Condensed Matter Seminar

February 5, 2025

Speaker: Dr. Ryan Poling-Skutvik, University of Rhode Island

Title: Dynamics-Centered Design of Soft Materials

Abstract: In contrast to traditional hard materials, such as metals and ceramics, soft materials are characterized by their deformability and viscoelastic relaxations. As a result, soft matter has the potential to replicate many characteristic properties of biological systems, including self-replication, hierarchical assembly, and functional signaling pathways. Unfortunately, synthetic soft matter systems often fail to achieve the performance required to fully mimic biological systems, in large part due to a lack of sufficient control over the interplay between dynamics and mechanics. Here, we demonstrate how dynamics must be integrated into the description of material properties and structure to approach biomimicry. With this approach, we investigate a novel class of soft matter building blocks – polymer-linked emulsions – that successfully replicate the structure, mechanics, and dynamics of soft biological tissue. On a fundamental level, this biomimicry arises from controlling the transition between the liquid-like dissipation of mechanical energy and the solid-like storage of elastic deformation. We further explore the implications of this stress-induced transition through the design of hydrogels with identical mechanics but different interparticle interactions. We demonstrate how soft materials can heal after failure and fully recover their elasticity through the rearrangement and restoration of interparticle associations. Overall, our findings provide insight into the relationship between structure and dynamics of soft materials, advancing our understanding of fundamental physics and achieving improved biomimetic control over their properties.

March 5, 2025

Speaker: Dr. Erin Teich, Department of Physics, Wellesley College

Title: Mechanical response in jammed materials: From local structure to control theory

Abstract: Amorphous and jammed particulate matter constitutes a wide range of natural and synthetic materials. Despite this ubiquity, the way in which these systems’ disordered microstructure couples to their often subtle and complex mechanical response to external forcing is not yet fully understood, with profound consequences for phenomena ranging from landscape evolution to cellular unjamming during tumor metastasis. In this talk, I will focus on two complementary methods we have used to elucidate the link between microstructure and mechanical response in jammed disordered systems. First, I will discuss previous work in which we explored the relationship between discrete local structure and rearrangement dynamics in jammed materials under oscillatory shear. Next, I will discuss our current efforts to bring linear network control theory to bear on the problem of predicting mechanical response in disordered systems. Our results indicate that node controllability in this context correlates strongly with particle rearrangement under stress, and generally demonstrate that network control theory is a promising mathematical framework for predicting and designing dynamical behavior in disordered media. 

March 26, 2025

Speaker: Dr. Joseph DuChene, Department of Chemistry, College of Natural Sciences, University of Massachusetts Amherst

Title: Sustainable Living through Electrochemistry

Abstract: Humanity is faced with the daunting task of meeting our ever-growing global demand for resources without causing irreparable ecological harm to our planet. Avoiding catastrophic climate change requires a swift and dramatic shift away from fossil fuels towards a more sustainable energy portfolio. Unfortunately, the storage of such intermittent sources of renewable energy from sunlight and wind remains a considerable challenge. If we are to eventually establish a sustainable society, we must learn to synthesize useful chemicals and fuels (H2, NH3, C2H4, etc.) from readily available small molecules (H2O, CO2, NO3, etc.) using renewable forms of energy. Taking inspiration from ecology, I will suggest strategies for tackling one of the greatest technological challenges of our time and outline some of the current limitations preventing humanity from realizing a sustainable future. I will also share the approach my lab is pursuing towards the synthesis of catalysts capable of converting common pollutants into useful chemicals and conclude by sharing our latest results involving the electrochemical conversion of nitrate into ammonia to produce fertilizers at ambient temperature and pressure.

April 2, 2025

Speaker: Emma Lejeune, Boston University

Title: Analyzing large datasets of time-lapse microscopy images to quantify and understand the behavior of mechanobiological systems

