upercapacitors store electrical energy physically in electrical double layers (EDLs) at the electrode/electrolyte interface.1 Despite their high power density and extra-ordinary cyclability, the widespread deploy-ment of supercapacitors... more
upercapacitors store electrical energy physically in electrical double layers (EDLs) at the electrode/electrolyte interface.1 Despite their high power density and extra-ordinary cyclability, the widespread deploy-ment of supercapacitors is limited by their moderate energy density. The current surge in interest in supercapacitors is driven by recent breakthroughs in developing novel electrode materials and electrolytes that promise to significantly improve their en-ergy density.26 In particular, a group of experiments showed that the area-normal-ized capacitance (or specific capacitance, hereafter referred to as capacitance) of sub-
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The invasion of gas into liquid-filled nanopores is encountered in many engineering problems but is not yet well understood. We report molecular dynamics simulations of the invasion of methane gas into water-filled mica pores with widths... more
The invasion of gas into liquid-filled nanopores is encountered in many engineering problems but is not yet well understood. We report molecular dynamics simulations of the invasion of methane gas into water-filled mica pores with widths of 2-6 nm. Gas invades into a pore only when the pressure exceeds a breakthrough pressure and a thin residual water film is left on the mica wall as the gas phase moves deeper into the pore. The gas breakthrough pressure of pores as narrow as 2 nm can be modeled reasonably well by the capillary pressure if the finite thickness of residual liquid water film and the liquid-gas interface are taken into account. The movement of the front of the liquid meniscus during gas invasion can be quantitatively described using the classical hydrodynamics when the negative slip length on the strongly hydrophilic mica walls is taken into account. Understanding the molecular mechanisms underlying the gas invasion in the system studied here will form the foundation f...
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Near-wall velocity of oscillating electroosmotic flow (OEOF) driven by AC electric field has been investigated by Laser-induced Fluorescence Photobleaching Anemometer (LIFPA). For the first time, an up to 3 kHz velocity response of OEOF... more
Near-wall velocity of oscillating electroosmotic flow (OEOF) driven by AC electric field has been investigated by Laser-induced Fluorescence Photobleaching Anemometer (LIFPA). For the first time, an up to 3 kHz velocity response of OEOF has been successfully measured experimentally, even though the oscillating velocity is as low as 600 nm/s. It is found that the oscillating velocity decays with forcing frequency f_f, as f_f^(-0.66). In the investigated range of electric field intensity (EA), below 1 kHz, the linear relation between oscillating velocity and EA is also observed. Since the oscillating velocity at high frequency is very small, the contribution of noise to velocity measurement is significant and discussed in this manuscript. The investigation reveals the instantaneous response of OEOF to the temporal change of electric fields, which exists in almost all AC electrokinetic flows. Furthermore, the experimental observations are important for designing OEOF-based micro/nanofl...
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Surface hydration-driven imbibition of water into strongly hydrophilic pores follows a diffusive scaling law and exhibits effective diffusion coefficients much higher than water molecules.
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CO2 adsorption regulates decane transport in inorganic nanopores by modulating interlayer mixing of interfacial fluids.
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We report the application of machine learning methods for predicting the effective diffusivity (De) of two-dimensional porous media from images of their structures. Pore structures are built using reconstruction methods and represented as... more
We report the application of machine learning methods for predicting the effective diffusivity (De) of two-dimensional porous media from images of their structures. Pore structures are built using reconstruction methods and represented as images, and their effective diffusivity is computed by lattice Boltzmann (LBM) simulations. The datasets thus generated are used to train convolutional neural network (CNN) models and evaluate their performance. The trained model predicts the effective diffusivity of porous structures with computational cost orders of magnitude lower than LBM simulations. The optimized model performs well on porous media with realistic topology, large variation of porosity (0.28–0.98), and effective diffusivity spanning more than one order of magnitude (0.1 ≲ De < 1), e.g., >95% of predicted De have truncated relative error of <10% when the true De is larger than 0.2. The CNN model provides better prediction than the empirical Bruggeman equation, especiall...
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Surface forces mediated by room-temperature ionic liquids (RTILs) play an essential role in diverse applications including self-assembly, lubrication, and electrochemical energy storage. Therefore, their fundamental understanding is... more
Surface forces mediated by room-temperature ionic liquids (RTILs) play an essential role in diverse applications including self-assembly, lubrication, and electrochemical energy storage. Therefore, their fundamental understanding is critical. Using molecular simulations, we study the interactions between two nanorods immersed in model RTILs at rod-rod separations where both structural and double layer forces are important. The interaction force between neutral rods oscillates as the two rods approach each other, similar to the classical structural forces. Such oscillatory force originates from the density oscillation of RTILs near each rod and is affected by the packing constraints imposed by the neighboring rods. The oscillation period and decay length of the oscillatory force are mainly dictated by the ion density distribution near isolated nanorods. When charges are introduced on the rods, the interaction force remains short-range and oscillatory, similar to the interactions betw...
