MEAM Seminar Series Summer 2015
For Spring 2015 Seminars, click here.
Seminars are held on Tuesday mornings, with coffee at 10:30 am in the Towne Building and the seminar beginning at 10:45 am in Towne Building, room 337 (unless otherwise noted).
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Philip Dames, PhD Candidate, University of Pennsylvania
Advisor: Vijay Kumar
3:45 pm, Towne 337
"Multi-Robot Active Information Gathering Using Random Finite Sets"
Information gathering tasks are becoming increasingly important in the modern world, ranging from infrastructure inspection to environmental monitoring to search and rescue. Teams of mobile sensor platforms have the ability to automate these information gathering tasks, which are often too dull, dirty, or dangerous for humans to perform. Additionally, robots are able to gather information that humans simply cannot gather, either by using mobility (e.g., flying overhead or crawling through rubble) or advanced sensors (e.g., multi-spectral cameras). In all of these tasks, the robot team must detect and localize an unknown number of objects of interest within the environment.
This talk will describe a unified estimation, control, and communication framework for active information gathering for multi-robot teams. There are many sources of uncertainty in these scenarios: in the number of objects of interest within the environment, in the locations of the objects of interest, in the sensor readings (e.g., false positive and false negative detections and noisy measurements), etc. We use the Probability Hypothesis Density (PHD) filter to simultaneously estimate the number of objects in the environment and their locations while taking into account these uncertainties. Using sets of potential actions generated at multiple length scales for each robot, the team selects the joint action that maximizes the expected information gain over a finite time horizon. We demonstrate the real-world applicability of the proposed autonomous exploration strategy through hardware experiments, exploring an office environment with a team of ground robots. We also conduct a series of simulated experiments, varying the planning method, target cardinality, environment, and sensor modality.
John Martin, PhD Candidate, University of Pennsylvania
Advisor: Robert Mauck
"Nanofibrous Disc-like Angle Ply Structures (DAPS) for Total Disc Replacement in a Small Animal Model"
The intervertebral discs are cartilaginous structures that impart multidirectional flexibility to the spine in support of routine physiologic loading. The discs of the lumbar spine are the largest avascular tissues in the body and, as a consequence, these discs predictably degenerate with age. Chronic low back pain is directly linked to intervertebral disc degeneration and is a significant socioeconomic burden, encompassing a yearly cost upwards of $100-$200 billion in the US.
My global hypothesis for this work is that cell-seeded synthetic discs can restore normal mechanical function to the spine after end-stage disc degeneration. To that end, our lab has developed what we call nanofibrous disc-like angle ply structures (DAPS) that replicate the natural structure, composition, and cellularity of the native intervertebral disc. In this talk, I will describe the optimization of the in vitro culture and fabrication processes of DAPS, the development of a small animal model of total disc replacement, and the evaluation of DAPS maturation and mechanical function in vivo.
Philip Dames, PhD Candidate, University of Pennsylvania
Advisor: Vijay Kumar
2:30 pm, Levine 307
"Multi-Robot Active Information Gathering Using Random Finite Sets"
Many tasks in the modern world involve collecting information, such as infrastructure inspection, security and surveillance, environmental monitoring, and search and rescue. All of these tasks involve searching an environment to detect, localize, and track objects of interest, such as damage to roadways, suspicious packages, plant species, or victims of a natural disaster. In any of these tasks the number of objects of interest is often not known at the onset of exploration. Teams of robots can automate these often dull, dirty, or dangerous tasks to decrease costs and improve speed and safety.
This dissertation addresses the problem of automating data collection processes, so that a team of mobile sensor platforms is able to explore an environment to determine the number of objects of interest and their locations. In real-world scenarios, robots may fail to detect objects within the field of view, receive false positive measurements to clutter objects, and be unable to disambiguate true objects. This makes data association, \ie matching individual measurements to targets, difficult. To account for this, we utilize filtering algorithms based on random finite sets to simultaneously estimate the number of objects and their locations within the environment without the need to explicitly consider data association. Using the resulting estimates they receive, robots choose actions that maximize the mutual information between the set of targets and the binary events of receiving no detections. This effectively hedges against uninformative actions and leads to a closed form equation to compute mutual information, allowing the robot team to plan over a long time horizon. The robots either communicate with a central agent, which performs the estimation and control computations, or act in a decentralized manner. Our extensive hardware and simulated experiments validate the unified estimation and control framework, using robots with a wide variety of mobility and sensing capabilities to showcase the broad applicability of the framework.
