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Circumferential gap propagation in an anisotropic elastic bacterial sacculus

Andrew Rutenberg Dalhousie University

December 05, 2013 PIRSA:13120014
Using the Higgs as a Direct Probe of New Physics

Michael Park Rutgers University
December 05, 2013 PIRSA:13120050
Scientific Area

Particle Physics
Slow Melting and Fast Crystals
December 05, 2013 PIRSA:13120015
Polymer translocation : alternative driving forces

Gary Slater University of Ottawa

December 05, 2013 PIRSA:13120016
From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar

Bianca Dittrich Perimeter Institute for Theoretical Physics
December 05, 2013 PIRSA:13120048
Scientific Area

Quantum Gravity
Inferring the spatiotemporal DNA replication program from noisy data
December 05, 2013 PIRSA:13120017
BBN and the Unnatural Weak Scale

David Pinner University of California
December 05, 2013 PIRSA:13120058
Scientific Area

Particle Physics
Dynamics of the magnetic disc in nematic liquid crystal under the action of magnetic field

Alena Antipova Western University

December 05, 2013 PIRSA:13120018
G_2 geometry and mirror triality

Conan Leung Chinese University of Hong Kong

November 20, 2004 PIRSA:04110021
The viscoelastic properties of the nematode C. elegans

Matilda Backholm McMaster University

December 05, 2013 PIRSA:13120019
Dynamic Force Patterns of an Undulatory Microswimmer

Rafael Shulman McMaster University

December 05, 2013 PIRSA:13120038
How are cell concentrations implicated in activity and selectivity of antimicrobial peptides?

Azadeh Bagheri University of Waterloo

December 05, 2013 PIRSA:13120020

Circumferential gap propagation in an anisotropic elastic bacterial sacculus

Andrew Rutenberg Dalhousie University

December 05, 2013 PIRSA:13120014

We 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.
Using the Higgs as a Direct Probe of New Physics

Michael Park Rutgers University
December 05, 2013 PIRSA:13120050
Scientific Area

Particle Physics
The discovery of the Higgs boson marks the first direct probe into the mechanism of electroweak symmetry breaking. All evidence currently points to the fact that electroweak symmetry is broken by at least one fundamental scalar, and naturalness remains the most compelling reason to expect additional degrees of freedom at the weak scale. This talk will describe some ideas for how to utilize powerful and proven experimental techniques, in conjunction with the fact that observables directly related to the Higgs boson are now experimentally accessible, to make concrete statements about the existence of possible new degrees of freedom at the weak scale.
Slow Melting and Fast Crystals
December 05, 2013 PIRSA:13120015

This talk will focus on the behavior of colloidal crystals, and will describe both the nucleation and growth of crystals and their melting. The nucleation and growth of colloidal crystals is experimentally observed to be much faster than expected theoretically or through simulation. The discrepancy can be as much as 10150! I will describe some new experiments that suggest a possible reason for this. I will also describe the melting of colloidal crystals formed with highly charged particles that form a Wigner lattice. I will show that this melting resembles a second-order phase transition, and follows the prediction of Born for a catastrophic collapse of the elastic constant.
Polymer translocation : alternative driving forces

Gary Slater University of Ottawa

December 05, 2013 PIRSA:13120016

TBA
From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar

Bianca Dittrich Perimeter Institute for Theoretical Physics
December 05, 2013 PIRSA:13120048
Scientific Area

Quantum Gravity
Spin foams provide models for quantum gravity and hence quantum space time. One of the key outstanding questions is to show that they reproduce smooth space time manifolds in a continuum limit.I will start with a very short introduction to spin foams and the structure of quantum space time they encode. I will explain how the investigation of the continuum limit via coarse graining and renormalization techniques led as to consider anyonic spin chains and a classification of ground states in systems with quantum group symmetries.I will then present new results on the continuum limit of spin net models, that allow us to draw first conclusions about the large scale dynamics of spin foams.
Based on: B.D., W. Kaminski, Topological lattice field theories from intertwiner dynamics, arXiv:1311.1798, B.D., S. Steinhaus, Time evolution as refining, coarse graining and entangling, to appear, B.D. M. Martin-Benito, S. Steinhaus, The refinement limit of quantum group spin net models, to appear
Inferring the spatiotemporal DNA replication program from noisy data
December 05, 2013 PIRSA:13120017

In eukaryotic organisms, DNA replication is initiated at “origins,” launching “forks” that spread bidirectionally to replicate the genome. The distribution and firing rate of these origins and the fork progression velocity form the “replication program.” With Antoine Baker, I generalize a stochastic model of DNA replication to allow for space and time variations in origin-initiation rates, characterized by a function I(x,t). We then address the inverse problem of inferring I(x,t) from experimental data concerning replication in cell populations. Previous work based on curve fitting depended on arbitrarily chosen functional forms for I(x,t), with free parameters that were constrained by the data. We introduce a model-free, non-parametric method of inference that is based on Gaussian process regression, a well-known inference technique from the machine-learning community. The method replaces specific assumptions about the functional form of the initiation rate with more general prior expectations about the smoothness of variation of this rate, along the genome and in time. Using this inference method, we can recover simulated replication schemes with data that are typical of current experiments without having to know or guess the functional form for the initiation rate I(x,t). I will argue that Gaussian process regression has many other potential applications to physics.
BBN and the Unnatural Weak Scale

