Program

Keynote speakers.

Title. Thin Films and Dry Eye

Clayton J. Radke

Chemical and Biomolecular Engineering

Berkeley College of Chemistry, University of California, US

Abstract

Dry eye, a burny, itchy feeling of dryness and discomfort, is a common malady that infects up to 30 % of the global population. It is especially prevalent in the elderly and women, and in arid, windy climates. During an interblink, randomly distributed ruptures can occur in the tear film. So-called “black spots” and/or “black streaks” appear in 15 to 40 s for normal individuals. For people who suffer from dry eye, tear-film breakup time can be less than a few seconds. Rapid tear breakup is widely believed a signature of dry-eye syndrome. In spite of decades of effort, there currently is no satisfactory explanation for how tear rupture gives rise to dry-eye symptoms nor is there a physically consistent explanation for the origin of tear rupture.

We propose local evaporative-driven tear rupture. Increased evaporation drives a hole in the tear film. As the hole deepens, local salinity increases. The growing hole is suppressed by curvature-driven healing flow and by osmotic-suction due to the local salinity increase. Rupture occurs only when the locally high evaporative flux outweighs the two healing flows. Quantitative evaluation of the evaporative-driven tear-breakup mechanism leads to significant increased salinities at the bottom of the rupture spot (or streak) that we coin salinity “hot spots”. Predicted roles of environmental conditions such as wind speed and relative humidity on tear-film stability agree with clinical observations. Most importantly, locally elevated evaporation leads to hyperosmolar spots in the tear film and, hence, vulnerability to epithelial inflammation and dry-eye symptoms. Tear-film rupture is more likely with contact-lens wear because initial tear-film thickness is reduced.

We provide the first (and only) physically consistent, quantitative explanation for black streaks and/or spots in the human tear film during interblink. Importantly, we explain the formation of “hot spots” of locally high concentration of solutes in the tear film. A tear film peppered with salinity hot spots activates corneal afferent nerve receptors (cold) causing pain sensation and eventually leading to dry eye.

Eye dryness on the cornea is well documented to correlate both with tear breakup time and with tear salinity. This presentation discusses the fundamental science that connects these two observations. By exposing the mechanisms underlying tear rupture, we discover that tear breakup and local salinity hot spots are consonant and lead to discomfort and pain. The root cause apparently is unhealthy meibum. With soft-contact-lens wear, salinity excursions in the post-lens tear film are damped compared to those in the pre-lens tear film, but they require further investigation. Discussion on the criteria for comfortable lens wear completes the presentation.

Title. Solute Partitioning and Diffusion in Hydrogels: Fundamentals of Drug Delivery

Clayton J. Radke

Chemical and Biomolecular Engineering

Berkeley College of Chemistry, University of California, US

Abstract

Hydrogels are biocompatible and, therefore, extensively applied, for example, in pharmaceutics, biomedicine, tissue engineering, and artificial organ scaffolds. Hydrogels also have application in a wide variety of bioseparation and biosensing processes. We focus specifically on hydroxyethyl-methacrylate (HEMA) /methacrylic acid (MAA) copolymer gels used in soft contact lenses to deliver drugs and comfort/wetting agents to the eye. In all applications, it is important to understand how aqueous solutes of varying size, molecular weight, charge, hydrophobicity, and configuration partition into and out of hydrogels which themselves are of differing water content, crosslink density (i.e., mesh size), and matrix charge density.

Two-photon confocal microscopy and back extraction with UV/Vis-absorption spectrophotometry quantify equilibrium partition coefficients, k , and diffusion coefficients, D, for prototypical drugs, polymers, polyelectrolytes, and proteins transporting in HEMA gels with varying MAA contents.

To express deviation from ideal partitioning, we define an enhancement or exclusion factor, E=k/phi, where phi is hydrogel water volume fraction. For solute i , E_i is derived as a product of individual enhancement factors for size exclusion (E_i,ex), electrostatic interaction (E_i,el), and specific adsorption (E_i,ad). To obtain the individual enhancement factors, we employ an extended Ogston mesh-size distribution for E_i,ex; Donnan equilibrium for E_i,el; and Henry’s law characterizing specific adsorption to the polymer chains for E_i,ad. Gels mesh sizes are obtained from measured linear oscillatory rheology; solute sizes are determined from measured bulk restricted-cell diffusion coefficients, D_o. Enhancement factors for various solutes vary between 10^-3 and 10² depending on gel charge and mesh size, and on solute size, charge, and chemistry. Predicted enhancement factors are in excellent agreement with experiment using no adjustable parameters.

