Events

The following are events associated with or of interest to the Center for Computation and Theory of Soft Materials community.

UPCOMING Events

Modeling Dynamics of Confined Hydrogels: Focus on Pattern Formation, Degradation and Erosion

Dr. Olga Kuksenok
Associate Professor, Materials Science and Engineering
Clemson University, Clemson, SC


March 3, 2021
10: 00 - 11: 00 am CST

Abstract:

Pattern formation and dynamic restructuring plays a critical role in a plethora of natural processes. Understanding and controlling pattern formation in synthetic materials is important for imparting a range of biomimetic functionalities. Further, degradation of polymer networks plays a vital role in applications ranging from regulating growth of complex tissues and neural networks to controlled drug delivery. I will focus on controlling dynamic restructuring in responsive hydrogels on two distinctly different length scales. On a continuum level, we use the three-dimensional gel Lattice Spring Model (gLSM), which allows us to focus on dynamics of pattern formation in mm-sized confined hydrogels under variations in external stimuli (temperature, light, mechanical forcing). We demonstrate dynamic restructuring of patterns under stretching and compression and characterize the hysteresis behavior. Our findings show that mechanical forcing can be harnessed to control the onset of pattern formation and hysteresis in gel-based systems.

In the second part of my talk, I will focus on the modeling of the photodegradation process of the confined hydrogel membrane on the mesoscale. Specifically, we develop a dissipative particle dynamics (DPD) approach that captures degradation and erosion in hydrogels. We focus on hydrogels formed by the end-linking of four-arm polyethylene glycol macromolecular precursors (tetra-PEG gels). We track the progress of the degradation via measuring the fraction of the degradable bonds intact, mass loss from the hydrogel films, and distributions of clusters formed during the degradation process. As the degradation proceeds, the hydrogel film undergoes reverse gelation. The reverse gel point calculated from simulations agrees well with the value obtained from the bond percolation theory. Our approach introduces first mesoscale framework capturing erosion and reverse gelation in degrading polymer networks.

Bio:

Dr. Kuksenok is currently an Associate Professor at the Materials Science and Engineering Department at Clemson University in Clemson, SC. Prior to joining Clemson University in 2015, she was appointed as a Research Associate Professor at the Chemical Engineering Department at the University of Pittsburgh, Pittsburgh, PA. Dr. Kuksenok  received her PhD in Physics and Mathematics from the Institute of Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine, in 1997, then she was appointed as a  research scientist in Theoretical Physics Department, Institute for Nuclear Research, Kiev, Ukraine, and as a postdoctoral research associate at the University of Pittsburgh. Dr. Kuksenok’s research interests and accomplishments span the following areas of computational materials science: elastodynamics of responsive polymer gels, dynamics of multi-component polymer blends, biomimetic and biological materials, pattern formation in non-equilibrium chemical systems, complex fluids dynamics, and theory of heterogeneous liquid crystalline systems. Dr. Kuksenok has co-authored over 90 peer-reviewed publications and 9 book chapters.

 


Past Events

Dielectric Response of Confined Polar Liquids: From Atomistic to Continuum Modeling

Dr. Mohammad Hossein Motevaselian
University of Illinois at Urbana-Champaign
February 17, 2021

Abstract:

Understanding the physics of the confined fluids and obtaining atomic-level insights into their unusual properties is essential to develop and design novel nanofluidic applications related to energy, water, and health. One of the most important properties of any polar liquid is the dielectric permittivity, as it affects the long-range electrostatic interactions. In this talk, molecular dynamics simulations, statistical-mechanical theories, and multiscale methods are adopted to unravel the effects of confinement on the dielectric response of polar liquids. Depending on the degree of confinement and different spatial directions (e.g., perpendicular, ε_⊥, or parallel, ε_∥, to a flat interface), polar liquids experience a universal reduction in the perpendicular permittivity, abnormally low out-of-plane screening ability and planar supper permittivity under extreme confinement of a few molecular diameters in width. Such effects have important consequences in developing accurate coarse-grained force fields, improving the solvent-implicit approaches often used in biology and continuum theories such as the Poisson-Boltzmann equation for accurate prediction of capacitance in the electric double-layer capacitors.

