
Prof. Dr. Hartmut Löwen
Institute for Theoretical Physics II
HeinrichHeineUniversität Düsseldorf
D40225 Düsseldorf, Germany


Charged colloidal suspensions: 100 years after DebyeHückel
Abstract: 100 years after the development of the linear screening theory by Debye and Hückel the research on highly charged colloidal dispersions is still ongoing [1] and reveals some results which are not expected within the linear DebyeHückel or the sizecorrection extended DejaguinVerweyLandauOverbeek theory. In my talk I shall treat two novel aspects of charged suspensions: First, recent computer simulations [2] have shown that mixtures of charged und neutral colloids exhibit phase separation which is not described within the effective Yukawa picture of linear screening. Second, we address breathing charged colloids for which the diameter changes harmonically as a function of time. This is a nonequilibrium problem for colloidal dynamics generalizing the dynamics of colloids from equilibrium to nonequilibrium. These breathing colloids can be considered as active particles [3]. They form twodimensional crystals at the airwater interface and the breathing process is induced by acoustic surface waves. It is shown by experiment and simulation [3] that breathing is able to annihilate crystalline defects significantly which establishes a new concept of ACDC (Acoustic Crystallization of twoDimensional Colloidal crystals).
References:
[1] G. Nägele, Physics Reports 272, 215 (1996).
[2] E. Allahyarov, H. Löwen, J. Chem. Phys. 157, 164902 (2022).
[3] C. Bechinger, R. di Leonardo, H. Löwen, C. Reichhardt, G. Volpe, G. Volpe, Reviews of Modern Physics 88, 045006 (2016).
[4] J. K. G. Dhont, An Introduction to Dynamics of Colloids, Elsevier, 1996.
[5] J. Menath, R. Mohammadi, J. C. Grauer, C. Deters, M. Böhm, B. Liebchen, H. Löwen, N. Vogel, Advanced Materials, 2022, 2206593 (2022).


Prof. Dr. Gerhard Nägele
Research Centre Jülich
WilhelmJohnenStraße D52428 Jülich, Germany


Deswelling, structure, and dynamics of ionic microgel suspensions
&
Conductivity of asymmetric electrolyte solutions
Gerhard Nägele^{1}, Alan R. Denton^{2}, Mariano E. Brito^{3} and Claudio Contreras – Aburto^{4}
^{1}Institute of Biological
Information Processing, IBI4, Forschungszentrum Jülich, Germany
^{2}Department of Physics,
North Dakota State University, Fargo, U.S.A.
^{3}Institute for
Computational Physics, Universität Stuttgart, Stuttgart, Germany
^{4}Facultad de Fisica, Universidad Veracruzana,
Xalapa, Mexico
Abstract: Ionic microgels are solvent and ion containing crosslinked polyelectrolyte networks of colloidal size. Suspensions of ionic microgels are both of fundamental and technological interest, owing to the sensitivity of their size to external control parameters such as concentration, ionic strength, and temperature. In the first part of the presentation, we report on a theoretical study of crowding effects on thermodynamic, structural, and dynamic properties of concentrated suspensions of ionic microgels [1]. Methods for calculating the crowdingdependent microgel radius are discussed and used for the calculations of an effective microgel pair potential from which the suspension properties are determined. It is shown that under lowsalinity conditions, counterioninduced microgel deswelling enlarges diffusion and osmotic pressure, lowers the suspension viscosity, and significantly shifts suspension crystallization to larger concentrations. For high salinity conditions, we explore how microstructure and diffusion of microgels are affected by their elasticity and (dynamic) solvent permeability.
In the second part, we describe a versatile theory for calculating dynamic properties of concentrated electrolyte solutions [2]. The theory accounts for solventmediated hydrodynamic interactions among the
dissolved ions, and allows for quantitative predictions of electrolyte conductance, diffusion properties and viscosity. At low concentrations, it reduces to the celebrated DebyeHückelOnsagerFuoss limiting law results for strong electrolytes. We discuss in particular a recent extension of the theory to the conductance of size and chargeasymmetric binary electrolytes [3].
[1] M.E. Brito, A.R. Denton and G. Nägele, J. Chem. Phys. 151, 224901 (2019)
[2] C. ContrerasAburto and G. Nägele, J. Chem. Phys. 139, 134110 (2013)
[3] F. PerezHernandez, G. Nägele and C. ContrerasAburto, work in progress (2023)


Dr. José Luis Arauz Lara
Institute of Physics
Autonomous University of San Luis Potosí
78290 San Luis Potosí, SLP, México.