From transient two-photon confocal-microscopy concentration profiles, and back-extraction histories with UV/Vis-absorption spectrophotometry, we measure the corresponding solute diffusivities in the gels. For large molecular-weight dextran polymers, whose molecular size is larger than the average gel mesh size (i.e., they are significantly size excluded with E_i,ex<< 1), the ratio D/D_ois near 10^-1indicating transport only through interconnected large mesh domains. We invent large-pore effective medium (LPEM) theory to account for solute size, hydrodynamic drag, and distribution of mesh sizes available for transport in the polymer network. For solutes that interact strongly with the polymer strands (i.e., those solutes with E_i,ad>> 1), D/D_ois reduced drastically due to specific association with the gel polymer network. Extension of LPEM to this case includes Henry-law constants to account for specific solute adsorption onto the polymer backbone. Again, using no adjustable parameters, diffusivities predicted from the proposed large-pore effective-medium model demonstrate good agreement with experiment. Our efforts provide a first step towards a priori design of hydrogels for uptake and delivery of specific water-soluble species by altering gel mesh size, polymer chemistry, and polymer backbone charge

Title. Multi-scale modeling in complex fluids through self-consistent descriptions: continuum-particle simulations

Juan Pablo Hernández Ortiz

Department of Materials and Minerals

National University of Colombia, Colombia.

Abstract

Computational modeling of complex systems, such as polymer solutions, liquid crystals, colloidal dispersions, cell, coacervates and proteins, involves a variety of length and time scales that need to be resolved to provide meaningful understandings and predictions. As requirements for material designs are increasing, computational modeling offers the opportunity to explore bigger and bigger systems that involve a mixture of discrete entities and complex physics. The challenge relies on how to resolve appropriately length and time scales, because it becomes computationally impossible to resolve them using multiple-particle molecular dynamics or Monte Carlo approaches. Concomitantly, the challenges behind material design rely on systems that are far-from equilibrium, and multi-scale frameworks to combine statistical and continuum analysis always consider dynamics and transient evolution equations. This is the rational behind the self-consistent continuum-particle simulation approach that we will discuss in the course, where the multi-scale challenge is resolved using the continuum approximation theory for the smallest molecular entity - the solvent - and stochastic calculus and statistical mechanics for the evolution of the discrete and bigger entities - polymers, proteins, cells, among others. For example, we will explore how the evolution of discrete Brownian particles in a continuum solvent is done through the mobility tensor (momentum equations) in a matrix-free and efficient algorithm. In general, the way we include continuum dynamics into discrete mechanics (or vice versa) is through balance equations: momentum, energy, mass, charge, etc. Other way to say this is that interacting forces from discrete particles will drive responses on the continuum, and that those responses are found by transport equations. Conversely, the continuum will drive forces on the discrete entities that enter the corresponding multi-particle evolution. In particular, we will study Green's function based methods for hydrodynamics and electrostatics interactions. In the course, we will cover the fundamentals and theory, and revisit some algorithms and examples.

Title. Macromolecule Mediated Colloidal Interactions, Dynamics & Assembly

Michael A. Bevan

Department of Chemical and Biomolecular Engineering

Whiting School of Engineering, Johns Hopkins University, USA.

Abstract

Designing functional colloids that can be deposited on target substrates, transported in a controlled manner, and assembled into desired microstructures is essential for a diverse range of applications and synthetic, natural, and biological material systems. To obtain desirable behaviors and properties in colloidal systems, it is necessary to design colloid surface chemistries and add adsorbing and non-adsorbing surfactants and macromolecules that are responsive to chemical and physical environments to enable processing and performance of such systems. Significant engineering challenges remain to design colloids and additives with physicochemical properties necessary to achieve optimal performance in technologically important complex fluid and particle based material applications.

