Nanoscale Transport Phenomena Research Articles

Chemotactic particles as strong electrolytes: Debye-Hückel approximation and effective mobility law.

We consider a binary mixture of chemically active particles that produce or consume solute molecules and that interact with each other through the long-range concentration fields they generate. We analytically calculate the effective phoretic mobility of these particles when the mixture is submitted to a constant, external concentration gradient, at leading order in the overall concentration. Relying on an analogy with the modeling of strong electrolytes, we show that the effective phoretic mobility decays with the square root of the concentration: our result is, therefore, a nonequilibrium counterpart to the celebrated Kohlrausch and Debye-Hückel-Onsager conductivity laws for electrolytes, which are extended here to particles with long-range nonreciprocal interactions. The effective mobility law we derive reveals the existence of a regime of maximal mobility and could find applications in the description of nanoscale transport phenomena in living cells.

The physics of complex systems: from traffic jams to nanoscale transport phenomena

The physics of complex systems: from traffic jams to nanoscale transport phenomena

Electroneutrality Breakdown and Perm-Selective End Groups in Carbon Nanotube Porins

Sub-nanometer carbon nanotube porins (n-CNTP) have been shown to achieve high water permeation with significant permselectivity precluding anion transport. Therefore, n-CNTPs are useful model systems to probe and optimize the nanoscale transport phenomena relevant for selective ionic separations. Surprisingly, these n-CNTPs demonstrate a unique conductance scaling behavior with ionic concentration, which has led to various continuum models being constructed in pursuit of determining the underpinning physics governing nanoscale ionic transport. Here, we formulate a continuum model to capture the conductance and permselectivity of n-CTNPs considering the breakdown of electroneutrality within the pore. Electroneutrality breakdown occurs when the ionic charge within the pore is not balanced by the charge on the pore walls. While electroneutrality is commonly assumed in the analysis of nanopore transport, we predict a departure from electroneutrality due to the extreme one-dimensional confinement in n-CNTPs, strongly influencing the conductance and selectivity of the pore. Furthermore, our analysis demonstrates the importance of the entrance groups in modulating ion transport through n-CNTPs. Lastly, we provide a framework for future refinements to incorporate additional microscopic physics into the phenomenological model.

Optimized Phonon Band Discretization Scheme for Efficiently Solving the Nongray Boltzmann Transport Equation

Abstract The phonon Boltzmann transport equation (BTE) is an important tool for studying the nanoscale thermal transport. Because phonons have a large spread in their properties, the nongray (i.e., considering different phonon bands) phonon BTE is needed to accurately capture the nanoscale transport phenomena. However, BTE solvers generally require large computational cost. Nongray modeling imposes significant additional complexity on the numerical simulations, which hinders the large-scale modeling of real nanoscale systems. In this work, we address this issue by a systematic investigation on the phonon band discretization scheme using real material properties of four representative materials, including silicon, gallium arsenide, diamond, and lead telluride. We find that the schemes used in previous studies require at least a few tens of bands to ensure the accuracy, which requires large computational costs. We then propose an improved band discretization scheme, in which we divide the mean free path domain into two subdomains, one on either side of the inflection point of the mean free path accumulated thermal conductivity, and adopt the Gauss–Legendre quadrature for each subdomain. With this scheme, the solution of the phonon BTE converges (error < 1%) with less than ten phonon bands for all these materials. The proposed scheme allows significantly reducing the time and memory consumption of the numerical BTE solver, which is an important step toward large-scale phonon BTE simulations for real materials.

Development of direct analysis method of conductivity by local electric field measurement

Recently, micro- and nanofluidics devices such as MEMS/NEMS and μ-TAS have attracted much attention. However, several unknowns have been remained in micro- and nanoscale transport phenomena unlike microscopic scale. In order to deepen understanding of such micro- and nanoscale phenomena, it is essential to develop a novel technique to measuer the small spaces. In this study, a localized and direct measurement method of electric fields is established by using a test section with a charcteristic length of 1 mm and a glass microelectrode. Using a KCl standard solution ranging from 0.56 to 100 mmol/L as a sample, the conductivity is successfully evaluated with a measurement error of less than 15%.

A large deviation theory perspective on nanoscale transport phenomena

Understanding transport processes in complex nanoscale systems, like ionic conductivities in nanofluidic devices or heat conduction in low dimensional solids, poses the problem of examining fluctuations of currents within nonequilibrium steady states and relating those fluctuations to nonlinear or anomalous responses. We have developed a systematic framework for computing distributions of time integrated currents in molecular models and relating cumulants of those distributions to nonlinear transport coefficients. The approach elaborated upon in this perspective follows from the theory of dynamical large deviations, benefits from substantial previous formal development, and has been illustrated in several applications. The framework provides a microscopic basis for going beyond traditional hydrodynamics in instances where local equilibrium assumptions break down, which are ubiquitous at the nanoscale.

