synthesis of Control Policies"
Advisor: Vijay Kumar & Daniel Koditschek

Read the Abstract

Using a group of robots in place of a single complex robot to accomplish a task has many benefits, including simplified system repair, less down time, and lower cost. Combining heterogeneous groups of these multi-robot systems allows addressing multiple subtasks in parallel, reducing the time it takes to address many problems, such as search and rescue, reconnaissance, and mine detection. These missions demand different roles for robots, necessitating a strategy for coordinated autonomy while respecting any constraints the environment may impose. We synthesize controllers for heterogeneous multirobot systems, a problem that is particularly challenging because of inter-robot constraints such as communication maintenance and collision avoidance, the need to coordinate robots within groups, and the dynamics of individual robots.

We present globally convergent feedback policies for navigating groups of heterogeneous robots in known constrained environments.  Provably correct by construction, our approach automatically and concurrently solves both the planning and control problems by decomposing the space into cells and sequentially composing local feedback controllers. The approach is useful for navigation and for creating and maintaining formations while maintaining desired communication and avoiding collisions. We also extend this methodology to large groups of robots by using abstractions to manage complexity. This provides a framework with which navigation of multiple groups in environments with obstacles is possible, and permits scaling to many groups of robots. Finally, we show that this automatic controller synthesis enables the design of feedback policies from user-specified high-level task specifications.


May 17

Tianxiang Su, Ph.D. Candidate
Advisor: Prashant Purohit
"Entropic Elasticity of Biopolymers and their Networks"