In this work, we report direct measurements to reveal fundamental mechanisms as well as mathematical models of colloidal interactions, dynamics, and assembly to provide interpretive/predictive capabilities for formal engineering of colloids and macromolecular additives. Using an integrated suite of optical microscopy, image analysis, and modeling tools, particle potentials and hydrodynamic interactions are directly measured and quantitatively related to particle dynamics and assembled microstructures as a function of particle properties and macromolecular additives. To demonstrate how such fundamental tools can be used to solve engineering problems, several different applications will be described in detail, including: (1) how interactions between synthetic macromolecules, proteins, carbohydrates, lipid bilayers, extracellular matrix, and cells control colloidal transport important to drug delivery, (2) how mixtures of macromolecules and surfactants and consumer product formulations control transport and deposition of different shaped fragrance capsules, and (3) how colloidal assembly on patterned surfaces can be controlled through a combination of electrostatic, van der Waals, and depletion interactions mediated by macromolecules. Each example illustrates how direct measurements and quantitative models of kT-scale colloidal interactions mediated by macromolecules can be used to design, control, and optimize colloidal behavior and properties and applications.

Title. External Field Mediated Colloidal Interactions, Dynamics & Assembly

Michael A. Bevan

Department of Chemical and Biomolecular Engineering

Whiting School of Engineering, Johns Hopkins University, USA.

Abstract

The autonomous and reversible assembly of colloidal nano- and micro- scale components into ordered configurations is often suggested as a scalable process capable of manufacturing meta-materials with exotic electromagnetic properties that could enable numerous emerging technologies. However, the inability to produce such ordered materials with a sufficiently low defect density has limited the development of the science and applications of such materials. As a result, there is strong interest in understanding how thermal motion, particle interactions, patterned surfaces, and external fields can be optimally coupled to robustly control the assembly of colloidal components into hierarchically structured functional meta-materials.

We approach this problem by directly relating equilibrium and dynamic colloidal microstructures to kT-scale energy landscapes mediated by colloidal forces, physically and chemically patterned surfaces, and gravitational and electromagnetic fields. 3D colloidal trajectories are measured in real-space and real-time with nanometer resolution using an integrated suite of evanescent wave, video, and confocal microscopy methods. Equilibrium structures are connected to energy landscapes via statistical mechanical models. The dynamic evolution of initially disordered colloidal fluid configurations into colloidal crystals in the presence of tunable field mediated interactions is modeled using a novel approach based on fitting the Smoluchowski equation to experimental microscopy and computer simulated assembly trajectories. This approach is based on the use of reaction coordinates that capture important microstructural features of crystallization processes and rigorously quantify both statistical mechanical (free energy) and fluid mechanical (hydrodynamic) contributions. With the ability to measure and tune kT-scale colloidal interactions and quantitatively model how such interactions are connected to dynamically changing microstructures, we demonstrate real-time control of assembly, disassembly, and repair of colloidal crystals using both open loop and closed loop control to produce perfectly ordered colloidal microstructures. This approach is demonstrated for close packed colloidal crystals of spherical particles and is being extended to anisotropic particles and non-close packed states.

Contributed talks

Revisiting fuel cell technology: from fundamental aspects to scale-up

Jorge Vázquez-Arenas [1]

[1] Centro Mexicano para la producción más limpia; IPN, México

Current demands of sustainable energy and environmental concerns related to fossil fuels have spurred the growth of clean power sources including batteries (BAT) and fuel cells (FC). Particularly, FC has been preferred for high energy density applications, although this technology has considerably moved toward the power demands unlike BAT. FC are electrochemical cells converting chemical energy in fuels (e.g. H₂, O₂) into electrical energy, with very high efficiency and low environmental impacts. To this concern, the Proton Exchange Membrane Fuel Cell (PEMFC) and the Solid Oxide Fuel Cell (SOFC) have become consolidated in numerous devices and penetrated untapped markets. This talk covers a comprehensively review of these devices [1-3], including material components (solids and liquid phases), reaction mechanisms in different FC types, catalytic aspects limiting performance and efficiency, scale-up of macro-scale cells, and electrochemical methods characterizing kinetics, performance and aging with the aim of envisaging major short and long term focuses of research and development.

