MEAM Seminar Series Summer 2018

For Spring 2018 Seminars, click here.

Seminars are held on Tuesday mornings beginning at 10:45 am in Room 337, in the Towne Building (unless otherwise noted).

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.

June 6

Zac Milne, Ph.D. Candidate, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania
Advisor: Robert Carpick

Title:

"Sliding and Stress Modulate Nanoscale Adhesion of Silicon-Silicon and Silicon-Diamond Contacts"

Abstract:

The Jost report estimates that approximately 3% of a developed nation’s GDP is wasted on friction and wear-related losses, whether through frictional energy loss, mechanical abrasion, machine downtime, and replacing failed parts. For the United States, this equates to hundreds of billions of dollars annually . Nanotribology, the nanoscale study of friction, wear, adhesion, and lubrication, seeks to understand the fundamental causes of these phenomena in an effort to not only reduce the immense costs involved, but to also find ways to manipulate and control frictional and adhesive properties of surfaces where desired.

In this talk, I present the results of experimental studies exploring nanoscale adhesion. Adhesion on the nanoscale is still not well understood and the typically strong adhesion on this scale currently prevents the manufacturing of MEMS devices with contacting and sliding surfaces. Silicon is the primary material used in nano lithography and MEMS development and diamond provides an ideally hard surface to test silicon adhesion against. Using our in situ transmission electron microscopy nanoindentation methodology, we find that cohesion of silicon and adhesion of silicon and diamond depend significantly on both the sliding speed and normal contact stress. Together, these results suggest that shear stress modulates the reactivity of the surfaces. This is the first time that tunable adhesion of hard contacts has been explored in situ.



June 12

NO SEMINAR



June 19

NO SEMINAR


June 26

NO SEMINAR

 

June 27 (Wednesday): MEAM PhD THESIS DEFENSE

Boyang Qin, Ph.D. Candidate, University of Pennsylvania
Advisor: Paulo Arratia

1:00 p.m., Room 309, Towne Building

"Flow Behavior and Instabilities in Viscoelastic Fluids: Physical and Biological Systems"

Abstract:

The flow of complex fluids, especially those containing polymers, is ubiquitous in nature and industry. From blood to plastic melts to airway mucus, the presence of particles, proteins, and polymers in fluids can impart nonlinear material properties not found in simple liquids like water. These fluid material properties, in particular viscoelasticity, can give rise to flow anomalies in industrial applications and intriguing transport dynamics in biological systems. The first part of my work focuses on the flow of viscoelastic fluids in physical systems where I investigate the dynamics of flow instabilities of viscoelastic fluids in three different geometries and configurations. Realized in microfluidic channels, these experiments mimic flows encountered in technology spanning the oil extraction, pharmaceutical, and chemical industries. In particular, by conducting high-speed velocimetry on the flow of polymeric solutions in a micro-channel, we report evidence of elastic turbulence in a parallel shear flow where the streamline is without curvature. These includes activation of the flow at many time scales, anomalous increase in flow resistance, and enhanced mixing associated with the polymeric flow. Moreover, the spectral characteristics and spatial structure of the velocity fluctuation are different from that in a curved geometry. Lagrangian trajectories show spanwise dispersion with transverse modulations, akin to the traveling waves in the turbulent pipe flow of Newtonian fluids. The second part of my talk investigates the motility and transport of active swimmers in viscoelastic fluids that are relevant to biological systems and human health. In particular, by analyzing the ciliary swimming of the bi-flagellated green algae Chlamydomonas reinhardtii in viscoelastic fluid, we show that fluid elasticity enhances the flagellar beating frequency and the wave speed. Yet the net swimming speed of the alga is hindered for fluids that are sufficiently elastic. The origin of this complex response lies in the complex response of flagellar kinematics or gait to elastic stresses. Numerical simulations show that such change in gait reduces elastic stress build up in the fluid and increases efficiency. These results suggest the complex coupling between fluid rheology and flagellar beating in biological processes such as mucociliary clearance in mammalian airways.

