"Kinetically-Arrested Nanostructures from Amphiphilic Heterografted Molecular Brushes in Solution"

Abelardo Ramirez

Solution self-assembly of heterografted molecular brushes offers a rich platform to create complex functional organic nanostructures. Recent studies have shown that kinetics, not just thermodynamics, plays an important role in defining the self-assembled structures that can be formed. In this talk, we present results from extensive molecular dynamics simulations that explored the self-assembly behavior of heterografted molecular brushes when the solvent quality for one of the side blocks is changed by a rapid quench. We have performed a systematic study of the effect of different structural parameters and the degree of incompatibility between side chains on the final self-assembled nanostructures in the low concentration limit. We found that kinetically-trapped complex nanostructures are prevalent as the size of the molecular brushes increases. We performed a quantitative analysis of the self-assembled morphologies by computing the radius of gyration tensor and relative shape anisotropy as the different relevant parameters were varied. Our results are summarized in terms of non-equilibrium morphology diagrams.

"Impact of mucus in the diffusivity and infectivity of viruses"

Antoni Luque

Mucus is a complex fluid that covers the organs of animals. An essential function of mucus is to protect and coordinate the interaction of animals with microbes. In this talk, I will discuss a phenomenon observed among viruses of bacteria in mucus. These viruses can attach to the molecular fibers that form mucus increasing their infectivity against their bacterial hosts. I will discuss our research modeling the biophysical mechanisms behind this phenomenon as well as its ecological implications. I will also introduce a general framework to investigate the diffusivity of viruses and other microscopic particles in mucus.

Further reading:

K. Joiner, A. Baljon, J. Barr, F. Rohwer, and A. Luque, "Impact of bacteria motility in the encounter rates with bacteriophage in mucus," Scientific Reports, 9, 16427 (2019): doi.org/10.1038/s41598-019-52794-2.

A. Cobarrubia, J. Tall, A. Crispin-Smith, and A. Luque, "Unifying framework for the diffusion of microscopic particles in mucus," bioRxiv, July 26, 2020: https://www.biorxiv.org/content/10.1101/2020.04.22.056689v1

"g(r) to u(r)"

Dirk Aarts

The pair distribution function g(r) plays a central role in liquid state theory, linking structure and thermodynamics. It is typically measured by constructing a histogram of the distances between all pairs of particles, which is used in simulations and experiments where single particle coordinates can be obtained. Here, we present a novel method based on Henderson’s method [1] for measuring the cavity distribution function, going beyond our recent work on particles with hard interactions [2]. The method measures g(r) in a highly efficient way; moreover, it allows us to obtain an effective pair potential between colloidal particles in experiment [3].

[1] Henderson, Mol Phys 1983

[2] Stones, Dullens, Aarts, JCP 2018

[3] Stones, Dullens, Aarts, PRL 2019

"Going multiscale: How to make soft-matter simulations feasible"

Florian Müller-Plathe

Polymers are a variant of soft matter that do not have a natural way of separating length and time scales. Their features are due to processes on different scales, and sometimes a single quantity depends on multiple scales. Therefore, all attempts to break them up according to scales are to some degree arbitrary. A common scheme is to coarse-grain particle models of polymers, i.e. to combine handfuls of atoms into "superatoms" and parameterise the interactions between those. There exist various protocols to do this in a systematic way. This approach of coarse-graining has been very successful when it come to reproducing the molecular structure of polymers, a little less successful for thermodynamic properties, and a complete loss for dynamical properties. There have been a few attempts to remedy the situation and to bequest realistic dynamics on coarse-grained models, as the potential benefits are considerable. In the case of polymer melts and solutions, for example, correct predictions of their rheology would be very much welcomed by polymer processors. This talk will review successes and challenges of coarse-graining polymers. In particular, I will talk about our recent attempts to improve coarse-grained polymer dynamics: excess-entropy scaling [1], Rough Mob [2], and slip-spring dynamics [3].