Stochastic transitions and anomalous diffusion of colloidal trimers in laser external fields

Juan Pablo Segovia[1], Manuel Escobedo[1], Erick Sarmiento[1], Stefan Egelhaaf[1]

[1] Condensed Matter Physics Laboratory, Heinrich Heine University, Düsseldorf, Germany

When a dielectric particle interacts with an inhomogeneous distribution of light, the change in momentum associated with such interaction produces a net force on the colloidal particle. A widely known example of this phenomena is the technique of optical tweezers. If the energy density is small enough, thermal motion can induce an escape of the particle from the potential well leading to anomalous diffusion associated with the confinement and further Markovian escape process. In this work, the particular case of planar trimers in random and periodic potentials is shown. As the distribution of light itself is inhomogeneous, the coupling between asymmetry of trimer and field gives a complex external potential, showing stochastic transitions and even diffusive, but not Gaussian, colloidal dynamics.

Effect of Meibomian lipids on the evaporation of the tear film in the presence of a meniscus

Bernardo Yáñez Soto[1,2], Rodrigo Vélez Cordero[1,2], Daniela Blanco Campoy[2], Enrique Graue Hernández[3]

[1] Conacyt, [2] Instituto de Física UASLP, [3] Instituto de Oftalmología Conde de Valenciana

The Tear Film Lipid Layer (TFLL) is a thin layer of lipid produced mainly by the Meibomian glands, located inside the lids. Its composition is 60-70% nonpolar lipids (wax esters, cholesterol and cholesterol esters), and 15% polar lipids (phospholipids and glycolipids) and has a thickness between 32-200 nm(1). One of the functions attributed to the TFLL is the retardation of evaporation. There has been a number of experimental studies trying to measure the specific influence of the TFLL on evaporation, but the results have been unimpressive and equivocal, especially in in vitro studies(2-3). In this work, we propose a system that relies on the evaporation induced by non-axisymmetric contact points(4). For the determination of the evaporation we used Meibomian lipids obtained from healthy controls and from patients suffering from varios Meibomian gland pathologies, and found that the evaporation of fluids covered with Meibomial lipid films can be as low as 50% of pure water.

Bibliography

(1) The Ocular Surface, 12(3):178-201(2014), (2) Curr Eye Res, 34:589-97(2009), (3) J Physics: Condensed Matter, 16:S2461(2004), (4) Langmuir, 32:8171-81(2016).

Diffusion and wall interactions of colloidal particles with biologically relevant complex surfaces

J. Manuel Hernández-Meza[1], J.R. Vélez-Cordero[1,2], B. Yáñez-Soto[1,2], A.Ramíres-Saito[1], S. Aranda-Espinoza[1], J.L. Arauz-Lara[1]

[1] Institute of Physics, UASLP, [2] Cátedras CONACyT

In this experimental work we obtained the diffusion and particle/wall interaction potential landscapes from the three-dimensional trajectories of particles moving above a glass wall. The particle tracking was realized by optical microscopy, using the point spread function of fluorescent particles in a range of several micrometers above the surface. The systems included particles diffusing above bare glass, glass coated with polyelectrolytes and glass covered with a lipid monolayer, all prepared in extremely low ionic strength conditions. Maximum antifouling properties was observed for the particle/hyaluronate and particle/lipid systems while eventual particle adsorption was detected in the particle/glass and particle/polyelectrolyte-bilayer systems. We also detected an increment in the weight of the particles due to the adsorption of molecules from the functionalized walls. For the levitating states, the diffusion coefficient components as a function of the particle/wall separation distance were below the theoretical hydrodynamic points. It turns out that the increment of the effective viscosity needed to match the theoretical curves is well predicted by the electroviscous constitutive equation, where the effective viscosity appears as a function of the conductivity of the media. Complementary AFM and zeta-potential measurements were also conducted in order to support the main results of this work.

Short- and long-time behaviour of a spherical tracer in a periodic external field: experiments and simulations

Daniela Pérez-Guerrero [1], Guillermo Iván Guerrero-García [2], Erick Sarmiento-Gómez [3], Jose Luis Araúz-Lara [1]

[1] Instituto de Física, Universidad Autónoma de San Luis Potosí, México., [2] CONACYT-Instituto de Física, Universidad Autónoma de San Luis Potosí, México., [3] Condensed Matter Physics Laboratory, Heinrich Heine University, Universitätsstr. 1, 40225 Düsseldorf, Germany.

