Session AP: Nanofluids I

Experimental Study of Fluid Structure Formation from the Linear to Non-Linear Regime in Polymer Nanofilms Subject to Benard-Like Instability

Researchers continue to seek novel nanopatterning techniques which unlike conventional photolithography can yield more rapid, less expensive pattern definition compatible with large areas, curved substrates and polymeric materials. Here we describe in-situ measurements of the spontaneous formation and growth of fluid elongations in molten polymer nanofilms. These films, confined in between two substrates held at different temperature, exhibit a free surface subject to thermocapillary instability. Unlike the typical Benard instability in which the fluid interface adopts an undulatory shape with small fixed amplitude, this system generates pillar-like features spaced tens of microns apart which evolve continuously toward the cooler substrate. The pillar shape and size therefore depend on the period of exposure to the thermal gradient. Optical microscopy images obtained through a sapphire window patterned with a transparent cylindrical protrusion are used to compute the pair correlation function, coordination number and Fourier transform of structure formation as a function of time. In-plane hexagonal symmetry is only observed at relatively late times and lamellar or square symmetry is all too easily obtained for tilted or curved substrates.   

Microscopic description of a liquid film on a solid substrate using density functional theory

We examine the wetting properties of planar and spherical substrates using a mean-field density functional theory. Equilibrium density profiles of a fluid close to an attractive wall are obtained by solving an integral equation resulting from the minimization of the grand potential. Using a novel pseudo-arc length continuation scheme, we compute the complete bifurcation diagram of the adsorption as a function of the chemical potential. For a spherical substrate we demonstrate a second unstable branch approaching saturation from the right, absent in the planar case. Our numerical results are in excellent agreement with analytical predictions obtained from a piecewise function approximation in which the density profile is assumed to be everywhere constant except near the wall-liquid and the liquid-gas interfaces. We also show that the sharp-interface approximation, used often to predict wetting behavior on planar substrates, is inadequate to describe wetting on a spherical substrate.   

Influence of Stabilizing van der Waals Forces on Benard Instability in Viscous Nanofilms

While best avoided in most practical applications, hydrodynamic instabilities in ultrathin films can provide a useful method for self-assembly of large area arrays. As one example, studies have confirmed that polymer nanofilms subject to a transverse thermal field gradient can undergo a Benard-like instability to produce arrays of pillar-like structures. For polymers with low glass transition temperatures, these structures solidify in place once the driving force is removed. During the actual formation process, the region in between pillars can thin substantially below 50 nm. Stabilizing van der Waals forces then become significant and can slow pillar growth and modify the instability wavelength. In this talk, we first discuss results of a linear stability analysis of an initially uniform film subject to thermocapillary, capillary forces and van der Waals forces. We then use analytic and numerical studies to explore the dynamics of film thinning in between pillar formation to better understand transitions in force balance and subsequent film deformation in non-uniform films. Results of investigations based on self-similarity and asymptotic analysis will be presented.   

The relationship between induced fluid structure and boundary slip in nanoscale polymer films: A molecular dynamics simulation study

The molecular mechanism of slip at the interface between polymer melts and weakly attractive smooth surfaces is investigated using MD simulations. In agreement with our previous studies, it is shown that the slip length passes through a local minimum at low shear rates and then increases rapidly at higher rates. We found that at sufficiently high shear rates, the slip flow over flat crystalline surfaces is anisotropic. It is demonstrated numerically that the friction coefficient (the ratio of viscosity and slip length) undergoes a transition from a constant value to the power-law decay as a function of the slip velocity. The characteristic velocity of the transition correlates well with the diffusion velocity of monomers in the first fluid layer. We also show that in the linear regime, the friction coefficient is well described by a function of a single variable, which is a product of the magnitude of surface-induced peak in the structure factor and the contact density of the adjacent fluid layer. The universal relationship between the friction coefficient and induced fluid structure holds for a number of material parameters of the interface: fluid density, chain length, wall-fluid interaction energy, wall density, lattice type and orientation, thermal or solid walls. Reference: cond-mat/1007.4534.   

The Saffman-Taylor Instability Without Walls

The Saffman-Taylor problem represents the unstable displacement of a more viscous fluid by a less viscous fluid under the action of an external pressure gradient. Small sinusoidal deformation of the separation front gives way to repeated fingering and tip-splitting. This repetitive process causes the transformation of an initially featureless front to a highly ramified curve whose fractal dimension is roughly 1.7. This instability requires that the fluids be confined by two substrates, such as in a Hele-Shaw cell, in order to enforce the pressure gradient which drives the flow. In this talk we describe experiments in which this instability is observed for the first time in nanofilms freely suspended in air for which there are no confining walls. The nanofilms consist of an aqueous surfactant solution containing hydrosoluble polymer. The fractal dimension D$_{F}$ ranges from 1 $<$ D$_{F} \quad <$2 and increases with the viscosity of the bulk solution. The expanding front appears to delineate between a surfactant-rich mobile phase and a polymer-rich less mobile phase. We describe a phenomenological model for linearly unstable flow in which the mobility contrast incorporates both modulation in film thickness from disjoining pressure variation as well as the viscosity contrast from phase segregation. This extension generalizes our understanding of this well known phenomenon.   

