Abstract: The distribution and mobility of electrolytes across electric double layers are important to countless processes or applications related to the geosciences, nanofluidics, energy storage, water purification, and biological systems. Partial insights into electric double layer properties can be inferred from high-resolution non-invasive experiments, but the interpretation of measurements typically relies on various assumptions and simple models, such as the basic Stern model. Atomistic simulations provide a powerful means to complement experiments and improve theoretical models. In this presentation, I will demonstrate a different way of looking at double layers on the basis of molecular simulations of a realistic silica-electrolyte interface. Specifically, I will focus on the charge inversion phenomenon and its influence on electrokinetic transport and I will investigate to what extend equilibrium and non-equilibrium simulation results can be predicted by theory in the case of charge inversion.

Abstract: In the theory of capillarity describing the shape of adsorbed fluid films, the liquid-vapor interface is characterized by a single parameter, namely, the interface tension. This yields the classical surfaces of constant curvature for the shape of the interface. For sufficiently thin films, this level of description is augmented by the introduction of an effective potential acting between the substrate and the liquid-vapor interface. In this talk we will discuss computer simulation experiments that probe the liquid-vapor interface of thin adsorbed films. In our study, we measured the spectrum of surface fluctuations, and the results indicate that the effective surface tension that is obtained becomes dependent on the film thickness for films on the sub-nanometer scale. Such behavior can be explained in terms of a simple microscopic theory, but the implication is that the classical Young-Laplace-Derjaguin-Frumkin theory of thin adsorbed films must be corrected for films at the nanometer scale. This finding could have implications in our understanding of line tensions, spinodal dewetting and droplet dynamics.

Key topics: A self-consistent generalized Langevin equation (SCGLE) of colloidal dynamics, SCGLE theory of dynamic arrest, dynamic arrest in multicomponent systems, non-equilibrium SCGLE theory of liquid dynamics, NE-SCGLE theory of aging, equilibration of dense liquids, and density-temperature-softness scaling of the dynamics of glass-forming soft-sphere liquids

Abstract:

Blood is a suspension of objects of various shapes, sizes and mechanical properties, whose distribution during flow is important in many contexts. Red blood cells tend to migrate toward the center of a blood vessel, leaving a cell-free layer at the vessel wall, while white blood cells and platelets are preferentially found near the walls, a phenomenon called margination that is critical for the physiological responses of inflammation and hemostasis. Additionally, drug delivery particles in the bloodstream also undergo margination – the influence of these phenomena on the efficacy of such particles is unknown.

In this talk a mechanistic theory is developed to describe segregation in blood and other confined multicomponent suspensions. It incorporates the two key phenomena arising in these systems at low Reynolds number: hydrodynamic pair collisions and wall-induced migration. The theory predicts that the cell-free layer thickness follows a master curve relating it in a specific way to confinement ratio and volume fraction. Results from experiments and detailed simulations with different parameters (flexibility of different components in the suspension, viscosity ratio, confinement, among others) collapse onto the same curve. In simple shear flow, several regimes of segregation arise, depending on the value of a ``margination parameter'' M. Most importantly, there is a critical value of M below which a sharp ``drainage transition'' occurs: one component is completely depleted from the bulk flow to the vicinity of the walls. Direct simulations also exhibit this transition as the size or flexibility ratio of the components changes. Results are presented for both Couette and plane Poiseuille flow. Experiments performed in the laboratory of Wilbur Lam indicate the physiological and clinical importance of these observations.