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It is discovered that a non-uniform alternating magnetic field can induce a translational motion of an anisotropic magnetic particle or cluster near a surface. Unlike a permanent magnet pulling a magnetic particle, the particle moves away... more
It is discovered that a non-uniform alternating magnetic field can induce a translational motion of an anisotropic magnetic particle or cluster near a surface. Unlike a permanent magnet pulling a magnetic particle, the particle moves away from the magnetic source with a periodic fluctuation in its trajectory that varies with a frequency that is twice that of the field frequency. The moving speed can be tuned by varying the magnetic field strength and gradient, its alternating frequency, and the particle size. A hydrodynamic model is developed that can qualitatively explain all of the phenomena observed. Such a simple particle manipulation method has a great potential in applications such as cell biology and microfluidics.
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The recent fabrication of liquid crystalline ion gels featuring rigid-rod polyanions aligned within room-temperature ionic liquids (RTILs) opens up exciting new avenues for engineering ion conducting materials. These gels exhibit an... more
The recent fabrication of liquid crystalline ion gels featuring rigid-rod polyanions aligned within room-temperature ionic liquids (RTILs) opens up exciting new avenues for engineering ion conducting materials. These gels exhibit an unusual combination of properties including high ionic conductivity, distinct transport anisotropy, and widely tunable elastic modulus. Using molecular simulations, we study the structure and dynamics of the ions in an ion gel consisting of rigid-rod polyanions and [C2mim][TfO] RTILs. We show that the ion distribution in the interstitial space between polymer rods exhibits the hallmarks of the RTIL structure near charged surfaces; i.e., cations (C2mim(+)) and anions (TfO(-)) form alternating layers around the polymer rods and the charge on the rod is overscreened by the ionic layer surrounding it. The distinct ordering of ions suggests the formation of a long-range "electrostatic network" in the ion gel, which may contribute to its mechanical c...
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The recent synthesis of organic molecular liquids with permanent porosity opens up exciting new avenues for gas capture, storage, and separation. Using molecular simulations, we study the thermodynamics and kinetics for the storage of... more
The recent synthesis of organic molecular liquids with permanent porosity opens up exciting new avenues for gas capture, storage, and separation. Using molecular simulations, we study the thermodynamics and kinetics for the storage of CH4, CO2, and N2 molecules in porous liquids consisting of crown-ether-substituted cage molecules in a 15-crown-5 solvent. It is found that the intrinsic gas storage capacity per cage molecule follows the order CH4 > CO2 > N2, which does not correlate simply with the size of gas molecules. Different gas molecules are stored inside the cage differently; e.g., CO2 molecules prefer the cage's core whereas CH4 molecules favor both the core and the branch regions. All gas molecules considered can enter the cage essentially without energy barriers and leave the cage on a nanosecond time scale by overcoming a modest energy penalty. The molecular mechanisms of these observations are clarified.
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ABSTRACT
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We report an integrated experimental and simulation study of ammonia recovery using microbial electrolysis cells (MECs). The transport of various species during the batch-mode operation of an MEC was examined experimentally and the... more
We report an integrated experimental and simulation study of ammonia recovery using microbial electrolysis cells (MECs). The transport of various species during the batch-mode operation of an MEC was examined experimentally and the results were used to validate the mathematical model for such an operation. It was found that, while the generated electrical current through the system tends to acidify (or basify) the anolyte (or catholyte), their effects are buffered by a cascade of chemical groups such as the NH3/NH4(+) group, leading to relatively stable pH values in both anolyte and catholyte. The transport of NH4(+) ions accounts for ~90% of the total current, thus quantitatively confirming that the NH4(+) ions serve as effective proton shuttles during MEC operations. Analysis further indicated that, because of the Donnan equilibrium at cation exchange membrane-anolyte/catholyte interfaces, the Na(+) ion in the anolyte actually facilitates the transport of NH4(+) ions during the ea...
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Molecular simulations of the diffusion of EMIM(+) and TFSI(-) ions in slit-shaped micropores under conditions similar to those during charging show that in pores that accommodate only a single layer of ions, ions diffuse increasingly... more
Molecular simulations of the diffusion of EMIM(+) and TFSI(-) ions in slit-shaped micropores under conditions similar to those during charging show that in pores that accommodate only a single layer of ions, ions diffuse increasingly faster as the pore becomes charged (with diffusion coefficients even reaching ∼5 × 10(-9) m(2)/s), unless the pore becomes very highly charged. In pores wide enough to fit more than one layer of ions, ion diffusion is slower than in the bulk and changes modestly as the pore becomes charged. Analysis of these results revealed that the fast (or slow) diffusion of ions inside a micropore during charging is correlated most strongly with the dense (or loose) ion packing inside the pore. The molecular details of the ions and the precise width of the pores modify these trends weakly, except when the pore is so narrow that the ion conformation relaxation is strongly constrained by the pore walls.
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Surfactant solutions typically feature tunable nanoscale, internal structures. Although rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius.... more
Surfactant solutions typically feature tunable nanoscale, internal structures. Although rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius. Here, we examine the structure and thermal transport in solution of AOT (Dioctyl sodium sulfosuccinate) in n-octane liquids using small-angle neutron scattering, thermal conductivity measurements, and molecular dynamics simulations. We report the first experimental observation of a minimum thermal conductivity occurring at the critical micelle concentration (CMC): the thermal conductivity of the surfactant solution decreases as AOT is added till the onset of micellization but increases as more AOT is added. The decrease of thermal conductivity with AOT loading in solutions in which AOT molecules are dispersed as monomers suggests that even the interfaces between individual oleophobic headgroup of AOT molecules and their surrounding non-polar octane ...