Heather Culbertson, PhD Candidate, University of Pennsylvania
Advisor: Katherine Kuchenbecker
2:00 pm, Towne 337
"Data-Driven Haptic Modeling and Rendering of Realistic Virtual Textured Surfaces"
The haptic sensations one feels when interacting with physical objects create a rich and varied impression of the objects, allowing one to gather information about the objects' physical characteristics such as their texture, shape, and compressibility. The human sense of touch excels at sensing and interpreting these haptic cues, even when the object is felt through an intermediary tool instead of directly with a bare finger. Dragging, pressing, and tapping a tool on the object allow you to sense the object's roughness, slipperiness, and hardness as a combination of vibrations and forces. Unfortunately, the richness of these interaction cues is missing from many virtual environments, leading to a less satisfying and less immersive experience than one encounters in the physical world. However, we can create the perceptual illusion of touching a real object by displaying the appropriate haptic signals during virtual interactions.
This thesis presents methods for creating haptic models of textured surfaces from acceleration, force, and speed data recorded during physical interactions. The models are then used to synthesize haptic signals that are displayed to the user during rendering through vibrotactile and/or kinesthetic feedback. The haptic signals, which are a function of the interaction conditions and motions used during rendering, must respond realistically to the user's motions in the virtual environment. We conducted human subject studies to test how well our virtual surfaces capture the psychophysical dimensions humans perceive when exploring textured surfaces with a tool.
Three haptic rendering systems were created for displaying virtual surfaces using these surface models. An initial system displayed virtual versions of textured surfaces on a tablet computer using models of the texture vibrations induced when dragging a tool across the real surfaces. An evaluation of the system showed that displaying the texture vibrations accurately captured the surface's roughness, but additional modeling and rendering considerations were needed to capture the full feel of the surface. Using these results, a second system was created for rendering a more complete three-dimensional version of the haptic surfaces including surface friction and event-based tapping transients in addition to the texture vibrations. An evaluation of this system showed that we have created the most realistic haptic surfaces to date. The force-feedback haptic device used in this system, however, was not without its limitations, including low surface stiffness and undesired inertia and friction. We developed an ungrounded haptic augmented reality system to overcome these limitations. This system allowed us to change the perceived texture and friction of a physical three-dimensional object using the previously-developed haptic surface models.
Alison Koser Patteson, PhD Candidate, University of Pennsylvania
Advisor: Paulo Arratia
10:45 am, Towne 321
"Living fluids and their interactions with particles and polymers"
Living fluids, or fluids that contain living matter such as swimming microorganisms, are of great technological and scientific interest. For instance, microorganisms colonize in the mucus of human stomachs and lungs, which contain particles and/or polymers. These fluid constituents impart nonlinear complex material properties, such as shear-thinning viscosity and elasticity. Although swimming microorganisms have been well studied in simple water-like fluids, the motility of bacteria in complex fluids is not fully understood. Many bacteria utilize a run and tumble swimming gait to seek food and flee toxins. This combination of straight swimming segments (runs) and sudden erratic rotations (tumbles) controls their spread and diffusion. We aim to provide a systematic experimental study to explore the interaction between the run and tumble dynamics of the model bacteria E. coli and complex polymeric solutions. By directly tracking the position and orientations of cells, we find that bacterial transport dramatically depends on the fluid properties. We show that the swimming velocity increases with elasticity and tumble frequency decreases with increasing fluid viscosity. Simultaneously, we observe that the swimming of E. coli can drive fluids out of equilibrium. This leads to fascinating features not possible in passive fluids. In particular, we find the anomalous size-dependent diffusion of hard spheres in suspensions of bacteria. Our results suggest novel ways in which the transport and dynamics of swimming microorganisms, flexible polymers, and hard spheres can be controlled.
David Gagnon, PhD Candidate, University of Pennsylvania
Advisor: Paulo Arratia
10:45 am, Towne 321
"Propulsion in Complex Fluids: Living & Artificial Swimmers"
Many microorganisms live and move in fluidic environments that contain polymers and particles. Such fluids often display complex rheological behavior such as shear-thinning viscosity and viscoelasticity. For example, the bacterium E. coli infects the viscoelastic mucus in the stomach, mammalian spermatozoa swim through shear-thinning cervical mucus, and the nematode C. elegans is found in wet, structured soil. In this talk, I investigate the swimming dynamics of the model biological organism, C. elegans, in dilute and non-dilute polymeric fluids with nonlinear material properties, including shear-thinning viscosity and viscoelasticity. Viscoelastic and gel-like polymeric solutions are found to significantly modify swimming behavior; on the other hand, shear-thinning viscosity seems to have little effect on swimming, but it does substantially modify the surrounding flow field. Finally, I will discuss two simple applications for swimming in complex fluids, inspired by the potential for artificial micro-swimmers to be used in drug delivery and self-assembly: one in a dilute viscoelastic solution and a second in a concentrated wormlike micellar solution. Nonlinear rheological properties are shown to break the so-called “scallop theorem” even when the swimming stroke is reciprocal – this greatly simplifies the potential design of artificial micro-swimmers.
