Ion Steric Effect Induces Giant Enhancement of Thermoelectric Conversion in Electrolyte-Filled Nanochannels.

Ionic thermoelectricity in nanochannels has received increasing attention because of its advantages, such as high Seebeck coefficient and low cost. However, most studies have focused on dilute simple electrolytes that neglect the effects of finite ion sizes and short-range electrostatic correlation. Here, we reveal a new thermoelectric mechanism arising from the coupling of the ion steric effect due to finite ion sizes and ion thermodiffusion in electric double layers, using both theoretical and numerical methods. We show that this mechanism can significantly enhance the thermoelectric response in nanoconfined electrolytes depending on the properties of electrolytes and nanochannels. Compared to the previously known mechanisms, the new mechanism can increase the Seebeck coefficient by 100% or even 1 order of magnitude enhancement under optimal conditions. Moreover, we demonstrate that the short-range electrostatic correlation can help preserve the Seebeck coefficient enhancement in a weaker confinement or in more concentrated electrolytes.

Structural features and electrostatic energy storage of electric double layers in confined polyelectrolyte solutions under low-salt conditions.

An electric double layer (EDL) in a polyelectrolyte solution plays a crucial role in diverse fields ranging from physical and life sciences to modern technologies. Due to the nonnegligible excluded volume effects, chain connectivity and complex intermolecular interactions, the EDLs in (confined) polyelectrolyte solutions display distinct features compared to those in simple electrolyte solutions. Here, we conducted a systematic study on the characteristics of EDLs in confined polyelectrolyte solutions for salt-free and low salt concentration systems using self-consistent field theory. Results suggest that the characteristic length scales measuring the EDL structures are different for positively and negatively charged surfaces. The former is the same as in the electrolyte solutions, while the latter is smaller due to the accumulation of oppositely charged polyelectrolytes near the surface. Furthermore, for low surface charge densities, a scaling law for the electrostatic energy stored in polyelectrolyte EDLs (in units of mJ m-2) was found to be U ∝ |σ|ν with ν ∼ 2-2.7, which differs from the electrolyte EDLs with ν ∼ 2; however, such a scaling law breaks down for high surface charge densities.

Effects of fluid slippage on pressure-driven electrokinetic energy conversion in conical nanochannels.

The effects of fluid slippage on the pressure-driven electrokinetic energy conversion in conical nanochannels are systematically investigated in this paper. We present a multiphysical model that couples the Planck-Nernst-Poisson equations and the Navier-Stokes equation with a Navier slip condition to fulfill this purpose. We systematically look into the variation of various performance indicators of electrokinetic energy conversion, for example, streaming current, streaming potential, generation power, energy conversion efficiency, regulation parameter, and enchantment ratio, with the conicity of nanochannels and the slip length for two pressure differences of the same magnitude but opposite directions. Particularly, enhancement ratios related to streaming current, streaming potential, generation power, and energy conversion efficiency are defined to comprehensively measure the enhancement of the performance of electrokinetic energy conversion due to the slip length. The results demonstrate that a combination of large slip length and small conicity enhances the electrokinetic energy conversion performance significantly. Furthermore, the fluid slippage-induced enhancement of the electrokinetic energy conversion in the backward pressure difference mode is stronger than that in the forward pressure difference mode. Our results provide design and operation guidelines for pressure-driven electrokinetic energy conversion devices.

Simultaneous thermoosmotic and thermoelectric responses in nanoconfined electrolyte solutions: Effects of nanopore structures and membrane properties.

HYPOTHESIS: Nanofluidic systems provide an emerging and efficient platform for thermoelectric conversion and fluid pumping with low-grade heat energy. As a basis of their performance enhancement, the effects of the structures and properties of the nanofluidic systems on the thermoelectric response (TER) and the thermoosmotic response (TOR) are yet to be explored. METHODS: The simultaneous TER and TOR of electrolyte solutions in nanofluidic membrane pores on which an axial temperature gradient is exerted are investigated numerically and semi-analytically. A semi-analytical model is developed with the consideration of finite membrane thermal conductivity and the reservoir/entrance effect. FINDINGS: The increase in the access resistance due to the nanopore-reservoir interfaces accounts for the decrease of short circuit current at the low concentration regime. The decrease in the thermal conductivity ratio can enhance the TER and TOR. The maximum power density occurring at the nanopore radius twice the Debye length ranges from several to dozens of mW K-2 m-2 and is an order of magnitude higher than typical thermo-supercapacitors. The surface charge polarity can heavily affect the sign and magnitude of the short-circuit current, the Seebeck coefficient and the open-circuit thermoosmotic coefficient, but has less effect on the short-circuit thermoosmotic coefficient. Furthermore, the membrane thickness makes different impacts on TER and TOR for zero and finite membrane thermal conductivity.