Structure and Brownian motion of colloidal species on an out of thermal equilibrium curved oil/water interface
José Luis ArauzLara and Maria de Jesus MartinezLopez
Instituto de Física, Universidad Autónoma de San Luis Potosí
Abstract: We investigate the spontaneous formation of water droplets and their motion at a spherical interface between water and oil. When water is put in contact with oil containing hydrophobic surfactants, water is spontaneously emulsified in the form of little droplets into the oil phase but remaining attached to the interface. The droplets, originally of submicron size, keep growing at the expense of the original water reservoir, to reach several microns in size, while at the same time they are able to move randomly along the interface.
Since the interface is curved, the action of gravity makes the droplets sediment towards the bottom, where they form an ordered structure. We trace the particles motion and determine the size of the droplets, the mean squared displacement and sedimentation velocity at various stages of development. Although the dynamic nature of the process, with both the interface and particles still changing, producing heterogeneities in the system, we do not observe anomalous diffusion, the motion of the droplets has a well identified Brownian component with a Gaussian distribution of steps due to the thermal agitation of the media surrounding the droplets and a drift component due to the effect of gravity.


Prof. Dr. Alan R. Denton
Department of Physics
North Dakota State University
Fargo, ND 581086050, U.S.A.


Modeling Particle Dispersions: From Charged Colloids to Ionic Microgels
Abstract: Colloidal dispersions have inspired fascination since the invention of microscopes and were fundamental in establishing the atomic nature of matter. Ranging from nanometers to microns in size, colloidal particles also vary in softness, from rigid microspheres to flexible polymer coils. The landmark DebyeHückel theory of electrolytes, which introduced the concept of electrostatic screening, opened a window on our understanding of chargestabilized colloids. Hard colloids can acquire charge in solution via surface dissociation of counterions. Microgels are soft and permeable colloidal particles, made of crosslinked polymer networks, that can ionize and swell in a good solvent [1, 2]. Sensitive responses to external fields and changes in environment give colloidal dispersions unique thermal, mechanical, and optical properties, enabling many applications in the biomedical, pharmaceutical, food, and consumer care industries. In this lecture, I will summarize our recent efforts to describe dispersions of charged colloids and ionic microgels within coarsegrained models that combine the dual colloidal and polymeric natures of the particles. After outlining practical implementations via PoissonBoltzmann theory and molecular simulations, I will present illustrative results for equilibrium thermal and structural properties of these remarkable soft materials.
[1] M. E. Brito, A. R. Denton, and G. Nägele, «Modeling deswelling, thermodynamics, structure, and dynamics in ionic microgel suspensions,» J. Chem. Phys. 151, 224901 (2019).
[2] P. S. Mohanty, S. Nöjd, M. J. Bergman, G. Nägele, S. ArreseIgor, A. Alegria, R. Roa, P. Schurtenberger, and J. K. G. Dhont, “Dielectric spectroscopy of ionic microgel suspensions,” Soft Matter 12, 9705 (2016).


Dr. Ramón Castañeda Priego
Science and Engineering Division Campus León
University of Guanajuato
37150 León, Guanajuato, México


Thermodynamics, effective interactions, structure, and transport
properties of chargestabilized colloidal suspensions
Abstract: In this talk, we will discuss on our scientific contributions of the last couple of decades to understand some of the most salient features of chargestabilized colloidal dispersions. By combining experimental evidence with meanfield approximations, mainly based on the PoissonBoltzmann description, and more demanding computer simulations, we will describe some thermodynamic, structural, and transport properties of charged colloids. We will also discuss the role of the effective charge to account for the interactions between colloids. Finally, we will point out some of the fruitful discussions with Gerhard Nägele on this contribution.


Prof. Dr. R.H.H.G. van Roij
Institute for Theoretical Physics
Utrecht University
3584 CS Utrecht, The Netherlands


TBA
TBA


Dr. Olegario Alarcón Waess
Department of Actuary, Physics, and Mathematics
Universidad de las Américas Puebla
72810 Cholula, Puebla, México


Beyond of dispersions of charged particles: A theoretical approach for nonspherical particles is proposed.
Abstract: Using the ideas involved in the wellknown DebyeHückel theory for spherical charged colloidal particles, a model for the projections of the pairinteraction potential for long and thin hard rods are developed. The self and collective diffusion coefficients, translational and rotational, are expressed in terms of these projections, with the use of the mode coupling theory up to lineal order in density. Under welldefined approaches the weak coupling limit of the diffusion coefficients are recovered.


Dr. José Miguel Méndez Alcaraz
Physics Departement
Center for Research and Advanced Studies of the
National Polytechnic Institute O7360 CDMX, México


TBA
TBA


Jonathan Josué EliseaEspinoza
Science Faculty
Autonomous University of San Luis Potosí
78290 San Luis Potosí, SLP, México.