The dynamic behaviour of a colloidal particle under the influence of a periodic external field is studied here experimentally via a two-dimensional array of fringes produced by two interfering lasers, and numerically via Brownian dynamics simulations. At short times, we estimate the maximum height barrier U0 of a simple potential of periodicity L (depending on the power laser) by fitting the short-time diffusion coefficient d0 obtained from experimental mean square displacement measurements, in the presence and in the absence of the periodic external field. With this information, our Brownian dynamics simulations are able to reproduce the experimental time- and position-dependent probability density profile of the tracer. At long times, we simulate the corresponding mean square displacement (MSD) for several U0 values. The corresponding long-time diffusion coefficient dm, the critical time t* at which a change of curvature of the MSD is observed, and the associated MSD(t*) are also reported. We observe that the Lifson and Jackson prescription provides a good approximation to the ratio of the short- and long-time diffusion coefficients d0/dm for the periodic potential used, whereas the simpler analytic Cecile-Ferrier et al formula predicts an accurate ratio d0/dm only for large values of U0.

Modeling rotational and translational diffusion of Brownian motion of ellipsoidal particles near glass transition using a nonvibrating granular system

Rosario Esperanza Moctezuma[1], Cecilio Tapia [2], Fernando Donado[2], Eric Weeks[3]

[1]CONACYT-Instituto de Física “Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosí, Alvaro Obregón 64, 78000 San Luis Potosí, S.L.P., México, [2]Instituto de Ciencias Básicas e Ingeniería de la Universidad Autónoma del Estado de Hidalgo-AAMF, Pachuca 42184, Pachuca, México,[3]Physics Department, Emory University, Atlanta, Georgia 30322, USA

We use granular systems to model the a glass-forming fluid. Our system consists of a monolayer of magnetic steels beads under an unsteady magnetic field. Each particle behaves as a molecule in a glass-forming fluid. Previously, we showed how the system can form a glass if the particle concentration is sufficiently high and/or if the magnetic forcing is sufficiently low. Now we use this system to study rotational and translational diffusion of Brownian motion of ellipsoidal tracer particles near the glass transition. We do this by placing a light ellipsoidal object above the monolayer of particles. In all experimental conditions we have studied, we observe that the motion is diffusive and its dynamic is slower than that of particles used to model the glass-forming liquid. As the granular system is “cooled” toward the glass transition, the rotational and translational motion of the tracer slows dramatically, by 2 – 3 orders of magnitude. However, the ratio between the translational diffusion constant and rotational diffusion constant of the tracer stays constant, in contrast to what is often observed in small molecule glasses and colloidal glasses. We also note that the ratio between translational and rotational diffusion depends on the ellipsoid aspect ratio, as expected for classical diffusion of ellipsoids.

Morphological study of drying complex fluids: a low resources technique for dengue virus detection

Daniel Gallardo Galaviz [1] Carmen Lucía Moraila Martínez [1]

[1] Parque de Innovación Tecnológica, Universidad Autónoma de Sinaloa

Drying of colloidal suspensions appears in many applications such as coatings, colloidal assembly, optoelectronics, etc,. The formation of stains of drying drops of any colloidal dispersion is known as the "coffee ring effect". The mechanism for particle deposition by drop drying and the study of the morphology of the deposits left, have been widely studied. Deegan et al., demonstrated that there were two conditions necessaries for deposits apparition: contact line pinning and evaporation from the edge of the drop. Nowadays, the coffee ring effect is used for an innovative approach for medical diagnostics. It is based on the comparison of the patterns of dried drops of biological liquids of people with different diseases with regard to people in healthy conditions. In the field of medicine, one of the world great challenges for health is the diagnosis of diseases in an economic way, particularly in countries with low resources. Therefore, the development of techniques that require a minimum use of energy and resources become a necessity. Recently, it has been found that different diseases (HIV, tuberculosis or malaria) can be diagnosed using techniques of biological fluids evaporation. During the drying of a biological fluid an evaporative flow carries particles (bacteria, cells, etc) to the interface and, in the wedge formed by the triple line, a particle segregation by size is presented. This segregation occurs only under certain conditions in which particles electrical charge, particle concentration, play a very important role. In the present work, we propose an indirect, rapid and low resources technique for Dengue virus detection by means of the drying of a blood drop and the study of it drying morphology.

