Eric Sembrat's Test Bonanza

Science Applications International Corporation (SAIC), a FORTUNE 500 company, has instituted the annual Georgia Tech Student Paper Competition for outstanding technical papers in the fields of engineering, physics, chemistry, applied mathematics, computer sciences, medicine, and science and technology policy written by students at the undergraduate, master’s, and Ph.D. levels. Senior Stefan Froehlich, double major in Physics and Mathematics, is one of the six winners of the 2010 SAIC $1,000 award for his single-author paper "Reducing continuous symmetries with linear slice" submitted to the competition. This research is supported by  a Georgia Tech President’s Undergraduate Research Award and Prof. Predrag Cvitanović's National Science Foundation grant DMR 0820054.

Student Nick Gravish and Assistant Professor Dan Goldman use plate drag to study the response of granular media to localized forcing as a function of volume fraction ϕ. A bifurcation in the force and flow occurs at the onset of dilatancy ϕ_c. Below ϕ_c rapid fluctuations in the drag force F_D are observed. Above ϕ_c fluctuations in F_D are periodic and increase in magnitude with ϕ. Velocity field measurements indicate that the bifurcation in F_D results from the formation of stable shear bands above ϕ_c which are created and destroyed periodically during drag. A friction-based wedge flow model captures the dynamics for ϕ>ϕ_c.

Physicists Nick Gravish and Daniel Goldman (Georgia Tech) and Paul Umbanhowar (Northwestern University) have conducted a systematic study of how the drag force on a vertical plate partially submerged in sand-sized glass beads depends on the beads’ packing fraction ϕ. Their study reveals a surprising phenomenon: For a dense packing—that is, when ϕ exceeds a critical value ϕ_c—the drag force oscillates as the plate moves horizontally. The crucial physics, argue the authors, hinges on the phenomenon of dilatancy: densely packed beads can become less dense when sheared. Dragging a plate through a dense packing creates a “shear plane” that runs from the bottom edge of the plate to the surface of the beads and makes an angle θ with the horizontal. Particles near the shear plane tend to move parallel to it, toward the surface; particles beyond the plane hardly move at all (see the figure). Shear forces arising at the plane cause the local packing fraction to decrease, which makes it easier to move the plate. When the packing fraction dips to ϕ_c, the shear plane remains stationary at the surface even as its bottom edge moves with the plate; thus θ increases, which causes the drag force to also increase. Once the drag force is high enough, a new low-θ, high-ϕ shear plane forms, and the cycle repeats. (N. Gravish, P. B. Umbanhowar, D. I. Goldman, Phys. Rev. Lett., in press.

Physics Today, Steven K. Blau

Hatchlings sea turtles must move quickly over a variety of terrain to reach the ocean.  Dr. Daniel Goldman along with Biology graduate student Nicole Mazouchova, Physics graduate student Nick Gravish and research technician Andri Savu studied in the field (Jekyll Island, GA) how Loggerhead hatchlings move on loose sand and on hard ground. The study revealed that on sand the turtles solidified the material behind their limbs and achieved performance comparable to that on hard ground. The results of this study could be valuable to biologists who seek to understand mechanics organisms use to move in the natural world as well as to roboticists who would like to build robots with performance comparable to animals. For more about this study see http://www.physics.gatech.edu/research/goldman/pages/research/projects/turtle.html.

The Society of Photo-Instrumentation Engineers (SPIE), an international society devoted to advancing light-based research, awarded Professor Rick Trebino and his company, Swamp Optics, the 2009 Prism Award in optics for his invention of the “BOA Pulse Compressor.”  Before the BOA, which stands for Bother-free Optimized Arrangement, compressing ultrashort laser pulses (which inconveniently expand as they propagate through optics) required multiple prisms, arranged extremely precisely relative to one another.  If the light source or prisms were off by the tiniest bit or the laser pulse was tuned to a slightly different wavelength, the output pulse would be badly distorted, and the entire device would require a tedious realignment.  Prof. Trebino’s invention uses only one prism, a corner cube, and a roof mirror.  It takes advantage of a clever new design and the extremely accurate manufacturing that occurs for corner cubes, which reflect a beam back essentially perfectly parallel to the beam incident on them.  As a result, the elegant device is automatically aligned and yields distortion-free pulses.  For further information, go to www.swampoptics.com.