Non-monotonic boundary resistivity for electron transport in metal nanowires

Boundary scattering is the most widely encountered size effect in nanoscale transport phenomena, and the scattering rate is usually regarded as a constant that is proportional to the ratio of carrier velocity to the characteristic size. Here, through combined experimental measurements and numerical modeling, we show non-monotonic variations of the boundary scattering rate for free electrons in metal nanowires as temperature escalates. This observation is attributed to the change in the electron-phonon (e-ph) scattering angle as temperature reduces, which alters the surface scattering rate. In particular, at low temperatures, electrons traveling along the wire axis have to be first relaxed by e-ph scattering before they collide with the nanowire surface. Theoretical analysis indicates a transition temperature of 0.29 times Debye temperature. A theoretical model considering the effects of the scattering angle is proposed that can fit the measured experimental data for both copper and silver nanowires over a wide temperature range.

Effects of solid/liquid interface on size-dependent specific heat capacity of nanoscale water films: Insight from molecular simulations

The thermophysical properties of an evaporative thin water film that is near the triple line may change because of the effects of size, geometry, interface, temperature, etc. However, the factor-dependence of the properties was seldom considered for the corresponding nanoscale transport phenomena or processes. In this work, the effects of the interface and temperature on the thickness-dependent specific heat capacities of nanoscale water films are investigated by comparing the values of the freestanding film between vacuum, the sessile film on a copper plate, and the confined film between two copper plates. It is found that the specific heat capacities of the thin water films decrease exponentially with reducing thickness, with the confined film having the highest value, followed by the sessile film and the freestanding film, at the same film thickness. The solid/liquid interface presents a substantial impact on the increase in the specific heat capacity, demonstrated by the analysis of density profile, vibration density of state of hydrogen atoms of water molecules, radial distribution function, and interface energy, compared between the films of different types. The analysis of the temperature effect by comparing the vibration densities of state in different regions of the films at different temperatures shows that the temperature has a negligible influence as compared to the size and interface effects. Therefore, the form of a thin water film will greatly affect its thermophysical properties, including specific heat capacity, which should be considered in applications such as evaporation of thin water film, nanoconfinement transfer in membrane, and interface separation of porous media.

(Invited) Porous Materials for PEM Fuel Cells and Electrolyzers: Heterogeneous and Interfacial Considerations

While in practical application, polymer electrolyte membrane (PEM) fuel cells and electrolyzers exhibit macroscale length scales spanning centimeters to meters, the inner workings of these energy conversion devices fundamentally rely on microscale and nanoscale transport phenomena and porous materials to facilitate their passive operation. In order to maximize the power density and minimize the cost of the PEM fuel cell or electrolyzer, we must optimize our surface area to volume ratios at the reaction sites; thus, state-of-the-art catalysts exhibit diameters on the order of nanometers. At the same time, we must consider the effective delivery of reactants to these small- catalyst sites, and porous materials provide a means to impart homogeneous distributions of reactants, in theory. In addition to laterally distributing reactants across the planar catalyst layer, porous materials also facilitate multiple pathways for the competing reactant and product pathways to co-exist. The PEM fuel cell and electrolyzer are composed of multiple porous materials, including the catalyst layer, microporous layer, and substrate. Commercial materials, whether by design or not, typically exhibit highly heterogeneous material and chemical properties. For example, in our past work, we extensively examined the heterogeneous microstructure and hydrophobicity of the gas diffusion layer (GDL). In order to reach cost targets for widespread commercial adoption, we must realize materials that enable more effective multiphase flow phenomena than what currently exists. Mass transport losses in PEM fuel cells and electrolyzers are both prohibitively significant. However, designing these materials requires the a priori knowledge of how the heterogeneous properties of the porous materials and their interfacial contacts influence electrochemical performance. These are factors that are currently not fully understood in the literature. In this work, I will discuss these critical design factors (heterogeneous porous materials and nature of interfacial contacts) and how they influence the flow and mass transport behaviour in PEM fuel cells and electrolyzers. I will also discuss the new materials we have designed and fabricated, informed by in-house numerical modelling and tested through a combination of in operando and ex situ X-ray and neutron beam characterization approaches.

Anomalous Trends in Nucleic Acid-Based Electrochemical Biosensors with Nanoporous Gold Electrodes.

