Session C29: Flow in Confinement and Porous Media

Colloidal jamming in nano-confinements observed with SESANS

The behavior of matter in nano-confinements is being investigated as a means for obtaining controlled highly-ordered nanomaterials. To understand this behavior a 3D structural characterization of the confined matter is necessary. Non-destructive probing of such samples challenges conventional microscopy techniques. On the other hand, the submicron size of a single confinement is impractical for neutron and x-ray scattering experiments but this dilemma can be overcome by using a confining matrix made up of an array of identical confinements, $e.g.$ the grooves of a diffraction grating. The caveat is that the periodicity of the sample amplifies dynamical scattering effects that are not accounted for in approximate scattering theories and a full dynamical theory (DT) calculation becomes unavoidable. Dynamical theory calculations, applied to neutron spin-echo small angle scattering (SESANS) measurements on nanostructured gratings, give good account of all the data sets we collected so far in reflection and transmission scattering geometries. Calculations on recent measurements performed on a silica suspension in contact with the grooves of a diffraction grating show colloidal jamming in the grooves.   

Numerical Studies into Flow Profiles in Confined Lubricant

This paper documents a computational study of flow profiles in confined fluids. The study is motivated by experimental evidence for deviation from Couette flow found by one of the authors (JSW). The computational study examines several possible stress-strain relations. Since a linear profile is the only possible solution for a constant stress layer even in presence of a power law, the study introduces a functional dependence of the fluid viscosity on the distance from the wall. Based on this dependence, a family of scaling laws for the velocity profile near the wall is derived which matches the measured profiles. The existence of this scaling law requires the viscosity of the fluid to increase at least linearly away from the wall. This behaviour is explained at a microscopic level by considerations on the mobility of long molecules near a wall. This behaviour is reminiscent of the variation of eddy length scales in near-wall turbulence.   

Through-Thickness Flow Profile Determination of Confined Lubricant

The knowledge of the through-thickness flow profile of lubricants confined between two rubbing surfaces is necessary for the friction prediction of lubricated engineering systems. While it is crucial to materials selection and engineering design, little work on the direct measurement of lubricant flow has been performed in elastohydrodynamic lubrication (EHL) regime as the nanoscopic film thickness bars the use of conventional techniques. Photobleached-fluorescence imaging was applied to obtain the first experimental flow profile of a $\sim $100 nm lubricant film within an EHL contact. Mapping of flow profiles was also carried out across the contact. The investigated lubricants show multiple flow phenomena. They do not follow the predicted Couette flow, often assumed in tribology theory. Two distinct flow conditions were observed: transition from Couette flow to a non-linear velocity profile; and shear banding, or dilation. Both conditions were shown to depend on position and normal stress experienced by the lubricant. Causes, such as pressure gradient and limiting shear stress, and the effect on traction, will be discussed.   

The Electrophoretic Mobility of a Polyelectrolyte within a Radially Confining Potential Well

We demonstrate that a polyelectrolyte electrophoresing while radially confined by a mechanical force has a conformationally dependent electrophoretic mobility that differs from its free-draining value. The mobility increases as a function of the confining harmonic potential and in the absence of solid walls. Mesoscale MPCD-MD hybrid simulations that include electro-hydrodynamics through a mean-field Debye H\"{u}ckel approximation will be presented for a variety of well widths and contour lengths, demonstrating that mobility increases with confinement after a critical point but remains independent of polymerization. For this reason, models based on a change of monomer friction coefficient at the confinement boundary (such as those recently put forward to explain experimentally measured mobility polyelectrolytes confined within nano- and microfluidic channels) are not sufficient to explain our observations. Since the potential acts perpendicular to the electric field and only on the monomers, the Electro-Hydrodynamic Equivalence Principle does not predict the mobility to differ. We present a course-grained theory explaining these findings in terms of hydrodynamic coupling within overlapping diffuse layers.   