Michael Norton, PhD Candidate, University of Pennsylvania
Advisor: Haim Bau
1:00 pm, Levine 307
"Dynamics and Statics of Liquid-Liquid and Gas-Liquid Interfaces on Non-Uniform Substrates at the Micron and Sub-Micron Scales"
Droplets and bubbles are ubiquitous motifs found in natural and industrial processes. In the absence of significant external forces, liquid-liquid and gas-liquid interfaces form constant mean curvature surfaces (common examples are cylinders and spheres) that locally minimize the free energy of a given system subject to constraints. However, even for sub-micron bubbles and droplets free of hydrodynamic or hydrostatic stresses (small Capillary, Weber, and Bond number), non-equilibrium at the contact line of sessile bubbles and droplets can influence geometries and dynamics. Motivated by electron microscopy observations of sub-micron bubbles in a liquid cell, a small mobile and growing bubble confined between two weakly diverging plates is considered theoretically. Scaling analysis suggests that observed bubbles move by continuously wetting and de-wetting the substrates they are adhered to, without a liquid film between them and the substrate, and that this process of moving the contact line is a dominant dissipater of energy in the system. 2D and 3D models are constructed around the Blake-Haynes mechanism, which relates the dynamic contact angle to contact line velocity. In 2D, the system is fully described by a set of non-linear differential equations that can be readily solved using standard software. In 3D, the equations of motions and constraints result in a non-linear partial differential equation, an integral constraint, and a point-wise constraint; decomposing the contact line geometry into a Fourier series and analytically integrating the equations of motion across the polar coordinate enables numerical integration in time using standard techniques. Both 2D and 3D models predict that in order for a doubly confined bubble to grow in a super-saturated solution it must first increase its curvature; this is in contrast to a free-floating bubble whose curvature always decreases with the addition of mass/volume. For a gaseous bubble, surface concentration is proportional to the internal pressure of the bubble; thus, this non-monotonic geometric change temporarily regulates the growth of the bubble. The model predicts growth rates like those observed experimentally that are several orders of magnitude lower than classical mass transfer driven growth theory (Epstein and Plesset). The framework developed is also used to explore the impact of partial contact line pinning on the geometry of growing bubbles. The presentation concludes with a look and electron beam-induced fluid dynamical phenomena relevant to electron microscopy.
Yi Yang, PhD Candidate, University of Pennsylvania
Advisor: Talid Sinno
2:00 pm, Levine 307
"Quantitative Modeling of Oxygen Precipitation in Silicon"
The vast majority of modern microelectronic devices are fabricated on single-crystal silicon wafers, which are produced predominantly by the Czochralski (CZ) melt-growth process. Important metrics that ultimately influence the quality of the silicon wafers include the concentration of impurities and the distribution of lattice defects (collectively known as microdefects). This thesis provides a multiscale quantitative modeling framework for describing physics of microdefects formation in silicon crystals, with particular emphasis on oxide precipitates.
Among the most prevalent microdefects found in silicon crystals are nanoscale voids and oxide precipitates. Oxide precipitates, in particular, are critically important because they provide gettering sites for highly detrimental metallic atoms introduced during wafer processing and also enhance the mechanical strength of large-diameter wafers during high-temperature annealing. On the other hand, like any other crystalline defect species, they are undesirable in the surface region of the wafer where microelectronic devices are fabricated. Although much progress has been made with regards to oxide precipitate prediction and optimization, it has been surprisingly difficult to generate a robust, quantitative model that can accurately predict the distribution and density of precipitates over a wide range of crystal growth and wafer annealing conditions.
In the first part of this thesis, a process scale model for oxide precipitation is presented. The model combines continuum mass transport balances, continuum thermodynamic and mechanical principles, and information from detailed atomic-scale simulations to describe the complex physics of coupled vacancy aggregation and oxide precipitation in silicon crystals. Results for various processing situations are shown and comparisons are made to experimental data demonstrating the predictive capability of the model.
