Baskaran Wins NSF-CAREER award to pursue research on active fluids

Dr. Aparna Baskaran of the Physics Department has been awarded the prestigious CAREER grant from the National Science Foundation that is a highly competitive development grant for early career tenure track faculty members. This grant will fund the research ongoing in Dr. Baskaran’s group on dynamics in active materials. Active materials are a novel class of complex fluids that are driven out of equilibrium at the level of individual entities. Examples of such systems include bacterial suspensions, cytoskeletal filaments interacting with motor proteins and inanimate systems such as self-propelled phoretic colloidal particles. The theoretical challenge in understanding these systems lies in the fact that, unlike traditional materials, we no longer have the scaffold of equilibrium on which to base the theoretical framework.  At the practical front, these materials exhibit novel properties not seen in regular materials.  Further, they form the physical framework of biological systems  in that regulatory mechanisms modulate the mechanical properties of this material in response to environmental stimuli.  Dr. Baskaran’s research in this field will be done in collaboration with the groups of Dr. Michael Hagan, Dr. Zvonimir Dogic and Dr. Bulbul Chakraborty. It will enhance and complement the MRSEC research activities in the active materials thrust.

Figure Caption : Videos of example systems for active materials. A) A fish school exhibiting complex collective swimming. B) Swarming at the edge of an E. Coli Bacterial Colony. C) Cytoplasmic streaming inside the yolk of a fertilized cell.

LHC announcement of new particle that could be the Higgs Boson

Today at CERN joint seminars were given by the two major experiments (ATLAS and CMS) at the Large Hadron Collider (LHC) in which they announced the observation of a new particle that could be the Higgs Boson. The mass of this particle is 126 GeV, roughly that of a Barium atom. The level of statistical significance of the new particle is five standard deviations, which is general considered the threshold level of observation needed to make a claim of discovery. Higgs Boson has been long sought to complete the Standard Model of Particle Physics. The Brandeis High Energy Physics Group, along with many colleagues from around the world, has been working for the last 18 years designing, building, commissioning, running and analyzing data from the ATLAS experiment (at CERN).

The Standard Model of Particle Physics is our best understanding of the laws of nature that govern the behavior of all the things in the universe that we can see. One outstanding question in this theory has been: “where does mass come from?”. One proposed solution to this problem (by Peter Higgs) was the addition a field to our picture of the universe that adds a drag on each particle, different for each kind of particle, which we interpret as inertia or mass. This theory predicts a new particle, the Higgs Boson. Finding this particle is considered proof of this version of the standard model. One comment on “physics speak”, a particle being “consistent with the Higgs Boson” is not the same as “discovery of the Higgs Boson”. The predicted properties of this particle are very specific and much more work needs to be done to establish the exact nature of this new object. Is it the object that completes that Standard Model or is it a slightly different object that will point to a new direction in the understanding of nature? This question will be hotly pursued in the future running of the experiments.

On a local note, in a more technical vein; one of the two decay modes that were used by the ATLAS experiment to look for the Higgs Boson decays into four leptons, either four muons or four electrons or two of each. The electron and muon being two of the three leptons (meaning these particles don’t participate in the strong or nuclear force) of the Standard Model. The Brandeis HEP group has been instrumental in the design, construction and operation of the system that identifies and measures muons produced in the collisions at the LHC.

Jim Bensinger
July 4, 2012

editor’s note: see also interview at Brandeis NOW

Video Poster: One Dimensional Rings of Coupled Oscillators

Brandeis Physics grad students (IGERT trainees) Michael Giver and Nathan Tompkins have a “video poster” in the NSF IGERT Video & Poster Competition on “One Dimensional Rings of Coupled Oscillators – Turing’s Theory Realized”. You can check out and comment on their poster on-line at http://posterhall.org/igert2012/posters/244.

award ribbonUpdate: Michael and Nathan’s poster received a Judge’s Choice award ($2,000.00) in the competition!

Six scientists secure fellowships

One current undergraduate, and five alumni, from the Brandeis Sciences were honored with offers of National Science Foundation Graduate Research Fellowships in 2012. The fellowships, which are awarded based on a national competition, provide three full years of support for Ph.D. research and are highly valued by students and institutions. These students are:

