Physics Office


List of Events

All seminars take place at 2 pm on Thursdays in Abelson 333, unless otherwise indicated.

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Fall 2017 IGERT Seminars

Thursday, September 28
Arjun Narayanan, MIT
Title:  A First Order Phase Transition Underlies the Formation of Sub-Diffractive Protein Aggregates in Mammalian Cells
Abstract: Failure in protein quality control can often lead to protein aggregation, yet in neuro-degenerative diseases, by the time aggregates can be seen, the cells have advanced well into the disease pathology. We develop a quantitative imaging approach to study the protein aggregation process in living mammalian cells. We find that sub-diffractive precursor aggregates may form even in untreated cells, and their size distribution is exactly as predicted for a system undergoing a first order phase transition. Practically, this implies that as soon as aggregates reach a critical size (Rc = 162 ± 4 nm in untreated cells), they will spontaneously grow into large inclusions. Our data suggest that a previously uncharacterized, RuvBL1 dependent mechanism clears aggregates above the critical size. Our study unveils the existence of sub-diffractive aggregates in living cells; and the strong agreement between cellular data and a nucleation theory, based on first order phase transition, provides insight into regulatory steps in the early stages of aggregate formation in vivo.
Abelson 333

Thursday, October 19
Eric Lowet, Boston University
Title: Cortical gamma-band synchronization is predicted by coupled oscillator principles
Abstract: Changes in neuronal gamma-band synchronization by stimulation or cognitive manipulations have been extensively documented, however a mechanistic framework for establishing gamma synchronization among cortical sites is currently lacking. Here, we expand on a theory of weakly coupled oscillators, originally proposed by Ermentrout and Kopell, to help explain an enigma: how can oscillators that are not rigid in their frequency subserve communication between neuronal ensembles? Using gamma-generating network models and recordings in awake macaque V1 we show that that the instability of instantaneous frequency in the gamma-band is not a problem, but rather an asset, as it allows dynamical patterns of neural communication. We found that the properties of these frequency fluctuations and the mean frequency difference predicted well the phase-locking strength and the phase-lag between cortical sites. We also mapped the Arnold tongue of V1 gamma synchronization. The findings support the view that gamma synchronization is involved in systematic coordination of cortical activity.
Abelson 333

Thursday, November 9
Ken Kamrin, MIT
Title: Granular Flow Continuum Modeling from Fundamentals to Applications
Abstract: Granular materials are common in everyday life but are historically difficult to model.  This has direct ramifications owing to the prominent role granular media play in multiple industries and terrain dynamics.  One can attempt to track every grain with discrete particle methods, but realistic systems are often too large for this approach and a continuum model is desired.  However, granular media display unusual behaviors that complicate the continuum treatment: they can behave like solid, flow like liquid, or separate into a "gas," and the rheology of the flowing state displays remarkable subtleties that have been historically difficult to model.  To address these challenges, in this talk we develop a family of continuum models and solvers, permitting quantitative modeling capabilities for a variety of applications, ranging from general problems to specific techniques for problems of intrusion, impact, driving, and locomotion in grains. 
To calculate flows in general cases, a rather significant nonlocal effect is evident, which is well-described with our recent nonlocal model accounting for grain cooperativity within the flow rule.  On the other hand, to model only intrusion forces on submerged objects, we will show, and explain why, many of the experimentally observed results can be captured from a much simpler tension-free frictional plasticity model.  This approach gives way to some surprisingly simple general tools, including the granular Resistive Force Theory, and a broad set of scaling laws inherent to the problem of granular locomotion.  These scalings are validated experimentally and in discrete particle simulations suggesting a new down-scaled paradigm for granular locomotive design, on earth and beyond, to be used much like scaling laws in fluid mechanics.
Abelson 333

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