2017-2018 Condensed Matter Seminar
The condensed matter seminar series brings together professors, graduate students, and undergrads interested in condensed matter physics on a biweekly basis. We invite young faculty, postdocs, and advanced graduate students from the greater Boston area to come in, present their research, and chat with their peers here at Tufts. Speakers and attendees cover a range of disciplines including physics, chemistry, biophysics, engineering, and applied mathematics.
Spring 2018
Unless otherwise noted, Spring 2018 seminars are held on Wednesdays at 1:45pm in Room 310 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.
January 31
Paleomagnetism and Nuclear Magnetic Resonance Spectroscopy with Nitrogen-Vacancy Defect Centers in Diamond
Pauli Kehayias, Harvard University
Abstract:
Nitrogen-vacancy (NV) color centers in diamond have generated much recent attention for magnetometry applications. NV centers work at ambient conditions and can be placed close to the diamond surface, allowing sub-micron spatial resolution and sample-sensor separation. I will review two NV magnetic sensing applications. First, I will introduce NV magnetic microscopy, where we image magnetic grains in rock samples for paleomagnetism applications. Together with geology collaborators, we use this tool to measure magnetism in meteorites and early-earth rocks. Next I will discuss NV nuclear magnetic resonance (NMR) spectroscopy from statistically-polarized magnetic nuclei in a picoliter sensing volume. This NMR spectroscopy approach circumvents the sensitivity and high-field limitations in conventional NMR. To conclude, I will summarize the ongoing challenges in these experiments and the planned future directions.
February 14
A Molecular Simulation Study of Crystal Nucleation from Flowing Polymer Melts
David Nicholson, MIT
Abstract:
Under typical processing conditions, the crystallization of polymer material occurs far from equilibrium. In particular, the application of a flow field is known to drastically accelerate the kinetics of crystallization, and in turn alter the morphology and properties of the resulting material. It remains a significant challenge to establish the processing-structure-property relationships that govern the processing of semi-crystalline polymer. Non-equilibrium molecular dynamics (NEMD) simulation has proven to be a useful investigative tool for the study of the early stages of flow-induced crystallization, known as flow-enhanced nucleation. Using this technique, nucleation studies were performed under steady shear and uniaxial extension for monodisperse melts of short (C20) and long (C150) alkanes, as well as for bimodal mixtures composed of both short and long chains. These studies are used to provide quantitative insight into the kinetic mechanism for flow-enhanced nucleation, including contributions from both an entropic driving force and diffusion. Additionally, these studies reveal how the acceleration in the nucleation rate correlates with macroscopic measurable quantities and conformational statistics of the flowing melt. The observed correlations are used to evaluate of the capacity of various models for flow-induced nucleation to describe the NEMD data, and identify promising new directions for modelling nucleation at larger spatiotemporal scales.
February 28
A Drastic Softening of Silica-PDMS Gels: Experiments and Theory
Yue Hu, Wellesley College
Abstract:
During a period of a few days, a mixture of silica powder and silicone oil (poly(dimethylsiloxane), or PDMS) can transform from a stiff gel to a free-flowing fluid. The rate of this gel-fluid transition depends on various factors, such as the PDMS molecular weight and end-groups, as well as surfactant additives. In this work, we conduct rheological measurements to systematically study the effects of these factors. We also propose a theoretical model to explain the mechanism of this gel-fluid transition. Our experimental results are in good agreement with our theoretical predictions.
March 14
Learning From Nature: Biomimetic Mechanisms for New Materials
Carl Goodrich, Harvard University
Abstract:
Biological structures exhibit a level of complexity, functionality, and hierarchy that, if fully understood at a mechanistic level, could usher in the next generation of complex designer materials. For example, biological hydrogels act as selective permeability barriers by filtering nano-scale particles based on size as well as biochemical and biophysical interactions. However, for a class of situations that includes the Nuclear Pore Complex, the mechanism of this filtering has proven challenging to untangle because large non-binding particles are caged by the surrounding polymer network while binding particles exhibit increased, not decreased, mobility. We present an equilibrium mechanism for this counter-intuitive filtering strategy that does not require energy consumption. We show that selective mobility can be achieved and controlled in a simple crosslinked polymer gel by coupling binding to crosslink dynamics. In addition to potentially explaining how the Nuclear Pore attains selectivity, our results lead to specific design rules for manufacturing complex selective gels.
