Eric Sembrat's Test Bonanza

Graphene has amazing mechanical properties, including high strength that could one day be useful in lightweight, robust structures. However the material does have flaws.  Researchers at Georgia Tech and the National Institute of Standards and Technology (NIST) have described a family of seven potential defect structures that may appear in sheets of graphene and imaged examples of the lowest-energy defect in the family.  According to a NIST researcher, the fabrication process plays a big role in creating the defects.  “As the graphene forms under high heat, sections of the lattice can come loose and rotate," he said. "As the graphene cools, these rotated sections link back up with the lattice, but in an irregular way. It's almost as if patches of the graphene were cut out with scissors, turned clockwise, and made to fit back into the same place. Only it really doesn't fit, which is why we get these flowers.” Georgia Tech Physics Professor Phil First noted that “…even with these defects, graphene is still spectacularly strong.” Professor First hopes the team can continue studying the defects, both to learn whether their formation can be controlled and to clarify the role of defects in the material's mechanical properties.  Read the article about this research in Physical Review B 83, 195425 (2011).

By studying the X-rays emitted when superheated gases plunge into distant and massive black holes, astrophysicists at the Georgia Institute of Technology have provided an important test of a long-standing theory that describes the extreme physics occurring when matter spirals into these massive objects.

Matter falling into black holes emits tremendous amounts of energy which can escape as visible light, ultraviolet light and X-rays. This energy can also drive outflows of gas and dust far from the black hole, affecting the growth and evolution of galaxies containing the black holes. Understanding the complex processes that occur in these active galactic nuclei is vital to theories describing the formation of galaxies such as the Milky Way, and is therefore the subject of intense research.

For the full article, go here.

The Highlights of 2010 is a special collection of papers that represent the breadth and excellence of the work published in the Physical Biology last year. The articles were selected for their presentation of outstanding new research, received the highest praise from our international referees, and had the highest numbers of downloads in 2010.  Adjunct Professor of Physics Joshua Weitz’s paper:  Quantifying enzymatic lysis: estimating the combined effects of chemistry, physiology, and physics was one of these highlighted articles.

As we are aware, the number of microbial pathogens resistant to antibiotics increases as the rate of discovery and approval of new antibiotic therapeutics decreases. Characterization of lytic enzymes using techniques based on synthetic substrates is often difficult because lytic enzymes bind to the complex superstructure of intact cell walls. Dr. Weitz and his group present a new standard for the analysis of lytic enzymes based on turbidity assays which allow scientists to probe the dynamics of lysis without preparing a synthetic substrate. The challenge in the analysis of these assays is to infer the microscopic details of lysis from macroscopic turbidity data. They proposed a model of enzymatic lysis that integrated the chemistry responsible for bond cleavage with the physical mechanisms leading to cell wall failure. Then they presented a solution to an inverse problem where they estimated reaction rate constants and the heterogeneous susceptibility to lysis among target cells. The ability to estimate reaction rate constants for lytic enzymes will facilitate their biochemical characterization and the development of antimicrobial therapeutics.

2011 Phys. Biol. 8 202010

Georgia Tech School of Physics assistant professor Daniel Goldman and his team were able to show that by tilting this undulating robot’s head up and down slightly, they could control the robot’s vertical motion as it traveled forward within a granular medium. The robot is built with seven connected segments, powered by servo motors, packed in a latex sock and wrapped in a spandex swimsuit.  Full article here.

Search and rescue missions have followed each of the devastating earthquakes that hit Haiti, New Zealand and Japan during the past 18 months. Machines able to navigate through complex dirt and rubble environments could have helped rescuers after these natural disasters, but building such machines is challenging.

Researchers at the Georgia Institute of Technology recently built a robot that can penetrate and "swim" through granular material. In a new study, they show that varying the shape or adjusting the inclination of the robot's head affects the robot's movement in complex environments.

"We discovered that by changing the shape of the sand-swimming robot's head or by tilting its head up and down slightly, we could control the robot's vertical motion as it swam forward within a granular medium,” said Daniel Goldman, an assistant professor in the Georgia Tech School of Physics.