In the second part of this thesis, atomistic simulations are performed to study the stress field and strain energy of oblate spheroidal precipitates in silicon crystals as a function of precipitate shape and size. Although the stress field of a precipitate in silicon crystals may be studied within a continuum mechanics framework, atomic scale modeling does not require the idealized mechanical properties (and precipitate shapes) assumed in continuum models and therefore provides additional valuable insight. The atomistic simulations are based on a Tersoff empirical potential framework for silicon, germanium and oxygen. Stress distributions and stress energies are computed for coherent germanium precipitates and for incoherent, amorphous silicon dioxide precipitates in a crystalline silicon matrix. The impacts of precipitate size and shape are considered in detail, and for the case of oxide precipitates, special emphasis is placed on the role of interfacial relaxation. Whenever possible, the atomistic simulation results are compared with analytical solutions.
James Paulos, PhD Candidate, University of Pennsylvania
Advisor: Mark Yim
10:45 am, Towne 321
"Underactuated Attitude Control for Rotocraft Micro Aerial Vehicles"
Unmanned aerial vehicles are now charged with missions very different from the traditional roles of manned aircraft. These applications challenge conventional vehicle formats on several fronts including scaling to small size, cost effective deployment, and human acceptance and safety. Reducing the required number of actuators and the mechanical complexity in micro aerial vehicles can help support these performance goals. I will present an approach for achieving conventional thrust, roll, pitch, and yaw control in a MAV with only two actuators. Attitude control is achieved by varying the blade pitch through every revolution of the propeller. Conventional helicopters kinematically proscribe this motion at the expense of a swashplate mechanism and additional servoactuators. In this work, cyclic variation of blade pitch is attained directly as the dynamic response of an underactuated hinged rotor to a modulated drive torque.
In this way two large drive motors may provide all thrust and attitude authority -- no secondary servomotors or control surfaces are required. The reduced number of actuators in contrast to a four-motor quadrotor or two-motor and two-servo helicopter may reduce cost, mechanical complexity, and actuator mass, in turn making increasingly small MAV more practical and effective.
Sheng Mao, PhD Candidate, University of Pennsylvania
Advisor: Prashant Purohit
"Continuum Modeling of Flexoelectricity"
A material is said to be flexoelectric when it polarizes in response to strain gradients. The phenomenon is well known in liquid crystals and biomembranes but has received less attention in hard materials such as ceramics. With the advent of nanotechnology, flexoelectric effect in solid structures is gaining increasing prominence. For instance, gradient-engineered epitaxial films, mechanical rotation of ferroelectric polarization, making piezoelectric structures from non-piezoelectric materials. It also inspires ways of enhanced energy harvesting. To better understand this effect, a systematic continuum model is built, starting from fundamental theory to computational techniques.
On the theoretical end, a model is established for a flexoelectric solid under small deformation and linear constitutive relation. Gradient raises the order of the governing PDE’s and a Navier equation is obtained for isotropic case, which resembles that of Mindlin’s in gradient elasticity. Based on this theoretical model, various boundary value problems can be solved with analytic solutions, including torsion, beam bending, pressurized disk/cylinder, etc. We predict size-dependent electromechanical properties and flexoelectric modulation of material behavior. We also look at the interplay of flexoelectricity with defects. Defects have a strong gradient field around them—so do we expect flexoelectric effect. We quantify this expectation by computing analytic solutions of displacement, stress and polarization fields near different types of defects in flexoelectric solids, namely point defects, line defects (dislocations) and cracks. Our solution can make connections to non-local piezoelectric theory, gradient elasticity theory and some well-known experimental results. Our crack asymptotic analysis can be used in classical criterion to make predictions on crack growth as well as material fracture.
Based on the above theoretical/analytic results, for the sake of studying more sophisticated problems of flexoelectric effect in solids, we derive a computational theory of simulating flexoelectric effect in solids. We circumvented the challenge of higher-order PDE’s by introducing a “mixed” formulation where both displacement and displacement gradients are treated as “independent” variables. Based on this idea, a weak formulation for flexoelectric solids can be derived. A new III9-87 element is used and clears the patch tests. The proposed technique gives excellent agreement to benchmark problems with known analytic solutions. Moreover, the technique is used to study the classical problem of a plate with a hole. We study the size-effect and the flexoelectric reduction of the stress concentration factor. We also study the shape effect. We demonstrate how non-centrosymmetric defects could help make piezoelectric nano-structures from centro-symmetric and even isotropic materials.