Abstract: From the beating heart to tissue assembly and repair, it is well accepted that mechanics plays an important role in the behavior of biological systems. Mechanical forces are not only fundamentally important to biological materials, but are also fundamental drivers of cellular behavior change. However, it is often difficult to determine mechanical state both in vitro and in vivo, and it is particularly challenging to determine how mechanical perturbations will change the mechanical state throughout the domain. Mathematical and computational modeling are important tools to bridge this gap, and image-based data is often the glue that connects mechanical theory to experimental data. In this talk, we will describe our work in curating and disseminating microscopy data and software tools for data analysis and modeling, with a particular focus on cardiac tissue engineering and wound healing. For cardiac applications, we have developed enhanced computational frameworks to analyze structural organization in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and automated tools to track mechanical forces in engineered cardiac microbundles. These tools enable quantitative assessment of tissue maturity and function across different experimental platforms. For wound healing, we are developing computational tools and mechanics-based understanding of three-dimensional tissue models. Our methodological framework has three essential components: (1) open access datasets of time-lapse movies of cells and tissue, (2) open source software to extract interpretable quantities of interest from these time-lapse movies, and (3) combined mechanistic and statistical models of biological behavior informed by these data. We are creating and releasing these datasets, software, and models under permissive licenses, having already published datasets containing over 1,500 cardiac microbundle movies and corresponding analysis tools. Looking forward, we anticipate that these large open access curated datasets combined with open source tools will enable significant advances in our understanding of, and ability to control, living systems. Through this talk, we hope to foster further discussion and collaborations at the interface of biomechanics, image analysis, and open science.

April 16, 2025

Speaker: Dr. Maijia Liao, College of Engineering, Northeastern University

Title: Branching Morphogenesis: Exploring the Scaling Law and its Underlying Molecular Determinants

Abstract: Dendrites, which serve as the antennae of neurons, are often highly branched so they can receive a large number of synaptic inputs, thereby supporting the high connectivity in the nervous system. The systematic narrowing of diameters in branched networks has tantalized physiologists and physicists since the discovery of da Vinci’s rule for trees: the sum of the cross-sectional areas of the daughter branches equals that of the mother branch. This can be written as d_1^p + d_2^p = d_m^p where d is the diameter and the exponent p = 2. In neurons, a scaling law with exponent 3/2, termed Rall's law, was proposed for diameter decrements in axons and dendrites. The challenge is that the finest dendritic processes are often beyond the diffraction limit and cannot be resolved using conventional approaches. I will describe recent progress in technology development and a new scaling law that was discovered due to the technical breakthrough. To gain mechanistic insight into the new scaling law, I combined multidisciplinary approaches including advanced imaging techniques and neurobiology for dissecting the molecular determinants underlying the scaling law. I will conclude by briefly discussing new opportunities and insights generated from this emergent field in biomedical engineering, physics, and neuroscience.

Unless otherwise noted, Fall 2024 seminars are held on Wednesdays at 3:00pm - 4:00pm in Room 402 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.

November 20, 2024

Advanced Manufacturing of Multi-Materials via Ultrafine Fibers and 3d Printing

Prof. Jay H. Park, Department of Plastics Engineering, University of Massachusetts-Lowell

While polymers have played vital roles in modern society, the emergence of nano-scale fabrication, additive manufacturing, and wearable technologies led to the importance of understanding processing-induced structures to tune properties, i.e., processing-structure-property. The Park research group aims to harness and engineer hierarchical structure of polymer, both at nonequilibrium and equilibrium induced by flow deformation and stress relaxation, respectively. To this end, this talk will focus on two form factors based on multi-layered multi-materials: i) ultrafine polymer scaffold with metal organic framework (MOF) nanopaticle, and ii) multi-material additive manufacturing. 

Firstly, polyvinyl alcohol (PVA) fibers electrospun in conjunction with simultaneous electrospraying UiO-66-NH₂ MOF particles that provides both breathable scaffold filters with highly reactive MOF sites with loading as high as 200 g/m² are presented.  Air-controlled electrospinning/electrospray has been implemented and its implication on loading, morphology, and throughput is discussed. The processing-structure-property relationships of the said method are examined with textile functionalities in mind. Ultimately though the reduction of collector speed and transverse direction; higher loadings were produced which enhances the capability of MOF to be used for gas separation. 

Secondly, co-extruded dual-material filaments that integrate materials with distinct glass transition temperature (T_g) and hardness into a single filament structure are presented. Primarily, these filaments pave the way for the fabrication of materials that have traditionally presented challenges in FFF, such as buckling. By integrating a stiffer core with soft thermoplastic elastomers, the structural integrity of the filament is preserved, facilitating the FFF process without compromising the material properties. Moreover, leveraging the differential T_g properties, the printed pars can be annealed at an intermediate temperature between the T_g of the core and the shell. This method effectively heals the shell-shell interfaces, enhancing the durability and longevity of the printed object. Simultaneously, the core, exhibiting a higher T_g, provides a scaffolding that maintains the original printed geometry, preventing warping and deformation commonly associated with annealing processes. Engineering applications that are unique to AM process are presented.
 