Results of the study will be presented on May 10 at the 2011 IEEE International Conference on Robotics and Automation in Shanghai. Funding for this research was provided by the Burroughs Wellcome Fund, National Science Foundation and Army Research Laboratory.

The study was conducted by Goldman, bioengineering doctoral graduate Ryan Maladen, physics graduate student Yang Ding and physics undergraduate student Andrew Masse, all from Georgia Tech, and Northwestern University mechanical engineering adjunct professor Paul Umbanhowar.

"The biological inspiration for our sand-swimming robot is the sandfish lizard, which inhabits the Sahara desert in Africa and rapidly buries into and swims within sand," explained Goldman. "We were intrigued by the sandfish lizard's wedge-shaped head that forms an angle of 140 degrees with the horizontal plane, and we thought its head might be responsible for or be contributing to the animal's ability to maneuver in complex environments."

For their experiments, the researchers attached a wedge-shaped block of wood to the head of their robot, which was built with seven connected segments, powered by servo motors, packed in a latex sock and wrapped in a spandex swimsuit. The doorstop-shaped head -- which resembled the sandfish's head -- had a fixed lower length of approximately 4 inches, height of 2 inches and a tapered snout. The researchers examined whether the robot's vertical motion could be controlled simply by varying the inclination of the robot's head.

Before each experimental run in a test chamber filled with quarter-inch-diameter plastic spheres, the researchers submerged the robot a couple inches into the granular medium and leveled the surface. Then they tracked the robot's position until it reached the end of the container or swam to the surface.

The researchers investigated the vertical movement of the robot when its head was placed at five different degrees of inclination. They found that when the sandfish-inspired head with a leading edge that formed an angle of 155 degrees with the horizontal plane was set flat, negative lift force was generated and the robot moved downward into the media. As the tip of the head was raised from zero to 7 degrees relative to the horizontal, the lift force increased until it became zero. At inclines above 7 degrees, the robot rose out of the medium.

"The ability to control the vertical position of the robot by modulating its head inclination opens up avenues for further research into developing robots more capable of maneuvering in complex environments, like debris-filled areas produced by an earthquake or landslide," noted Goldman.

The robotics results matched the research team's findings from physics experiments and computational models designed to explore how head shape affects lift in granular media.

"While the lift forces of objects in air, such as airplanes, are well understood, our investigations into the lift forces of objects in granular media are some of the first ever," added Goldman.

For the physics experiments, the researchers dragged wedge-shaped blocks through a granular medium. Blocks with leading edges that formed angles with the horizontal plane of less than 90 degrees resembled upside-down doorstops, the block with a leading edge equal to 90 degrees was a square, and blocks with leading edges greater than 90 degrees resembled regular doorstops.

They found that blocks with leading edges that formed angles with the horizontal plane less than 80 degrees generated positive lift forces and wedges with leading edges greater than 120 degrees created negative lift. With leading edges between 80 and 120 degrees, the wedges did not generate vertical forces in the positive or negative direction.

Using a numerical simulation of object drag and building on the group’s previous studies of lift and drag on flat plates in granular media, the researchers were able to describe the mechanism of force generation in detail.

"When the leading edge of the robot head was less than 90 degrees, the robot's head experienced a lift force as it moved forward, which resulted in a torque imbalance that caused the robot to pitch and rise to the surface," explained Goldman.

Since this study, the researchers have attached a wedge-shaped head on the robot that can be dynamically modulated to specific angles. With this improvement, the researchers found that the direction of movement of the robot is sensitive to slight changes in orientation of the head, further validating the results from their physics experiments and computational models.

Being able to precisely control the tilt of the head will allow the researchers to implement different strategies of head movement during burial and determine the best way to wiggle deep into sand. The researchers also plan to test the robot's ability to maneuver through material similar to the debris found after natural disasters and plan to examine whether the sandfish lizard adjusts its head inclination to ensure a straight motion as it dives into the sand.

This material is based on research sponsored by the Burroughs Wellcome Fund, the National Science Foundation (NSF) under Award Number PHY-0749991, and the Army Research Laboratory (ARL) under Cooperative Agreement Number W911NF-08-2-0004. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of NSF, ARL or the U.S. government.

