Session AC: Microfluidics I: Nanoscale

Chirality Effects on Water Transport through Single-Walled Carbon Nanotubes

Using quantum partial charges, computed from 6-31G**/B3LYP density functional theory (DFT), in molecular dynamics (MD) simulations, we found that water inside (6,6) and (10,0) single-walled carbon nanotubes (SWCNTs) with similar diameters but with different chiralities has remarkably different structural and dynamical properties. The quantum partial charges, which capture the molecular electrostatic potential, induce relatively stronger wall-water electrostatic interactions at the ends of the tube and weaker electrostatic interactions at the center. The partial charges at the ends of a (10,0) tube are around 4.5 times higher than those of a (6,6) tube. Molecular dynamics simulations with the partial charges show different water dipole orientations. The water diffusion coefficient is found to increase in the presence of the partial charges. From PMF analysis, we found that a larger energy fluctuation inside the partially charged (10,0) tube induces a slower diffusion coefficient when compared with the partially charged (6,6) tube.   

Predicting Frictionless Flows

Nearly drag-free flow through carbon nanotubes (CNTs) was recently demonstrated.\footnote{ M. Majumder et al. \textit{Nature} \textbf{438}: 44 (2005), J. K. Holt et al. \textit{Science} \textbf{312}: 1034 (2006) \par \par } Flow rates orders of magnitude above that predicted by applying the no-slip boundary condition were measured. We formulate an analytical model which reproduces the high-speed flow observed. As inferred from the experiments, we find extremely large slip lengths. Unlike the no-slip condition which applies equally to all materials, the amount of slip is dependent on material parameters. CNTs can be made with different diameters and lattice structure. We show how CNT properties affect the amount of slip. The properties of the liquid also affect the amount of slip, and we show how the orientation of water molecules affects the amount of slip. We also show how to design CNT flows with the maximum amount of slip.   

Electrorheology and field-induced percolation of carbon nanotube suspensions under AC fields

Dilute ($\sim$ 0.003\% by volume fraction) suspensions of SWNTs in oil exhibit large variations in viscosity under electric fields. We report on an experimental investigation of the characteristics and mechanisms of electrorheology in SWNTs suspended in silicone oil. Shear thinning is observed in which the apparent viscosity at fixed field strength decreases with higher shear rates. The apparent viscosity increases with higher field strengths, until a threshold field strength is reached. Above that critical electric field, the apparent viscosity makes a large, sudden jump. This phenomena is associated with the formation of gap-spanning chains (percolation networks) of SWNTs under dipole-dipole interactions. The role of individual nanotubes versus bundles of nanotubes is assessed by comparing the electrorheology of centrifuged and un-centrifuged suspensions.   

Slippage of water past superhydrophobic carbon nanotube forests in microchannels

We present an experimental characterization of liquid flow slippage over superhydrophobic surfaces made of carbon nanotube forests, incorporated in microchannels. We make use of a $\mu$-PIV (Particule Image Velocimetry) technique to achieve the submicrometric resolution on the flow profile necessary for accurate measurement of the surface hydrodynamic properties. We demonstrate boundary slippage on the Cassie superhydrophobic state, associated with slip lengths of a few microns, while a vanishing slip length is found in the Wenzel state, when the liquid impregnates the surface. Varying the lateral roughness scale $L$ of our carbon nanotube forest-based superhydrophobic surfaces, we demonstrate that the slip length varies linearly with $L$ in line with theoretical predictions for slippage on patterned surfaces.   

Ionic and Biomolecular Transport in Nanochannels

In this work both steady and transient ionic and biomolecular transport in nanochannels is considered. Electroosmotic Flow (EOF) has been analyzed for both steady and transient two and three ionic component in a nanochannel. The sudden introduction of a species at the inlet of a channel generates a short transient regime followed by fully developed and steady state EOF in which the concentrations, potential and velocity are independent of the streamwise coordinate. In a channel with negatively charged walls and the cathode on the upstream side, a negatively charged species may move in a direction opposite to the direction of bulk fluid flow. A positively charged species is transported in the direction of fluid flow and there is a significant decrease in transit time as compared to an uncharged or negatively charged species. Results for concentration and species flux are presented for both charged and uncharged species. The steady state model is compared with a number of experimental results and the comparisons are extremely good.   

Flow-enhanced Mixing in Nanofluidic Channels.

We study mixing in nanoscale channels created by patterned buckling in compressed films on silicone substrates. The buckled film displays the telephone cord morphology with a characteristic configuration of zig-zag shape along the length direction. Also, the cross-sectional shape, which is almost semi-circular, has a periodic asymmetry. The channel heights are in the range of 20 -- 2000 nm. In this study, buckling channels are used as molds for PDMS stamping, which produced flow channels with height of 400 -- 800 nm and widths of 5 -$10\mu m$, and these channels are then integrated into a microfluidic experimental system. The flow behavior in the buckling channels shows laminar streams with Reynolds numbers $Re\approx 10^{-2}$. A series of mixing experiments was performed with various flow velocities. Observations with confocal microscopy reveal that the interface is strongly perturbed due to the periodic asymmetric profile of the buckling geometries. The axial mixing distance is proportional to ln (\textit{Pe}), indicative of a flow enhanced dispersion.   