Bio:

Mohammad Hossein Motevaselian received his B.S. in mechanical engineering from University of Tehran in 2012, M.Sc. in mechanical engineering from University of Illinois at Urbana-Champaign in 2015 and Ph.D. in mechanical engineering from University of Illinois at Urbana-Champaign in 2020. His Ph.D. thesis was titled “Bridging the Gap Between Atomistic and Continuum Models to Predict Dielectric and Thermodynamic Properties of Confined Fluids” under the supervision of Dr. Narayana Aluru. His research activity is focused on developing multiscale methods using coarse-graining techniques, statistical mechanical theories, and atomistic simulations to develop force-fields and study the physics of interfacial molecular fluids, with the primary focus on the role of dielectric permittivity in altering the electrostatic interactions between charged species. Applications of his research include energy storage devices such as supercapacitors, ionic liquids, electrochemical reduction of carbon dioxide, ion transport across desalination membranes and biophysics.

 


Active particles with aligning interactions: aggregation vs. flocking

Dr. Demian Levis
Assistant Professor, Condensed Matter Physics
University of Barcelona, Spain


October 14, 2020
10: 00 - 11: 00 am CST

Abstract:

Unlike passive systems, active systems, made of interacting self-propelled particles, are driven out-of-equilibrium at the level of each constituent. As such, they exhibit rich and novel collective behaviours. In particular, activity may trigger clustering and phase separation in the absence of cohesive forces, or the spontaneous emergence of collective directed motion.

To gain an understanding of the general mechanisms underlying such phenomena,  simplified model systems have been introduced and analyzed in-depth. Among them, the Active Brownian Particle (ABP) and Vicsek model has become paradigmatic systems to elucidate the role played by the nature (or symmetry) of the interactions between agents. ABP accounts for the competition between excluded volume interactions and self-propulsion, giving rise to the so-called Motility-Induced Phase Separation (MIPS), while the Vicsek model describes particles tending to align towards the mean direction of its neighbours, triggering a flocking phase transition.

Here I'll discuss the interplay between these different interaction's mechanisms: excluded volume and velocity alignment. I'll show how different kind of alignment affects the stability of the Motility-Induced Phase Separated state, and, conversely, the role played by steric effects in the emergence of coherently moving structures. If time allows I'll also mention how different self-propulsion strategies, in particular circle swimming, provide different routes to non-equilibrium pattern formation.   

Bio:

Demian Levis earned a master's degree in Condensed Matter Physics in 2009 from the University of Orsay, France and a PhD in 2012 in Statistical Mechanics from the University of Pierre et Marie Curie, France. During his doctorate, under the supervision of Prof. L. Cugliandolo, he worked on the role played by hard constraints in the critical phenomena and out-of-equilibrium dynamics of a geometrically frustrated magnet, spin-ice. After that, he joined the group of Dr L. Berthier at the University of Montpellier 2, France where he started to work on active matter using simple particle models. In 2015 he obtained the Marie Curie Individual Fellow to continue studies in the collective behaviour of chiral active matter. Recently he has been appointed as Lecturer (Assistant Professor) Condensed Matter Physics Department at the University of Barcelona, Spain.

 


CCTSM Distinguished Lecturer Co-sponsored with CBES: Surprises in the Self-assembly of Particles in Spherical Confinement

Alfons van Blaaderen
Professor, Soft Condensed Matter
Debye Institute for Nanomaterials Science
Utrecht University

Monday, July 1, 2019

Abstract: About 6 years ago our group started research at developing methodologies to structure matter at multiple length scales by Self-Assembly (SA). Presently, we see the induced SA of particles inside slowly drying droplets dispersed in an emulsion system and the resulting supraparticles (SPs) as a powerful generally applicable methodology of hierarchical SA. We found that making the shape of the particles the dominant factor in the SA is the most versatile way to use this route also for complex particle shapes and mixtures of particles. One of our first findings by both experiments and computer simulations was that spherical particles self-assembled inside a spherical confinement do not have their equilibrium bulk face centered cubic, close packed, crystal arrangement, but instead adopt an icosahedral symmetry. In recent work, we have extended our results to include the effects of particles shape (e.g. using rounded cube shaped particles), rod-shaped particles, plate-shaped and binary particle systems. We will discuss how these changes affect the SA and how such SPs can be analyzed quantitatively on the single particle level in 3D by electron microscopy tomography. We will also show our first more applied work on creating SPs with tunable light emission.


