Circumferential gap propagation in an anisotropic elastic bacterial sacculus
Andrew Rutenberg Dalhousie University
PIRSA:13120014
9949 - 9960 of 16890 Results
Format results
-
Predicting plasticity with soft vibrational modes: from dislocations to glasses
Joerg Rottler University of British Columbia
PIRSA:13120013
Supersymmetry, Non-thermal Dark Matter and Precision Cosmology
Scott Watson Syracuse University
Modelling Materials Microstructure Across Scales using Phase Field Methods
Nikolas Provatas McGill University
PIRSA:13120012
Transition Pathways Connecting Stable and Metastable Phases
An-Chang Shi McMaster University
PIRSA:13120010
Monte Carlo Field-Theoretic Simulations Applied to Block Copolymer Melts
Mark Matsen University of Waterloo
PIRSA:13120009
The Planet Within: Caves from Earth to Mars and Beyond
Penelope Boston National Cave and Karst Research Institute
PIRSA:13120045
The Galactic Real Estate Market: The Physics and Chemistry of Habitability
Penelope Boston National Cave and Karst Research Institute
PIRSA:13120044
Jets Without Jets
Daniele Bertolini Massachusetts Institute of Technology (MIT)
-
Circumferential gap propagation in an anisotropic elastic bacterial sacculus
Andrew Rutenberg Dalhousie University
PIRSA:13120014We have modelled stress concentration around small gaps in anisotropic elastic sheets, corresponding to the peptidoglycan sacculus of bacterial cells, under loading corresponding to the effects of turgor pressure in rod-shaped bacteria. We find that under normal conditions the stress concentration is insufficient to mechanically rupture bacteria, even for gaps up to a micron in length. We then explored the effects of stress-dependent smart-autolysins, as hypothesised by Arthur L Koch. We show that the measured anisotropic elasticity of the PG sacculus can lead to stable circumferential propagation of small gaps in the sacculus. This is consistent with the recent observation of circumferential propagation of PG-associated MreB patches in rod-shaped bacteria. We also find a bistable regime of both circumferential and axial gap propagation, which agrees with behavior reported in cytoskeletal mutants of B. subtilis. We conclude that the elastic anisotropies of a bacterial sacculus, as characterised experimentally, may be relevant for maintaining rod-shaped bacterial growth. -
Predicting plasticity with soft vibrational modes: from dislocations to glasses
Joerg Rottler University of British Columbia
PIRSA:13120013We show how to utilize soft modes in the vibrational spectrum as a universal tool for the identification of defects in solids. Perfect crystals with isolated dislocations exhibit single phonon modes that localize at the dislocation core, and their polarization pattern predicts the motion of atoms during elementary dislocation glide in two and three dimensions in great detail. A superposition of soft modes can be used to construct a population of soft spots that predict the location of local plastic rearrangements at the grain boundaries of polycrystals and in amorphous solids. Additionally, we find a significant correlation between the soft directions of the polarization fields and the atomic displacements that result from elementary shear events. -
Supersymmetry, Non-thermal Dark Matter and Precision Cosmology
Scott Watson Syracuse University
Within the Minimal Supersymmetric Standard Model (MSSM), LHC bounds suggest that scalar superpartner masses are far above the electroweak scale. Given a high superpartner mass, nonthermal dark matter is a viable alternative to WIMP dark matter generated via freezeout. In the presence of moduli fields nonthermal dark matter production is associated with a long matter dominated phase, modifying the spectral index and primordial tensor amplitude relative to those in a thermalized primordial universe. Nonthermal dark matter can have a higher self-interaction cross-section than its thermal counterpart, enhancing astrophysical bounds on its annihilation signals. I will review recent progress in this program, and discuss how we can constrain the contributions to the neutralino mass from the bino, wino and higgsino using existing astrophysical bounds and direct detection experiments for models with nonthermal neutralino dark matter. Using these constraints we will then see how expected changes to inflationary observables result from the nonthermal phase. -
Modelling Materials Microstructure Across Scales using Phase Field Methods
Nikolas Provatas McGill University
PIRSA:13120012Phase field crystal models and their recent extension will be summarized. Their application to non-equilibrium kinetics and phase transformations in materials will be reviewed. In particular, we review new results from applications of this modeling paradigm to solute trapping during rapid solidification of alloys, defect-mediated solid-state precipitation, and magneto-crystalline interactions. We close with a discussion of new complex amplitude representations of PFC models and how these can be used for multi-scale simulations using adaptive mesh refinement methods. -
Building Colloidal Crystals in Anisotropic Media
Colin Denniston Western University
PIRSA:13120011Colloids in a liquid crystal matrix exhibit very anisotropic interactions. Further, these interactions can be altered by both properties of the colloid and of the liquid crystal. This gives a potential for creating specific colloidal aggregates and crystals by manipulating the interactions between colloids. However, modelling these interacting colloids in a liquid crystal is very challenging. We use a hybrid particle-lattice Boltzmann scheme that incorporates hydrodynamic forces and forces from the liquid crystal field. I will discuss configurations that we have studied, including chains and a potentially stable colloidal crystal with a diamond lattice structure. -
Transition Pathways Connecting Stable and Metastable Phases
An-Chang Shi McMaster University