November 6, 2024

Markus Nemitz, Tufts University

Title and Abstract TBA

September 25, 2024

Emergent Physical Learning

Sam Dillavou, University of Pennsylvania 

Biological systems (such as brains or slime molds) learn in a collective, bottom-up manner. Local rules and physics define the evolution of constituent elements (e.g. neurons), and the system evolves towards useful functionality as an emergent phenomenon. In contrast, man-made computers perform machine learning in a centralized manner: learning is enforced on each element from the top-down.

I’ll discuss a new class of systems, Contrastive Local Learning Networks (CLLNs), which are ensembles of all-analog elements that perform machine learning. However they do so using physics and local, bottom-up evolution rules, eschewing digital logic and processors entirely. As a result, these systems are fast like computers, energy-efficient like the brain, and are divisible like materials. Understanding the manner in which these systems learn connects machine learning, biology, and out of equilibrium material properties in a new, malleable, experimental framework.

Unless otherwise noted, Spring 2019 seminars are held on Wednesdays at 3:00pm - 4:00pm in Room 316 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.

February 6, 2019

An Eulerian method for mixed soft and rigid body interactions in fluids
Xiaolin Wang, Harvard University

Note location change: This seminar will be held in Room 204.

Abstract: Fluid-solid interaction problems are encountered in many engineering and biological applications, but are challenging to simulate due to the coupling between the two material phases. Typically, solids are simulated using a Lagrangian approach with a grid that moves with the material, whereas fluids are simulated using an Eulerian approach with a fixed spatial grid, requiring some type of interfacial coupling between the two different perspectives. Here, we present a fully Eulerian method for simulating structures immersed in fluids. By introducing a reference map variable to model finite-deformation constitutive relations in the structures on the same grid as the fluid, the interfacial coupling problem is highly simplified. The method is particularly well suited for simulating soft, highly-deformable materials and many-body contact problems. We also extend the technique to simulate rigid solids in an incompressible fluid, using a projection step formulated as a composite linear system that simultaneously enforces the rigidity and incompressibility constraints. Several examples including single deformable/rigid objects, multiple objects.

February 20, 2019

High-Throughput Simulations to Design Optimal Electrolytes for Energy Storage Devices
Nav Nidhi Rajput, Tufts University

Abstract:Today, the pursuit of transformative gains in the performance of electrical energy storage(EES) systems and industrial technologies are intrinsically a material’s problem, which requires the development of novel electrode materials, electrolytes, and architecture. Such innovations of novel electrodes as well as electrolytes for future EES systems lie inherently in the computationally driven design of materials by obtaining a fundamental understanding of the interplay between events scaling over wide spatial and temporal ranges. The composition of electrolytes has critical implications for the performance of current and future energy storage systems; from the formation and stability of the electrode-electrolyte interface to the transport properties, speciation, and viscosity of the bulk electrolyte. In this work, I present a high-throughput multi-scale modeling approach for screening salts and solvents important to multivalent (e.g., Mg2+, Ca2+ and Zn2+), chemical transformation (e.g., Li-S), and redox flow batteries. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode. We find that both Mg and Zn electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg2+→Mg+), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. We also study the effect of salt anion and the solvent on the solvation structure and dynamics of Li-polysulfide in the solution for application in Li-S battery. It is observed that the polysulfide chain length, solvent, and anions have a significant effect on the ion-ion and ion-solvent interaction as well as on the diffusion coefficient of the ionic species in solution. We also considered ionic liquid tethered ferrocene catholyte as redox center utilizing the Fc/Fc+ reaction for improving the performance of non-aqueous redox flow battery materials. It was observed that at solubility limit, the precipitation of solute is initiated through agglomeration of contact-ion pairs due to overlapping solvation shells. This works shows that the combination of modeling with experimental techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties of electrolytes, which is crucial in designing optimal electrolytes for beyond Li-ion batteries.

February 27, 2019

APS Talk Practice

Abstract: Students in condensed matter physics and related areas will practice their APS talks (may take two hours).