Computer Simulations of Heterochromatin Segregation in Eukaryotic Cell Nucleus

Aykut Erbas
Assistant Professor, Material Science and Nanotechnology Program

Bilkent University, Ankara, Turkey

Wednesday, June 19, 2019

Abstract:One of the unresolved puzzles in biological sciences is the 3D packing of the meters-long DNA molecule in the confinement of micron-scale cell nucleus while regulating fundamental cellular activities, from protein transcription to replication. Although the underlying 3D conformation of the genome is a complex phenomenon resulting from the dynamic interactions between nuclear proteins and negatively charged DNA, relatively simple computational models can guide us about the large-scale and long-time behavior of the nuclear ingredient. For instance, extensive computer simulations can provide an explanation of how changes in heterochromatin (a histone rich version of the chromosome) content of the nucleus can alter global nuclear organization. Similarly, the model system can allow us to manipulate the interactions between confining nuclear shell and chromatin, thus, help us elucidate the rules of genomic organization. Furthermore, activities of structural-maintenance-of-chromosomes proteins such as topoisomerase II can be considered by relaxing of certain topological constraints in computational models.

Biography: Dr. Aykut Erbas received his Ph.D. in Physics from Technical University Munich (TUM) - Germany in 2011. During his graduate studies, he worked on peptide-surface interactions, modeled single-molecule experiments. Later for his postdoctoral studies, he moved to the University of North Carolina - Chapel Hill, Chemistry Department, to focus on polymer dynamics. Between 2014 and 2018, he was a research fellow at Northwestern University, Material Science & Engineering Department and Biomolecular Sciences. Dr. Erbas has recently accepted a faculty position at Bilkent University, Material Science and Nanotechnology Program (UNAM) in Ankara-Turkey and explores complex phenomena in polymeric systems, including hydrogels, ionic liquids, and amphiphilic peptides and DNA-protein interactions.


Water At Nanoscale solid-liquid interfaces

Pietro Asinari
Professor, Energy Department and Director, Multiscale modeling Laboratory, Politecnico Di Torino, Italy

Wednesday, May 8, 2019

Abstract: In this talk, we will discuss three applications, where the behavior of water at nanoscale solid-liquid interfaces has a prominent significance to predict the performance of biomedical phenomena or to guide the rational design of engineering devices.

First, we use molecular dynamics simulations to compute the self-diffusion coefficient of water within na-nopores, around nanoparticles, carbon nanotubes and proteins. For almost 60 different cases, the diffusion coefficient is found to scale linearly with a dimensionless parameter, which represents the confinement degree of the water molecules [1]. Such relationship has accurately predicted the response of contrast agents for magnetic resonance imaging [1]. Later on, the Oak Ridge National Laboratory (ORNL) has validated experi-mentally and independently this relationship, beyond biomedical applications [2].

Second, experiments and atomistic simulations are used to elucidate the non-trivial interplay between na-nopore hydrophilicity and the overall water transport through zeolite crystals. A poor correspondence between the experiments and simulations has revealed the presence of a surface diffusion resistance at the interface between the zeolite porous matrix and water [3]. This suggests future experimental works to address these surface imperfections, as an essential prerequisite for improving water permeability of such membranes.
Finally, the complexity of water-solid interfaces is fully revealed by surfactants wrapping nanoparticle (NP) in aqueous solutions. Despite the large use of nanoparticle suspensions, tuning NP interactions and identifying desired NP assembly processes, in presence of surfactants, still represent a challenge for the design of nano-suspensions. We present a multiscale model for investigating nanoscale interfacial phenomena, stability, and aggregation of nanoparticles in aqueous solutions, including the dynamics of realistic surfactants [4]. The key idea is to combine the traditional DLVO (by Derjaguin-Landau-Verwey-Overbeek) theory with the kinetic theo-ry of aggregation. As a future perspective, we plan now to apply such approach for studying the aggregation of engineered nanomaterials and hence to get insights for their risk management (http://www.nanoinformatix.eu).


The Architecture of Biological Matter: Amino Acids, Proteins and Nucleic Acids

Jean-Louis Sikorav
High Council for Economy & Sorbonne Université

Tuesday, December 4, 2018

Abstract: DNA is the solution found by Nature for the storage and transmission of hereditary information. Why is DNA such as it is and not otherwise? This question will be addressed through a theoretical construction of the genetic material, using the language and the methods introduced in order to build the foundations of biology (1). The construction shows the necessity of molecular chirality for living processes; it establishes the unique and ideal character of DNA and improves our understanding of the structure of amino acids and proteins (2). 


