MEAM Seminar Series Summer 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).
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Tianxiang Su, Ph.D. Candidate
Advisor: Prashant Purohit
"Entropic Elasticity of Biopolymers and their Networks"
The elastic energy for many biopolymer systems is comparable to the thermal energy at room temperature. Therefore, biopolymers and their networks are constantly under thermal fluctuations. From the point of view of thermodynamics, this suggests that entropy is playing a crucial role in determining the mechanical behaviors of these filaments.
In this talk, we will discuss the entropic elasticity of a polymer and polymer network. A polymer chain under room temperature has higher tendency to be in its curved configurations, instead of in its straight configuration. This is because curved states are overwhelmingly more probable, with higher entropy. The consequence is that one needs to apply forces to “stretch” the polymer in order that it is in the straight configuration. We will talk about how much thermal fluctuations are there in a polymer network and how much force we need to apply to stretch out the fluctuations and to cause deformation [1-3]. We will also discuss the mechanical properties of a polymer network, including its expansion, shear, tension and buckling behaviors, which reveal the non-trivial effects of the thermal fluctuations. We combine structural mechanics and statistical mechanics to tackle these problems theoretically. The network will be viewed as a mechanical structural characterized by a stiffness matrix. Statistical mechanics is built upon this. We will show that thermal fluctuations of a polymer network are governed by (1) thermal energy, and (2) inverse of the stiffness matrix of the structure [1-3].
For applications, thermal fluctuations are useful in several cases. We will show that entropy itself can drive a biopolymer to migrate in a non-uniform nano-channel . The effective entropic driving force is derived based on a random walk model, and the migration and deformation of the polymer in the channel will be discussed. But, thermal fluctuations are not always desirable. For example, to do genome mapping in a nano-channel, one would like the thermal fluctuations to be as small as possible. We will discuss the internal fluctuations of DNA in nano-channels and show a length-dependent transition between de Genne’s and Odjik’s regimes . If there is time, we will briefly discuss how the fluctuating chain theory can be applied to protein forced unfolding problems .
(1) T Su and PK Purohit (2010) Thermomechanics of a heterogeneous fluctuating chain. Journal of the Mechanics and Physics of Solids, 58:164-186.
(2) T Su and PK Purohit (2009) Mechanics of heterogeneous fluctuating elastic rods. The Proceedings of the ASME-IDETC conference. (8 pages)
(3) T Su and PK Purohit (2011) Fluctuating elastic filaments under distributed loads. Accepted to appear in Molecular and Cellular Biomechanics
(4) T Su and PK Purohit (2011) Entropy driven motion of polymers in non-uniform nanochannels. Accepted to appear in Physical Review E.
(5) T Su, SK Das, X Ming, and PK Purohit (2011) Transition between two regimes describing internal fluctuation of DNA in a nanochannel. PLoS One, 6: e16890
(6) T Su and PK Purohit (2009) Mechanics of forced unfolding of proteins. Acta Biomaterialia, 5:1855-1863.
Paul White, Ph.D. Candidate
Advisor: Mark Yim
"Miniaturization Methods for Modular Robotics"
Neil Zuckerman, PhD Candidate
Advisor: Dr. Jennifer Lukes
"Propagation and Scattering of Mechanical Vibrations in Semiconductor
To simulate phonon-inclusion scattering events I have developed a new computational method using Molecular Dynamics (MD). It is capable of determining the amount and spatial distribution of vibrational energy scattered by granular nanometer-scale inclusions in anisotropic bulk crystals. This information is then incorporated into a new Kinetic Monte Carlo model which performs statistical tracking of phonon scattering to calculate a material's thermal conductivity at and beyond the micron scale. I will also present a straightforward MD-based method to compute optical phonon lifetimes, simulating phonon-phonon collisions in hot spots on computer chips.