Nathan Ip, PhD Candidate, University of Pennsylvania
Advisor: Kevin Turner
"Characterization of the Adhesion and Separation Behaviors of Elastomers Using a Blister Contact Test"
The characterization of interfacial adhesion is important in a wide variety of applications such as microtransfer printing of semiconductor components and pressure sensitive adhesives. The properties of adhesive contacts during both formation and the separation of the contact are of interest in many applications. However, most standard tests, such as the peel test, only provide information on the separation behavior. In this talk, a blister contact test to simultaneously measure the elastic and adhesive properties of a flexible flat sheet pressed into contact with a much stiffer reference surface is presented. The test consists of a circular flat flexible specimen that is held in place at the edge and loaded with a time-varying pressure to achieve controlled loading and unloading configurations. The stiffer reference surface is placed a short distance from the specimen and constrains the out-of-plane deformation. The in-plane displacements are tracked via digital image correlation prior to contact so that the elastic properties of the specimen can be determined. Once the specimen is pressurized into contact with the reference surface, the contact area is tracked optically and the strain energy release rate can be calculated from the experimental data using a mechanics model. Contact formation and separation are both observed in the test, allowing for complete characterization of the adhesion behavior of the contact.
This talk will discuss the details of the experimental setup as well as finite element and analytical models for calculating the strain energy release rate from the experimental data. The effect of boundary conditions, friction, and pre-stress on the specimen mechanics will be discussed. Results from two series of experiments will be discussed: (1) millimeter thick PDMS sheets in contact with glass and (2) thin PET sheets in contact with PDMS-coated glass substrates. In both of these systems, it was found that: (1) the critical strain energy release rate increases with increasing separation rate, consistent with previous results, and (2) the critical strain energy release rate decreases with increasing contact rate during contact formation. The rate-dependence during contact formation has not been investigated extensively in previous work, thus a new model is developed to describe this rate dependence. In addition, the effect of relative humidity and repeated contact were investigated. The results of this study provide useful data on PDMS contacts, which are important in transfer printing and bioinspired adhesives, and also show the potential for using the blister contact test as a means to measure adhesive properties of flat sheet specimens.
Moeketsi Mpholo and Teboho Nchaba
"Evolution of Wind Speeds Over Subtropical Southern Africa"
10:45 am, Towne 337
An appreciation of the historical wind climate is essential to provide context for developing projections of the possible scenarios of the resource. Changes in wind speeds have significant impacts on a number of fields, such as, energy output of wind farms, wind turbines designs and wave climate. ERA-Interim, CFSR, and MERRA reanalysis models products at 10 m and at 850 hPa geopotential height have been analysed over the period 1979 to 2010 for part of the subtropical Southern Africa domain. A statistically significant decrease in speeds is mainly observed over the south west of the domain in the summer months (December, January, and February) and an increase over the south in the winter months (June, July, and August). The uncertainty in the results is quantified by the percentage error between the models wind climatology and trend results. Principal component analysis, performed on mean sea level pressure data for each of the models, has been used to describe the observed changes and their link to the most important mode of large scale circulation over the southern hemisphere, the Southern Annular Mode (SAM).
Helen Minsky, PhD Candidate, University of Pennsylvania
Advisor: Kevin Turner
"Composite Posts to Achieve Tunable and Enhanced Adhesion"
Surfaces with enhanced and tunable adhesion have a variety of applications, including microtransfer printing of semiconductor elements, material handling in manufacturing, and gripping surfaces on climbing or perching robots. Traditionally, schemes to achieve tunable adhesion have relied on fabricating arrays of posts or fibers with complex hierarchical geometries, including angled posts terminated by wider caps, that are difficult to fabricate. This work focuses on alternatives to post structures with complex geometries through the use of composite posts that consist of a stiff core and a compliant shell. Posts consisting of a stiff core and compliant shell have enhanced adhesion under normal loading and the pull-off can be reduced via the application of shear.
The adhesion mechanics of composite posts are demonstrated here through a combination of finite element (FE) simulations and experimental measurements. Experiments have been carried out on mm- and micro-scale composite posts to demonstrate their adhesion behavior. On the mm-scale a 3x enhancement in adhesion was observed, while on the micro-scale the improvement in adhesion is up to 8x. At both scales, shear displacements can be used to significantly decrease the effective adhesion. FE simulations were used to understand the experimental results and to examine key features of the mechanics that cannot be measured experimentally, such as interface stress distributions, which. Finally, a parametric study of the effect of the geometry of the insets, including the height, radius and shape of the surfaces (e.g. flat, concave, and convex), was carried out using FE modeling to optimize the adhesion of post structures.