March 28
The Influence of pH on the Motility and Chemotaxis of Helicobacter Pylori
Clover Su, Boston University
Abstract:
Helicobacter pylori is a bacterium that colonizes the human stomach and can cause gastric diseases such as ulcers and cancer. While extensive studies have been published on the motility of H. pylori in various media, the majority of the studies were carried out in homogeneous environments. The gastric mucosa where H. pylori lives exhibits various chemical and mechanical gradients, including a pH gradient varying from 2 to 7 across the mucus layer. We present a live cell tracking study of the motility of H. pylori in Brucella broth at homogeneous pH levels as well as with a pH gradient created using a microfluidic channel. We found that H. pylori swims faster at lower pH in homogeneous environments over pH 3 to 6.3. The bacteria either became immotile or died in a pH below 3. We also noted the bacteria body shape became more coccoidal as the pH decreased below pH 4. The cell body rotation frequency appeared to peak at pH5. In response to the presence of a pH gradient, H. pylori travelled in directed trajectories. They can detect the boundary between the regions of neutral and low pH, and were observed to reverse and move away from the low pH region. Analysis of the bacteria swimming and reversals in porcine gastric mucins and in presence of chemoattractants are underway to examine the chemotactic responses and the influence of a gradient in viscoelasticity on the bacteria motility.
April 4
Symmetric Informationally Complete Quantum Measurements
Blake Stacey, Brandeis University
Abstract:
The SIC problem has a classic feel. It is pretty easy to pose, it has proven fiendishly difficult to solve, and the partial results obtained to date reveal conceptual wormholes between topics that nobody had anticipated were connected. I will explain what these SICs are, the senses in which they constitute optimal quantum measurements, and how thinking about them with the tools of a statistical physicist leads to surprises for quantum computation and thermodynamics
April 11
Intradomain Phase Transitions in Block Copolymers With Orientational Segment Interactions
Chris Burke, University of Massachusetts-Amherst
Abstract:
Block copolymers (BCPs) — i.e. polymer molecules made up of sections of chemically distinct monomers — form a variety of microphase separated structures depending on properties like monomer fraction, monomer immiscibilities, and chain topology. Here we turn our attention to the effect of orientational interactions among chain segments in the limit of flexible chains. Based on previous studies, we know that even in the absence of orientational interactions, chain segments in BCP mesophases exhibit orientational order which couples to the mesophase domain structure. Explicit orientational interactions will induce further orientational order, which will couple to this phase-separation induced ordering in nontrivial ways. Here I will present a self-consistent field theory (SCFT) of flexible block copolymers with orientational interactions. In the context of lamellar phases, we explore the effect of a local preference for segment alignment. We observe a new type of phase behavior: intradomain phase transitions between regions with distinct types of orientational order. I will discuss the origin of this behavior, as well as prospects for further studies of orientational interactions in flexible BCPs.
April 25
Flexible Sensors and Bioelectronics
Sameer Sonkusale, Tufts University
Abstract:
This talk will explore the new realm of making flexible sensors and bioelectronics for applications in healthcare and medicine. The talk will cover fabrication, processing, materials, devices and CMOS integrated circuits for this new and exciting field. Applications range from smart wound dressings for chronic wounds to point of care diagnostics for the developing world. In the first part of the talk, I will focus on the development of sensors and diagnostics on paper and textile as an unconventional substrate. Fabrication of these devices rely on low cost, room temperature processing using a combination of screen and wax printing using locally sourced materials. This is used to make flexible smart bandage that can monitor biomarkers of wound healing and deliver drugs on demand. Another application in point of care diagnostics for early screening of stomach cancer will also be discussed. In the second part of the talk, I will discuss our work on on thread as an unconventional substrate for the realization of microfluidics, sensors and electronics for the future of medical diagnostics. Threads serves as an ideal platform for wearable and implantable application because of its flexibility, stretchability and the possibility for an intimate interface with organs and tissues without the need for any carrier substrate. Application in chronic wound monitoring and surgical suture will be shown for thread-based platforms.
May 16
Interrogating Single Proteins with a Nanopore: Challenges and Opportunities
Liviu Movileanu, Syracuse University
Abstract:
A single nanopore represents an amazingly versatile single-molecule probe that can be employed to reveal several important features of proteins, such as their folding state, backbone flexibility, mechanical stability, binding affinity to other interacting ligands, and enzymatic activity. This information can be obtained using high-resolution single-channel electrical recordings. Moreover, groundwork in this area using engineered protein nanopores demonstrated new opportunities for discovering the biophysical rules that govern the transport of proteins through transmembrane protein pores. With further development and adaptations to a microfabricated chip platform, the outcomes of these approaches will provide a new generation of research tools in nanomedicine for examining the details of complex recognition events in a quantitative manner.
Fall 2017
Unless otherwise noted, Fall 2017 seminars are held on Wednesdays at 3:00pm in Room 401 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.
September 13
Student Research Soundbites
Abstract:
Graduate and undergraduate students in condensed matter physics and related areas will present short synopses of their current research projects.
October 4
Threading through the Basis of Life: Drug-DNA Interactions: A View with Optical Tweezers
Thaya Paramanathan, Bridgewater State University
Abstract:
Studies of small molecule—DNA interactions are essential for developing new drugs for challenging diseases like cancer and HIV. The main idea behind developing these molecules is to target the DNA and inhibit the reproduction of the tumor cells and infected cells. We mechanically manipulate single DNA molecules using optical tweezers to investigate potential drugs that have complex and multiple binding modes. Mononuclear ruthenium complexes have been extensively studied as a test for rational drug design. Potential drug candidates should have high affinity to DNA and slow dissociation kinetics. To achieve this, design of the ruthenium complexes are altered. I will be presenting how combining two mononuclear ruthenium complexes alter the binding of these drugs to DNA and how we use optical tweezers to investigate these dumb-bell shaped binuclear ruthenium complexes threading through the DNA bases.