Molecular diagnostics have significantly advanced the early detection of diseases, where electrochemical sensing of biomarkers has shown considerable promise. For a nucleic acid-based electrochemical sensor with signal-off behavior, the performance is evaluated by percent signal suppression (% ss), which indicates the change in current after hybridization. The % ss is generally due to more redox molecules (e.g., methylene blue) associating with the probe DNA bases in the single-strand form than the double-strand form upon hybridization with the target nucleic acid. Nanostructured electrodes generally enhance electrochemical sensor performance via several mechanisms, including increased number of capture probes per electrode volume and unique nanoscale transport phenomena. Here, we employ nanoporous gold (np-Au) as a model electrode material to study the influence of probe immobilization solution concentration on sensor performance and the underlying mechanisms. Unlike planar gold (pl-Au) electrodes, where % ss reaches a steady state with increasing concentration of the grafting solution, the % ss displays peak performance at certain grafting solution concentrations followed by rapid deterioration and reversal of the % ss polarity, suggesting an unexpected case of increased charge transfer upon hybridization. Fluorometric assessments of electrochemically desorbed nucleic acids for different electrode morphologies reveal that a significant amount of DNA molecules (unhybridized and hybridized) remain within the nanopores posthybridization. Analysis of electrochemical signals (e.g., square wave voltammogram shape) suggests that the large unbound nucleic acid concentration may be altering the modes of methylene blue interaction with the nucleic acids and charge transfer to the electrode surfaces.

Meeting U.S. Solid Oxide Fuel Cell Targets

Michael M. Whiston is a Postdoctoral Research Associate in the Department of Engineering and Public Policy at Carnegie Mellon University. As a postdoctoral researcher, Michael has elicited the assessments of over 100 fuel cell experts and presented his findings on Capitol Hill and at international conferences. Michael has published research in the Proceedings of the National Academy of Sciences, Journal of Power Sources, and Journal of Fuel Cell Science and Technology. Michael received his Ph.D. from the University of Pittsburgh in Mechanical Engineering. During his graduate work, Michael performed cost analyses of SOFC systems and developed a dynamic SOFC model. Professor Ines M.L. Azevedo is an Associate Professor of Energy Resources Engineering at Stanford University. Her work focuses on the transitions to sustainable and low-carbon energy systems. She has published 70+ journal publications and graduated 26 Ph.D. students. Since 2010, she has been serving as PI/co-PI of the NSF sponsored Climate and Energy Decision Making Center (CEDM). Professor Azevedo participated as a committee member and co-author on several reports from the U.S. National Academy of Sciences. She was awarded the “40 Scientists under 40” award by the World Economic Forum (WEF) (2014) and the C3E Research Award for Women in Energy (2017). Shawn Litster is a Professor in the Department of Mechanical Engineering at Carnegie Mellon University in Pittsburgh, PA. He received his Ph.D. in Mechanical Engineering from Stanford University and his bachelor’s and master’s degrees from the University of Victoria in Canada. His current research focus is micro- and nano-scale transport phenomena in energy conversion technologies where electrochemistry and electrokinetics play a dominant role, including fuel cells, batteries, and ultra-capacitors. He is the author of over 60 journal publications on fuel cells and batteries. Constantine Samaras is an Associate Professor of Civil and Environmental Engineering and a Fellow in the Wilton E. Scott Institute for Energy Innovation at Carnegie Mellon University. His research spans energy, automation, and climate change. He has contributed to U.S. National Academies of Sciences reports, the Fourth National Climate Assessment, and the Global Energy Assessment. He received a Ph.D. in Civil and Environmental Engineering and Engineering and Public Policy from Carnegie Mellon University and a M.P.A. in Public Policy from New York University. He was named 2018 Professor of the Year by the American Society of Civil Engineers Pittsburgh Section. Kate S. Whitefoot is an Assistant Professor of Engineering and Public Policy and of Mechanical Engineering at Carnegie Mellon University. Professor Whitefoot's research bridges engineering design theory and analysis with that of economics to understand how product and process design affects enterprise and social-welfare objectives, such as expected profits, customer adoption, productivity, energy efficiency, emissions, and consumer welfare. Her work is featured in the Wall Street Journal, Washington Post, Popular Mechanics, and Bloomberg Business and referenced in the 2017–2025 Corporate Average Fuel Economy rulemaking. Professor Jay F. Whitacre started his career at the California Institute of Technology/Jet Propulsion Laboratory. Since joining Carnegie Mellon University in 2007, Dr. Whitacre has focused on the synergistic fields of energy storage and energy system techno-economic assessment. He has developed a novel battery chemistry/design that is manufactured and sold by Aquion Energy, a company he founded in 2008. He is the recipient of multiple awards, including the 2017 Leigh Ann Conn Prize for Renewable Energy, the 2015 Lemelson-MIT Prize, and the 2014 Caltech Resnick Institute Resonate Award, and has been named a Fellow of the National Academy of Inventors.