Bibliography

(1)Deegan et al., Nature 389, 827–829 (1997), (2)Diego Noguera-Marín, C.L. Moraila-Martinez, M. Cabrerizo Vilchez, M. Rodriguez-Valverde. Langmuir, (2015), (3)Diego Noguera-Marín, C.L. Moraila-Martinez, M. Cabrerizo Vilchez, M. Rodriguez-Valverde. SOFT MATTER, (2015), (4) GULKA, et al, ACS APPL. MATER. INTERFACES (2014), (5) SEFIANE, K. ,206, 372-81. ADV. COLLOID INTERFACE SCI. (2014).

A code based on the Lattice Boltzmann Method for the simulation of complex fluids

Venecia Chávez Medina[1], Francisco S. Guzmán[1]

[1]Instituto de Física y Matemáticas, Universidad Michoacana de San Nicolás de Hidalgo

We present a code capable of simulating the 3 dimensional evolution of a two phase fluid using the Shan-Chen flavor of the Lattice Boltzmann Method. The work presents the pertinent mathematical analysis that enables a fair understanding of the method and its validity as well as the mathematical derivation and physical concepts of the hydrodynamic equations. By focusing on the development of the computational code, the work also presents a non-dimesionalisation analysis, boundary conditions and more importantly, all basic tests any computational method of this nature should verify in order to be reliable. As a particular application, the influence of confined geometries over the dynamics of standing capillary waves is analyzed, specially the effects of the boundaries parallel to the interface between the two phases of the fluid. We also describe the potential and limitations of the code.

Brownian Attractive Correspondence in the Hard-sphere System

Leticia Lopez Flores [1], Martín Chavez Páez [1], Honorina Ruiz Estrada [2] and Magdaleno Medina Noyola[1]

[1] Instituto de Física "Manuel Sandoval Vallarta", UASLP, [2] Facultad de Ciencias Físico Matemáticas, BUAP

The hard-sphere colloids provide an excellent illustration of the difficulties involved in understanding the equilibrium states and the mechanisms by which systems evolve. However, not all the particles are hard spheres systems, we can found another system that interacting the different manner, for example repulsive, attractive forces or both. In this way, we perform systematic simulation experiments on model systems with Lennard-Jones attractive interactions to test the dynamic equivalence known to exists between soft-sphere liquids with similar static structure [1,2]. For this, we compare the simulated dynamics (mean squared displacement, intermediate scattering function, α-relaxation time, etc.) of LJ systems in a wide range of conditions with the hard-sphere liquid. In addition, we verify another more recently-proposed dynamic equivalence, this time between the dynamics of a Brownian fluid and the long-time dynamics of the corresponding atomic liquid, i.e., the atomic system whose particles interact with the same interaction potential.

Bibliography

(1) Phys. Rev. Lett. 107, 155701 (2011), (2) Phys. Rev. E 88, 042301 (2013).

Building a fundamental approach to glass transition and glassy behavior

Pedro E. Ramírez-González[1] and Magdaleno Medina-Noyola[2]

[1] CONACYT-Instituto de Fı́sica ”Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosı́, [2] Instituto de Fı́sica ”Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosı́

A non-equilibrium extension of Onsager’s canonical theory of thermal fluctuations is employed to derive a self-consistent theory for the description of the statistical properties of the instantaneous local concentration profile of a colloidal liquid in terms of the coupled time evolution equations of its mean value and of the covariance of its fluctuations (1). These two coarse grained equations involve a local mobility function which, in its turn, is written in terms of the memory function of the two-time correlation function. This theory, known as the Non-Equilibrium Self-Consistent Generalized Langevin Equation (NE-SCGLE) theory, also provides a general theoretical framework to describe irreversible processes associated with dynamic arrest transitions, such as aging, and the kinetics of glass and gel formation (2,3). A detailed study of glass formation as a function of the cooling rate is also presented in this talk.

Bibliography

(1) Phys. Rev. E, 82, 061503 (2010), (2) Phys. Rev. E, 87, 052306 (2013), (3) J. Chem. Phys., 143, 174505 (2015).

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