Further reading:

[1] “Predicting the Mobility Increase of Coarse-Grained Polymer Models from Excess Entropy Diffe-ren¬ces”, G. Rondina, M. C. Böhm, and F. Müller-Plathe, J. Chem. Theor. Comput. 16, 1431−1447 (2020). [DOI:10.1021/acs.jctc.9b01088]

[2] “Loss of Molecular Roughness upon Coarse-Graining Predicts the Artificially Accelerated Mobility of Coarse-Grained Molecular Simulation Models”, M. K. Meinel and F. Müller-Plathe, J. Chem. Theor. Comput. 16, 1411−1419 (2020). [DOI: 10.1021/acs.jctc.9b00943]

[3] “A Multi-chain Slip-Spring Dissipative Particle Dynamics Simulation Method for Entangled Polymer Solutions”, Y. Masubuchi, M. Langeloth, M.C. Böhm, T. Inoue, and F. Müller-Plathe, Macromolecules 49, 9186–9191 (2016). [DOI: 10.1021/acs.macromol.6b01971]

"Solvent mediated interactions between model nanoparticles: is water special?"

Robert Evans

Classical density functional theory (DFT) is employed to investigate how confining a simple liquid between a pair of blocks with (microscopic) cross section affects its structure and thermodynamic properties. We consider identical blocks that are solvophobic or solvophilic and blocks that have solvophobic and solvophilic patches. For sufficiently solvophobic blocks, at small separations and for reservoir states close to bulk liquid-gas coexistence, we observe a strongly attractive and near-constant solvent mediated force between blocks, stemming from (local) capillary evaporation of the liquid, i.e. the presence of gas-like intrusions between the blocks[1]. New measures, the local compressibility and thermal susceptibility, quantify the (pronounced) density fluctuations occurring at the incipient gas-liquid interfaces. Comparing our results with those of subsequent MD simulations of SPC/E water confined by nano-slabs [2], we argue that the form of the density profiles and solvent-mediated force found for ‘water’ are essentially the same as in the simple liquid we investigate, i.e. our simple model system captures all the key physical phenomena associated with confinement.

[1] B. Chacko, A. J. Archer and R. Evans J. Chem. Phys. 146, 124703 (2017).

[2] J. Lam and J.F. Lutsko J. Chem. Phys. 149, 134703 (2018).

"Unveiling the complex dynamics of biological systems by Differential Dynamic Microscopy"

Roberto Cerbino

Dynamic light scattering (DLS) techniques probe entities whose size can be orders of magnitude smaller than the wavelength of light; for this reason, they are routinely used in biology laboratories, for instance to size proteins and nucleic acids or to probe biomolecular interactions and binding. In recent years, Differential Dynamic Microscopy (DDM) emerged as an effective way to perform DLS-like experiments by simply analyzing movies acquired with a microscope in various imaging modes. In this talk, I will give a brief introduction to DDM and describe how it has been successfully used to determine the collective dynamics of a variety of biological systems, including proteins, DNA, bacteria, epithelial cells, motile cilia, vesicles and actin.

"Predicting phase behavior of coacervates of oppositely charged polyelectrolites"

Ronald G. Larson, Ali Salehi, and Mohsen Ghasemi

We develop a modeling framework for phase behavior in oppositely charged polyelectrolyte complexes (PECs). The core of the phase behavior model is the development of a free energy model that includes free energies for ion pairing, counterion condensation, charge regulation, electrostatic free energy, as well as elastic energy of the network and Flory Huggins entropy and enthalpy. We apply this model to “doping” of PECs, in which ion-pairs between polyelectrolytes in the PEC are gradually broken up by and replaced with salt ions upon increase of the salt concentration in the solution in contact with the PEC. The predictions of our model agree well with the experimental data for doping of stoichiometric PECs, made of poly(diallyldimethylammonium) PDADMA and poly(styrene-sulfonate) PSS, with KBr as well as for salt partitioning between the PEC and the co-existing solution. Further, we model “overcharging” of PECs by putting a stoichiometric PEC in contact with a solution, containing an excess of polycation. We rationalize the adsorption of the excess polycation by the stoichiometric PEC in terms of two entropic forces: counterion release and combinatorial binding entropy. Using the same parameters used to model doping of PDADMA/PSS PECs with KBr, we find semi-quantitative agreement between our predictions and the equilibrium overcompensation experiments of multilayer films of the Schlenoff group. Given the diverse library of salt-polyelectrolyte candidates for PECs, development of such theories is crucial in rational design of applications of PECs.

