MEAM Seminar Series Summer 2012
Seminars are held on Tuesday mornings, with coffee at 10:30 am in the Levine Hall Mezzanine and the seminar beginning at 10:45 am in Wu and Chen Auditorium (unless otherwise noted).
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Tong Gao, Ph.D. Candidate, University of Pennsylvania
Advisor: Howard Hu
"Dynamics of soft elastic particles in viscous flows"
Qiwei Shi. Ph.D. Candidate, University of Pennsylvania
Advisor: John Bassani
"Diffusional Aggregation with Material Anisotropy"
Tong Gao, Doctoral Defense, University of Pennsylvania
Levine Hall 307, 11:00 am
Advisor: Howard Hu
"Dynamics of soft elastic particles in viscous flows"
Quentin Lindsey, Doctoral Defense, University of Pennsylvania
Levine Hall 315, 10:00 am
Advisor: Vijay Kumar
"Robotic Consrtuctuion 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 wella 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.
This thesis will develop general methods for automated robotic construction. In order to test these approaches, a robotic construction infrastructure was constructed at the GRASP Lab of the University of Pennsylvania. This system is comprised of several quadrotor helicopters with simple grippers, simple modular construction elements, and a motion capture system. Using the limitations and constraints of this infrastructure, several algorithms for constructing truss-like cubic structures using these modular elements are developed. First, an algorithm, which can construct a class of cubic structures without holes, is described. This algorithm is extended to several new algorithms capable of hole closure, which effectively expands the class of cubic structures to those that contain holes. Finally, these approaches are generalized for a larger class of lattice types including tetrahedral lattices.
Vahid Vahdat, Ph.D. Candidate, University of Pennsylvania
Advisor: Robert Carpick
"Mechanics of Interactions and Atomic-Scale War of Tips in Amplitude Modulation Atomic Force Microscopy Probes"
William McMahan, Ph.D. Candidate, University of Pennsylvania
Advisor: Katherine Kuchenbecker
"Providing Haptic Perception to Telerobotic Systems via Tactile Acceleration Signals"
This talk presents a suite of methods we have developed for enabling robots to be more aware of their physical interactions in operating conditions ranging from direct teleoperation to full autonomy. We will focus on the design and control of a haptic system capable of accurately recreating tactile acceleration signals experienced by a teleoperated robot in real time. This system has been implemented on the Intuitive Surgical da Vinci Surgical System, an FDA-approved telerobotic system that natively provides no haptic feedback. Building on prior work, we use MEMS-based accelerometers to provide real-time measurement of the high frequency accelerations experienced by the robot as a result of environmental contact. We use a dedicated linear voice coil actuator to generate high fidelity recreations of the tactile acceleration signals for the user to feel at the operator interface. This approach involves signal processing methods used to enhance the measured accelerations and modeling to carefully control the acceleration output of the voice coil actuator. The provided feedback feels natural and promises to reduce the operator's cognitive load and increase their situational awareness.
Tianxiang Su, Postdoctoral Fellow, Harvard University
"Enhancing stability of a rod by friction and elastic confinement"
Slender rods are ubiquitous in nature and widely used in engineering construction. Although most of these slender structures are mechanically supported, under large compressive load, they will still buckle and cause catastrophic structural failure. It is so far not well understood how Coulomb friction caused by the supporting medium affects the stability of rods. In the first part of the seminar, we will discuss the distinct mechanisms when compressed rods in frictional and frictionless contact lose stability: a frictionless rod buckles as its stiffness becomes negative; a rod in frictional contact, in contrast, can bear substantial amount of perturbation without buckling after its stiffness turns negative. Buckling is initiated as perturbation tolerance decreases below the level set by the environment, at a much higher critical load. In the second part of the seminar, we will discuss the 1D to 2D to 3D configuration transitions of the constrained rods as they buckle. We will see that this transition can be tuned by and is highly sensitively to the supporting matrix stiffness. This property may be useful for future photonic devices.
This research is under the supervision of Dr. Katia Bertoldi at Harvard University and Dr. Pedro M. Reis at MIT. The study for the first part of the talk is funded by Schlumberger.
Ian Cosden, Ph.D. Candidate, University of Pennsylvania
Advisor: Jennifer Lukes
"A Hybrid Atomistic-Continuum Model for Non-Isothermal Fluid Flow"
Graham Wabiszewski, Ph.D. Candidate, University of Pennsylvania
Advisor: Robert Carpick
"Understanding the Degradation of Nanoelectromechanical Contacts: Gigacycle Interrogation of Platinum Interfaces Using Atomic Force Microscopy"
Nanomechanical logic based on nanoelectromechanical systems (NEMS) switches promises a significant reduction in total switching energy over conventional solid-state transistors. This energy savings is achieved using electrodes that are brought in and out of contact to form an electrically-controlled mechanical switch. Unlike the leaky dielectric conduction channels employed in fully-electronic transistors, no current flows when the NEMS switch is off (open). However, the reliability of the electrical contact interface in NEMS switches is a crucial barrier to their commercialization. The adhesiveness and reactivity of conventional contact materials (i.e. metals) results in permanent adhesion, or the buildup of insulating tribofilms, at the contact. Careful selection and testing of next-generation contact materials, and expanding the scientific understanding of electromechanical contact degradation, is imperative to achieve lifetimes above a quadrillion cycles, a requirement for the commercialization of NEMS logic.