Radial basis function interpolation supplemented lattice Boltzmann method for electroosmotic flows in microchannel.

Large gradients of physical variables near the channel walls are characteristic of EOF. The previous numerical simulations of EOFs with the lattice Boltzmann method (LBM) utilize uniform lattice and are not efficient, especially when the electric double layer (EDL) thickness is significantly smaller than the channel height. The efficient LBM simulation of EOF in microchannel calls for a nonuniform mesh which is dense in the EDL region and sparse in the bulk region. In this article, we formulate a radial basis function (RBF)-based interpolation supplemented LBM (ISLBM) to solve the governing equations of EOF, that is, the Poisson, Nernst-Planck, and Navier-Stokes equations, in a nonuniform mesh system. Unlike the conventional ISLBM, the RBF-ISLBM determines the prestreaming distribution functions by using the local RBF-based interpolation over circular supporting regions and is particularly suitable for nonuniform meshes. The RBF-ISLBM is validated by the EOFs in infinitely long and finitely long microchannels. The results show that the RBF-ISLBM possesses excellent robustness and accuracy. Finally, we use the RBF-ISLBM to simulate the EOFs with the hitherto highest electrokinetic parameter, κa, defined by the ratio of channel height a to EDL thickness κ-1 , in LBM simulations of EOF.

Numerical investigation of the solute dispersion in finite-length microchannels with the interphase transport.

This paper utilizes a combined approach of the convection-diffusion theory and the moment analysis to conduct a comprehensive investigation of the solute dispersion under the influence of the interphase transport in finitely long inner coated microchannels. The present work has threefold novel contributions: (1) The 2D solute concentration contours in the stationary phase are calculated for the first time to facilitate the understanding the role of the interphase transport in the solute dispersion in the mobile phase. (2) The skewness of the elution curves is investigated to guide the control of solute band shape at the channel outlet. (3) The 2D diffusion-convection theory and zero-dimensional (0D) moment analysis complement each other to present a characterization of the solute dispersion behaviors more comprehensive than that by either of the two methods alone. Parametric studies are performed to clarify the effects of four major parameters related to the interphase transport (i.e., stationary phase Péclet number, interphase transport rate, partition coefficient, and stationary phase thickness) on the solute dispersion characteristics. The results from this study provide a straightforward understanding of the effects of interphase transport on the solute dispersion in finitely long microchannels and are of potential relevance to the design and operation of the microfluidics-based analytical devices.

Confinement Effect of Graphene Interface on Phase Transition of n-Eicosane: Molecular Dynamics Simulations.

Phase change materials (PCMs) are widely used in thermal management and energy storage systems. Investigations on the thermophysical properties enhancement of organic PCMs by introducing carbon-based frameworks have received much attention in recent years. Studies of the phase transition in nanoconfinement are still in controversy with divergent opinions among researchers. In this article, the phase transition behavior of n-eicosane in slit-shaped pores between sheets of graphene is investigated by molecular dynamics simulation. It is found that the graphene interface makes the phase transition temperature of n-eicosane increase, under the initial slit widths of 1.5-5.3 nm. Impacted by interaction and size effects, the distribution and orientation of n-eicosane molecules are quite different from those of the bulk state. In the confinement of graphene, the molecules turn to a reversible layered distribution parallel to the graphene sheets after solidification. The contact layers are found in all the confined systems, which is harder to melt and easier to solidify compared with the main part of the systems. The melting points of different systems are obtained by analysis of the liquid ratio. Finally, the relationship between the dimensionless phase transition point and slit width is discussed.

Electrokinetic power generation in conical nanochannels: regulation effects due to conicity.

Electrokinetic power generation is a promising clean energy production technology, which utilizes the electric double layer in a nanochannel to convert the hydrodynamic energy to electrical power. Previous research largely focused on electrokinetic power generation in nanochannels with a uniform cross-section. In this work, we perform a systematic investigation of electrokinetic power generation in a conical nanochannel. For this purpose, a multiphysical model consisting of the Planck-Nernst-Poisson equations and the Navier-Stokes equation is formulated and solved numerically. In particular, we discover various regulation effects in electrokinetic power generation in conical nanochannels, which manifest as the difference in the power generation characteristics (streaming potential, streaming current and current-voltage relationship) between two opposite pressure differences of the same magnitude. These regulation effects are found to originate from the conicity of the nanochannel. Furthermore, the regulation parameters are defined to quantify the observed regulation effects. Various regulation parameters can be up to severals tens of percent under extreme conditions (e.g., large pressure difference, high surface charge density or large conicity), indicating the substantial significance of the regulation effects in electrokinetic power generation. The conclusions from this work can serve as an important reference for the design and operation of nanofluidic electrokinetic power generation devices.

