Session Y50: Focus Session: Micro and Nano Fluidics III: Microtransport and Thermophysical Properties

Cavitation in confined water: ultra-fast bubble dynamics

In the hydraulic vessels of trees, water can be found at negative pressure. This metastable state, corresponding to mechanical tension, is achieved by evaporation through a porous medium. It can be relaxed by cavitation, i.e. the sudden nucleation of vapor bubbles. Harmful for the tree due to the subsequent emboli of sap vessels, cavitation is on the contrary used by ferns to eject spores very swiftly. We will focus here on the dynamics of the cavitation bubble, which is of primary importance to explain the previously cited natural phenomena. We use the recently developed method of artificial tress, using transparent hydrogels as the porous medium. Our experiments, on water confined in micrometric hydrogel cavities, show an extremely fast dynamics: bubbles are nucleated at the microsecond timescale. For cavities larger than 100 microns, the bubble ``rings'' with damped oscillations at MHz frequencies, whereas for smaller cavities the oscillations become overdamped. This rich dynamics can be accounted for by a model we developed, leading to a modified Rayleigh-Plesset equation. Interestingly, this model predicts the impossibility to nucleate bubbles above a critical confinement that depends on liquid negative pressure and corresponds to approximately 100 nm for 20 MPa tensions.   

Multiphase flow within 3D porous media

Multiphase flow through porous media is important for a diverse range of processes including aquifer remediation, CO2 sequestration, and oil recovery. Despite its enormous importance, exactly how flow proceeds within a porous medium is unknown; the opacity of the medium typically precludes direct imaging of the flow. Here, we present an experimental technique to directly visualize multiphase flow within porous media. Using this approach, we show how heterogeneity strongly affects flow behavior during the drainage of porous media.   

Modeling capillary filling of micropores with nanoparticle-filled binary fluid

We examine the behavior of binary fluids containing nanoparticles that are driven by capillary forces to fill well-defined pores of microchannels. To carry out these studies, we use a hybrid computational approach that combines the lattice Boltzmann model for binary fluids and a Brownian dynamics model for the nanoparticles. The hybrid model allows us to capture the interactions between the binary fluids and the nanoparticles, as well as model the interactions among the fluid, the nanoparticles and the pore walls. We show that the nanoparticles dynamically alter both the interfacial tension between the two fluids and the contact angle on the pore walls; this, in turn, strongly affects the dynamics of the capillary filling. We demonstrate that by tailoring the properties of the nanoparticles, such as their affinity to the fluid components and their interaction with the pore walls, one can effectively control both the filling velocities and the deposition of nanoparticles on the pore walls. Our findings provide fundamental insights into the dynamics of this complex system, as well as potential guidelines for technological processes involving capillary filling with nanoparticles in microchannels with differing geometries.   

Label-free Screening of Multiple Cell-surface Antigens Using a Single Pore

Microfluidic pores have emerged as versatile tools for performing highly sensitive measurements. Pore functionalization can result in slower particle transit rates, thereby providing insight into the properties of particles that travel through a pore. While enhancing utility, functionalizing with only one species limits the broader applicability of pores for biosensing by restricting the insight gained in a single run. We have developed a method of using variable cross-section pores to create unique electronic signatures for reliable detection and automated data analysis. By defining a single pore into sections using common lithography techniques, we can detect when a cell passes through a given pore segment using resistive-pulse sensing$_{.}$ This offers such advantages as 1) the ability to functionalize each portion of a pore with a different antibody that corresponds to different cell surface receptors, enabling label-free multianalyte detection in a single run; and 2) a unique electronic signature that allows for both an accelerated real-time analysis and an additional level of precision to testing. This is particularly critical for clinical diagnostics where accuracy and reliability of results are crucial for healthcare professionals upon which to act.   

Tracking rotation and translation simultaneously in confined liquids

At the same spatially-resolved spots when fluid is confined to molecularly-thin spacings between atomically-smooth mica crystals, we track simultaneously, using fluorescence correlation spectroscopy and time correlated single photon counting, the translational and rotational diffusion of small dyes suspended in octamethylcyclotetrasiloxane (OMCTS). The spatially-resolved quantification of both dynamical quantities gives insight, as it does in bulk glasses, into the origins of dynamical heterogeneity in confined fluids.   