PIRSA:13120010Phase transitions are ubiquitous in nature. Understanding the kinetic pathways of phase transitions has been a challenging problem in physics and physical chemistry. From a thermodynamics point of view, the kinetics of phase transitions is dictated by the characteristics of the free energy landscape. In particular, the emergence of a stable phase from a metastable phase follows specific paths, the minimum energy paths, on the free energy landscape. I will describe the characteristics of the minimum energy paths and introduce an efficient method, the string method, to construct them. I will use self-assembled phases of block copolymers as examples to demonstrate the power of the method. In particular, I will show how precisely determined transition pathways provide understanding and surprises when we try to connect the different ordered phases of block copolymers. -
Monte Carlo Field-Theoretic Simulations Applied to Block Copolymer Melts
Mark Matsen University of Waterloo
PIRSA:13120009Monte Carlo field-theoretic simulations (MC-FTS) are performed on melts of symmetric diblock copolymer for invariant polymerization indexes extending down to experimentally relevant values of N=104. The simulations are performed with a fluctuating composition field, W-(r), and a pressure field, W+(r), that follows the saddle-point approximation. Our study focuses on the disordered-state structure function, S(k), and the order-disorder transition (ODT). Although short-wavelength fluctuations cause an ultraviolet (UV) divergence in three dimensions, this is readily compensated for with the use of an effective Flory-Huggins interaction parameter, ce. The resulting S(k) matches the predictions of renormalized one-loop (ROL) calculations over the full range of ceN and N examined in our study, and agrees well with Fredrickson-Helfand (F-H) theory near the ODT. Consistent with the F-H theory, the ODT is discontinuous for finite N and the shift in (ceN)ODT follows the predicted N-1/3 scaling over our range of N. -
O Topology
Randall Kamien University of Pennsylvania
PIRSA:13120008Yes, quite. But also with some applications. -
The Planet Within: Caves from Earth to Mars and Beyond
Penelope Boston National Cave and Karst Research Institute
PIRSA:13120045We can set foot on faraway planets, in a sense, by exploring the world beneath our
feet. Underground caves provide unique insights into what we might find beneath
alien landscapes. We are studying caves on Earth to understand how they
form, the spectacular minerals they produce, and the unusual creatures – from
microbes to vertebrates – that thrive in them.
By understanding the caves of our own planet, we can use them as models for the
subsurfaces of other planets. This work provides insights into the lava tubes
on celestial bodies including Mars and our Moon, as well as possible dissolved
caves on Titan, which orbits Saturn. There are many possibilities surrounding
cave formation on practically every type of object in the Solar System.
Some of the most extreme cave environments on Earth are inhabited by an amazing array
of microorganisms. Some of these creatures eat their way through bedrock,
some live in hyperacid conditions, some produce unusual biominerals and rare
cave formations, and many produce compounds of potential pharmaceutical and
industrial significance. We study these unique organisms and the physical
and chemical biosignatures they leave behind. Such traces can be
used to provide a “Field Guide to Unknown Organisms” for developing life-detection
space missions. Additionally, the lava tubes clearly present on Mars
and the Moon can provide the basis for future human habitations on those
planets. 
Direct Detection of Classically Undetectable Dark Matter through Quantum Decoherence
Jess Riedel NTT Research
Although various pieces of indirect evidence about the nature of dark matter have been collected, its direct detection has eluded experimental searches despite extensive effort. If the mass of dark matter is below 1 MeV, it is essentially imperceptible to conventional detection methods because negligible energy is transferred to nuclei during collisions. Here I propose directly detecting dark matter through the quantum decoherence it causes rather than its classical effects such as recoil or ionization. I show that quantum spatial superpositions are sensitive to low-mass dark matter that is inaccessible to classical techniques. This provides new independent motivation for matter interferometry with large masses, especially on spaceborne platforms. The apparent dark matter wind we experience as the Sun travels through the Milky Way ensures interferometers and related devices are directional detectors, and so are able to provide unmistakable evidence that decoherence has galactic origins.-
The Galactic Real Estate Market: The Physics and Chemistry of Habitability
Penelope Boston National Cave and Karst Research Institute
PIRSA:13120044Exoplanets, planets circling distant stars, are proving to be an extraordinary source of new thinking about the potential for life beyond Earth. Until recently, we have assumed that our Solar System and its planets were probably representative of such systems elsewhere. But the amazing array of very odd exoplanets that are being uncovered have stimulated a renaissance of thought on the subject of potential homes for life in the universe. Combined with work on extreme lifeforms here on Earth and intensive study of Mars and several other planets and moons in our system, new paradigms for life search missions are emerging. Science fiction has long drawn from and extrapolated out from science, but the cross-fertilization has gone both ways. Some of the more outrageous planets incorporated into fiction in the past may not be so outrageous after all. I will discuss what we think we know about exoplanets so far, how they are detected, how we are beginning to characterize their environments, and ideas about what this means for our search for living neighbors in our galaxy, whether they be microbes or folks we can actually chat with some day. -
Jets Without Jets
Daniele Bertolini Massachusetts Institute of Technology (MIT)
Jets are key tools for physics at the LHC. Usually, jets are identified through a jet algorithm. In this talk, I will present an alternative way of thinking about jets, by showing how a broad class of inclusive jet-based observables can be replaced by event shapes. These event shapes do not require any jet clustering, but they still implement a jet-like pT cut on "jets" with an R-like radius. I will discuss various applications, including event selection at trigger-level, event-wide trimming, and alternative definitions for boosted objects identifiers.