Specific Heat Capacity of Physically Confined Ethylene glycol in Nano Pores

Sensible heat is a cheap and effective means of storing solar energy where energy storage density can be improved by enhancing the specific heat capacity of the heat transfer materials. Formulating composite materials of heat transfer fluids is a mechanism by which the bulk specific heat capacity can be altered and preferably increased. Traditionally, the specific heat capacity of composite material is evaluated from the weighed average of the individual specific heat capacities of the constituents. This, however, does not take into account the effect of interfacial atoms and molecules. The effect of interfacial atoms and molecules becomes increasingly significant when one of the constituents has dimensions in nano meters. In this study, we evaluate the role of interfacial molecules on the specific heat capacity of composite systems. In order to systematically control the interfacial molecules, we have measured the specific heat capacity of ethylene glycol when it is physically confined in nano pores.   

Diffusion in a soft confining environment: Dynamic effects of thermal fluctuations

A dynamical model of a soft, thermally fluctuating two-dimensional tube is used to study the effect of thermal fluctuations of a confining environment on diffusive transport. The tube fluctuations in both space and time are driven by Brownian motion and suppressed by surface tension and the rigidity of the surrounding environment. The dynamical fluctuations modify the concentration profile boundary condition at the tube surface. They decrease the diffusive transport rate through the tube for two important cases: uniform tube fluctuations (wave vector, $q = 0$ mode) for finite tube lengths and fluctuations of any wave vector for infinitely long tubes.   

Probing The Dynamics Of Flow Within A 3D Porous Medium, From The Pore Scale Up

Flows through micro and nano scale pores are ubiquitous; they arise in everyday situations, such as in forcing fluid out of a wet sponge, to important technological applications, including oil recovery, groundwater remediation, geological CO$_{2}$ storage, and even nutrient transport through mammalian tissues. Such flows are typically modeled using a simple continuum approach, which neglects local, pore scale variations in the flow. Here, we present an experimental technique to directly visualize flow within a 3D porous medium over a broad range of length scales, from the scale of individual pores to that of the entire medium. We quantify the dynamics of the flow, both without and with residual trapping of an additional, immiscible fluid within the medium. The pore space is highly complex and interconnected; nevertheless, we find excellent agreement between our measurements and a dramatically simplified mean-field picture of flow.   

Forced drainage and imbibition in microfluidic porous media

We present an experimental study on the dynamics of two-phase flow in microfabricated porous media. In particular we focus on pressure-driven imbibition and drainage in two-dimensional networks of microchannels. We vary the geometrical features of the network, viscosity of the non-wetting fluid and surface chemistry of the microchannels. The rate of displacement and entrapment of the liquid are studied. A simple model that accounts for capillary and viscous effects is compared with the experimental results.   

Unusual Properties of Water Confined in Nanoporous Silica Glasses

The structure and dynamics of water confined in nanoporous silica are different from that of bulk water, and insight into the properties of confined water is important for our understanding of many geological and biological processes. We use reactive molecular dynamics simulations to study the structure and dynamics of nanoconfined water between 100 and 300K. The simulations are based on experimentally validated force fields for silica and water. These force fields allow dissociation of water molecules. We prepare nanoporous silica systems with pore sizes in the range 1-6 nm, and study the behavior of water in the nanopores. We observe a mixture of high-density and low-density water in the pores, and hysteresis in the energetics of water upon cooling and heating. We will present results for the structure and diffusion of water near surfaces and in the interior regions of nanopores as a function of temperature.   

Analysis of gas transport in polymer electrolyte fuel cells using porous structure constructed from X-ray nano CT

This paper describes the analysis of gas transport in micro porous layers of polymer electrolyte fuel cells based on the three-dimensional structure obtained from X-ray nano computed tomography (CT). The polygonal surface representation of the porous structure was constructed from the cross-sectional CT images using the marching tetrahedrons algorithm. The diffusion flux through the porous layer was evaluated by the direct simulation Monte Carlo method since the characteristic pore size is comparable to the mean free path of gas molecules. The numerical simulation well reproduces the experimentally observed pressure dependence of diffusion resistance originating from the transition between Knudsen and molecular diffusion regimes. The effect of porous media morphology on gas transport was examined by an analysis of the trajectories of transmitted molecules through the porous layer.   