October 11
State-resolved measurements of methane reactivity on metal single crystal catalysts
Eric High, Tufts University
Abstract:
State-resolved molecular beam experiments are used to study the reactivity of methane on transition metal surfaces due to the significance of C-H bond cleavage in industrial steam reforming. Non-statistical bond-selectivity and mode-specificity resulting from eigenstate selective excitation of CH4 and it's isotopologues have established the significance of rovibrationally excited molecules in the overall reactivity of this industrial process. Recent results for CH2D2 on Ni(111) excited to the symmetric (v1) and antisymmetric (v6) C-H stretching states indicate the role of limited intramolecular vibrational energy redistribution (IVR) between similar oscillators. Additionally, researchers have demonstrated the importance of surface atom motion in C-H bond cleavage by modulating surface temperature across of wide range of translational energies. These findings provide mechanistic insight into this industrially significant reaction.
November 1
Rafts and tunable interactions in colloidal membranes
Joia Miller, Brandeis University
Abstract:
Colloidal membranes composed of micron-long rods are a rich test system for studying membrane properties. Here we study membrane-mediated interactions between self-assembled rafts of shorter rods suspended in the membrane. These rafts, made up of chiral rods, display strongly repulsive interactions when in a background membrane of the opposite chirality due to the twist deformation they cause. However, we find that lowering the net chirality of the membrane allows rafts to bind together into groups by stabilizing an alternate raft state with unfavorable internal twist that minimizes membrane deformation.
November 15
Watching E.coli DNA polymerase III adding and removing DNA bases, one molecule at a time
M. Nabuan Naufer, Northeastern University
Abstract:
Molecular motors are biological machines that harness chemical energy and use it for mechanical work. They play critical roles in many processes such as cell division, cargo transportation, and propagation of genomic information. Replicative DNA polymerases are nucleic acid molecular motors that primarily replicate the DNA in the cell to accurately and efficiently propagate the genomic information. We investigate the force-dependent polymerization (addition of DNA bases) and proofreading exonucleolysis (removal of DNA bases) of pol III core, the three-subunit subassembly of the E. coli replicative DNA polymerase III holoenzyme. We probe these two catalytic activities by exerting force on a single DNA molecule, using optical tweezers to manipulate the pol III core to switch between these two processes. By examining the force and concentration dependence, we demonstrate that the process of switching between polymerase and exonuclease substrates is governed solely by primer stability, which changes with temperature, force, and the presence of mismatches.
November 29
Exploring ultrafast carrier dynamics in energy materials with terahertz spectroscopy
Lyubov Titova, Worcester Polytechnic Institute
Abstract:
Application of new materials in solar energy conversion needs to be guided by the understanding of the ultrafast dynamics of photoinjected carriers and optical excitations. Terahertz (THz) spectroscopy is an all-optical, contact-free tool to probe carrier dynamics over nanometer length scales with sub-picosecond time resolution. In this talk, I will give two examples of using THz spectroscopy to unravel the complex behavior of photoexcited carriers. In one case, we apply time-resolved THz spectroscopy to elucidate carrier transport mechanisms in BiVO4 photoanode material. In another example, we have used THz emission spectroscopy to study photoinduced ultrafast currents in GeS nanosheets. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third generation photovoltaics based on shift current, or bulk photovoltaic effect.
December 6
Microorganism locomotion in viscoelastic fluids
Becca Thomases, University of California-Davis
Abstract:
Many important biological functions depend on microorganisms' ability to move in viscoelastic fluids such as mucus and wet soil. The effects of fluid elasticity on motility remain poorly understood, partly because, the swimmer strokes depend on the properties of the fluid medium, which obfuscates the mechanisms responsible for observed behavioral changes. In this study, we use experimental data on the gaits of the algal cell C. reinhardtii swimming in Newtonian and viscoelastic fluids as inputs to numerical simulations that decouple the swimmer gait and fluid type in order to isolate the effect of fluid elasticity on swimming. In viscoelastic fluids, cells employing the Newtonian gait swim faster but generate larger stresses and use more power, and as a result the viscoelastic gait is more efficient. Furthermore, we show that fundamental principles of swimming based on viscous fluid theory miss important flow dynamics: fluid elasticity provides an elastic memory effect which increases both the forward and backward speeds, and (unlike purely viscous fluids) larger fluid stress accumulates around flagella moving tangent to the swimming direction, compared to the normal direction.
For more information, contact Nelaka Govinna. [NEED UPDATED CONTACT OR REMOVE]