Thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries

A systematic theoretical study of thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries is presented. The study is based on a semianalytical model developed by simultaneously solving the non-isothermal Poisson-Nernst-Planck-Navier-Stokes equations with the lubrication theory. Particularly, this paper clarifies the interplay and relative importance of the thermoelectric mechanisms due to (a) the convective transport of ions caused by the fluid flow, (b) the dependence of ion electrophoretic mobility on temperature, (c) the difference in the intrinsic Soret coefficients of cation and anion. Additionally, synergy conditions for the three thermoelectric mechanisms to fully cooperate are proposed for thermo-phobic/philic electrolytes. The temperature-gradient-driven electrokinetic flow is shown to be a nearly unidirectional flow whose axial velocity profiles vary with the axial location. Also, the flow can be regarded as a consequence of the counteraction or cooperation between a thermoelectric-field-driven electroosmotic flow and a thermo-osmotic flow driven by the osmotic pressure gradient and dielectric body force. Moreover, the Seebeck coefficient and the fluid average velocity are demonstrated to be affected by electrolyte-related parameters. The results are beneficial for understanding the temperature-gradient-driven electrokinetic transport in nanocapillaries and also serve as theoretical foundation for the design of low-grade waste heat recovery devices and thermoosmotic pumps.Comment: 17 pages, 11 figures, with some correction

Ion steric effect induces giant enhancement of thermoelectric conversion in electrolyte-filled nanochannels

Ionic Seebeck effect has received increasing attention because of its advantages such as high Seebeck coefficient and low cost. However, theoretical study on the ionic Seebeck coefficient is still in its infancy and mainly focuses on diluted simple electrolytes excluding the contributions of ion steric effects and short-range electrostatic correlation. Here, we show that the coupling of the steric effects due to finite ion sizes and ion thermodiffusion in electric double layers can significantly enhance the thermoelectric response in confined electrolytes via both theory and simulation. The Seebeck coefficient can reach 100% or even one order of magnitude enhancement as compared to previous theoretical models depending on the degree of the ion steric effects and the sum of ion Soret coefficients. In addition, we demonstrate that the short-range electrostatic correlation is beneficial to achieving the maximum Seebeck coefficient at weaker confinement or more concentrated electrolytes. These findings can provide a strategy for achieving high Seebeck coefficient and high electric conductivity simultaneously to improve the efficiency of the ionic thermoelectric conversion.Comment: 12 pages, 6 figure

Extracting local surface charges and charge regulation behavior from atomic force microscopy measurements at heterogeneous solid-electrolyte interfaces

We present a method to determine the local surface charge of solid–liquid interfaces from Atomic Force Microscopy (AFM) measurements that takes into account shifts of the adsorption/desorption equilibria of protons and ions as the cantilever tip approaches the sample. We recorded AFM force distance curves in dynamic mode with sharp tips on heterogeneous silica surfaces partially covered by gibbsite nano-particles immersed in an aqueous electrolyte with variable concentrations of dissolved NaCl and KCl at pH 5.8. Forces are analyzed in the framework of Derjaguin–Landau–Verwey–Overbeek (DLVO) theory in combination with a charge regulation boundary that describes adsorption and desorption reactions of protons and ions. A systematic method to extract the equilibrium constants of these reactions by simultaneous least-squared fitting to experimental data for various salt concentrations is developed and is shown to yield highly consistent results for silica-electrolyte interfaces. For gibbsite-electrolyte interfaces, the surface charge can be determined, yet, an unambiguous identification of the relevant surface speciation reactions is not possible, presumably due to a combination of intrinsic chemical complexity and heterogeneity of the nano-particle surfaces

AC electrokinetic phenomena over semiconductive surfaces: effective electric boundary conditions and their applications

Electrokinetic boundary conditions are derived for AC electrokinetic (ACEK) phenomena over leaky dielectric (i.e., semiconducting) surfaces. Such boundary conditions correlate the electric potentials across the semiconductor-electrolyte interface (consisting of the electric double layer (EDL) inside the electrolyte solutions and the space charge layer (SCL) inside the semiconductors) under AC electric fields with arbitrary wave forms. The present electrokinetic boundary conditions allow for evaluation of induced zeta potential contributed by both bond charges (due to electric polarization) and free charges (due to electric conduction) from the leaky dielectric materials. Subsequently, we demonstrate the applications of these boundary conditions in analyzing the ACEK phenomena around a semiconducting cylinder. It is concluded that the flow circulations exist around the semiconducting cylinder and are shown to be stronger under an AC field with lower frequency and around a cylinder with higher conductivity.Comment: 29 pages, 4 figure

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.Comment: 38 pages, 10 figure

Induced charge effects on electrokinetic entry flow

Electrokinetic flow, due to a nearly plug-like velocity profile, is the preferred mode for transport of fluids (by electroosmosis) and species (by electrophoresis if charged) in microfluidic devices. Thus far there have been numerous studies on electrokinetic flow within a variety of microchannel structures. However, the fluid and species behaviors at the interface of the inlet reservoir (i.e., the well that supplies the fluid and species) and microchannel are still largely unexplored. This work presents a fundamental investigation of the induced charge effects on electrokinetic entry flow due to the polarization of dielectric corners at the inlet reservoir-microchannel junction. We use small tracing particles suspended in a low ionic concentration fluid to visualize the electrokinetic flow pattern in the absence of Joule heating effects. Particles are found to get trapped and concentrated inside a pair of counter-rotating fluid circulations near the corners of the channel entrance. We also develop a depth-averaged numerical model to understand the induced charge on the corner surfaces and simulate the resultant induced charge electroosmosis (ICEO) in the horizontal plane of the microchannel. The particle streaklines predicted from this model are compared with the experimental images of tracing particles, which shows a significantly better agreement than those from a regular two-dimensional model. This study indicates the strong influences of the top/bottom walls on ICEO in shallow microchannels, which have been neglected in previous two-dimensional models. Published by AIP Publishing