Linking the Colloidal Cluster Morphology With the Extended Law of Corresponding States

Nestor Valadez-Perez
Postdoc, Illinois Institute of Technology

Monday, November 26, 2018

Abstract: The clustering of spherical particles interacting with a short-range attraction has been extensively studied due to its relevance to many applications, such as the large-scale structure in amorphous materials, protein crystallization, aggregation and phase separation of protein inside cells, among others. The parameters that determine the interaction potential among particles affect the way they assemble and the morphology of clusters. It was widely accepted that the range of the attraction solely controls the fractal dimension of clusters, however recent experimental results challenged this concept by also showing the importance of the strength of attraction. In this talk it is presented a quick review of the applications of the extended law of corresponding states in colloidal systems, including the relation of local properties to this law. This physical escenario is confirmed with the reanalysis of experimental results on colloidal-polymer mixtures, previously reported in the literature.


Asymmetric Breathing Motions of Nucleosomal DNA and the Role of Histone Tails

Sharon M. Loverde
Associate Professor, Chemistry Department,
CUNY - Staten Island, New York

Wednesday, October 10, 2018

Abstract: Being the basic packaging unit of DNA, the nucleosome core particle (NCP) stabilizes as well as controls DNA accessibility during vital cellular processes. Recently reported structures of the NCP suggest that the histone octamer undergoes conformational changes during the process of DNA translocation (Bilokapic, Nature Communications, 2018). The mechanism of repositioning of the DNA around the histone octamer has been suggested to be sequence-dependent. Furthermore, the histone talks have been suggested to play a key role in this repositioning. We demonstrate with long-time all-atomistic molecular dynamics simulations (5 microseconds) that the histone tails do indeed play a critical role in nucleosome repositioning through formation of a loop of nucleosomal DNA around the histone core at high salt concentrations (2 M NaCl) (Chakraborty, Kang, and Loverde, in review).  

Biography: Sharon M. Loverde, PhD, is an Associate Professor in the Chemistry department at College of Staten Island, a senior campus at the City University of New York (CUNY).  She is also a faculty member of the Graduate Center of CUNY in the areas of Chemistry, Biochemistry, and Physics.  Dr. Loverde’s research focuses on molecular dynamics simulations of soft and biological systems.  In 2014, she received an ACS PRF New Investigator Award and in 2018 she received an NSF CAREER Award.  Dr. Loverde was an NIH NRSA Postdoctoral Fellow who worked with Dennis E. Discher (UPenn) and Michael L. Klein (Temple).  She received her PhD in Materials Science and Engineering from Northwestern University working with Monica Olvera de la Cruz. 


Soft Matter Physics at the Nanoscale

John T. King
Center for Soft and Living Matter, Institute for Basic Science,
South Korea

Friday, September 28, 2018

Abstract: A significant challenge in experimental soft matter physics is overcoming the reliance on bulk-level measurements that average over spatial and temporal heterogeneities. Rapidly advancing microscopy and spectroscopy techniques, including super-resolution fluorescence microscopy, provide access to the spatial and temporal dimensions that often govern macroscopic structure and dynamics. In this talk, two examples will be discussed: 1) nanoscale molecular transport inside protein-DNA liquid-liquid phase separated droplets, and 2) interfacial dynamics of polymers at a non-adsorbing wall.

Biography: John King leads a group at the Institute for Basic Science, Center for Soft and Living Matter (S. Korea), which focuses on understanding how molecular and macromolecular structure and dynamics manifest in the bulk-level behavior of soft materials. He joined the Institute for Basic Science following a post-doc at the University of Illinois, Urbana-Champaign (2013-2016), where he worked on super-resolution microscopy and its application to polymer physics. He received a PhD in chemistry from the University of Michigan (2009-2013) studying ultrafast multidimensional spectroscopy of liquids. 


Assembly engineering of soft-matter-based nanomaterials using molecular dynamics simulations

Vikram Jadhao
Professor, Intellegent Systems Engineering -
School of Informatics, Computing and Engineering at Indiana University, Bloomington

Monday, September 17, 2018

Abstract: Self-assembly is a common process in generating biological materials and provides inspiration for engineering synthetic materials with tailored structure and functionality. I will present results from coarse-grained molecular dynamics simulations of the self-assembly of soft-matter-based nanoparticles (NPs); these systems have important applications in nanomedicine and in the design of active functional materials. In the first part of this talk, I will discuss the results on the assembly of bacteriophage P22 virus-like particles (VLPs) into three-dimensional ordered array materials in the presence of oppositely-charged dendrimers via electrostatic control. Equilibrium structures (including VLP-bound dendrimer distributions) as a function of the ionic strength and VLP surface charge will be identified and correlated with experiments. I will then present results on controlling the shape of deformable nanocontainers via the modulation of surface design patterns and ionic environment, which show that surface tension modulates the relative favorability of oblate disk to prolate rod morphologies. Finally, I will present some preliminary results on the nanoscale self-assembly of deformable building blocks, in which we explore the self-assembly of Hepatitis B viral capsids based on a model involving soft capsomeres in order to understand the role of deformability in the capsid assembly and overgrowth.

