Chakraborty lab provides new understanding on the physics of granular materials

By Kabir Ramola, Ph.D

In the late 1980’s Sir Sam Edwards proposed a framework for describing the large scale properties of granular materials, such as sand or salt. In this description, similar to the well-established framework of statistical mechanics, the global properties of a complex system are determined by an average over all possible microscopic configurations consistent with a given global property. This is usually attributable to the very fast dynamics of the constituent particles making up the system. The extension of such treatments to granular systems where particles are static or ‘jammed’ represents a fundamental challenge in this field. Even so, Edwards’ conjecture postulated that for given external parameters such as volume, all possible packings of a granular material are equally likely. Such a conjecture, like Boltzmann’s hypothesis in statistical mechanics, can then be used as a starting point to develop new physical theories for such materials based on statistical principles. Indeed, several frameworks have been developed assuming this conjecture to be true.

Figure 1 : Snapshot of the system studied and illustration of the associated energy landscape at different volume fractions.

A simple illustration of this conjecture would be, if one were to pour sand into a bowl, and not bias the preparation in any way, then all the trillion trillions of configurations allowed for the grains would be equally likely. Clearly such a conjecture is utterly infeasible to test experimentally.  In a recent paper that appeared in Nature Physics, we instead performed detailed numerical computations on a theoretical system of soft disks (in two dimensions) with hard internal cores. We focused on a system of 64 disks which already pushed the limits of current computational power. We found that if one fixes the density of a given system of disks, the probability of a packing occurring depends on the pressure, violating Edwards’ proposition. However, at a critical density, where particles just begin to touch or ‘jam’, this probability remarkably becomes independent of the pressure, and all configurations are indeed equally likely to occur. This jamming point is in fact very interesting in its own right since most granular materials are found at the threshold of being jammed and ‘unjammed’. To be fair to Edwards, the hypothesis was made for ‘hard’ grains in which particles are precisely at this threshold, and therefore our numerics seem to confirm the original statement. This is the first time that this statement has been out to a direct test and will no doubt lead to many interesting directions in the future.

Links to news sources describing this article:

doi: 10.1038/nphys4168
Numerical test of the Edwards conjecture shows that all packings are equally probable at jamming.
Stefano Martiniani, K. Julian Schrenk, Kabir Ramola, Bulbul Chakraborty & Daan Frenkel.
Nature Physics
2017

 

Colleagues and Students Gather for Astrophysics Symposium

by Roopesh Ojha (PhD ’98)

Radio Galaxy NGC 4261. (credit: Teddy Cheung)

From June 28th through 30th, about fifty former and current students, colleagues and friends of Brandeis astrophysics Professors John Wardle and David Roberts gathered in the Physics building for a symposium titled “When Brandeis met Jansky: astrophysics and beyond.” This event was organized to celebrate their achievements in astrophysics and their impact on generations of students. Their work has established Brandeis as a major player in radio astronomy.

The symposium title refers to Karl Jansky who is credited with starting an entirely new means of studying the cosmos using radio waves. Radio astronomy arrived at Brandeis with Professor Wardle in 1972. He was joined in 1980 by Professor Roberts and together they pioneered a very powerful observational technique called Very Long Baseline Polarimetry. This involves the use of telescopes separated by thousands of kilometers to produce the sharpest images available to astronomers. Their methods allow astronomers to map the magnetic fields in and near celestial objects. With their students and colleagues, John and Dave have exploited this technique to study the magnetic fields in quasars and active galaxies, and near super massive black holes far outside our Milky Way galaxy as well as black holes closer to home.

Physics Conference Group

Professors John Wardle and David Roberts (front right) with former students and colleagues on the steps of the Abelson physics building (photo: Mike Lovett)

The reach of John and Dave’s work was reflected in the content of the presentations and the composition of the attendees, some of whom had traveled from as far afield as South Korea, India, and Europe. All major centers of radio astronomy were represented. At the conference dinner, several former students expressed their appreciation for the roles Dave and John have played as their mentors.

In their presentations, Dave and John described their current projects and highlighted the work of their undergraduates, graduate students and postdoctoral fellows, who have all gone on to successful careers in academia and industry.

The nineteen PhD theses produced by the Brandeis Radio Astronomy group

Professor Roberts has decided to retire at the end of August, though his retirement plans include a huge program of continuing research into unusual-shaped radio galaxies. These may represent galaxy mergers and the possible merger of their central black holes, and is being carried out with colleagues in India. Professor Wardle has no intention of retiring and is expanding his horizons so to speak — he is part of the Event Horizon Telescope collaboration, an international team of astronomers that is attempting to make the first image of the ‘event horizon’* of a black hole!

