Session A08: Nano Flows

Slip at the liquid-solid interface reveals soliton propagation and solute partitioning

The classical no-slip condition of fluid mechanics does not apply under certain circumstances. Rather, there is a finite liquid velocity at the solid surface that depends on the nature of the solid and liquid. It is poorly understood how this slip occurs, in particular, how liquid molecules move over the substrate. Of particular interest is propagation of nonlinear waves--solitons--conveying liquid molecules. Here we report that the slip velocity of a Lennard-Jones fluid over a cubic lattice solid substrate is due to soliton propagation. The angle between the direction of soliton propagation relative to that of the applied force depends on the type of liquid molecule. Consequently, different species of molecules can be spatially separated as they move over the substrate. Propagation of solitons is a fundamental process leading to slip at liquid-solid interfaces. The directional sensitivity of solitons yields insight into designing low friction surfaces and new technologies for chemical separations.    

Mo' Mobilities, No Problems: An Excess Entropy Scaling Relation for Diffusivity of an Active Fluid

Active matter systems feature discrete particles that convert energy into motion. Such systems are intrinsically out of equilibrium, and exhibit a wide array of unusual transport phenomena, with many potential engineering applications. In this work, we demonstrate that excess-entropy scaling (a principle that has been shown to describe transport properties for a wide range of inactive fluids) can be adapted to also describe diffusivity in a simple model for active fluids. In support of this claim, we perform extensive molecular-dynamics (MD) simulations of an active fluid, in which we vary the total fraction of active particles and the active particles’ degree of activity. Our MD simulation results are consistent with a Rosenfeld-type scaling between active matter diffusion and excess entropy. This scaling relationship constitutes a novel connection between structure and transport in active matter, and is a promising step forward for predictive and generalizable models of other transport phenomena in active systems. We close by briefly describing the features of a system (whether inactive or active) that render its transport properties amenable to excess-entropy-type approaches.   

Correlations Between the Kapitza Resistance and Wall Induced Structure at Quiescent Liquid/Solid Interfaces

Continuous miniaturization of electronic components for various space applications has led to high density chips prone to deleterious  thermal runaway effects. Thermal extraction of enormous waste heat is therefore being replaced by liquid-based cooling strategies, although these too at ultimately limited by the Kapitza effect which induces a temperature jump at the interface between any dissimilar materials. In this work, we focus on quiescent liquid/solid interfaces and employ molecular dynamics simulations to elicit correlations between the Kapitza resistance and configurational aspects of stratified liquid atoms in the vicinity of the solid surface. Our results indicate that obvious measures, such as the liquid contact density at the wall, don't necessarily guarantee efficient thermal extraction. We will discuss insights obtained by varying the wall temperature and parameters controlling the liquid/solid intermolecular potential. These results, some intuitive and others not, yield useful correlations incorporating the collective response of atoms within the first liquid layer.   

On the Role of Fluid-Solid Interaction Strength in Anomalous Fluid Diffusion under Nanoscale Confinement

The transport properties of a nanoconfined fluid can differ significantly as compared to that same fluid under bulk conditions. Previous studies have reported that under certain circumstances, the diffusivity of a nanoconfined fluid can be higher than its bulk counterpart. In this work, we discuss the relationship between the diffusivity of nanoconfined water and the magnitude of interactions between water and its confining solid. We perform extensive molecular-dynamics (MD) simulations in which we vary the energy scale of interactions between water and solid, as well as the aspect ratio and size of the system. We show (following meticulous assessment of associated uncertainties) that the diffusivity of confined water can be a remarkably non-monotonic function of the water-solid interaction strength, with extremal values that differ considerably from the bulk diffusivity. To rationalize this finding, we also present results on direction-decomposed diffusivity. This work highlights a subtle finite-size effect that can appear in MD simulations of nanoconfined fluids, and also underscores the importance of careful uncertainty quantification in such studies.    