Biography: Vikram Jadhao is an assistant professor in Intelligent Systems Engineering at the School of Informatics, Computing, and Engineering at Indiana University in Bloomington. Prior to joining IU, he held postdoctoral fellowships in the Department of Physics and Astronomy at Johns Hopkins University as well as the Department of Materials Science and Engineering at Northwestern University. He received his Ph.D. and M.S. in Physics from the University of Illinois at Urbana-Champaign, and his B.S. in Physics from the Indian Institute of Technology at Kharagpur. His current research interests are in soft materials, self-assembled nanostructures, bio-inspired systems, ionic solutions, and flow of polymeric materials, with applications in nanomedicine, energy devices, and electronics. He is a recipient of the NSF CAREER award.


Studies in Bio-Inspired Physics

Yitzhak Rabin
Professor, Physics Department; Institute for Nanotechnology and Advanced materials (BINA), Bar Ilan University, Israel

Thursday, August 16, 2018

Abstract:

We devise and study simple physical models that address several questions inspired by biology:

  1. Do non-specific interactions assist or hinder the formation of specific complexes?
  2. How can one suppress aggregation of intrinsically disordered proteins?
  3. How do active processes affect polymer (DNA) dynamics?

Starting With Copolymer Phase Behavior and Moving Away From Equilibrium

Wei Li
Center for Nanophase Materials Sciences Oak Ridge National Laboratory

Monday, April 16, 2018

Abstract: The abundant phase behaviors make block copolymer systems quite intriguing for theoretical and simulation studies. According to the combination of different numbers and types of the constituting blocks, block copolymer can have unlimited varieties. How the architecture at the molecular scale affects the self-assembly morphology at the meso-scale remains an interesting question.  We present a conceptual framework to describe and categorize the phase behavior of two-component block copolymer systems. Based on self-consistent field theory studies, three major universality classes for the topology of the equilibrium phase diagram are identified. Following a bottom-up approach, we analyze the relationship between chain architecture and copolymer mean-field phase behavior.

The precise control of self-assembly over morphology and size at the nanoscale brings a new era for the applications of block copolymers. Combining with other properties, the versality of block copolymers offers a unique platform for material design. To achieve rational design, many fundamental problems need to be addressed. We will present two case studies based on coarse-grained molecular dynamics simulations. In one study, we examined effects of dipolar interactions on the thermosdynamics of microphase separation in diblock copolymer melts and compared the results with the prediction from a field theory approach. In the other study, we investigated responses of thin films containing microphase separated ionic diblock copolymers to applied electric fields.  A plausible picture was provided to interpret the experimental findings from neutron reflectometry measurements.


Mechanics of a Bacterial Biofilm Formed at the Air-Liquid Interface

Dr. Eric Raspaud
Laboratoire de Physique des Solides, Université Paris

Friday, November 10, 2017

Abstract: Bacterial biofilms consist of a complex network of biopolymers embedded with microorganisms, and together these components form a physically robust structure that enables bacteria to grow in a protected environment. This structure can help unwanted biofilms persist in situations ranging from chronic infection to the biofouling of industrial equipment, but under certain circumstances can allow the biofilm to disperse and colonize new niches. Mechanical properties are therefore a key aspect of biofilm life. We study the physical forces acting on the floating pellicles at the air-liquid interface formed by a wild type strain of Bacillus subtilis. We have recently discovered the existence of a growth-induced compressive stress present within the biofilm and we have performed mechanical experiments in which Bacillus subtilis pellicles were subjected to elongational deformations using a custom built apparatus while simultaneously tracking the force response and macroscopic structural changes. Overall, our results indicate that we must consider not only the viscoelastic, but also the viscoplastic and mechanically heterogeneous nature of these structures to understand biofilm dispersal and removal.