The symposium was organized by Teddy Cheung (PhD ’05, now at the Naval Research Laboratory) and Roopesh Ojha (PhD ’98, now at NASA, Goddard Space Flight Center), with generous help and support from the Physics Department.

* The boundary around a black hole beyond which nothing can escape.

Summer SciFest 2017 to Showcase Undergrad Research on August 3

SciFest 2016Brandeis Summer Scifest, an Undergraduate Research Poster Session, will be held on Thursday, August 3. The poster session will be 1:00 to 3:00 pm in the Shapiro Science Center atrium.

SciFest is an annual poster session for undergraduates who have spent their summers working in both on-campus and off-campus labs doing scientific research, usually alongside grad students, postdocs and faculty members. It an opportunity for undergraduates from across the Division of Science, including summer visitors and Brandeis students, to present posters summarizing their research.

There were 106 posters presented last year. Prospective presenters for this year should note that the deadline to register for this event is July 25. Early registrants will get the prime locations for their posters!

The public is invited to attend and to discuss research with the students. As always, refreshments will be served.

Learning to see

How do we learn to see? Proper visual experience during the first weeks and months of life is critical for the proper development of the visual system. But how does experience modify neural circuits so that they exhibit the proper responses to visual stimuli? Knowledge of the mechanisms by which the brain is constructed early in development should inspire new therapies for repairing the brain if it develops improperly or is damaged by disease or injury.

At the present time, it is not possible to directly view all or even most connections within a living neural circuit. Therefore, neuroscientists often build computational models to study how these circuits may be constructed and how they may change with experience. A good model allows scientists to understand how these circuits may work in principle, and offers testable predictions that can be examined in the living animal to either support or refute the model.

Undergraduate Ian Christie ’16 was interested in understanding how neural circuits in the ferret visual system become selective to visual motion. At the time of eye opening, neurons in ferret visual cortex respond to an object moving in either of two opposite directions. With about a week of visual experience, each neuron develops a preference for only one of these directions, and greatly reduces its responses to the opposite direction.

Previous models of this process posited that the primary source of the change was in the organization and pattern of inputs to the cortex. But, recent experiments from the Van Hooser lab (Roy/Osik/Ritter et al., 2016) showed that stimulating the cortex by itself was sufficient to cause the development of motion selectivity, which suggests that some changes within the cortex itself must be underlying the increase in selectivity, at least in part. Further, other experiments in the lab of former Brandeis postdoc Arianna Maffei (Griffen et al., 2012) have shown that the cortex becomes less excitable to focal stimulation over the first weeks after eye opening.

Ian constructed families of computational models that could account for both of these observations. In the model, columns of neurons in the cortex already receive input that is slightly selective for motion in one of two opposite directions, but the connections between these cortical columns are so strong that both columns respond to both directions. However, the activity that is caused by simulated visual experience activates synaptic plasticity mechanisms in the model, that served to greatly reduce the strength of these connections between the columns, allowing motion selectivity to emerge in the cortical columns. The project was supervised by faculty members Paul Miller and Stephen Van Hooser, and the results were published in Journal of Neurophysiology (Christie et al., 2017).

Future experiments will now look for evidence of weaker connectivity between cortical neurons with visual experience.

This work was supported by the “Undergraduate and Graduate Training in Computational Neuroscience” grant to Brandeis University from NIH, and by the National Eye Institute grant EY022122. It also used the Brandeis University High Performance Computing Cluster.

Physics department mourns passing of Professor Emeritus Sam Schweber

Sam SchweberSam Schweber, Professor Emeritus of Physics, died May 14th at the age of 89. A theoretical physicist and historian of science, Sam was among that first generation of Brandeis faculty whose genius turned a fledgling institution into a university of the first rank. He published his first book in 1956, when not yet thirty, and his last in 2012, in his mid-eighties. His was an extraordinary life and career.

Sam was born in Strasbourg and came to this country at the age of 14. Like many immigrants and children of immigrants, he attended college at City College of New York, and he then went on to earn an M.S. from the University of Pennsylvania and a Ph.D. from Princeton. A postdoctoral fellowship at Cornell gave him the special opportunity to work under Hans Bethe (whose biography he wrote, many years later). Sam came to Brandeis in 1955 as associate professor of physics and quickly became involved in building the young department. In 1957, the Physics Department started a graduate program, and the following year it established, at Sam’s initiative, a summer institute in theoretical physics, bringing to campus leading physicists as well as selected graduate students and postdocs, for weeks of seminars and colloquia. The institute ran annually for fifteen years, until the federal funding ceased.