"Polyelectrolyte Complex Hydrogels: Microstructure and Properties"

Samanvaya Srivastava

The utility of hydrogels in bioadhesion and other biomedical (drug delivery, tissue engineering) applications requires fundamental mappings interrelating the hydrogel mesoscale structures, nanoscale relaxation processes and bulk material properties. In this talk, I will discuss the development of a family of polyelectrolyte complex (PEC) hydrogels with tunable microstructure and material properties, and our progress on establishing a materials framework for polyelectrolyte complex-based robust bioadhesives. In the first part of my talk, comprehensive structure-property interrelations of PEC hydrogels that form upon complexation of oppositely charged block polyelectrolytes will be presented. X-ray scattering investigations of these materials highlight large-scale ordering of the nanoscale PEC domains, characterized by a disorder-order transition, followed by morphological transitions with increasing polymer loading. The PEC domains, composed of the charged blocks of the oppositely charged polyelectrolytes, can be designed to encapsulate and release charged therapeutics, genetic materials and proteins, thus enhancing the utility of PEC hydrogels for diverse biomedical applications. The influence of key parameters such as polymer block lengths and concentration on the PEC domain morphology and hydrogel rheology will also be reviewed.

In the second part, I will present our investigations on double-network hydrogels comprising electrostatic (PEC) and covalent networks. Minimal influence of incorporation of the covalent network on the equilibrium hierarchical structure of PEC networks will be demonstrated, conserving the gel’s ability to encapsulate biomolecules and other charged cargo. At the same time, we will highlight marked improvements in the shear and the tensile strengths of the PEC hydrogels upon incorporation of the covalent network, even as a minor component, in the hybrid hydrogels. The decoupling of the microstructure of the double-network hydrogel with their mechanical properties and rheology, along with improved resistance to salt and controllable swelling, facilitates precise tuning of microstructure and shear moduli of the gels prior to formation of covalent networks, and of the tensile strength, toughness, swelling and chain relaxation dynamics in the double-network hydrogels. These attributes will be argued to enable the development of a platform for polyelectrolyte complex-based fast-curing and robust bioadhesives.

"Universal reshaping of arrested colloidal gels via active doping"

Stewart A. Mallory

Colloids that interact via a short-range attraction serve as the primary building blocks for a broad range of self-assembled materials. However, one of the well-known drawbacks to this strategy for self-assembly is that these building blocks rapidly and readily condense into a metastable colloidal gel. Using computer simulations, I will discuss how the addition of a small fraction of purely repulsive self-propelled colloids, a technique referred to as active doping, can prevent the formation of this metastable gel state and drive the system toward its thermodynamically favored crystalline target structure. The simplicity and robust nature of this strategy offers a systematic and generic pathway to improving the self-assembly of a large number of complex colloidal structures. I discuss in some detail the process by which this feat is accomplished and provide quantitative metrics for exploiting it to modulate self-assembly. On the whole, this talk highlights the pivotal role active forces can play in directing colloidal self-assembly.