  • Samuel McCandlish ’12 (Physics) , a current student who did research with Michael Hagan and Aparna Baskaran, resulting in a paper “Spontaneous segregation of self-propelled particles with different motilities” in Soft Matter (as a junior). He then switched to work with Albion Lawrence for his senior thesis research. Sam will speak about “Bending and Breaking Time Contours: a World Line Approach to Quantum Field Theory” at the Berko Symposium on May 14.  Sam has been offered a couple of other fellowships as well, so he’ll have a nice choice to make. Sam will be heading to Stanford in the fall to continue his studies in theoretical physics.
  • Briana Abrahms ’08 (Physics). After graduating from Brandeis, Briana followed her interests in ecological and conversation issues, and  in Africa as a research assistant with the Botswana Predator Conservation Trust, Briana previously described some of her experiences here in “Three Leopards and a Shower“. Briana plans to pursue as Ph.D. in Ecology at UC Davis.
  • Sarah Robinson ’07 (Chemistry). Sarah did undergraduate research with Irving Epstein on “Pattern formation in a coupled layer reaction-diffusion system”. After graduating, Sarah spent time with the Peace Corps in Tanzania, returning to study Neurosciene at UCSF.
  • Si Hui Pan ’10 (Physics) participated in a summer REU program at Harvard, and continued doing her honors thesis in collaboration with the labs at Harvard. Her award is to study condensed matter physics at MIT.
  • Elizabeth Setren ’10 was a Mathematics and Economics double major who worked together with Donald Shepard (Heller School) on the cost of hunger in the US. She has worked as an Assistant Economist at the Federal Reserve Bank of New York and her award is to study Economics at Harvard.
  • Michael Ari Cohen ’01 (Psychology) worked as a technology specialist for several years before returning to academia as  PhD student in the Energy and Resources Group at UC Berkeley.

Congratulations to all the winners!

Barry awarded Joseph Katz Fellowship from Argonne Natl Lab

Edward Francis Barry (PhD ’11) has recently been awarded the prestigious Argonne Scholar-Joseph Katz Postdoctoral Fellowship at Argonne National Laboratory. Ed began his scientific career studying the self-assembly of fd virus with Zvonimir Dogic, during the latter’s Junior Fellowship at the Rowland Institute at Harvard University. When Dogic joined the physics faculty at Brandeis, Ed also came to Brandeis as a Ph.D. student and helped to start the Dogic lab. Ed published seven papers describing various novel assemblages found in the fd system. Most notably, his 2010 Proceedings of the National Academy of Sciences paper describing the physical properties of colloidal membranes won the 2010 Cozzarelli Prize for scientific excellence. As the Katz fellow, Ed will be working between Argonne National Laboratory and the University of Chicago, where he is working with Experimental Condensed Matter Professor Heinrich Jaeger studying the self-assembly of monolayers composed of nanoparticles.


The Higgs boson – what it is and how you would find it

The Higgs Boson is an elementary particle predicted by the Standard Model of particle physics. It is associated with the Higgs mechanism that was developed with the contributions of physicists Brout, Englert, Guralnik, Hagen, Kibble and Higgs in 1964. This mechanism proposed an elegant solution to one of the standing problems in particle physics – how particles acquire mass. The elementary particles of nature have masses varying by many orders of magnitude. For instance, the top quark, the heaviest known particle yet, has a mass that is roughly 340000 times that of the electron. Neutrinos on the other hand, have masses that are constrained by experiment to only a small fraction of the electron mass. The Higgs mechanism suggests that all particles acquire their mass by the strength of their couplings to a field that permeates the whole universe – the Higgs field. The top quark does not get its higher mass because it is bigger in size; in fact it is probably no larger than an electron. It simply couples more strongly to the Higgs field to get its enormous mass at around 173 GeV. Photons do not couple to this field at all and hence are massless. The much sought after Higgs boson is the quanta of this hypothetical field, very similar to the photon being the quanta of the electromagnetic field. One of the main goals of the LHC physics program is to find evidence of this quanta, the Higgs boson.

Higgs bosons, if they exist, can be created in proton – proton collisions at the LHC but would decay instantaneously. Any search for them has to be carried out by analyzing data for signatures of Higgs decay. This is not a trivial task however, because such decays are impossible to distinguish from some other well known Standard Model processes. Collision data from LHC would have to be analyzed, and deviations from known background processes would point towards some form of new physics and possibly the existence of the Higgs boson. One would then have to compare the rates in different Higgs decay channels to see if the observed rates match those from predictions for a Higgs decay.

In December 2011, ATLAS and CMS, the two main experiments at LHC, announced their 2011 Higgs search results which led to much hype in the physics world and the media – both experiments were indeed seeing some excess in several Higgs decay channels around 125 GeV. The mass of a proton is about 0.94 GeV, so the finding suggests a Higgs boson mass that is about 133 times heavier than the proton (notice the bump in the above plot from ATLAS at around 125 GeV).

However, it is still very early to come to a conclusive statement about a discovery. Standard Model backgrounds are subject to statistical fluctuations which show themselves at various levels in experiments. A discovery would only be claimed if the deviations from expected backgrounds are over 5 standard deviations. The results announced by CMS and ATLAS in December 2011 were 1.9 and 2.6 standard deviations above expected backgrounds respectively. Though this is interesting, it is far from sufficient to provide a conclusive answer about the existence of the Higgs boson. The search will have to continue in 2012 as more data from LHC becomes available, and physicists will finally be able to give a conclusive answer to either confirm or exclude the Standard Model Higgs boson, ending a puzzle that has been around for several decades.

Serdar Gozpinar is a 5th year graduate student in the high-energy physics group at Brandeis working on the ATLAS experiment at CERN.