(Invited) Thermal and Thermoelectric Transport in Semiconductor Nanowires

During the last two decades we have witnessed tremendous advancement in the characterization and understanding of thermal transport in nanostructures, particularly in the ‘phonon boundary scattering’ regime where the characteristic size of nanostructures ranges from tens to hundreds nanometers and is comparable to phonon mean free path. This understanding is one of the key underlying reasons for improved performance observed in nanostructured thermoelectric materials. However, the boundary scattering mechanism may reach its limit and new mechanisms for improving thermoelectric performance need to be explored. One of the opportunities may arise from the study of thermal transport beyond the boundary scattering regime, where the characteristic size of nanostructures is below 10-20 nanometers and is comparable to phonon wavelength. However, there is little experimental work on thermal transport in nanostructures in this regime. In this talk, we will discuss our recent work on using rationally designed nanostructures to study nanoscale thermal and thermoelectric transport phenomena in this regime. We will describe the instrumentation we developed that enables the characterization of thermal and thermoelectric properties of these nanostructures and discuss and interpret the measurement results.

Fast nanofluidics by travelling surface waves

In this paper, we investigate the fast flow in nanochannels, which is induced by the travelling surface waves. The nanoscale fluid mechanism in nanochannels has been influenced by both amplitude and frequency of travelling surface waves, and the hydrodynamic characteristics have been obtained by molecular dynamics simulations. It has been found that the flow rate is an increasing function of the amplitude of travelling surface waves and can be up to a sevenfold increase. However, the flow rate is only enhanced in the limited range of frequency of travelling surface waves such as low frequencies, and a maximum fivefold increase in flow rate is pronounced. In addition, the fluid–wall interaction (surface wettability) plays an important role in the nanoscale transport phenomena, and the flow rate is significantly increased under a strong fluid–wall interaction (hydrophilicity) in the presence of travelling surface waves. Moreover, the friction coefficient on the wall of nanochannels is decreased obviously due to the large slip length, and the shear viscosity of fluid on the hydrophobic surface is increased by travelling surface waves. It can be concluded that the travelling surface wave has a potential function to facilitate the flow in nanochannels with respect to the decrease in surface friction on the walls. Our results allow to define better strategies for the fast nanofluidics by travelling surface waves.

Quantifying Liquid Transport and Patterning Using Atomic Force Microscopy.

Atomic force microscopy (AFM) provides unique insight into the nanoscale properties of materials. It has been challenging, however, to use AFM to study soft materials such as liquids or gels because of their tendency to flow in response to stress. We propose an AFM-based technique for quantitatively analyzing the transport of soft materials from an AFM probe to a surface. Specifically, we present a method for loading an AFM probe with a single 0.3 to 30 pL droplet of liquid and subsequently measuring the mass of this liquid by observing the change in the vibrational resonance frequency of the cantilever. Using this approach, the mass of this liquid was detected with picogram-scale precision by a commercial AFM system. Additionally, sub-femtoliter droplets of liquid were transferred from the probe to a surface with agreement found between the real-time change in mass of the liquid-loaded probe and the volume of the feature written on the surface. To demonstrate the utility of this approach in studying nanoscale capillary and transport phenomena, we experimentally determine that the quantity of liquid transported from the tip to a surface in a given patterning operation scales as the mass of liquid on the probe to the 1.35 power. In addition to providing new avenues for studying the dynamics of soft materials on the nanoscale, this method can improve nanopatterning of soft materials by providing in situ feedback.

Transport Phenomena of Water in Molecular Fluidic Channels.

In molecular-level fluidic transport, where the discrete characteristics of a molecular system are not negligible (in contrast to a continuum description), the response of the molecular water system might still be similar to the continuum description if the time and ensemble averages satisfy the ergodic hypothesis and the scale of the average is enough to recover the classical thermodynamic properties. However, even in such cases, the continuum description breaks down on the material interfaces. In short, molecular-level liquid flows exhibit substantially different physics from classical fluid transport theories because of (i) the interface/surface force field, (ii) thermal/velocity slip, (iii) the discreteness of fluid molecules at the interface and (iv) local viscosity. Therefore, in this study, we present the result of our investigations using molecular dynamics (MD) simulations with continuum-based energy equations and check the validity and limitations of the continuum hypothesis. Our study shows that when the continuum description is subjected to the proper treatment of the interface effects via modified boundary conditions, the so-called continuum-based modified-analytical solutions, they can adequately predict nanoscale fluid transport phenomena. The findings in this work have broad effects in overcoming current limitations in modeling/predicting the fluid behaviors of molecular fluidic devices.