Abstract:

When water is confined in nanostructures, or forced to pass thorough them, it reveals some peculiar behaviour due to its underlying molecular nature. Taking into account such effect may be problematic, but it offers also novel opportunities [1]. In this talk, I will discuss two examples based on zeolites. First of all, we will investigate the possibility to use zeolites for thermal storage. I will present a hybrid (molecular dynamical + Monte Carlo) numerical approach, validated against experimental data from literature, for predicting both adsorption and infiltration isotherms of zeolite-water pairs. The approach is general and it can be used for other pairs as well. Adsorption isotherm data are critical for designing optimal sorption heat storage cycles [2]. Secondly, I will present the challenges for molecular separation by zeolite-based membranes. Infiltration isotherms are essential for assessing the performances of molecular sieve for next-future desalination (or other separation) processes. Numerical evidences show that a simple Dubinin-Astakhov model can be safely used (and physically interpreted) for describing the infiltration process. However, from the practical point of view, the role of surface barriers remains elusive [3].

 

References:

1. E. Chiavazzo, M. Fasano, P. Asinari, P. Decuzzi. Scaling behaviour for the water transport in nanoconfined geometries. Nature Communications 5, 3565 (2014)
2. M. Fasano, D. Borri, E. Chiavazzo, P. Asinari. Protocols for atomistic modeling of water uptake into zeolite crystals for thermal storage and other applications. Applied Thermal Engineering 101, 762-769 (2016)
3. M. Fasano, T. Humplik, A. Bevilacqua, M. Tsapatsis, E. Chiavazzo, E. Wang, P. Asinari. Interplay between hydrophilicity and surface barriers on water transport in zeolite membranes. Nature Communications 7, 12762 (2016)

Abstract:

Water management is a major technological problem with Polymer Electrolyte Membrane (PEM) Fuel Cells. Bad performance is seen with membranes operating at low relative humidity or with liquid water flooding the fuel cell. Employing model fuel cell reactors that elucidate the basic physics of Polymer Electrolyte Membrane (PEM) fuel cell reactors we have been able to demonstrate that the water produced at the cathode can auto- humidify the fuel cell and operate with dry feeds to 130°C. PEM fuel cells reactors are shown to be conceptually identical to exothermic reactions, e.g. flames!

Model fuel cells are employed to elucidate the mechanism of liquid water motion through the PEM fuel cell. Water slugs and drops form in the gas flow channels of fuel cells giving rise to spatial-temporal current density fluctuations. The current fluctuations were reduced by redesigning the flow channels to exploit gravity to assist in water drainage. Water emerging from the porous electrode grows into drops that eventually occlude the open area of the channel. Drops detach and move when the force for gas flow through the porous electrode is greater than the force to advance the solid/liquid contact lines of the drop. The shape and surface properties of the flow channel can be engineered to minimize blockage effects of drops and slugs. In this talk, we shall demonstrate the use of reaction engineering principles to improve PEM fuel cell design.

Abstract:

Stochasticity is a defining feature of biochemical reaction networks, with molecular fluctuations influencing cell physiology. In principle, master probability equations completely govern the dynamic and steady state behavior of stochastic reaction networks. In practice, a solution had been elusive for decades, when there are second or higher order reactions. A large community of scientists has then reverted to merely sampling the probability distribution of biological networks with stochastic simulation algorithms. Consequently, master equations, for all their promise, have not inspired biological discovery.

We recently presented a closure scheme that solves chemical master equations of nonlinear reaction networks [1]. The zero-information closure (ZI-closure) scheme is founded on the observation that although higher order probability moments are not numerically negligible, they contain little information to reconstruct the master probability [2]. Higher order moments are then related to lower order ones by maximizing the entropy of the network. Using several examples, we show that moment-closure techniques may afford the quick and accurate calculation of steady-state distributions of arbitrary reaction networks.

With the ZI-closure scheme, the stability of the systems around steady states can be quantitatively assessed computing eigenvalues of the moment Jacobian. This is analogous to Lyapunov’s stability analysis of deterministic dynamics and it paves the way for a stability theory and the design of controllers of stochastic reacting systems.

In this seminar, we will present the ZI-closure scheme, the calculation of steady-state probability distributions, and discuss the stability of stochastic systems.