David Pinner University of California
December 05, 2013 PIRSA:13120058
Scientific Area

Particle Physics
The discovery of a perturbatively-coupled, 125 GeV Higgs, together with the absence of LHC signals for supersymmetry, places the principle of naturalness under tension. In this talk I will discuss the possibility that the weak scale is unnatural, with its value determined environmentally in the landscape. In particular, this environmental selection may be driven by BBN: as the weak scale is increased, the abundance of Hydrogen in the early universe is rapidly depleted. That our own universe contains an O(1) fraction of primordial Helium arises as a conspiracy among the weak scale, the neutron-proton mass difference, and the planck scale.
Dynamics of the magnetic disc in nematic liquid crystal under the action of magnetic field

Alena Antipova Western University

December 05, 2013 PIRSA:13120018

We simulated Ni disc immersed in a liquid crystal using a lattice Boltzmann algorithm for liquid crystals. In the absence of external torques discs with homeotropic anchoring align with their surface normal parallel to the director of the nematic liquid crystal. In the presence of a weak magnetic field (<10G) the disc will rotate to equilibrate the magnetic torque and the elastic torque due to the distortion of the nematic director. When the magnetic field rotates the disc so that the angle between normal to the surface of the disc and director of the liquid crystal becomes greater than the disc goes through the transition in which . The analysis of this behavior was performed.
G_2 geometry and mirror triality

Conan Leung Chinese University of Hong Kong

November 20, 2004 PIRSA:04110021
The viscoelastic properties of the nematode C. elegans

Matilda Backholm McMaster University

December 05, 2013 PIRSA:13120019

Undulatory motion is utilized by crawlers and swimmers such as snakes and sperm at length scales spanning more than seven orders of magnitude. The understanding of this highly efficient form of locomotion requires an experimental characterisation of the passive material properties of the organism as well as of its active force output on the surrounding medium. The millimeter-sized nematode Caenorhabditis elegans provides an excellent biophysical system for both static and dynamic biomechanical studies. Here we present a novel experimental technique where the deflection of a force-calibrated micropipette is used to directly probe the viscoelastic material properties of C. elegans. The worm was shown to have a self-similar elastic structure as well as a surprising shear-thinning viscous component (M. Backholm W. S. Ryu and K. Dalnoki- Veress PNAS 110 (2013)). The excellent force (pN) and time (ms) resolution provided by the micropipette deflection technique also enables measurements of the drag forces experienced by the microswimmer as it moves through a liquid. This direct experimental characterisation of C. elegans provides guidance for theoretical treatments of undulatory locomotion in general.
Dynamic Force Patterns of an Undulatory Microswimmer

Rafael Shulman McMaster University

December 05, 2013 PIRSA:13120038

C. elegans is a millimeter-sized nematode which has served as a model organism in biology for several decades primarily due to its simple anatomy. Using an undulatory form of locomotion this worm is capable of propelling itself through various media. Due to the small length scales involved swimming in this regime is qualitatively different from macroscopic locomotion because the swimmers can be considered to have no inertia. In order to understand the microswimming that this worm exhibits it is crucial to determine the viscous forces experienced during its motion. Using a micropipette deflection technique in conjunction with high speed imaging we have directly measured the time-varying forces generated by C elegans during swimming. Furthermore by analyzing the bodys kinematics over time and applying a simple model of locomotion we can compute the theoretical force curves. We observe excellent agreement between the measured and calculated forces. The success of this simple model has important implications in the understanding of microswimming in general.
How are cell concentrations implicated in activity and selectivity of antimicrobial peptides?

Azadeh Bagheri University of Waterloo

December 05, 2013 PIRSA:13120020

Antimicrobial peptides (AMPs) are known to be active against a wide range of microbes. Cell selectivity is an important quality of AMPs which enables them to preferentially bind to and kill the microbes over host cells. Despite its significance in determining the cell selectivity however the cell-concentration dependence of AMP activity has not been criticality examined. Here we present a coarse-grained model for describing how cell concentrations are implicated in AMP's membrane-perturbing activity and selectivity.

Title	Speaker(s)	Date	Collection	Type	Info	Relevance
Circumferential gap propagation in an anisotropic elastic bacterial sacculus	Andrew Rutenberg	2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
Using the Higgs as a Direct Probe of New Physics	Michael Park	2013‑12‑05	Particle Physics	Scientific Series	View details	1
Slow Melting and Fast Crystals		2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
Polymer translocation : alternative driving forces	Gary Slater	2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar	Bianca Dittrich	2013‑12‑05	Quantum Gravity	Scientific Series	View details	1
Inferring the spatiotemporal DNA replication program from noisy data		2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
BBN and the Unnatural Weak Scale	David Pinner	2013‑12‑05	Particle Physics	Scientific Series	View details	1
Dynamics of the magnetic disc in nematic liquid crystal under the action of magnetic field	Alena Antipova	2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
G_2 geometry and mirror triality	Conan Leung	2004‑11‑20	Thematic Program on The Geometry of String Theory 2004 - 2005: Mirror Symmetry	Other	View details	1
The viscoelastic properties of the nematode C. elegans	Matilda Backholm	2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
Dynamic Force Patterns of an Undulatory Microswimmer	Rafael Shulman	2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1
How are cell concentrations implicated in activity and selectivity of antimicrobial peptides?	Azadeh Bagheri	2013‑12‑05	Waterloo Soft Matter Theory	Conference	View details	1

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