Neutron Scattering Applications for Characterizing Phase Behavior and Dynamics of Confined Fluids in Nanoporous Materials

Fluid-solid interactions in natural and engineered porous solids underlie variety of technological processes, including sequestration of anthropogenic greenhouse gases, hydrogen storage, membrane separation, and catalysis. The size, distribution and interconnectivity of pores, the chemical and physical properties of the solid and fluid phases collectively dictate how fluid molecules migrate into and through the micro- and mesoporous media, adsorb and ultimately react with the solid surfaces. Due to the high penetration power and relatively short wavelength of neutrons, small-angle neutron scattering (SANS) as well as quasi elastic neutron scattering (QENS) techniques are ideally suited for \textit{in situ} studies of the structure and phase behavior of confined fluids under pressure as well as for evaluating structure of pores in engineered and natural porous systems. It has been demonstrated recently that SANS and USANS can also be used for evaluating the volume of closed pores as a function of pore sizes in the range from micrometer to sub-nanometer pores. In this talk I will overview some recent developments in the SANS and QENS methodology and give several examples of how it can be used for in-situ studies of the adsorption and dynamics of carbon dioxide and methane in porous fractal silica and carbon aerogels as well as characterizing the abnormal densification of hydrogen in activated carbons at ambient temperatures.   

A mathematical model for the transport of a solute through a porous-walled tube

Predicting the distribution of solutes or particles in flows within porous-walled tubes is essential to inform the design of cross-flow filtration devices. Here we use Taylor-dispersion theory to derive a radially averaged model for solute transport in a tube with porous walls, where the wall Darcy permeability may vary both spatially and in time. Crucially, this model includes solute advection via both radial and axial flow components, as well as diffusion, and the advection, diffusion and uptake coefficients in the averaged equation are explicitly derived. The model is used to explore the specific example of a hollow-fibre membrane bioreactor for tissue engineering applications - here membrane fouling and cell population expansion mean that the effective membrane permeability is intrinsically coupled to both fluid flow and nutrient transport. We conclude by presenting design considerations that promote spatially uniform cell population growth.   

Biased and flow driven Brownian motion in periodic channels

In this talk we will present an expansion of the common Fick-Jacobs approximation to hydrodynamically as well as by external forces driven Brownian transport in two-dimensional channels exhibiting smoothly varying periodic cross-section. We employ an asymptotic analysis to the components of the flow field and to stationary probability density for finding the particles within the channel in a geometric parameter. We demonstrate that the problem of biased Brownian dynamics in a confined $2$D geometry can be replaced by Brownian motion in an effective periodic one-dimensional potential $\Psi(x)$ which takes the external bias, the change of the local channel width, and the flow velocity component in longitudinal direction into account. In addition, we study the influence of the external force magnitude, respectively, the pressure drop of the fluid on the particle transport quantities like the averaged velocity and the effective diffusion coefficient. The critical ratio between the external force and pressure drop where the average velocity equals zero is identified and the dependence of the latter on the channel geometry is derived. Analytic findings are confirmed by numerical simulations of the particle dynamics in a reflection symmetric sinusoidal channel.   

Crossover from the Hydrodynamic to the Kinetic Regime in Confined Nanoflows

We present an experimental study of a confined nanoflow. The nanoflow is generated in a simple fluid by a sphere oscillating in the proximity of a flat solid wall. Varying the oscillation frequency, the confining length scale and the fluid mean free path over a broad range provides a detailed map of the flow. We use this experimental map to construct a scaling form, which seamlessly describes the nanoflow in both the hydrodynamic and the kinetic regimes. Furthermore, our scaling form unifies previous theories based on the slip boundary condition and the effective viscosity.   

Viscosity of ultrathin water films confined between oxide surfaces -- ab initio and classical molecular dynamics simulations

We compare estimates based on ab initio (DFT/PBE) and on classical molecular dynamics simulations of the viscosity of 2, 3 and 5-layer water films confined between hydrophilic kaolinite surfaces. Results were obtained by constraining the confining surfaces to move in +x and -x directions at equal speeds of 1-200 m/sec and loads up to 1 GPa. In neither simulation approach did the calculated viscosity of the confined water exceed that of bulk water by more than an order of magnitude. Thus neither supports the idea that nano-confinement dramatically enhances water's viscosity.   