March 13, 2019

Swarms and Cells: Collective Behavior via Local Interactions in Biological Systems
Andrea Welsh, Georgia Tech

Abstract: Swarming is a ubiquitous self-organization phenomenon which occurs in many biological systems such as flocks of bird and insect, schools of fish, and collections of bacteria. This sort of behavior emerges spontaneously, arising without any sort of centralized control or leadership. Many crustaceans such as brine shrimp produce swarms in which individuals cluster together rather than spreading out uniformly in their environment. The size and distribution of these swarms are governed by local interactions between individuals. We will discuss the three-dimensional patterns that can be observed in brine shrimp swarms, specifically of the Great Salt Lake strain of Artemia franciscana, at high concentration. These patterns can be easily observed with simple tabletop experiments; however, the causes of these patterns are unknown. We experimentally test the effects of certain environmental conditions on the development of these swarms and discuss the mechanisms behind these swarms. Patterns can also develop in systems where individuals stay in fixed position and change state in time. This is true for systems like the cells in cardiac tissue where the local interactions between cells transfer electrical impulses and calcium ions. We particularly look at the spatial extensions of the FitzHugh-Nagumo model, a reaction diffusion system that is used to model cardiac cells and firing neurons like relaxation oscillators. We see the formation of two distinct types of dynamics despite oscillators being identical and locally coupled.

April 3, 2019

Title TBD
Michael Dimitriyev, Georgia Tech

April 10, 2019

Title TBD
Kerstin Nordstrom, Mount Holyoke College

April 17, 2019

Title TBD
Emily Davidson, Harvard University

April 24, 2019

Title TBD
Speaker TBD

Unless otherwise noted, Fall 2018 seminars are held on Wednesdays at 3:00pm - 4:00pm in Room 316 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.

September 19, 2018

Research Soundbites

Abstract: Students in condensed matter physics and related areas will present short synopses of their current research projects.

September 26, 2018

Driving Forbidden Vibrational Overtones in Trapped Molecular Ions
David A. Hanneke - Amherst College

Abstract: Quantum control of atomic states has enabled understanding and tests of fundamental physics, produced incredibly accurate timekeeping, and advanced quantum computation and simulation. Molecules have extra degrees of freedom that bring both challenges and opportunities if they can be controlled at the quantum level. Active development of molecular control techniques has already produced impressive results. After a brief overview of the state of the art with small numbers of simple molecules, I will focus on one particular application: searches for new physics through time-variation of fundamental constants. Some models of quantum gravity and some classes of dark matter predict temporal changes in the proton-to-electron mass ratio. Molecular vibrations are sensitive to these changes. I will describe our work at Amherst with singly ionized oxygen molecules, both in a beam and a trap, and its prospects for testing physical laws.

October 3, 2018

Unexpected Tools for Programmed Properties and Switchable Fluorescence in Conjugated Molecules
Seth Sharber, Tufts University

Abstract: Advancing the frontiers of optoelectronic and responsive materials with conjugated molecules enables "bottom-up" control over material function from chemical structure, ideally where rational design of molecular components could dictate properties on-demand. However, predicting and directing properties in the solid state remains a central challenge due to the interplay of weak, non-covalent interactions that drive molecular conformation and assembly, which are integral to material properties but difficult to control. Our group has employed side chains, which traditionally function only to increase solubility, as non-covalent control units to achieve programmed assembly in conjugated materials, thereby tuning fluorescence and stimuli-responsive behavior in solids. Fluoroaromatic side chains in three-ring phenylene ethynylene (PE) oligomers introduce significant twisting into the conjugated backbone through fluoroarene-arene (ArF-ArH) interactions with the main chain, resulting in blue-shifted shifted optical properties compared to typical PE solids. Moreover, PEs with this twisted motif show mechanofluorochromic (MFC) response, in which mechanical force leads to a shift in fluorescence that is reversible with heat or solvent fuming. In this "side-chain engineering" domain, we have elucidated two unexpected tools that can tune optical and material properties. Terminal alkyl chains in the PE backbone can tune MFC response in PEs with green-to-orange and blue-to-green force-induced transitions. In addition, the regiochemical arrangement of fluorine atoms in the side chains significantly alters fluorescence and can invert the direction of the MFC response. These tools do not impact electronics of the system, but crystal structures show clear effects on molecular assembly with direct consequences for solid-state behavior, lending fundamental insight to these supramolecular structure-property relationships.

October 17, 2018

HIP Materials: Harnessing Interfacial Phenomena to Design New Soft Materials
Laura Bradley, UMass Amherst

Abstract: Fluid interfaces are fundamentally unique environments for materials synthesis and chemical operations that form the core of this exciting research. For example, interfacial mass transport provides avenues for exploitation in catalysis and separations. Interfacial structure and composition dictate the interactions between phases, giving rise to soft materials such as foams and emulsions. Interfaces are inherently open systems, in contact with bulk phases. Delivery from these phases provides versatile routes to engineer functional materials at fluid interfaces.