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Media Relations Contacts: Abby Robinson (abby@innovate.gatech.edu; 404-385-3364) or John Toon (jtoon@gatech.edu; 404-894-6986)

Writer: Abby Robinson

Panelists David Ballantyne, Assistant Professor in the School of Physics and the Center for Relativistic Astrophysics (CRA); Ashley Korzun, graduate student in the Daniel Guggenheim School of Aerospace Engineering; John Krige, Kranzberg Professor in the School of History, Technology and Society; and David Spencer, Professor in the School of Aerospace Engineering and Director of the Center for Space Systems. discussed the future of NASA after the end of the space shuttle program, touching on topics such as the future of payload delivery, the exploration of Mars, the commercialization of space, funding issues and much more.

To view the presentation, please go to http://smartech.gatech.edu/handle/1853/38548.

The sandfish, a type of desert lizard, can vanish into a sandy substrate in a blink of an eye. Approaches that draw on mathematics, physics, and engineering provide complementary insights into how the animal achieves this feat.  In a paper in the Journal of the Royal Society Interface, graduate student Ryan Maladen, Assistant Professor Dan Goldman, and their colleagues describe how they have used a refinement of their resistive force theory to show that a sand-swimming lizard moves forward about as fast as it can. Their model will allow biologists and engineers to explore loco¬motion in granular solids with unprecedented ease and speed. (Nature, 472:177, April 14, 2011)

Assistant Professor David Ballantyne and his co-authors, J.R. McDuffie (Center for Relativistic Astrophysics) and J.S. Rusin (South Cobb High School student) published an article entitled:  “A Correlation Between the Ionization State of the Inner Accretion Disk and the Eddington Ratio of Active Galactic Nuclei” in The Astrophysical Journal.  The paper addressed supermassive black holes in the centers of galaxies grow by ‘accretion’ -- that is, they suck in material from their surroundings which swirls into the black hole like water going down the drain. Now, the actual physics involved in how an accretion disk works is complicated, and, because these accreting black holes are so very far away, it is very difficult to test accretion disk theories by observations with telescopes. However, these accretion disks get so hot as they swirl around the black hole that they produce X-rays and these X-rays interact with the accreting gas leaving ‘fingerprints’ of accretion physics in the X-ray radiation that astronomers can detect. This paper uses these fingerprints from a number of different accreting black holes and describes the discovery of a relationship between the ionization state of the accreting gas close to a black hole and how rapidly the black hole is being fed. Basically, we see that the more rapidly a black hole is gobbling material, the more highly ionized its accretion disk. The exact implication of this relationship is unclear, but further study will allow new tests of our theories of how black holes grow in the universe.

Scientists have discovered a method to control the gas-phase
selective catalytic combustion of methane, so finely that if done at room
temperature the reaction produces ethylene, while at lower temperatures it
yields formaldehyde. The process involves using gold dimer cations as catalysts
— that is, positively charged diatomic gold clusters. Being able to catalyze
these reactions, at or below room temperature, 
may lead to significant cost savings in the synthesis of plastics,
synthetic fuels and other  materials. The
research was conducted by scientists at the Georgia Institute of Technology and
the University of Ulm. It appears in the April 14, 2011, edition of The Journal of Physical Chemistry C.

“_­The beauty of this process is that it allows us
to selectively control the products of this catalytic system, so that if one
wishes to create formaldehyde, and potentially methyl alcohol, one burns
methane by tuning its reaction with oxygen to run at  lower temperatures, but if it’s ethylene  one is after, 
the reaction can be tuned to run at room temperature,” said Uzi Landman,
Regents’ and Institute Professor of Physics and director of the Center for
Computational Materials Science at Georgia Tech.

Reporting last year in the journal Angewandte Chemie International Edition, a team that included
theorists Landman and Robert Barnett from Georgia Tech and experimentalists
Thorsten Bernhardt and Sandra Lang from the University of Ulm, found that by using
gold dimer cations as catalysts, they can convert methane into ethylene at room
temperature.