The young Sam Schweber had clearly impressed Hans Bethe. In 1955 he co-authored with Bethe (and a third physicist) the two-volume Mesons and Fields, and in 1960, the same three authors published Quantum Theory of Fields. A year after that, in his foreword to Sam’s new book, An Introduction to Relativistic Quantum Field Theory, Bethe observed, “It is always astonishing to see one’s children grow up, and to find that they can do things which their parents no longer fully understand.” This book remains in print five decades after its initial publication.

Sam continued to conduct research and publish in the field of quantum field theory, while also playing an integral part in the growth of Brandeis University. His scholarly interests then started to shift. Volunteering to teach a course on how probability entered the sciences, he became fascinated with the history of science and chose to spend his next sabbatical in the History of Science Department at Harvard. In the third decade of his career, Sam became a historian of science. He joined our interdepartmental program in History of Ideas, and in 1982 was appointed to the Koret Chair in the History of Ideas.

Sam became equally eminent in his new field, publishing a series of significant books and helping to found and then lead the Dibner Institute for the History of Science and Technology at MIT. Sam brought to his writing not only rigorous historical research and a deep understanding of science, but also a strong interest in the human dimension and social consequences of scientific research and discovery. Among his many books were Einstein and Oppenheimer: The Meaning of Genius, In the Shadow of the Bomb: Oppenheimer, Bethe and the Moral Responsibility of the Scientist, and Nuclear Forces: The Making of the Physicist Hans Bethe. Describing another of Sam’s books, Freeman Dyson wrote that “he has produced a lively and readable narrative history, with a lightness of touch than can come only to one who is absolute master of his subject.”

Sam continued to be an active scholar and author after his retirement from Brandeis in 2003. In 2011, he won the Abraham Pais Prize for History of Physics. The citation spoke of “his sophisticated, technically masterful historical studies” and his “broadly insightful biographical writing on several of the most influential physicists of the 20th century.” Sam was a Fellow of the American Physical Society, the American Association for the Advancement of Science, and the American Academy of Arts and Sciences. A further measure of his stature and influence came in the past few days, from the Max-Planck-Institut fur Wissenschaftsgeschichte: “It is with deep regret that we announce the passing on May 14, 2017 of the distinguished historian of science, Professor Sam S. Schweber. Sam was a dear colleague and mentor of many at the Institute and will be sorely missed by all those who had the great fortune and pleasure of knowing him.”

That sentiment will surely be echoed by the many former Brandeis colleagues and students who greatly admired Sam and learned from him.

Brandeis’ Pioneering Science Posse Program

Photo: Mike Lovett

Samia Tamazi ’20

BrandeisNow has posted an article about the history and accomplishments of the Brandeis’ Science Posse program. Read the following excerpt or the entire article:

In June, Macareno and his posse, all Class of 2020, get off Amtrak’s Acela Express train and take a shuttle bus to Brandeis for science boot camp. On the first day, they gather in a classroom in the Abelson physics building […]

(Melissa) Kosinski-Collins, who earned a PhD at MIT, tells them college science is profoundly different from high-school science. With equal parts candor and caring, she sets high expectations, describing the intense workload. The students know that they will be held to lofty standards and that she will support them.

Later in the day, they gather around a long lab table in the Shapiro Science Center, in an area Kosinski-Collins calls Hufflepuff — a nod to one of the houses at Harry Potter’s Hogwarts School. An array of equipment is scattered before them — pipettes, balances, bottles of acetic acid (vinegar) and sodium bicarbonate (baking soda). There are also aluminum foil, Kimwipes, Scotch tape and Ziploc bags.

The students’ assignment is to build an air bag. When acetic acid combines with sodium bicarbonate, they produce carbon dioxide. The students must figure out how much of each chemical to add to fully inflate a quart-size Ziploc bag. But they also have to protect an egg placed inside the bag. This is where the foil, tape and extra bags come in. Along with the cushion of air, these items can be used to keep the egg from cracking when they drop the bag from the Science Center steps, about 15 feet above the ground.

There’s an important catch. Several months earlier, at a meeting in New York, the students got the same assignment. They also completed lab reports describing the quantities of chemicals they used and how they arranged the materials inside the bag to protect the egg. These lab reports are now handed out to different students. They have 10 minutes to repeat the earlier experiment using the reports as a guide […]

Read more at BrandeisNow

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