Thermal and Thermoelectric Transport in Semiconductor Nanostructures

During the last decade or so we have witnessed tremendous advancement in characterization and understanding of thermal transport in nanostructures, particularly in the ‘phonon boundary scattering’ regime where the characteristic size of nanostructures ranges from tens to hundreds nanometers and is comparable to phonon mean free path. This understanding is the key underlying reason for improved performance observed in nanostructured thermoelectric materials. However, it is increasingly clear that the boundary scattering mechanism may soon reach its limit and new mechanisms for improving thermoelectric performance need to be explored. One of the opportunities may arise from the study of thermal transport in the so-called ‘confinement’ regime, where the characteristic size of nanostructures is below 10-20 nanometers and is comparable to phonon wavelength. In this work, we will discuss our recent work on using rationally designed nanostructures to study nanoscale thermal and thermoelectric transport phenomena in this regime. We will describe the instrumentation we have been developing that enables the characterization of thermal transport and related properties of these nanostructures and discuss and interpret the measurement results.

Combined Laser-Based Measurements for Micro- and Nanoscale Transport Phenomena

The present paper summarizes our recent research in combined laser-based measurement techniques for investigating micro- and nanoscale transport phenomena. Micrometer-resolution particle image velocimetry has been combined with the laser-induced fluorescence (LIF) technique in order to simultaneously analyze velocity and scalar fields. The measurement system is based on confocal microscopy to realize a depth resolution of approximately 2 m, and we have applied this technique to liquid–liquid mixing flows, gas–liquid two-phase flows, gas permeation phenomena through membranes, and surface-modified microchannel flow. Furthermore, in order to evaluate the electrostatic potential at a solid–liquid interface (i.e., zeta potential), the LIF technique was extended by evanescent wave illumination, and only the fluorescent dye within approximately 100 nm of the microchannel wall was irradiated. The extended LIF technique was applied to microdevices with a surface modification pattern, and the zeta-potential distribution was successfully visualized. The proposed techniques will contribute to novel applications related to microscale multiphase flows or electrokinetics.

Nanoscale Transport Phenomena in PEM of PEFC by Large Scale Molecular Dynamics Simulations

In this paper, we analyzed the nanoscale transport phenomena in membrane electrode assembly (MEA) of polymer electrolyte fuel cell (PEFC) by large scale molecular dynamics (MD) simulations. Especially, (1) transport phenomena of proton and water in polymer electrolyte membrane (PEM), (2) oxygen permeability of ionomer in catalyst layer (CL), and (3) transport phenomena of water droplet in a nano pore were simulated, and the nanoscale transport characteristics were analyzed in detail. In the analysis of proton transfer in PEM, the simulation results were compared with some experimental results to evaluate the validity of our simulation. With regards to the oxygen permeability of ionomer, the dependence of water content on the permeability was estimated and the difference of characteristics between ionomer and bulk membrane was discussed. In the analysis of transport phenomena of a water droplet in a nano pore, we compared the results of our simulation with macroscopic governing equation.

Nanotherapeutics and Nano-Imaging in Biotransport

Nanotherapeutics and Nano-Imaging in Biotransport

Report on the Seventh U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena—Science and Engineering

The seventh U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena was held in Shima, Japan, from December 11 to 14, 2011. The goals of this joint seminar were to provide a critical assessment of the state of the art and future directions in the field of nanoscale transport phenomena and energy conversion processes, to foster U.S.–Japan collaborations, and to provide international exposure to a new generation of scientists in this field. Issues discussed in the joint seminar were organized in 10 topical sessions, including (1) nanoscale thermophysical measurements; (2) optical characterization; (3) thermal and molecular transport; (4) phonon transport modeling; (5) energy storage and conversion; (6) nanoscale fluidics and phase change phenomena; (7) biological and organic systems; (8) interfacial thermal transport; (9) novel thermoelectric and thermal management materials; and (10) nanocarbon materials and devices. In addition to these topical sessions, the joint seminar featured an opening plenary session and a closing plenary session as well as an expert panel, where leading experts provided critical assessment of the past progress and addressed future directions in the field. In addition, an evening poster session provided opportunities for graduate and postdoc students to present their latest research results. About 35 researchers from Japan and 31 researchers from the United States participated in the meeting. The meeting was organized by S. Maruyama, K. Fushinobu, L. Shi, and J. Lukes together with about 20 other participants who served as session chairs. Summaries of different sessions of the seminar were prepared by the session and conference chairs and are collected into this report.