Theoretical description of the spherical and the planar electrical double layer for a mixture of n ionic species with arbitrary size and chargeasymmetry.
Jonathan Josué EliseaEspinoza^{a)}, Enrique GonzálezTovar^{a)}, and Guillermo Iván
GuerreroGarcia ^{b)}
^{a)} Instituto de Física de la Universidad Autónoma de San Luis Potosí, Alvaro Obregón 64,
78000 San Luis Potosí, San Luis Potosí, México.
^{b) }Facultad de Ciencias de la Universidad Autónoma de San Luis Potosí, Av. Chapultepec
1570, Privadas del Pedregal, 78295 San Luis Potosí, San Luis Potosí, México.
Abstract: Coulombic fluids can be found in nature in a wide variety of forms, for instance, as aqueous electrolytes supporting charged biomolecules, such as the DNA or lipid bilayers, macroion/nanoparticle solutions, ionic liquids, molten salts, or plasmas, just to mention a few. In this talk, we would like to present a theoretical description of the ionic profiles of a mixture of n species of spherical charged particles dissolved in implicit solvent, with arbitrary size and chargeasymmetry, bathing either a spherical macroion or an infinite planar electrode. This theoretical formalism is numerically solved via the robust finite element method and aims to close the gap between the nano and the microscale in macroion solutions, taking into account ion correlations and ionic excluded volume effects consistently. When these last two features of Coulombic fluids are neglected, the classical nonlinear PoissonBoltzmann theory for n ionic species– with different ionic closest approach distances to the colloidal surface–is recovered as a limit case. The present theoretical finite element approach is general, fast, easily scalable, and allow us to observe interesting phenomena such as the surface charge amplification, charge inversion, and charge reversal in mixtures of colloidal solutions either in bulk or under the influence of an electric field produced by an infinite planar electrode.


Dr. Magdaleno Medina Noyola
Institute of Physics
Autonomous University of San Luis Potosí
78290 San Luis Potosí, SLP, México.


Aging of the linear viscoelasticity of glasses and gels
Orlando JoaquínJaime, Ricardo PeredoOrtiz, Magdaleno
MedinaNoyola
Instituto de Física, Universidad Autónoma de San Luis Potosí
Luis Fernando ElizondoAguilera
Instituto de Física, Benemérita Universidad Autónoma de Puebla
Leticia LópezFlores
Department of Materials Science and Engineering, Northwestern
University
Abstract: In this talk, we announce the proposal of the first and only firstprinciples statisticalmechanical theory of the nonequilibrium aging processes of the linear viscoelasticity of glass and gelforming liquids after a sudden cooling or compression. This theory is based on a nonequilibrium modecouplinglike approximate expression for the shear stress relaxation function η(τ;t), in terms of the structure factor S(k;t) and of the intermediate scattering function F(k,τ;t) (where t is the waiting time after cooling/compression). The nonequilibrium properties S(k;t) and F(k,τ;t), on the other hand, are provided by our theory of irreversible processes in liquids, referred to as NESCGLE. As an illustrative exercise, the resulting methodology is applied to simple models (soft spheres, square well, etc.) subjected to instantaneous cooling, exhibiting a remarkable agreement with available experimental results.


Prof. Dr. Jan Karel George Dhont
Research Centre Jülich
WilhelmJohnenStraße D 52428 Jülich, Germany


SingleParticle Thermophoresis
and
ElectricField Induced Phases/States
Abstract: In the first part of this presentation, I will discuss colloidal mass transport induced by temperature gradients (commonly referred to as thermophoresis) resulting from the electric double layer of charged spherical colloids. There are three contributions to the thermophoresis of charged colloids. The temperature dependence of the internal energy of the electric double layer leads to migration from high to low temperatures. The temperaturegradient induced asymmetry of the double layer gives rise to an electrostatic force onto the surface charges of the colloid. Finally, the asymmetry of the double layer leads to an electro osmotic flow, that acts with a friction force onto the core of the colloid. All three contributions will be discussed, and the theoretical results will be compared to experiments.
In the second part, the phases and dynamical states that are induced by external electric fields in a system of very long and thin and highly charged rodlike colloids will be discussed. The experimental phase/state diagram, for a concentration within the twophase isotropicnematic coexistence region, will be presented.
Depending on the electric field strength and the frequency, several phase/statetransitions are induced: a transition from nematic to chiral nematic, from a nematic to a homeotropic state, and a transition to a dynamical state where nematic domains persistently melt and reform. An explanation of these phenomena is presented, both on an intuitive level and based on the Smoluchowski equation, which is an equation of motion for the probability density function for the positions and orientations of the rods.