Hydrophobic Hydration and the Effect of NaCI Salt in the Adsorption of Surfactants on Clathrate Hydrates

Felipe Jimenez Angeles
Reservoir Engineering Research Institute

Wednesday, September 20, 2017

Abstract: Clathrate hydrates are crystalline structures of water forming cages with guest molecules. Adsorption of hydrocarbons and functional molecules on the surface of hydrates is a key in the mechanisms of hydrate inhibition. The thinking in the literature is that hydrocarbons may not adsorb at the hydrate interface, surfactants adsorb through the headgroup, and ionic surfactants adsorb more strongly than non-ionic surfactants. Hydrophobic hydration is the ordering of water molecules around hydrophobic molecules in structures similar to clathrate hydrates. Here we establish a connection between hydrophobic hydration and the affinity of molecules such as n-decane, a non-ionic surfactant, and an ionic surfactant at the surface of hydrates. The hydrophobic hydration is investigated by means of hydrogen bonding and tetra-hedral structure of water. The affinity for the hydrate surface is investigated through the free energy profile of adsorption on the hydrate surface. We observe the molecules that are favor-ably adsorbed on the surface of hydrates induce hydrophobic hydration in the bulk. On the other hand, the hydrophilic ionic surfactants disrupt hydrophobic hydration and are not favora-bly adsorbed on hydrate surfaces. We show that the idea often presented in the literature that the ionic ammonium groups are hydratephilic may not be correct. We also find that salt disrupts the hydrophobic hydration of hydrocarbons and urfactants in the aqueous phase and increases the propensity of these molecules to be adsorbed on hydrate surfaces. We find a major difference in the adsorption of ionic and non-ionic surfactants on hydrate surfaces. The non-ionic surfactants can be adsorbed by headgroup and tail while the ionic surfactants have less affinity for adsorption and cannot adsorb through the head. Our study is performed by means of molecular dynamics and steered simulations.


Salt Entropy Effects on Protein Solubility and the Hofmeister Series


Wednesday, June 21, 2017Dr. Yuba Dahal
Department of Physics, Kansas State University


Instabilities at Soft Interfaces: From Elastocapillary Snap-Through to Wetting Dynamics on Elastomers

Aurelie Hourlier-Fargette
PhD student at the Université Pierre et Marie Curie in Paris

Thursday, November 17, 2016

Abstract: Capillary forces are often too weak to deform a substrate, but are able to dominate elasticity at small scales, on soft materials. A water droplet can interact in a spectacular way with slender flexible structures such as thin elastic sheets or beams, inducing wrinkling, static or dynamic folding, aggregation of fibers, or buckling. On thicker but softer materials, a droplet that does not deform macroscopically the substrate can raise a microscopic ridge at the triple line. 

The role of surface tension in the mechanics of deformable solids is a question raising a growing interest in the soft matter community. In this context, we will present two examples of instabilities featuring liquids on soft materials.

We will begin by revisiting the snap-through instability from an elastocapillary point of view. Snap-through, which is present in systems ranging from carnivorous plants to MEMS, is a well-known phenomenon in solid mechanics: when a buckled elastic beam is subjected to a transverse force, above a critical load value the buckling mode is switched. We show that capillary forces are strong enough to trigger a snap-through instability at small scales, and even counterbalance gravity for a droplet deposited below a downward buckled elastic strip. We investigate the statics and dynamics of this phenomenon, and compare droplet-induced snap-through to dry point force indentation on a buckled thin strip.

In a second part, we will focus on the dynamics of droplet sliding on elastomers. The motion of droplets on stiff surfaces has been investigated for a long time, both experimentally and theoretically, while recent studies have shown the interesting physics underlying the sliding of droplets on soft surfaces. We focus on the dynamics of water-glycerol mixture droplets sliding down vertical plates of commercial silicone elastomers, highlighting an unexpected behavior: the droplet dynamics on such a surface includes two regimes with different constant speeds. Different candidates can be responsible for the sudden speed change: bistability, chemical interaction with the substrate, softness of the material, etc. Our experiments reveal an unexpected link between microscopic phenomena at the scale of the polymer matrix and the macroscopic dynamics of a droplet. 

Short Bio: Aurelie Hourlier-Fargette is a last year PhD student at the Université Pierre et Marie Curie in Paris (Institut Jean Le Rond d’Alembert), and has a teaching position in the Physics Department at the Ecole Normale Supérieure in Paris. Her research interests are mainly experimental soft matter physics, and her PhD work focuses on elastocapillarity and wetting at soft interfaces.