Microfluidic Techniques for Analytes Concentration

Microfluidics has been undergoing fast development in the past two decades due to its promising applications in biotechnology, medicine, and chemistry. Towards these applications, enhancing concentration sensitivity and detection resolution are indispensable to meet the detection limits because of the dilute sample concentrations, ultra-small sample volumes and short detection lengths in microfluidic devices. A variety of microfluidic techniques for concentrating analytes have been developed. This article presents an overview of analyte concentration techniques in microfluidics. We focus on discussing the physical mechanism of each concentration technique with its representative advancements and applications. Finally, the article is concluded by highlighting and discussing advantages and disadvantages of the reviewed techniques

Dynamic Electroosmotic Flows of Power-Law Fluids in Rectangular Microchannels

Dynamic characteristics of electroosmosis of a typical non-Newtonian liquid in a rectangular microchannel are investigated by using numerical simulations. The non-Newtonian behavior of liquids is assumed to obey the famous power-law model and then the mathematical model is solved numerically by using the finite element method. The results indicate that the non-Newtonian effect produces some noticeable dynamic responses in electroosmotic flow. Under a direct current (DC) driving electric field, it is found that the fluid responds more inertly to an external electric field and the steady-state velocity profile becomes more plug-like as the flow behavior index decreases. Under an alternating current (AC) driving electric field, the fluid is observed to experience more significant acceleration and the amplitude of oscillating velocity becomes larger as the fluid behavior index decreases. Furthermore, our investigation also shows that electroosmotic flow of power-law fluids under an AC/DC combined driving field is enhanced as compared with that under a pure DC electric field. These dynamic predictions are of practical use for the design of electroosmotically-driven microfluidic devices that analyze and process non-Newtonian fluids such as biofluids and polymeric solutions

Induced-charge nonlinear electrokinetic phenomena and applications in micro/nano fluidics

Induced-charge nonlinear electrokinetic phenomena have drawn increasing attention not only due to their fundamental importance but also due to their potential applications for manipulating fluid flows and particles in microfluidics. Such type of nonlinear electrokinetic phenomena is jointly driven by the external electric field and the surface charge induced by the same field on polarizable or conducting surfaces, and is also frequently referred to as induced-charge electrokinetic phenomena. The fluid or particle velocity generated by induced-charge electrokinetics is proportional to the square of the external electric field strength. This is strikingly different from the conventional linear electrokinetics for which the fluid or particle velocity is linearly proportional to the external electric field strength. As a result, the induced-charge electrokinetics can generate larger flow rates and even allows for net flows under AC driving electric fields. Based on the basic theories of electrokinetics and electrostatics, effective electric boundary conditions between liquid-solid interfaces are derived for induced-charge electrokinetics under two situations. These boundary conditions are capable of predicting the induced zeta potentials over surfaces of solids with finite electric properties which are crucial for theoretical characterization of induced-charge electrokinetics. The applications of these two types of boundary conditions are demonstrated by analyzing the DC field driven induced-charge electroosmosis in a slit microchannel embedded with a pair of dielectric blocks and the AC field driven induced-charge electroosmosis around a leaky-dielectric cylinder, respectively. The calculations show that the basic flow patterns for induced-charge electroosmosis are the flow vortices which get stronger as the polarizability and /or the conductivity of solids increase. A complete numerical model is then developed to describe dynamic characteristics of the charging of electric double layer and the associated flows around polarizable dielectrics. The presented model does not invoke various assumptions that can be easily violated in practical applications but usually are made in existing analyses. The comparison with a benchmark solution ensures the validity of the complete model. It is shown that the complete model corroborates the two time scales during the EDL charging revealed in former asymptotic analyses. More importantly, the detailed information inside the EDL during the transient charging is resolved for the first time, which provides insight into the induced-charge electrokinetic phenomena with finite thickness of EDLs. Furthermore,  the  concept  of  induced‐charge  electrokinetics  is  extended  to  nanofluidics. Two nanofluidic systems, i.e., a straight nanochannel and a tapered nanochannel, are proposed  for  flexible modulations of both ionic  transport and fluid flow. For the straight channel, the modulations are achieved by th control  of gate voltage (i.e., the voltage applied on the conducting walls of nanochannel).  For  the  tapered channel,  the modulations are achieved by varying  the direction  and magnitude of external electric field and the taper angel of the channel walls.  Both  systems  are  advantageous  over  other  nanofluidic  systems  driven  by  the  conventional  linear  electrokinetics  which  usually  exhibit  poor  control  of  both  ionic transport and fluid flow. Finally,  a  novel  method  relying  on  induced‐charge  electrokinetics  is  developed for particle trapping. The proposed technique has been demonstrated experimentally  for  high‐throughput  trapping  and  concentration  of  particles  ranging  from  submicron  to  several microns.  In  addition,  a  theoretical model is  formulated  to  explain  the  experimental  observations  and  the  trapping mechanisms. DOCTOR OF PHILOSOPHY (MAE