Numerical investigation of mist/air impingement cooling on ribbed blade leading-edge surface.

The working gas turbine blades are exposed to the environment of high temperature, especially in the leading-edge region. The mist/air two-phase impingement cooling has been adopted to enhance the heat transfer on blade surfaces and investigate the leading-edge cooling effectiveness. An Euler-Lagrange particle tracking method is used to simulate the two-phase impingement cooling on the blade leading-edge. The mesh dependency test has been carried out and the numerical method is validated based on the available experimental data of mist/air cooling with jet impingement on a concave surface. The cooling effectiveness on three target surfaces is investigated, including the smooth and the ribbed surface with convex/concave columnar ribs. The results show that the cooling effectiveness of the mist/air two-phase flow is better than that of the single-phase flow. When the ribbed surfaces are used, the heat transfer enhancement is significant, the surface cooling effectiveness becomes higher and the convex ribbed surface presents a better performance. With the enhancement of the surface heat transfer, the pressure drop in the impingement zone increases, but the incremental factor of the flow friction is smaller than that of the heat transfer enhancement.

Predictive model of solute transport with reversible adsorption in spatially periodic hierarchical porous media.

Solute transport in hierarchical porous media with reversible adsorption is predicted by the volume averaging method. A transient macroscopic advection-diffusion equation is derived to describe the multiscale solute transport problem. The theoretical expression of the dispersion tensor is obtained which is the function of pore-scale velocity profile. Steady closure equations are derived to calculate the dispersion tensor. With the aid of pore-scale simulation in unit cells of the hierarchical porous media, the dispersion tensor can be calculated. The model is verified by comparing its predictions and obtaining favorable agreement with results of direct numerical simulations and with experimental data for columns comprised of ordered, porous pillars. It is straightforward to use the model to predict the solute transport behavior in fixed beds packed with particles.

Theoretical tools for predicting optimal cross-sectional shapes in micro-gas chromatography.

It is meaningful to explore the possibility of improving the micro-GC column performance by adjusting the column cross-sectional shape. The objective of this study was to seek the column cross-sectional shape that results in larger plate number per meter than other shapes with the same cross-sectional area and the same flow resistance coefficient. We applied a model based on the volume averaging method to derive the expression of plate height for columns with arbitrary cross-sectional shapes, and conducted the shape optimization by combining the model and an optimization tool. By varying flow resistance coefficient, we obtained a series of optimal shapes. It is found that, the optimal shape with larger flow resistance coefficient is shallower and the related plate number per meter is larger. We predicted and optimized the performance of a micro-GC column reported in literature. The prediction agrees reasonably with experimental data. More than twice the plate number per meter of the original column was predicted by using a hypothetical column with one optimal cross-sectional shape.

Numerical investigation into the effects of ordered particle packing and slip flow on the performance of chromatography.

The pressure drop and the plate height of chromatography columns packed with particles in the face-centered cubic, the body-centered cubic and the simple cubic configurations are calculated by a volume averaging method model. It is found that the Kozeny-Carman equation provides a reasonable prediction of the pressure drop when particles are in the face-centered cubic configuration, but overestimates the pressure drop when particles are in the body-centered cubic and the simple cubic configurations. The face-centered cubic configuration has the advantage to provide a smaller longitudinal dispersion coefficient than the body-centered cubic, the simple cubic, and the random configurations. The pressure drop and the plate height for slip flow through particles in the face-centered cubic configuration are lower than that for no-slip flow. The values of the smallest reduced plate height of columns packed with particles in the face-centered cubic configuration for no-slip flow and slip flow are about 0.084 and 0.059, respectively. The plate height of the ordered particle packing structures is smaller and the effect of slip flow on the plate height is less remarkable than results reported in literature.

Dispersion in retentive pillar array columns.

The method of volume averaging is applied to estimate the Taylor-Aris dispersion tensor of solute advected in columns consisting of ordered pillar arrays with wall retention of the type used in chromatographic separation. The appropriate closure equations are derived and solved in a unit cell with periodic boundary conditions to obtain the dispersion tensor (or the reduced plate height) as a function of the Peclet number (reduced velocity); pillar pattern, shape and size; partition coefficient; and resistance to mass transfer. The contributions of the velocity profile, the wall adsorption, and the mass transfer resistance to the dispersion tensor are identified and delineated. The model is verified by comparing its predictions and obtaining favorable agreement with results of direct numerical simulations and with experimental data for columns containing ordered pillars. The model is then used to study the effect of pillars' shape and pattern on the longitudinal dispersion coefficient (plate height).