Nanofluidics: Ionic transport through a nanochannel

Nanopore based membranes find nowadays interesting applications such as water treatment [1] and power energy conversion [2]. We consider a single micrometric glass channel. We show how surface effect can modify the channel trasport properties. We measure the ionic conductance through the channel as a function of the electrolyte concentration showing a saturation effect in the very low concentration regime. Such effect is due to the surface correction to the channel conductance [3]. The effective surface charge obtained by the conductance measurements, is largely higher than what commonly measured for glass surfaces [4]. Introducing a moderate hydrodynamic slippage of the fluid at the solid-fluid interface allows us to obtain surface charge values in agreement with what presented in literature. A quantitave agreement with independent measurement of electro-osmotic mobility and streaming current allows us to fully characterize the slippage effect on the channel electokinetic properties.\\[4pt] [1] H. Liu et al., 64 (2009)\\[0pt] [2] F.H.J. van der Heyden et al. Nanoletters 7, 4, 1022 (2007)\\[0pt] [3] D. Stein et al. Phys. Rev. Lett. 93, 3, 035901 (2004)\\[0pt] [4] L. Bocquet et al. Chem Soc, Rev. (2010)   

Hydronium-dominated ion transport in carbon-dioxide-saturated electrolytes at low salt concentrations in nanochannels

Nanochannel ion transport is known to be governed by surface charge at low ionic concentrations. In this talk, we show that this surface charge is dominated by hydronium ions arising from dissolution of ambient atmospheric carbon dioxide. By refining the electrokinetic model of the nanochannel conductance for low salt concentrations, we identify a minimum conductance value before saturation at a value independent of salt concentration in the dilute limit. Our model self-consistently couples chemical equilibrium models of the silica wall and the electrolyte bulk, and is parameterized by only the surface reaction equilibrium constant for silica/hydronium reactions. The model describes our experimental data with aqueous potassium chloride solutions in 165-nm-high silica nanochannels well, and furthermore, by comparing model predictions with measurements in bulk and in nanochannels with hydrochloric acid solutions, we verify its predictive power.   

Electrochemical surface properties of bare- and silane-coated silica nanochannels

We present a combined theoretical and experimental analysis of the solid-liquid interface of fused-silica nanofabricated channels with and without a hydrophilic cyanosilane coating. Our theoretical model consists of three parts: (1) a chemical equilibrium model of the wall, (2) a chemical equilibrium model of the bulk electrolyte, and (3) a self-consistent Gouy--Chapman--Stern triple-layer model of the electrochemical double layer coupling (1) and (2). To validate our model, we used both pH-sensitive dye based capillary filling experiments and electro-osmotic current-monitoring measurements. Our model shows that the important fitting parameters are the inner Stern capacitance $C_1$ and the surface reaction constant p$K_+$. We also find that changing the outer Stern capacitance $C_2$ with surface composition results in more accurate fits of experimentally determined $\zeta$ potentials. This model is of value to predict experimentally observed phenomena in nanofluidic systems.   

Physical Origins of Enhanced Interfacial Viscosity in Electroosmotic Flows

Electroosmotic flows (EOF) is widely used in micro/nanofluidic systems for fluid manipulation. The driving force for EOF exists only within the electrical double layers (EDLs) near charged substrates. While EOFs with thick EDLs are now well-understood, current knowledge on EOFs with EDLs thinner than a few nanometers remains limited. Specifically, experimental evidence suggests that the viscosity of interfacial fluids in EOF is higher than that of bulk fluids, but the physical origins of this universal phenomenon remain elusive. Many mechanisms such as layering of interfacial fluids, high ion concentration in EDL, and polarization of fluids in the EDL have been proposed, but a universal mechanism that encompass the breadth of experimental evidence has not been firmly established. In this work, we use molecular dynamics simulations to compute the effective viscosity of interfacial fluids in carefully controlled EOFs. We examine many mechanisms in the literature and suggest a mechanism that is capable of explaining the enhanced viscosity of interfacial fluids in EOFs regardless of the nature of the solid substrates.   

An investigation of nucleation-growth of bubbles using molecular dynamics simulation

Microscopic phase transition phenomena have been applied in medical or industrial area recently. However, the behavior of bubble nucleation in nano-scale has not been clarified yet. In this study, a bubble nucleation-growth process was investigated in a decompressed Lennard-Jones fluid using an NVT ensemble molecular dynamics simulation. As a result, as was reported in the NVE ensemble, a competitive growth similar to Ostwald ripening was also observed. On the other hand, more bubble nuclei were formed than those in the NVE ensemble, and the growing/shrinking speed of each bubble nucleus became slower. It was confirmed that the temperature rises in NVE ensemble when bubble nuclei are generated while it does not rise in NVT. Therefore, the pressure is lower in NVT and this pressure difference causes the results mentioned above. The growth exponent for the mean radius of bubble nuclei was also evaluated, and it was confirmed that the value of the exponent became smaller than that in the NVE ensemble, at least in the present time scale. However, since the competitive growth is similar to the result of the NVE simulation, the exponent value could be equal to that of the NVE by investigating the longer time behavior.