Microfluidics of ordered fluids

Flow of ordered fluids (e.g. liquid crystals) is inherently complex due to the coupling between the flow and the long-range orientational order. Experiments carried out with nematic liquid crystals at micro scales further reveal the influence of surface properties on the static and dynamic outcomes. Microfluidics provide a convenient platform to tune one or more of the above competing components, and explore the resulting equilibrium states. The delicate but intricate balance between the viscous, elastic and surface forces was consequently used to devise optofluidic and micro-scale-transport applications. On one hand the novel applications complement the conventional microfluidic capabilities, and on the other hand, broaden the reach of \textit{isotropic} microfluidics by offering competitive advantages. Standard microfluidic techniques and a combination of polarizing optical microscopy, fluorescence confocal polarizing microscopy and particle tracking methods were employed for the investigations.   

The Casimir effect in microfluidics

We describe predictions of unusual Casimir and light-induced interactions between bodies immersed in fluids, including tunable and highly temperature-dependent stable suspensions of compact microspheres. We exploit recently developed, sophisticated computational techniques for modeling Casimir interactions in arbitrary geometries to study fluid deformations in corrugated surfaces with features on the scale of the fluid-layer thickness, and quantify the contributions of non-additive electromagnetic effects in those geometries. Unlike previous calculations of wetting and dewetting effects based on the Lifshitz formula, our approach is fully general and allows studies of complex microfluidic environments with no uncontrolled approximations. Time permitting, we present preliminary experimental results.   

Molecular dynamics simulation for vapor-liquid coexistence of water in nanocylinder

Molecular dynamics simulation was conducted in order to investigate the vapor-liquid coexistence of the water molecules in nanopore. In this research, the Lennard-Jones energy parameter between a water molecule and an atom of nanopore was optimized so as to model the contact angle between a water droplet and the carbon material in the fuel cell. The TIP4P/2005 as the model of a water molecule was used; this model produces well the vapor-liquid coexistence line. All of the systems were equilibrated by Nos\'e-Hoover thermostat. The electrostatic interaction between water molecules was calculated through smooth particle mesh Ewald method. First, we equilibrated a water plug in the single-wall atomistic nanocylinder as a model of nanopore in the fuel cell with radius 1.3nm. Water molecules burst from an interface of the water plug in equilibration. Then, the equilibrium densities both in dense and dilute region ware sampled over 1 ns. The vapor-liquid coexistence line, density profile, free energy profile will be presented in the session.   

On-demand generation of aqueous two-phase microdroplets with reversible phase transitions

Aqueous two-phase systems contained within microdroplets enable a bottom-up approach to mimicking the dynamic microcompartmentation of biomaterial that naturally occurs within the cytoplasm of cells. Here, we demonstrate the on-demand generation of femtolitre aqueous two-phase droplets within a microfluidic oil channel. Gated pressure pulses were used to generate individual, stationary two-phase microdroplets with a well-defined time zero for carrying out controlled and sequential phase transformations over time. Reversible phase transitions between single-phase, two-phase, and core-shell microbead states were obtained via evaporation-induced dehydration and on-demand water rehydration. In contrast to other microfluidic aqueous two-phase droplets, which require continuous flows and high-frequency droplet formation, our system enables the controlled isolation and reversible transformation of a single microdroplet and is expected to be useful for future studies in dynamic microcompartmentation and affinity partitioning.   

H2O and CO2 confined in cement based materials: an ab initio molecular dynamics study with van der Waals interactions

Although the cement has been widely used for a long time, very little is known regarding the atomistic mechanism behind its functionality. Particularly, the dynamics of molecular systems at confined nanoporous and water hydration is largely unknown. Here, we study the dynamical and structural properties of H$_2$O and CO$_2$ confined between Tobermorite 9\AA (T9) surfaces with Car-Parrinello molecular dynamics with and without van der Waals (vdW) interactions, at room temperature. For H$_2$O confined, we have observed a broadening in the intra and intermolecular bond angle distribution. A shift from an ice-like to a liquid-like infrared spectrum with the inclusion of vdW interactions was observed. The bond distance for the confined CO$_2$ was increased, followed with the appearance of shorter (larger) intramolecular (intermolecular) angles. These structural modifications result in variations on the CO$_2$ symmetric stretching Raman active vibration modes. The diffusion coefficient obtained for both confined H$_2$O and CO$_2$ were found to be lower than their bulk counterparts. Interestingly, during the water dynamics, a proton exchange between H$_2$O and the T9 surface was observed. However, for confined CO$_2$, no chemical reactions or bond breaking were observed.