In gas—liquid interface engineering, deposition from the vapor-phase provides an important new path for materials synthesis. Here, this concept is explored and developed for vapor-phase deposition of functional polymers onto liquid substrates. A variety of morphologies are formed, ranging from particles, to planar films, to microstructured films, depending on wetting properties and polymer growth rates. Delivery and reaction of precursors in the bulk liquid provide additional degrees of freedom enabling formation of polymer—liquid gels, layered composite films, and encapsulating shells.

Materials design can also be advanced by imparting functionality to interfaces through structured colloids. Amphiphilic Janus particles of various shape have long been endorsed in this context as colloidal analogues to molecular surfactants. However, their widespread utilization has been impeded by the absence of scalable synthesis methods suitable for diverse chemistries and shapes. Here, I describe a newly developed synthesis platform to address this need: Clickable Janus particles that can be modified through thiol-yne click reactions with commercially available thiols which widen the palate of chemical compositions. The extent of modification can be used to control the particle morphology and thus the type of emulsion stabilized. I demonstrate the versatility of this method for generating Janus particles with pH- responsive properties to control emulsions in a tunable fashion.

October 31, 2018

Engineering Defects in Smectic Liquid Crystals: A Toolkit to Direct Assembly of Advanced Materials
Mohamed Gharbi, UMass Boston

Abstract: The opportunities for guiding assembly using energy stored in soft materials are wide open. The emerging scientific frontiers in this field show an exceptional promise for significant new applications. Since soft materials can be readily reconfigured, there are unplumbed opportunities to make responsive devices with tunable properties. Liquid crystals (LCs) constitute a fascinating class of matter characterized by the counterintuitive combination of fluidity and long-range order. These materials are known for their exceptionally successful applications in displays, smart windows, and biosensing applications. When the order of molecules in some local regions of LCs is not well defined, topological defects form. These defects are strong trapping sites for colloidal inclusions and have been widely used to control their assembly. In this talk, I will focus on defects in smectic liquid crystals: the focal conic domains (FCDs). I will show how these defects self-assemble into highly ordered structures named the "flower textures", when created on curved interfaces. These patterns are capable of focusing light and act as micro-lenses similar to an insect's compound eye. I will present recent progress in understanding the mechanisms that govern the formation of theses assemblies and will report how defects can inspire strategies for making a new generation of advanced materials.

November 7, 2018

Fast-moving bacteria self-organize into active two-dimensional crystals of rotating cells
Alexander Petroff, Clark University

Abstract: We investigate a new form of collective dynamics displayed by Thiovulum majus, one of the fastest-swimming bacteria known. Cells spontaneously organize on a surface into a visually striking two-dimensional hexagonal lattice of rotating cells. As each constituent cell rotates its flagella, it creates a tornado like flow that pulls neighboring cells towards and around it. In the first part of the talk, we describe the earliest stage of crystallization, the attraction of two bacteria into a hydrodynamically-bound dimer. In the second part of the talk, we present the dynamics of bacterial crystals, which are composed of 5--200 hydrodynamically bound cells. As cells rotate against their neighbors, they exert forces on one another, causing the crystal to rotate and cells to reorganize. We show how these dynamics arise from hydrodynamic and steric interactions between cells. We derive the equations of motion for a crystal, show that this model explains several aspects of the observed dynamics, and discuss the stability of these active crystals.

December 5, 2018

Engineered biomaterials to improve human health
Gulden Camci-Unal, UMass Lowell

Abstract: Regeneration of tissues that are damaged due to disease or trauma represents a major medical need. Although surgical replacement can be performed to address this issue, insufficient number of donors limits the applicability of the approach. There is an unmet demand for development of tissue replacements. My research aims to control and modulate cellular behavior for directing repair and regeneration of tissues. To achieve this goal, I use diverse tools from chemistry, cell biology, materials science, and engineering. In my seminar, I will talk about new biomaterial platforms to generate multicellular and compartmentalized tissue-mimetics for clinical applications including endothelialization of cardiovascular tissues, regeneration of bone, and invasion of tumors. To overcome the limitations with the conventional methods, we have developed novel approaches to assemble tissue-like structures. This strategy offers unique opportunities ranging from understanding fundamental biology to development of disease models for personalized medicine and organ assembly. The ultimate goal of my research is to improve human health and quality of life.

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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, 2018

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.

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, 2017

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, 2017

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, 2017

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, 2017

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, 2017

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, 2017

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, 2017

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]