Structure Prediction in Nanoparticle Superlattices

Alex Travesset, professor in the Department of Physics and Astronomy at Iowa State University

Date: Friday, July 22, 2016

 

Abstract: Materials whose fundamental units are nanoparticles instead of atoms or molecules offer major opportunities to overcome many of the technological challenges of our century. They display unique structures and properties that are not found in other types of materials, usually exhibiting deep underlying geometry and topological constraints. In this talk, I will focus on crystalline assemblies of nanoparticles, i.e. supercrystals. I will discuss the efforts in my group to predict the structure and dynamics of three very successful experimental strategies for the rational design of nanoparticle materials: DNA programmable self-assembly, Evaporation of solvent in nanoparticles with hydrocarbon capping ligands, and more recently, a new technique developed at Ames lab consisting in electrostatic crystallization of nanoparticles with grafted neutral (uncharged) polymers.

Short Bio: Alex Travesset is professor in the Department of Physics and Astronomy at Iowa State University and Associate Research scientist at the Ames Lab. His research interests are in the area of theoretical and computational soft condensed matter. He received his PhD from Universitat de Barcelona and had postdoc positions at Syracuse University and the University of Illinois at Urbana-Champaign.


The maximally crumpled state: crumpling dynamics and the evolution of damage networks

Shmuel Rubinstein, Ph.D.,
Physics Department, Harvard University

Monday, May 23, 2016, 4:00pm,

For further information see http://esam.northwestern.edu

When describing the dynamics of a sheet of paper being crumpled one may be tempted to only take the elastic response of the thin sheet into account and consider only those deformations which minimize the elastic energy of the crumpled sheet. However, most materials yield and deform plastically, leaving permanent scars in the thin sheet. Indeed, the simple process of crumpling a sheet of paper with our hands results in a complex network of interconnected permanent creases of many sizes and orientations, along which the sheet preferentially bends. Thereby, history dependence is introduced into the process. I will present an experimental study of the dynamics of crumpling. Specifically, we investigate how a crease network evolves when a thin elastoplastic sheet is repeatedly crumpled, opened up and then re-crumpled. Is there a maximally crumpled state after which the sheet can be deformed into a sphere without further plastic deformations?


Effective Electrostatic Interaction

Pedro González Mozuelos, Ph.D., 
Physics Department, Cinvestav Av. Instituto Poletécnico Nacional

Monday, April 18, 2016

We discuss the general theory applicable to the determination of the parameters regulating the screened Coulomb interactions among spherical macroions immersed in an electrolyte. This approach comes from implementing the precise methodology of the dressed ion theory to the proper definition of the effective direct correlation functions, which emerge from tracing-out the degrees of freedom of the microscopic ions, and thus provides rigorous definitions for the corresponding screening length, effective permittivity, and renormalized charges. Moreover, it can be profitably employed for precise and reliable calculations of these effective parameters within any practical scheme for the determination of the bulk correlations among the components of an ionic solution.


Chemical Microswimmers: From Cargo Delivery to Motion Control

Mykola Tasinkevych, Max Planck Institute for Intelligent Systems

Monday, April 4, 2016

Microscopic agents which can self-propel through confined, liquid-filled spaces are envisioned as a key component of future lab-on-a-chip and drug delivery systems. Chemically active Janus colloids offer a realization of such agents. A Janus microswimmer works by catalytically activating, over a fraction of its surface, chemical reactions in the surrounding solution. The resulting anisotropic distribution of reactants and products drives a surface flow in a thin layer surrounding the particle leading to a directed motion. Depending on the systems, various self-propulsion mechanisms emerge such as self-electrophoresis or self-diffusiophoresis. Here only the last mechanism shall be considered.


First, the self-propulsion of a spheroidal particle which is covered by a catalyst over a cap-like region will be discussed. I will describe how the velocity of the particle depends on its shape and on the size of the catalytic cap. Next, I will show how such particles can be used as carriers of microcargo. As a model for a carrier-cargo system, I will consider a microswimmer connected by a thin rod to an inert cargo particle. It turns out that the velocity of such composite strongly depends on the relative orientation of the carrier-cargo link. Accordingly, there is an optimal configuration for the linkage. The subtlety of such carriers is underscored by the observation that a spherical particle completely covered by catalyst, which is motionless when isolated, acts as a carrier once attached to a cargo.