Using optically dense, ultra-cold clouds of rubidium atoms, researchers have made advances in three key elements needed for quantum information systems -- including a technique for converting photons carrying quantum data to wavelengths that can be transmitted long distances on optical fiber telecom networks.  See the full press release here.

Assistant Professor Dan Goldman has won one of the Sigma Xi Young Faculty Awards given to faculty of rank no higher than assistant professor for the outstanding paper (or papers all in a single subject area) published in scholarly journals between January 1, 2008, and December 31, 2008, based on research performed at Georgia Tech. Dan’s interest focuses on the mechanics of locomotion of organisms and models of organisms as they scurry on and within matter like sand. The foot steps and body-undulations of his study subjects (lizards, crabs, cockroaches and robots) push him to understand interaction with materials which can display complex fluid and solid properties in response to perturbation.

Professor Kurt Wiesenfeld was given the Sigma Xi Award for Sustained Research in recognition of outstanding sustained research in a given area. Kurt has maintained an active level of research and his interests focus on the field of nonlinear dynamics. Lately he has turned his attention to so-called complex systems, which are nonlinear but also involve many degrees of freedom.

The Georgia Tech Observatory will host its final Public Night of the semester on Thursday.

“We try to schedule the Public Night
at the best phases of the moon, which is first quarter or half full,” said observatory assistant for the semester Nicole Cabrera. “We stay away from the full moon, so people can see the detail between
the light and the dark of the terminator [the line dividing the dark and light
on the moon’s surface]. In Atlanta, there’s so much light pollution, so we
regularly view the moon and the brighter planets.”

Cabrera adds a festive atmosphere
will accompany the evening, as food and other items will be available to those
who attend the April 22 event, from 8:30 to 11 p.m. atop the Howey Physics
Building. A short, 15-minute information session will precede the event,
speaking to the crowd about what objects they will be viewing. “The Orion
nebula, Saturn and—of course—the moon will be visible, weather permitting,”
Cabrera said. “If it’s a really good night, we can look at some other objects:
a star cluster, binary system and other nebulae.”

Directed by Senior Academic
Professional James Sowell, the Georgia Tech Observatory hosts an “open house”
to Tech students and other visitors each month. On a clear night, Cabrera says,
the observatory may host anywhere from 80 to 100 people, but the average number
of visitors is roughly 60. During the Fall 2008 Family Weekend, 556 people
visited the observatory. While in the past more off-campus visitors have
attended public nights, this academic year has seen an increase in more student
attendants.

WREK Radio hosted a Sunday
Special on April 18 with deejay Daniel Mitchell at 7 p.m. in recognition of
the event. Cabrera answered students’ questions, submitted in advance, during the segment.

“People are usually really
interested in black holes, supernovae,” said observatory assistant Nicole Cabrera, who recently received her
undergraduate degree from the School of Physics. “But questions can be about
anything related to astronomy—the solar system, the sun, comets or even
asteroids.”

While a good way to promote Public Night, Cabrera said the radio appearance was also a good way to clear up common misconceptions
people may have about astronomy. In an undergraduate class she attended, for
example, people were confused how the revolution of the earth around the sun
caused the seasons. “Some people think the earth has an eccentric or elliptical
orbit, and depending on whether it’s further or closer to the sun is how the
seasons work,” she said. “That isn’t true—the earth has a nearly circular
orbit. The tilt of the earth’s axis is responsible for the seasons.”

While a resource to the Georgia
Tech Astronomy Club and School of Physics students, Sowell has used the
observatory as an opportunity for outreach. Groups are able to book private
events. Most recently, the Georgia Tech Environmental Health and Safety Group,
as well as fraternities, sororities, residence halls and other student groups,
have attended viewings at the observatory.

Sowell also takes this outreach to
off-campus audiences. Area school groups have attended private events, and more
than 200 Boy Scouts in the past 2 ½ years have received their astronomy merit
badges through the observatory. Reaching out across the globe, Sowell and the
Georgia Tech Observatory have held high-definition video conferencing with
schools for astronomy outreach, including seven elementary schools in Texas and
a girls’ school in Sydney, Australia.