Studying alternative electrical contact materials typically requires costly and time-consuming fabrication of a new device for each material under investigation. In the present work, a new rapid prototyping method was developed to interrogate the electrical robustness and adhesiveness of any contact material without the need for new device fabrication. The methodology is based on dynamic atomic force microscopy (AFM) to perform quantitative studies of single asperity contacts. Candidate contact materials can be effectively probed in conditions representative of NEMS switches (e.g., nanometer dimensions, nanonewton applied forces, relevant applied voltages across the contacts, and billions of contact cycles), allowing physical processes dominating the performance of nanoscale switches to be elucidated. This methodology can be performed in most commercial AFM systems
The new methodology was used to investigate the reliability of platinum-platinum electrical contacts for two billion cycles of contact. Increases in contact resistance up to four orders of magnitude were observed, similar to microscale/multi-asperity investigations, validating that this approach is representative of device-level behavior. Further insights into the physical and chemical phenomena leading to the degradation of platinum electrical contacts will be provided. Alternate contact materials with potential for superior performance will then be discussed.
Xiaoning Shen, Ph.D. Candidate, University of Pennsylvania
Advisor: Paulo Arratia
"Undulatory Swimming in Complex Fluids"
Life immersed in a fluid is nothing unusual for an organism. They cope and take advantage of water or wind currents to move, feed, and reproduce. These fluidic environments range from simple, Newtonian fluids that flow like water to complex fluids that possess shear-rate dependent viscosity and viscoelasticity. Understanding the main mechanisms that govern the locomotion/motility of microorganisms in simple and complex fluids is of much practical interest with potential impact on drug delivery, robotics, medicine, and surveillance. The main question to be addressed in this talk is: how does the fluidic environment affect the motility behavior of micro-organisms?
In this talk, I will investigate the swimming behavior of the nematode Caenorhabditis (C.) elegans in both Newtonian and complex fluids. The nematode Caenorhabditis elegans is a small (1 mm long) roundworm that generates traveling waves to propel itself. In addition, C. elegans are an attractive organism since its genome has been completely sequenced and their complete cell lineage established. Using optical microscopy and a high-speed camera, we develop a set of methods to simultaneously track the nematode and trace the flow fields in order to characterize the motility behavior of C. elegans and the induced flow fields. In Newtonian fluids, we found that: (i) flow fields in Newtonian fluids exhibit an exponential decay trend, which agrees with the theoretical results in the low-Reynolds-number regime; (ii) calculations of propulsive forces from both resistive force theory and slender body theory agree well with the experimental data. As the nematode swims in viscoelastic fluids, we find that the presence of elasticity in the fluid can decrease the swimming speed and efficiency of C. elegans by up to 35% compared to Newtonian fluids of the same shear viscosity. As elasticity in the fluid is increased, the swimming speed also decreases. The flow fields generated by the swimmer may hold the answer to this phenomenon.
This work is supported by NSF-CAREER (CBET)-0954084 and Army Research Office.
Lichao Pan, Ph.D. Candidate, University of Pennsylvania
Advisor: Paulo Arratia
"A subcritical elastic instability in channel flows at low Reynolds number"
Fluids containing polymers are of much interest spanning the petroleum, semiconductor, pharmaceutical, and chemical processing industries. They are also frequently encountered in everyday life in foods, paints, and cosmetics. Fluids containing polymer molecules do not flow like water. Even when flowing slowly, these fluids can exhibit hydrodynamic instabilities and a new type of turbulence - the so-called purely elastic turbulence even at low Reynolds numbers (Re) where linear viscous forces dominate non-linear inertial forces. These phenomena, driven by the extra elastic stresses in the flow due to the presence of polymer molecules in the fluid were experimentally observed in flows around objects (cylinders), Couette cells, and curved microchannels. A common feature of the above-mentioned geometries is the presence of curved streamlines, which are necessary for infinitesimal perturbations to be enhanced by the normal stress imbalances in viscoelastic flows. Thus, it is a common assumption that in the absence of curvature and inertia, the flow of viscoelastic fluids is linearly stable.
Here we present experimental results that suggest the existence of a nonlinear instability in flows with parallel streamlines at low Re. We perform experiments in a long, straight microchannel that is 100 μm deep, 100 μm wide and 3.3 cm long. The channel is divided into two main regions: a short (~ 0.3 cm) region where a linear array of cylinders (0 ≤ n ≤ 15) is positioned in order to introduce perturbations to the flow, and a long (~ 3.0 cm) parallel flow region in which the fate of an initial disturbance is monitored; a channel devoid of cylinders is also used for control. The flow is investigated using both dye advection and particle tracking velocimetry. Results show that the initial disturbance is sustained far downstream in the parallel shear geometry above certain Wissenberg number (Wi), and increase non-linearly with Wi even at vanishing small Re. Above a critical Weissenberg number (Wi > 5.4) and a critical number of obstacles (n ≥ 2), a sharply increase of velocity fluctuations together with a hysteresis loop indicate presence of a subcritical elastic instability. This scenario is akin to the transition from laminar to turbulent flow of Newtonian fluids in pipe and channel flows, except that the instability is caused by the nonlinear elastic stresses and not inertia.