 



July 3

NO SEMINAR



July 9 (Monday): MEAM PhD THESIS DEFENSE

Mohammed Asaduzzaman, Ph.D. Candidate, University of Pennsylvania
Advisor: Howard H. Hu

"Interfacial Wave Dynamics of Core-annular Flow of Two Fluids"

10:00 a.m., Room 307, Levine Hall

Abstract:

In core-annular flow of two different fluids, for a set of suitable flow conditions, various shapes of saturated waves such as bamboo, snake and corkscrew waves are observed. Some of the dominant parameters such as thickness ratio of the fluid, Reynolds number, viscosity ratio, density ratio, interfacial surface tension, and the direction of gravitational forces determine the final shape of the saturated wave and their ultimate stability in a nonlinear regime.

When the flow rate ratio is high, sometimes it is difficult to determine the differences between the final shape of the waves for up-flow and down-flow. For some combinations of thickness ratio, viscosity ratio, density ratio, Reynolds number and surface tension, waves tend to break down and bubbles start to form. Interfacial surface tensions between these two fluids play a very important role to stabilize the waves from breaking down.

In this study, new sets of waves were discovered for core-annular flow, which modulate at certain flow parameter ranges. The critical parameter ranges are identified where the waves shift from saturated bamboo waves and bifurcate into modulated bamboo waves. A thorough analysis is performed for the first time to depict the windows of these critical parameters at which this transition takes place. A bifurcation diagram is constructed to capture the regime. A detailed wave shape analysis is performed to characterize these wave shapes and their periods of oscillation.

Due to challenges associated with large computational domain and an enormous computational power requires to resolve the interfacial instability, a three-dimensional true non-axisymmetric model was never studied before. For the first time, effort is being undertaken to construct a viable 3-D Core-annular flow. A general purpose computational fluid dynamics package ANSYS Fluent is used for this analysis. Three dimensional models for both up-flow and down-flow were constructed and a novel explanation is presented to distinguish between the Bamboo waves, Cork-Screw waves, and Snake waves. The sensitivity of down-flow on initial conditions was also verified with 3-D models on some parameter space from selected publication.

July 17: MEAM PhD THESIS DEFENSE

Chenchen Liu, Ph.D. Candidate, University of Pennsylvania
Advisor: Celia Reina

"Dynamic Behavior of Elastic Metamaterials: Multiscale modeling, Simulation, and Design"

11:00 a.m., Room 309, Towne Building

Abstract:

Elastic/Acoustic metamaterials have exhibited a rapid increase of interest due to their surprising dynamic properties, such as sub-wavelength bandgaps and negative effective elastic constant and/or density. The design and optimization of these novel architectures call for an enhanced understanding of the microstructure-properties relations when micro-inertia effects may start to play a macroscopically observable role. In addition, these metamaterials often come with very complex microstructure, resulting in extremely time-consuming simulations, which is beyond the capability of direct numerical simulations.

The thesis starts with the fundamental theorems in homogenization problems, such as Hill's theorems and Hill-Mandel condition. We proved that these theorems hold exactly under finite element discretization, i.e. no additional numerical error is introduced by the finite element discretization during the scale transition. Then, in order to take micro-inertia effect into consideration, we present a variational coarse-graining framework for heterogeneous media under dynamic loading conditions, which is applicable to general material behavior as well as to discrete or continuous representations of the material and its deformation, e.g., finite element discretization or atomistic system. The proposed theoretical framework can be used to perform multiscale numerical simulations in the spirit of multilevel finite element method (FE^2), which has been implemented with the help of an open-source finite element library deal.II. Various time integration algorithms, i.e. explicit and implicit Newmark methods, have been employed for different applications in layered and locally resonant structures with space/time modulation. Also, the multiscale solver has been enabled to run in parallel and the results are compared with direct numerical simulations, showcasing the efficiency and accuracy. Furthermore, we report a material architecture design based on locally resonant hierarchical structures to deliver phononic crystals with enhanced bandwidth. The theoretical analyses based on lattice systems are then certified using continuum models and finite element simulations.