A new twist on interfacial tension

In a mixture of two molecular components, the surface tension is defined as the energetic cost per unit area of moving molecules from the bulk and bringing them to the interface. The higher the magnitude of the surface tension, the greater the tendency of two components to demix. Surface tension allows trees to carry nutrients from the roots out to the branches, and water striders to walk on the surface of water.

The interface between hydrophobic and hydrophilic components has very high interfacial tension. A common way to adjust the magnitude of surface tension is to add amphiphilic molecules (like soaps), which contain both hydrophilic and hydrophobic components. These amphiphilic molecules prefer to be at the interface between the two components, and effectively lower the interfacial tension, allowing the components to mix more easily. This is how detergent causes oily stains to dissolve in water.

In a recently published article in Nature, an interdisciplinary team of researchers at Brandeis headed by Zvonimir Dogic, and consisting of experimental, theoretical, and computational physicists as well as biologists, has demonstrated a new way of controlling interfacial tension using a molecular property called chirality, or lack of mirror symmetry. The study was performed on a model system of two-dimensional colloidal membranes composed of the rod-like bacteriophage virus fd, which are about one micrometer in length and 7 nanometers in diameter. The electrostatically repulsive virus particles are condensed into membranes through the depletion mechanism by adding non-adsorbing polymer to a virus suspension. Because the fd rods are chiral, they tend to twist by a small angle with respect to neighboring rods. However, the geometry of the membrane prevents twisting in the structure’s interior; only along the perimeter can the rods twist. Thus, increasing the strength of chirality of the rods both lowers the energy of the rods along the membrane’s edge and increases the frustration of untwisted rods in the bulk, lowering the interfacial tension. This contrasts the standard method of controlling interfacial tension using amphiphilic molecules, since the rod-like particles are completely homogenous, and do not contain any hydrophilic components.

The strength of chiral interactions in fd is temperature sensitive; the rods are achiral at 60o C, and the strength of chirality increases with decreasing temperature. By increasing the strength of chiral interactions in-situ, the team of researchers was able to dynamically vary the membrane’s interfacial tension in order to drive a dramatic transition from a membrane to several twisted ribbon structures (Movie 1). The twisted ribbons have much more interfacial area than the membranes, but are much “twistier” structures, and are therefore favored when the strength of chirality is relatively high. Additionally, the team was able to drive the same membrane-to-ribbon transition using optical tweezers, as shown in Movie 2. Membranes and ribbons are only two of a myriad of structures that were observed in the fd system. This work presents a powerful new method to control the assembly of materials by tuning interfacial tension with chirality.

Shear-induced jamming

From breakfast cereals to sand on a beach, granular materials are all around us. Under different conditions, these materials can exhibit liquid-like behavior (flowing) as well as solid-like behavior. The transition between solid and liquid phases has been known as the jamming transition.

The basic concept of jamming is pretty intuitive. A simple example of what can induce jamming is the following: compacting loose sand inside a container increases its density. When the container is removed, the sand can form a self-supporting pile, hence becoming jammed. Jamming has been studied extensively in numerical simulations of systems composed of idealized grains without frictional forces.  These studies find a critical density at which jamming occurs. Since these idealized granular materials are non-cohesive (no attractive forces between them)  they can become solids only through externally imposed pressure, such as through compaction, and therefore a critical density makes sense.  Real granular materials, however, have friction, and how this affects jamming is not well understood.

An experimental image of typical Shear Jammed state in a 2-D frictional granular material. The shear strain is applied in the horizontal direction. Red colored grains form the backbone of the system, which provides rigidity with respect to external shear

Newly published in Nature, are results of a collaboration between Prof. Bulbul Chakraborty’s group at Brandeis and Prof. Behringer’s group at Duke University, which show a new class of jammed states in frictional granular materials. This new class of “Shear-Jammed” states exhibits a richer phenomenology than previously seen. An initially unjammed or loose granular material can become jammed not just by increasing its density, but by applying shear strain on it while holding the density fixed. Shear-Jammed states are inherently anisotropic in their stress and grain-to-grain contact network (see photo above). The transition from an unjammed to shear-jammed state is clearly marked by a percolation of the strong force chains in all directions (see video below). The phenomenon of shear-jamming does not currently have a fundamental theoretical description. Ongoing work in Prof. Chakraborty’s group attempts to construct a theoretical framework for this non-equilibrium phase transition using a generalization of equilibrium statistical ensembles.

This video shows the evolution of the strong force cluster and transition from unjammed to fragile and eventually to SJ. The video shows experimental states created under pure shear. Green colored grains form the strong force cluster defined in the paper. Initially, the system is unjammed. As the fraction of force bearing grains increases with increasing strain, the strong force cluster percolates in the compressive (vertical) direction and we call the state fragile.  Eventually the system becomes percolated in all directions with sufficient number of force bearing grains. We call these states Shear Jammed.

see also:

Protected by Akismet
Blog with WordPress

Welcome Guest | Login (Brandeis Members Only)