Submicron flows of polymer solutions

We study flow properties of high molecular weight polymer solutions below the micron scale. Fluorescence photobleaching is used as a non-invasive technique to evaluate the velocity of pressure-driven flows in channels from 100 to 4000 nm height. We observe a striking reduction of the effective viscosity of polyacrylamide solutiuons in the semi-dilute regime. This effect increases with molecular weight and concentration. Using a Rabinovitch-like approach, we correlate the data at different thicknesses to obtain the wall slip velocity and the flow curve at sub-microscale. Those properties are also evaluated using particle imaging velocimetry close to similar surfaces and standard rheometry. Comparing the measurements in bulk and in confined geometries, we conclude that the observed viscosity reduction can not be solely explained by slippage. We discuss the possible reasons of this effect that are size-dependant filtration and shear-thinning enhancement due to the confinement.   

Nanomechanics and dynamics of confined water and other liquids

From oil recovery to molecular biology, nanoconfined water plays an important role in many areas of research. However, the mechanics and dynamics of nanoconfined water are not well understood. Over the last ten years, a number of groups have measured the mechanics of confined water using atomic force microscopy (AFM) or surface force apparatus (SFA) - often with contradictory results. At Wayne State University, we have developed high resolution AFMs for ultra-small amplitude, linear measurements of the mechanics and dynamics of confined liquids. We have shown that water shows a distinct slow-down in dynamics under confinement (PRB 2004), co-discovered a dynamic ``solidification'' in a model liquid (Langmuir 2006), and showed that normal and shear stiffness are closely related in confined liquids (Rev. Sci. Instr. 2008). Recently, we found dynamic solidification also in water layers (PRL 2010), a finding that explains the contradictory findings in earlier measurements and points to surprisingly complex behavior in this seemingly simple system. Here we will review these findings, as well as present new findings that show the profound effects of ion concentration on these dynamical effects, as well as measurements of colloidal systems, which illustrate that some findings at the molecular scale can be understood from purely geometric considerations and are not dependent on molecular-scale interactions.   

Investigation of Nanoscale Structure Using Spin-Echo Small-Angle Neutron Scattering (SESANS)

Spin-Echo Small-Angle Neutron Scattering (SESANS) is a new technique for probing structural correlations in real space over distances ranging from $\sim $20 nm to several microns. The measured SESANS correlation function is a projection of the normal Patterson correlation function on to a particular spatial direction. A framework to theoretically calculate this correlation function is laid out, followed by a general discussion of the features of the SESANS correlation function for colloidal systems with different interaction potentials. Our calculations for a system of monodisperse spherical particles, show that SESANS is much more sensitive to the intercolloid potential than conventional Small Angle Neutron Scattering. We have used SESANS to study the correlations between 300-nm-diameter surfactant-stablized poly(methyl methacrylate) (PMMA) spheres suspended in a good solvent, with and without an added polymeric depletant. Below a PMMA volume fraction of $\sim $30{\%} we find good agreement between the experimental data and theoretical prediction based on the Percus-Yevick approximation. With a small amount of polymer added to the suspension (less than 0.2{\%} by weight of 110 kD polymer), the short-range correlations between PMMA spheres are enhanced because of the presence of polymer depletant. The magnitude of the change is roughly as expected on the basis of calculations of a mixture of spherical particles of different sizes.   

Non local rheology and near wall fluctuations in microgel jammed suspensions

We study flows of concentrated suspensions of soft nanoparticles in microchannels, over smooth hydrophilic and hydrophobic surfaces, using nano-PTV and $\mu $PIV techniques. With hydrophobic walls, the flow curves are in good agreeement with bulk rheology. With hydrophilic walls, substantial deviations from bulk rheology are observed. In the meantime, large velocity oscillations close to the wall are detected. We couple these observations by introducing a local rheology based on an energy barrier. As a whole, our work confirms the existence of non local rheological behavior in glassy systems.