"Surface Forces and Stratification in Micellar Foam Films & Soap Bubbles"

Vivek Sharma

Foam films and soap bubbles typically consist of fluid sandwiched between two surfactant-laden surfaces that are ~ 5 nm -10 microns apart, and the drainage in films occurs under the influence of viscous, interfacial and intermolecular forces, including disjoining pressure. Ultrathin foam films of soft matter containing supramolecular structures like micelles, nanoparticles, smectic liquid crystals,lipid bilayers, and polyelectrolytes undergo drainage via stratification, manifested as step-wise thinning in interferometry-based measurements of average thickness.In this study, we focus exclusively on stratification in micellar foam films formed with aqueous solution of sodium dodecyl sulfate (SDS) above the critical micelle concentration (CMC).In reflected light microscopy, stratifying films (thickness < 100 nm) display regions with distinct shades of grey implying that domains and nanostructures with varied thickness coexist in the thinning film. Understanding and analyzing such nanoscopic thickness transitions and variations have been long-standing experimental challenge due to the lack of technique with the requisite spatio-temporal resolution, and theoretical challenge due to the absence of models for describing hydrodynamics and thermodynamics in stratified thin films. Using IDIOM(interferometrydigital imaging optical microscopy) protocols we developed recently, we show that the nanoscopic thickness variations in stratifying films can be visualized and analyzed with an unprecedented spatial (thickness ~ 1 nm, lateral ~500 nm) and temporal resolution (< 1 ms). Stratification proceeds by formation of thinner domains that grow at the expense of surrounding films. Using the exquisite thickness maps created using IDIOM protocols, we provide the first visualization of nanoridges as well as mesas that form at the moving front around expanding domains. We contrast the step size measured instratification studies with intermicellar distance obtained from scattering measurements, and explicitly measure the non-DLVO supramolecular oscillatory surface force contribution to disjoining pressure. Most significantly, we develop a self-consistent theoretical framework, a nonlinear thin film equation model that explicitly accounts for the influence of supramolecular oscillatory surface forces (using expressions we developed as a part of this study), as well as the physicochemical properties of surfactants. We show the complex spatio-temporal evolution of domains, nanoridges, nanoridge-to-mesa instability and mesasin stratifying foam films can be modeled quantitatively, and we elucidate how surfactant type and concentration can be manipulated and controlled for molecular engineering of micellar foams.

"Equilibrium and Non-equilibrium Assembly of Spherical Colloids at Fluid Interfaces"

Anna Kozina

Spherical colloids confined to fluid interfaces behave differently from their counterparts in quasi-2D and 3D. One of the reasons is the presence of additional inter-particle interactions, mainly long-range dipole-dipole repulsion and capillary attraction. Many times, one type of interaction dominates, which drives the system out of equilibrium. The resulting structure is often more disordered than expected due to arrested particle dynamics. However, if both types of interactions are moderate, their interplay results in an equilibrium structure formation, for instance, equilibrium clustering at very low packing fractions. Such a behavior should be reflected in the presence of a secondary minimum in the interaction potential, which was theoretically suggested at the beginning of 2000s. However, non-equilibrium behavior did not allow application of structure inversion to experimental systems in order to confirm theoretical prediction up to the present work. In this contribution we will discuss the details and consequences of equilibrium and non-equilibrium assembly of spherical colloids confined to fluid interfaces.

"Complex systems through simple models"

Gerardo Odriozola

In this talk, we present simple models which, to some extent, capture the behavior of a couple of complex systems. These are key and lock colloids and DNA phospholipid-membrane complexes. Both systems share that self-assembly from smaller entities, following an entropy-driven process. The process is a consequence of the action of forces on the relatively large assembling particles, which arise from their direct interactions and also from more complex solvent-mediated interactions. A possible way to deal with these contributions is through Monte Carlo simulations, where we fix the positions of the assembling particles, and the small particles are allowed to sample from configuration space. This way, the total force acting on the fixed particles can be split into electrostatic and contact (or depletion, or entropic) contributions. By obtaining the net force as a function of the inter-particle distance, we can then integrate them to get an overall pair-potential map. The outcome aids in understanding the self-assembling of complex systems.