July 24

Gavin Kenneally, Ph.D. Candidate, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania
Advisor: Daniel Koditschek

“Actuator Transparency and Legged Robot Design”

10:45 a.m., Room 309, Towne Building

Abstract:

In the field of haptics, conditions for mechanical “transparency” entail such qualities as “solid virtual objects must feel stiff” and “free space must feel free”, suggesting that a suitable actuator is able both to do work and readily have work done on it. In this context, seeking transparency or backdrivability has come to mean a preference for actuators exhibiting minimal dynamics or no impedance. While such general notions seem satisfactory for a haptic interface, actuators with good mechanical transparency are now being used in high-performance robots where once again they must be able to do work, but are now also expected to perceive their environment by processing signals related to contact forces in the leg or manipulator when an explicit force sensor is not present.

In this talk, I will address the importance of transparency in the context of legged robot design, including the insights leading to the creation of the first power-autonomous direct-drive legged robots. In addition, more recent work seeks to empirically characterize actuator transparency in an effort to characterizing transparency as revealed by comparing the energetic cost of feeling the environment.


July 31

NO SEMINAR

August 2 (Thursday)

John Cortes, Ph.D. Candidate, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania
Advisor: Igor Bargatin

"Knudsen Pump Powered Micro-Hovercrafts"

10:45 a.m., Room 337, Towne Building

Abstract:

The term “Micro-flyers” has generally been associated with centimeter-scale devices such as robotic insects or miniaturized drones. Due to manufacturing and aerodynamics challenges, propulsion at even smaller, sub-centimeter scales has historically been very difficult. In this work, we have utilized the Knudsen pump effect to create micro-hovercrafts a few millimeters in size, which can hover hundreds of microns above an engineered low-stiction substrate at atmospheric pressure conditions. The Knudsen Pump effect, also known as thermal transpiration, can be described as the gas flow in a tube or channel whose one end is hotter than the other. This gas flows from the cold end to the hot end and then creates an overpressure underneath the micro-hovercraft, which causes it to lift-off and maintain a hovering gap.

The micro-hovercrafts consist of a microfabricated plate metamaterial called nanocardboard, which is made from two face sheets interconnected by channels. These ultralight devices have wall thicknesses in the 35-100 nm range, giving them areal densities as low as 0.3 g/m2. The temperature gradient is created by shining a high-powered LED onto the device, which includes a light-absorbent carbon nanotube (CNT) coating. Testing has shown that the micro-hovercrafts can consistently reach heights in the 300-500 µm range above the substrate. Additionally, we present a theoretical model which accurately predicts the results of hovering heights achieved during testing.



August 7

NO SEMINAR


August 14

Teboho Nchaba, Ph.D. Candidate, University of Cape Towne, South Africa
Advisor: Chris Lennard

“Synoptic Drivers of Wind Speeds Over Southern Africa”

10:45 a.m., Room 337, Towne Building

Abstract:

Observational and climate reanalyses data show a decrease in wind speeds over Southern Africa and it’s adjacent South Atlantic Basin and Indian Ocean. The slow down in circulation is predominately observed in austral summer over the satellite era. The changes are largely attributed to the positive inclination of the most important mode of intra- and inter-annual variability over the Southern Hemisphere, the Southern Annular Mode (SAM)/ Antarctic Oscillation (AAO). In this study, we use third generation reanalyses, the CFSR, ERA-I, and MERRA2, to characterize qualitatively and quantitatively the major synoptic tropospheric and stratospheric drivers of circulation over Southern Africa. The study focuses on the trends in the pressure gradients within and between the South Atlantic Anticyclone (SAA) and the continental thermal low, and trends in the intensity of the stratospheric polar vortex. These drivers are associated with the observed circulation changes over the sub-continent’s south to west coasts and adjacent Atlantic and Indian Oceans between 1979 and 2005 in austral summer. The characterization by the reanalyses is used to benchmark the performance of 39 Climate Model Intercomparison Project 5 (CMIP5) General Circulation Models (GCMs) in capturing and reproducing circulation features over Southern Africa.


August 21

TBA