Acoustogeometric streaming for manipulation and mixing of 100 femtoliter droplets in nanoslit channels

Surface and viscous forces make manipulation and mixing of fluids difficult at the nanoscale. Finding broad adoption in microfluidics, MHz-order vibration produces even more powerful effects at the nanoscale, with the use of a new form of acoustic streaming---acoustogeometric streaming---that leads to an ability to split, merge, transport, and mix 200-fL droplets of water and other fluids. By using 40-MHz surface acoustic devices in lithium niobate bonded to a second lithium niobate layer, we are able to produce a channel from 120 nm to as little as 5 nm in height extending over 5 mm in length. The width is shaped to produce 130 micron wide droplet traps along 20 micron wide channels, Y-channels and other features. By forming traps as locally widened regions along such a channel, individual fluid droplets may be propelled from one trap to the next, split between them, mixed, and merged. A simple theory is provided to describe the mechanisms of droplet transport and splitting.    

Mesoscale Hydrodynamic Fluctuations in Nonequilibrium Gas Mixtures Across a Transpiration Membrane

Various separation technologies involve the transport of gas mixtures across species-selective membranes. At mesoscopic length scales, where the gas mean free path is much larger than the membrane pore size, the long-ranged hydrodynamical fluctuations that are thermally intrinsic in the transporting gas can get significantly modified. In this talk, I will describe our investigations into the spatial structure of hydrodynamical fluctuations in gas mixtures that are driven by concentration gradients across a transpiration membrane. To simulate thermal fluctuations in mesoscale gas mixtures, we utilize a fluctuating hydrodynamics framework, which extends the deterministic compressible Navier-Stokes equations by incorporating stochastic fluxes. The transpiration membrane is modeled using a Langevin equation implementation of the fluxes crossing the interface with a given transmission probability. The role of species-dependent transmission probability and concentration gradient on non-equilibrium hydrodynamical fluctuations will be described.   

Anomalously enhanced moisture-vapor-transport rate in vertically aligned 0.8nm-diameter carbon-nanotube membranes

Membranes with small-diameter, vertically aligned carbon-nanotube (VA-CNT) pores provide a promising path toward highly permeable membranes for applications such as breathable yet protective garments, desalination membranes, and highly efficient filters.  However, it has not been previously possible to fabricate VA-CNT membranes with sub-nanometer pores because of the difficulty in growing aligned forests of CNTs of such small diameters.  Here, we describe the transport properties of macroscopic VA-CNT membranes made from 0.8nm diameter CNTs grown in bulk with a specialized catalyst.  The membranes are made by dispersing nanotubes in a liquid prepolymer and aligning them with an electric field before locking them in place in a polyurethane matrix by UV curing.  After oxygen-plasma etching to open the CNT pores, the VA-CNT membranes were permeable and demonstrated cation selectivity in KCl solution, and sublinear conductance scaling with salt concentration at low molarities.  These characteristics indicate that the membranes were defect-free, with small-diameter CNTs serving as the primary pathway for transport.  Moisture-vapor-transport-rate (MVTR) measurements revealed that water vapor diffused many orders of magnitude faster than either Knudsen diffusion, or the rates reported in literature for larger-diameter VA-CNT membranes.  These scalably fabricated VA-CNT membranes with sub-nanometer pores may present new opportunities for fundamental studies of transport in 1D pores, and to engineer highly permeable membranes for a variety of applications.   

Lifshitz theory for a wedge

We develop the Lifshitz theory of van der Waals forces in a wedge of a dielectric material. The non-planar geometry of the problem requires determining point-wise distribution of stresses. The findings are relevant to contact line motion, in particular. First, the stresses prove to be anisotropic as opposed to the classical fluid mechanics treatment of the contact line problem. Second, the wedge configuration is always unstable with its angle tending either to collapse or unfold. The presented theory unequivocally demonstrates quantum nature of the forces dictating the wedge behavior, which cannot be accounted for with the classical methods.