"A multi-scale approach for nonequilibrium simulations of liquid crystal colloids"

Humberto Híjar

Liquid crystals are fluids composed of anisotropic molecules with long-range orientation order. They become enormously interesting when doped with solid particles ranging from nanometer to micronmeter size since, then, they have potential applications in, e.g., biological sensors and manipulation of light [1]. Simulations of liquid crystal colloids could play an essential role in validating theoretical assumptions and understanding behavior which could be difficult to resolve experimentally. However, this behavior results from the interaction of phenomena that operate at widely separated scales: topological defects at the nanoscale, Brownian motion at the mesoscale, and flow and orientation patterns at the continuum scale. In this talk, I will describe a numerical method that could be used for bridging the gap between molecular and mesoscopic scales in the simulation of nematic colloids. It operates in a range where molecular motions demonstrate an average behavior and treats the solvent as composed of parcels of molecules. These parcels are grouped in cells where dynamics follows simplified stochastic collisions which preserve momentum and energy and promote collective alignment. This allows to satisfy hydrodynamic behavior after long simulation times. The interaction with guest colloids is treated on Molecular Dynamic bases. The method will be illustrated through the study of the nonequilibrium dynamics of spherical and elongated colloids in nemtaic liquid crystals under flow [2].

[1] I. Muševič, Liquid Crystal Colloids (Springer, 2017).

[2] D. Reyes-Arango, et al. Physica A 547, 123862 (2020).

"Dynamics of the zooplankton microswimming and its ecotoxicological applications"

Jesús Alvarado-Flores¹, Jorge Adrián Perera-Burgos¹, Ramón Castañeda-Priego² and Francisco Alarcón²

Zooplankton is a set of microscopic free-living organisms that live in fresh and saltwater, with sizes ranging from 0.1 mm to more than 3.0 mm. In general, such motion is make to feed, reproduce, and to escape from natural predators. Its behavior in both natural microenvironments and laboratory culture is affected by physical and chemical changes in water: temperature, salinity, light, pH, and contaminants. On a global scale, its distribution in the ecosystem is due to its passive and active dispersion, migration phenomena, nomadism, and tendencies to remain at rest for long periods. In general, two behaviors in their movement have been identified, free swimming with spiral patterns and adhesion to a substrate. Zooplankton is currently used by current regulations to study water pollution, with species being used to assess its toxicity using national and international protocols. Traditional toxicological tests evaluate the mortality of test organisms as an endpoints within 24 to 48 hours, also performing bioaccumulation tests due to prolonged exposure to different types of contaminants present in the aquatic environment, such as heavy metals (lead, cadmium, etc.) and pollutants of anthropogenic origin (pesticides, antibiotics, caffeine, etc.), among others. However, despite its normative use as toxicity indicators, there are no studies to quantify changes in their movement patterns due to environmental pollutants. Because of this, the study of zooplankton motion in response to toxic contaminants in aquatic systems is proposed to generate a fast indicator at low concentrations of chemicals, since such motility can be a perceptible indicator of fast response due to the polluting chemical.

Keywords: active matters, zooplankton ecology, water pollution.

1.- Unidad de Ciencias del Agua - Centro de Investigación Científica de Yucatán, A.C.

2.- División de Ciencias e Ingenierías, Campus León, Universidad de Guanajuato.

"Grain Boundary Engineering and Multiaxial Deformation of Soft Crystals: From Liquid Interfaces to Liquid Crystal Kaleidoscopes"

José Adrián Martínez González

Liquid crystals (LCs) are states of matter where the molecules exhibit fluid properties while follow a preferential order that can be orientational and/or positional. Chiral liquid crystals, for instance, can form cubic crystalline structures, the so called Blue Phases, where the unit cells are orders of magnitude larger than the atomic crystals. The optical response of a liquid crystal is related to its internal structure which can be changed by small cues from outside making them ideal materials not only for the well-known LCD-display technologies, but for detection of particles and structures of biological interest, among other intriguing phenomena. In this talk, we are going to explore some interesting facts about the directed self-assembling of chiral liquid crystals and their amazing optical response that can be induced by geometrical deformation.