1. Smadbeck P, Kaznessis YN. A closure scheme for chemical master equations, Proc. Natl. Acad. Sci. USA, 110(35):14261-5, 2013.
2. Smadbeck P, Kaznessis YN. Efficient moment matrix generation for arbitrary chemical networks, Chem. Eng. Sci., 84:612–618, 2012.

Abstract:

Spatial confinement has a strong impact in the motion of liquids and suspensions. Although the presence of solid surfaces hinders, in first instance, the motion of a nearby fluid and its suspended particles, the specific coupling of the fluid and the solid can give rise to a rich variety of scenarios. In this talk I will discuss the interplay between geometrical confinement and the motion of self-propelled particles as well as actuated colloids forced externally. I will address different theoretical and computational approaches that provide flexible frameworks to gain insight in the behaviour of such complex scenarios and the new concepts that are useful to quantify the interplay between geometrical variability and particle transport.

Abstract:

We examine a diluted gas with inter-molecular attraction. It is argued that the attraction force amplifies random density fluctuations by pulling molecules from lower-density regions into high-density regions and, thus, may give rise to an instability. Using a kinetic equation with the attraction force taken into account under the approximation of self-consistent field, we demonstrate that the instability occurs when the temperature T is lower than a certain threshold value Ts depending on the gas density. It is further shown that, even if T is only marginally lower than Ts, the instability generates ‘clusters’ with density much higher than that of the gas. These results suggest that the instability should be interpreted as a gas–liquid phase transition, with Ts being the temperature of saturated steam and the high-density clusters representing liquid droplets.

Abstract:

Controlling the advancement of a fluid inside a microchannel is one of the main issues in microfluidics. At ever smaller scales the surface to volume ratio becomes increasingly large and system frontiers play a crucial role. In this situation surface properties of the microchannel, such as roughness and wetting, are of fundamental importance since they would largely determine the large resistance to flow. We will discuss properties of fluid front advancement in a microchannel with natural roughness, such as pinning and avalanches. We will also describe wetting-induced fluid entrainment by advancing contact lines on surfaces. A new phenomenon that we dub superconfinement will be present.

Abstract: Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years ago, based on stepwise features in force curves between two hydrophobic surfaces, eventually leading to the first atomic force microscopy (AFM) image in 2000. While in the early years it was suspected that they may be an artefact caused by AFM, meanwhile their existence has been confirmed with various other methods, including through direct optical observation. Their existence seems to be paradoxical, as a simple classical estimate suggests that they should dissolve in microseconds, due to the large Laplace pressure inside these nanoscopic spherical-cap-shaped objects. Moreover, their contact angle (on the gas side) is much smaller than one would expect from macroscopic counterparts. This review will not only give an overview on surface nanobubbles, but also on surface nanodroplets, which are nanoscopic droplets (e.g. of oil) on (hydrophobic) substrates immersed in water, as they show very similar properties and can easily be confused with surface nanobubbles and as they are produced in a very similar way, namely by a solvent exchange process, leading to local oversaturation of the water with gas or oil, respectively, and thus to nucleation.

We will briefly report how surface nanobubbles and nanodroplets can be made, how they can be observed (both individually and collectively), and what their properties are. We will then explain the long lifetime of the surface nanobubbles. The crucial element is pinning of the three-phase contact line at chemical or geometric surface heterogeneities. The dynamical evolution of the surface nanobubbles then follows from the diffusion equation, Laplace’s equation, and Henry’s law. In particular, one obtains stable surface nanobubbles when the gas influx from the gas-oversaturated water and the outflux due to Laplace pressure balance. This is only possible for small enough surface bubbles. It is therefore the gas oversaturation ζ which determines the contact angle of the surface nanobubble or nanodroplet and not the Young equation.

The talk reports on joint work with Xuehua Zhang.