This time around, the team has discovered that, by using the
same gas-phase gold dimer cation catalyst, methane partially combusts to
produce formaldehyde at temperatures below 250 Kelvin or -9 degrees Fahrenheit.
What’s more, in both the room temperature reaction-producing ethylene, and the
formaldehyde generation colder reaction, the gold dimer catalyst is freed at
the end of the reaction, thus enabling the catalytic cycle to repeat again and
again.

The temperature-tuned catalyzed methane partial combustion
process involves activating the methane carbon-to-hydrogen bond to react with
molecular oxygen. In the first step of the reaction process, methane and oxygen
molecules coadsorb on the gold dimer cation at low temperature.  Subsequently, water is released and the
remaining oxygen atom binds with the methane molecule to form formaldehyde. If
done at higher temperatures, the oxygen molecule comes off the gold catalyst,
and the adsorbed methane molecules combine to form ethylene through the
elimination of hydrogen molecules.

In both the current work, as well as in the earlier one,
Bernhardt’s team at Ulm conducted experiments using a radio-frequency trap,
which allows temperature-controlled measurement of the reaction products under
conditions that simulate realistic catalytic reactor environment. Landman’s
team at Georgia Tech performed first-principles quantum mechanical simulations,
which predicted the mechanisms of the catalyzed reactions and allowed a
consistent interpretation of the experimental observations.

In future work, the two research groups plan to explore the
use of multi-functional alloy cluster catalysts in low temperature-controlled
catalytic generation of synthetic fuels and selective partial combustion
reactions.

Thanks to videoconferencing equipment and a few large-screen televisions, Jennifer Curtis is reaching out to students beyond Tech’s Midtown campus.  

Curtis, an assistant professor in the School of Physics, participates in the Direct to Discovery program, a Georgia Tech Research Institute program that brings research labs into K-12 classrooms with a little help from technology. 

The program’s goal is to help students better understand various areas of science and mathematics in a way that fosters ongoing interest in these areas.   

“Since my lab is so interdisciplinary, we can tie into the curriculum of a physics, chemistry or biology class,” she said. 

According to Kimm Bankston, the Winder-Barrow high school teacher Curtis has worked with, the demos have been quite successful and have stimulated student discussions about science that extend beyond the classroom.

“I think the program is an excellent way to inspire the next generation of engineers and scientists,” Curtis added. 

Recently, The Whistle had an opportunity to learn more about Curtis and her approach to teaching. Here’s what we learned:  

How did you get to Georgia Tech? 

In 2006, both my husband and I were seeking tenure-track academic positions. In the end, it was clear that Tech was the best fit for our combined interests both professionally and personally.

How did you become interested in your area of teaching and research?    

When I started out as an undergraduate at Columbia University, I wanted to pursue photography and writing. But I experienced a major creative block, which led me back to my first love, science and mathematics. The next semester, I started taking physics classes and the rest is history. As for becoming a biophysicist, I always loved biology and after observing that some of the most interesting work done by physicists was in the area of biophysics, I knew where I needed to be.    

In a few sentences, tell us a little bit about your research focus.

My research group studies the mechanics of cells and biomaterials. Also, we invent or develop unique tools to help answer questions about, for example, the coating of a  cell.       

What is your greatest challenge as an instructor, and how have you dealt with it? 

Helping students figure out how to learn and study effectively is always a challenge. For example, there is always a large group of students who work very hard and spend vast amounts of time studying for my introductory physics course. Yet, their performance on tests does not reflect their efforts. I am experimenting with how to instruct students to get to the point where they can internalize and comprehend the difference between deeply understanding how and why they solve problems a certain way versus superficially memorizing or accepting a concept or problem-solving strategy in physics.  

What piece of technology could you not live without as an instructor?

I think a tablet PC works wonders for large classroom lecture halls.   

Where is the best place to grab lunch and what do you order? 

My favorite place used to be Bobby and June’s, but it recently closed. I’d order the Salisbury steak with a side or two of vegetables.

Tell me something unusual about yourself. 

When I was younger, I was a competitive épée fencer and trained several hours a day while I was in high school and for part of my time in college.