Finally, I will demonstrate that a chemical microswimmer approaching a planar wall reveals novel motion scenarios, including a steady “sliding” at fixed orientation and height above the wall, and motionless steady “hovering”. Hovering particles create recirculating regions of flow, and can be used to mix fluid or to trap other particles.  Sliding states, on the contrary, provide a starting point to engineer a stable and predictable motion of microswimmers. Thus, for topographically patterned walls, novel states of guided motion along the edges of the patterns emerge when the parameters of the microswimmers are such that a sliding state occurs in the absence of the patterns. Such topography-guided states emerge from a complex interplay between chemical activity of the particle, hydrodynamic interactions and the confinement of the chemical and hydrodynamic fields by the topography.

Co-sponsored by CBES 


Co(non)solvency or the puzzle of polymer properties in mixed good or poor solvents

Professor Kurt Kremer
Director, Max Planck Institute for Polymer Research

Wednesday, February 17, 2016

The relation between atomistic structure, architecture, molecular weight and material properties is of basic concern of modern soft matter science. Here computer simulations on different levels of resolution play an increasingly important role. To progress further adaptive schemes are being developed, which allow for a free exchange of particles (atoms, molecules) between the different levels of resolution.

The extension to open systems MD (grand canonical MD) as well as recent Hamiltonian based molecular dynamics and Monte Carlo adaptive resolution methods will be explained. Typical examples include the solvation of polymers in mixed solvents, especially PNIPAM in water alcohol mixtures, which reveals an interesting coil-globule-coil transition. This conformational transition cannot be explained within the classical Flory-Huggins picture, which is the standard mean field theory for polymer solutions and mixtures. The results point towards a general design of 'smart stimuli responsive polymers'.

This work has been performed in collaboration with D. Mukherji and C. Marques.

M. Praprotnik et al. Ann. Rev. Phys. Chem. 59, (2008)

S. Fritsch et al. Phys. Rev. Lett. 108, 170602 (2012)
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D. Mukherji and K. Kremer Macrom. 46, 9158 (2013)
D. Mukherji, C. M. Marques, K. Kremer, Nat. Comm. 5, 4882 (2014)
D. Mukherji, C. M. Marques, T. Stuehn, K. Kremer, J. Chem. Phys. 142, 114903 (2015)  


Self-Consistent Field Modeling of the Polyelectrolyte Charged Nanoparticles Mixtures

Monday, October 19, 2015

The effective interactions between charged hydrophilic nanoparticles (CNP) in polyelectrolyte (PE) solutions are computed using a Self-Consistent Field model. The electrostatic interactions are described within the Poisson-Boltzmann approximation. The dielectric constant is chosen to be a function of the local composition of the species. Local PE and CNP’s charges are dependent on the local concentration of ionizable groups and local degrees of dissociation. A hierarchy of two- , three- and multi-body interactions between CNP’s are computed. By reducing interactions to pairwise only one can access the phase diagram of such PE/CNP systems. For such systems only gas and crystal like phases are thermodynamically stable; the fluid-like phase is metastable. MC modelling demonstrates the suppression of the phase segregation by the formation of highly anisotropic clusters of CNPs.

Victor A. Pryamitsyn (born in 1961) obtained his M.S. in Physics in 1984 from Leningrad State University, USSR and PhD in Physics and Mechanics of Polymers in 1987 from the Institute of Macromolecular Compounds, Russian Academy of Sciences. From 2002 to present he has worked as a research associate in the Department of Chemical Engineering of the University of Texas at Austin. Before coming to Texas he was working at the Department of Chemistry and Institute of Theoretical Science at University of Oregon; IRC in Polymer Science and Technology, Department of Physics and Astronomy, University of Leeds, UK; School of Chemistry University of Bristol, UK; Institute of Problems of Mechanical Engineering and Institute of Macromolecular Compounds of Russian Academy of Sciences.


The Geometry and Assembly of the Rough Endoplasmic Reticulum

Jemal Guven

The rough endoplasmic reticulum (RER) is a large intracellular membrane consisting of a number of more or less regular stacks, an architecture that reflects its function as the principal site of protein synthesis within the cell.  Recently it was discovered that adjacent sheets within these stacks are connected by helicoidal ramps.  Guven will argue that stacks are organized about a number of  dipole defects, or “parking garages”, formed by two parallel gently-pitched ramps of opposite chiralities. Viewed more closely, individual sheets are composed of a pair of lipid bilayers enclosing a lumen. Membrane-shaping proteins are concentrated along the bilayer fold which forms the inner boundary of a ramp. These same proteins also play a role in establishing the tubular geometry of the adjoining smooth ER (SER). 

This coincidence suggests a mechanism for parking garage assembly via the nucleation of dipoles at the center of three-way tubular junctions within the SER.