Speakers:

• Prof. Steve Davis, Northwestern University: Open problems and challenges in surfactant laden drops moving on substrates.
• Dr. Ory Schnitzer, Mathematics, IC: Problems in electrohydrodynamics.
• Dr. Prasun Ray, Mathematics, IC: Instabilities in polymeric shear flows.
• Dr. Peter Yatsyshin, Chemical Engineering, IC: Microscopic approach to surface tension.
• Mr. Andreas Nold, Chemical Engineering, IC: Nanoscale fluid structure in the vicinity of a contact line.
• Dr. Thibault Dairay, Aeronautics, IC: Direct numerical simulation of a turbulent wake generated by an irregular object.
• Dr. Alice Thompson, Mathematics, IC: Mathematical problems in electrostatically induced patterning with applications in soft lithography.

Abstract: The natural frequencies and mode shapes for an inviscid free drop vibrating due to surface tension were reported by Rayleigh (1879). In many applications, however, drops are constrained by a solid substrate which significantly influences both the frequencies and shapes of the Rayleigh spectrum, as we have recently reported. In this talk, we focus on the influence of weak viscosity (small Ohnesorge number) and its role, in competition with dissipation arising from contact-line mobility, on the damping of forced oscillations of the sessile drop. We report observations of a mechanically-excited sessile drop, driven in the plane-normal direction. We compare experiment against prediction by a viscous potential theory extension of the inviscid linear stability results. Spectrum splitting and the broadening of resonance peaks of the inviscid spectrum are highlighted. 

Abstract: A capillary surface is an interface between two fluids whose shape is determined primarily by surface tension. Sessile drops, liquid bridges, rivulets, and liquid drops on fibers are all examples of shapes influenced by contact with a solid. Capillary shapes can reconfigure spontaneously or exhibit natural oscillations, reflecting static or dynamic instabilities, respectively. Both instabilities are closely related and a review of static stability precedes the dynamic case. The focus of the dynamic case is the hydrodynamic stability of capillary surfaces subject to constraints of i) volume conservation, ii) contact-line boundary conditions and iii) geometry of the supporting surface

Abstract: When two drops come into contact they will rapidly merge and form a single drop. Here we address the coalescence of drops on a substrate, focussing on the initial dynamics just after contact. For very viscous drops we present similarity solutions for the bridge that connects the two drops, the size of which grows linearly with time. Both the dynamics and the self-similar bridge profiles are verified quantitatively by experiments. We then consider the coalescence of water drops, for which viscosity can be neglected and liquid inertia takes over. Once again, we find that experiments display a self-similar dynamics, but now the bridge size grows with a power-law $t^{2/3}$. We provide a scaling theory for this behavior, based on geometric arguments. The main result for both viscous and inertial drops is that the contact angle is important as it determines the geometry of coalescence.

Abstract: The process of nucleation plays an important role in a wide variety of sciences including chemistry, materials science, nanoscience and cosmology. In this talk, an overview of the theory of nucleation will be given highlighting the outstanding issues and unresolved questions. This will be followed by the presentation of a recently developed approach based on fluctuating hydrodynamics that treats microscopic free energy models, fluctuations and transport within a single framework. A survey of results will be given including the recovery of Classical Nucleation Theory (CNT) as a limit of the general framework, extensions of CNT, nucleation within confined geometries and non-classical, strong-noise effects. One unanticipated result is the result that even liquid-vapor nucleation is a more complex processes than is assumed within CNT.

Abstract: We discuss the use of free energy lattice Boltzmann algorithms as tools to investigate multiphase flow at surfaces with chemical and topographic patterning. Putting together arguments based on the Gibb’s criterion for interface pinning and the numerical approaches, we show how capillary filling and imbibition can be controlled by surface patterning. We then consider the link between patterned surfaces and superhydrophobic behaviour and discuss the extent to which the dynamics of drops rolling across superhydrophobic surfaces is understood. We identify a novel, partially suspended, superhydrophobic state and suggest an interpretation of the anisotropic motion of water drops on butterfly wings.