Multiscale Simulation of Ionic Flux through a Nanopore under Applied Electric Potential

This talk discusses the development of an atomic/continuum multiscale model for the simulation of electrophoretically driven ionic fluxes through a nanopore of an electrodesalination membrane. The ion transport in this single pore model is simulated using Brownian Dynamics computations, with parameter fields that are obtained from targeted Molecular Dynamics simulations. The Brownian Dynamics simulations couple with a fine-grained Poisson-Nernst-Planck continuum model in the pore vicinity. The model is calibrated against patch clamp measurements of current through track-etched nanoporous membranes under an applied electric potential. Simulations are performed to investigate the effects of pore size, membrane wall charge and applied potential on the pore throughput and selectivity.   

Electrophoresis of a polyelectrolyte through a nanopore

Translocation of polyelectrolytes (such as DNA) through natural and artificial nanopores can be detected with single molecule resolution by monitoring the resistivity of the pore ({\it Nature Biotechnology} (2001) {\bf 19}, pp. 248). The technique could evolve into a technology for sequencing DNA at speeds that are orders of magnitude faster than what is currently possible. Here a hydrodynamic model to determine the electrophoretic speed of a polyelectrolyte through a nanopore is presented. It is assumed that the speed is determined by a balance of electrical and viscous forces arising from within the pore and that classical continuum electrostatics and hydrodynamics may be considered applicable. An explicit formula for the translocation speed as a function of the pore geometry and other physical parameters is obtained and is shown to be consistent with experimental measurements on DNA translocation through nanopores in silicon membranes. Secondary effects such as the hydrodynamic friction on the part of the polymer outside the nanopore must also be considered to explain the weak dependence of the translocation speed on the polymer length.   

Selective Adhesion Rate Control of Micron-Scale Objects Using NanoPatterned Surfaces

Dynamic particle adhesion from flowing solution onto collecting surfaces, heterogeneous at the submicron scale, occurs in important natural scenarios and current technologies. Potential new applications for sensing, separating, and sorting objects in the 200 nm to 5 $\mu$m range will stem from our ability to manipulate selectively their dynamic adhesion in flowing conditions. We describe micron-scale particle adhesion from suspensions flowing over surfaces tailored at the 10-50 nm lengthscale. Our collecting surfaces were generally repulsive (electrostatically) towards 500 nm and 1 $\mu$m flowing silica particles, but the collectors also contained randomly distributed polymeric or proteinaceous entities that produced spatially fluctuating attractions. With these systems we observed a dependence of the particle capture rate on the density of adhesive groups and, more importantly, an adhesion threshold or lower limit on the feature density that gave rise to a curvature-based selectivity. A semiquantitative fluctuation treatment captures the essential observations, while hydrodynamic simulations also predict the adhesion threshold and particle dynamics near the collecting surfaces.   

Modeling the Dynamics of Micron-Scale Objects in Flow Over Nano-Textured Surfaces

The manipulation of dynamic particle adhesion in flow over heterogeneous collecting surfaces is an important aspect of microfluidic sensing technologies. Selective tuning of the sizes, surface densities, and chemistries of 10-nanometer scale heterogeneities on planar surfaces that interact with micron- scale particles in shear flow at low Reynolds number allows control of the particle dynamics and selective adhesion. Adhesion is reversible in a substantial portion of parameter space, and surface features can give rise to particle skipping and rolling on the surfaces. These dynamics are captured by a new model that incorporates hydrodynamic forces and the spatially varying physicochemical interactions between the particles and heterogeneous surfaces. The adhesion behavior is reminiscent of pattern recognition, although the patch distribution on the collector is random. Spatial fluctuations in the patch density are shown to play a critical role in the dynamic adhesion, rolling, and skipping (e.g., allowing adhesion on a net-repulsive surface), which prevents this behavior from being predicted by a mean-field approach. Good agreement is found between model predictions and experimental results.   

Dissipative particle dynamics simulations of breakup of liquid nanocylinders and nanojets

Dissipative particle dynamics (DPD) is a coarse-grained mesoscopic method which can be used for modeling sub-micron fluid flows. We use a DPD two-phase model to simulate systems where thermal fluctuations play a significant role. In this work, we present results from a thermally induced breakup of liquid nanocylinders and nanojets. In the former case, we compare the DPD results with Rayleigh's stability criterion. In the latter case, we compare DPD results with molecular dynamics simulations presented in literature.   

Probing the Nanohydrodynamics at Liquid-Solid Interfaces Using Thermal Motion

We report on a new method to characterize nanohydrodynamic properties at the liquid-solid interface relying solely on the measurement of the thermal motion of confined colloids. This equilibrium measurement of surface properties—equivalent in spirit to the passive microrheology technique used for bulk properties—is able to achieve nanometric resolution on the slip length measurement. Exploring the ``zero shear rate'' limit, it rules out shear rate threshold to slip effects and extends the range over which slip lengths are shown to be flow independent. Avoiding the nucleation of gas pockets (nanobubbles) through external forcing, it validates the theoretical picture for intrinsic liquid-solid interfaces, reporting nanometric slip lengths (b~18 nm) only in nonwetting situations, opening the route to quantitative study on more complex surfaces with combined effects of nonwettability and roughness.