I will then discuss the fabrication methods I use to build micron-scale silicon devices for the creation and study of vibrations. The presentation will show new plate wave devices and the associated MEMS manufacturing procedure. These devices are made from single crystals using cleanroom lithographic processes originally developed for microelectronics. The devices function using surface-mounted piezoelectric transducers that generate mechanical waves. The waves propagate through a thin silicon membrane, scatter off of carefully-etched perforations, and are detected by output transducers. This allows creation and detection of a transient mechanical wave scattering event. I will discuss laboratory measurements using these sensors, as well as limitations and advantages of this experimental method.
Michael Shomin, MSE Candidate
Advisor: Dr. Vijay Kumar
"Penetration-based Grasping for Aerial Robot Perching and Manipulation"
much attention as of late. Rotorcraft such as quadrotors have the ability to operate indoors, hover in place for surveillance, and still have the advantages of longer flights outdoors. This makes quadrotors a
very interesting research platform with much potential for applications.
For many years, almost all UAV research has focused on surveillance, mapping, localization, and other areas with no physical interaction with the world. This work addresses and explores the areas of manipulation and perching with a quadrotor in an effort to increase the utility of UAVs. To this end, a series of grippers has been designed to give a quadrotor the ability to pick up objects for transportation as well as perch on surfaces to carry out perch-and-stare objectives, or set up radio relays. The design of the gripper takes inspiration from the area of climbing robots and penetration into surfaces. Using a fabrication technique called Shape Deposition Manufacturing, the gripper is made with compliant joints, suspension, and embedded claws. Shape Deposition manufacturing is a form of lost-wax casting that allows for composite parts with embedded components, variable-compliance and multi-material parts to be fabricated together. A significant feature of the gripper is passive actuation, where penetration into a surface is triggered simply by contact with a surface, allowing the quadrotor to perch without precise timing of an actuator. In total, this work discusses the design strategy for the penetration-based gripper, the manufacturing process, and the control of the robot to achieve manipulation and perching tasks.
Alan Rosenwinkel, PhD Candidate
Advisor: Dr. John L. Bassani
"Understanding Stress at the Atomic Scale Through the Principle of Equivalence of Virtual Work"
Daniel Mellinger, Ph.D. Candidate
Advisor: Vijay Kumar
"Control Methods for Quadrotor Helicopters"
NOTE: CHANGE IN LOCATION - BERGER AUDITORIUM, 10:45 A.M.
Quentin Lindsey, Ph.D. Candidate
Advisor: Dr.Vijay Kumar
"Robotic Construction of Truss-like Structures"
Trussed structures are prevalent in everyday life. We use truss construction for the interior structural support of large buildings (Eiffel Tower or power transmission towers) as well as for temporary scaffolding to support workers and materials along the exterior. These structures are usually assembled on-site by human workers. However, there are many instances when construction of these structures is better suited for robots. These situations include environments with extreme temperature, high toxicity (e.g. nuclear incidents) or structurally unsafe areas (e.g. collapsed buildings). In these instances, more automated approaches are needed to build these structures.
In this presentation, I will describe the robotic construction infrastructure used in the GRASP Lab. With the limitations and constraints of this infrastructure, I will develop an algorithm for constructing truss-like cubic structures without holes from simple modular elements. In order to expand the class of cubic structures to those that contain holes, I will introduce another algorithm, which is capable of hole closure. Finally, I will generalized this approach for a larger class of lattice types including tetrahedral lattices.
Subhrajit Bhattacharya, Ph.D. Candidate
Advisor: Vijay Kumar
"Topological and Geometric Techniques in Graph-search Based Robot Planning and Exploration Problems"
Joseph Romano, Ph.D. Candidate
Advisor: Katherine Kuchenbecker
"Enhancing Autonomous Robotic Manipulation via Haptic Perception"
This talk presents several of the advances we have made in interpreting and using haptic cues to improve robot task performance. Information from a variety of mechanical sensors is carefully integrated to synthesize signals that resemble the haptic sensing channels commonly found in the human hand. By analyzing this information in the context of the manipulation task and the kinesthetic state of the system, we have been able to demonstrate high-performance sensing and acting with autonomous robots such as the Willow Garage PR2.