arXiv subjects

Kyle C. Smith

Publications and source records attributed to Kyle C. Smith.

15 recordsLinked to original sources

arXiv2025

An Omni-Temporal Theory for Hydrodynamic Dispersion and Reaction in Porous Media

A frequency-based omni-temporal dispersion theory is developed to capture the transient interplay between diffusion, advection, and reaction during solute transport through porous media. Unlike classical asymptotic dispersion theories, which commonly rely on long-time approximation, the proposed framework simultaneously captures both fast and slow components of dispersion. The theory is formulated by volume averaging the Fourier-transformed pore-scale advection-diffusion equation, yielding four frequency-dependent upscaled transport coefficients for a periodic unit cell: a dispersion tensor, an advection-suppression transfer function, a spectral Sherwood number, and a reactivity-bias vector. These coefficients act as transfer functions that relate microscopic driving forces to corresponding effective fluxes in the frequency domain, enabling prediction of transient transport dynamics in the time domain through inverse Fourier transformation. The utility of the proposed framework is demonstrated by deriving analytical expressions for the transfer functions in Poiseuille flow between parallel plates and through circular tubes, and subsequently using them within a Fast Fourier Transform framework to obtain breakthrough curves. For fast solute pulses between inactive parallel plates, the proposed theory produces breakthrough curves in close agreement with direct numerical simulations, whereas conventional asymptotic theory overpredicts propagation rates by orders of magnitude. Finally, the framework is applied to reactive and non-reactive porous media consisting of periodic arrays of square rods under cross flow, demonstrating the generality and versatility of the proposed omni-temporal theory.

physics.flu-dyn↗

arXiv2023

Embedded, micro-interdigitated flow fields in high areal-loading intercalation electrodes towards seawater desalination and beyond

Faradaic deionization (FDI) is a promising technology for energy-efficient water desalination using porous electrodes containing redox-active materials. Herein, we demonstrate for the first time the capability of a symmetric FDI flow cell to produce freshwater (<17.1 mM NaCl) from concentrated brackish water (118mM), to produce effluent near freshwater salinity (19.1 mM) from influent with seawater-level salinity (496 mM), and to reduce the salinity of hypersaline brine from 781 mM to 227 mM. These remarkable salt-removal levels were enabled by using flow-through electrodes with high areal-loading of nickel hexacyanoferrate (NiHCF) Prussian Blue analogue intercalation material. The pumping energy consumption due to flow-through electrodes was mitigated by embedding an interdigitated array of <100 $μ$m wide channels in the electrodes using laser micromachining. The micron-scale dimensions of the resulting embedded, micro-interdigitated flow fields (e$μ$-IDFFs) facilitate flow-through electrodes with high apparent permeability while minimizing active-material loss. Our modeling shows that these e$μ$-IDFFs are more suitable for our intercalation electrodes because they have >100X lower permeability compared to common redox-flow battery electrodes, for which millimetric flow-channel widths were used exclusively in the past. Total desalination thermodynamic energy efficiency (TEE) was improved by more than ten-fold relative to unpatterned electrodes: 40.0% TEE for brackish water, 11.7% TEE for hypersaline brine, and 7.4% TEE for seawater-salinity feeds. Water transport between diluate and brine streams and charge efficiency losses resulting from (electro)chemical effects are implicated as limiting energy efficiency and water recovery, motivating their investigation for enhancing future FDI performance.

physics.chem-ph↗

arXiv2019

Biophysics-based critique of the assisted discharge mechanism hypothesis

Cell experiments with large, short electric field pulses of opposite polarity reveal a remarkable phenomenon: Bipolar cancellation (BPC). Typical defining experiments involve quantitative observation of tracer molecule influx at times of order 100 s post pulsing. Gowrishankar et al. BBRC 2018 503:1194-1199 shows that long-lived pores and altered partitioning or hindrance due to inserted occluding molecules can account for BPC. In stark contrast, the Assisted Discharge (AD) hypothesis, Pakhomov et al. CellMol- LifeSci 2014 71(22):4431-4441; Fig. 6, only involves early times of a microsecond down to nanoseconds. Further, well established terminology for cell membrane discharge relates to membrane potential decays shortly after pulsing. Discharge is silent on molecular or ionic transport, and does not address the fact that tracer molecule uptake vs time is measure at about 100s after pulsing ceases. Our critique of AD notes that there can be an association of AD with BPC, but associations are only necessary, not sufficient. A BPC mechanism hypothesis must be shown to be causal, able to describe time-dependent molecular influx. The two hypotheses involve very different time-scales (less than a microsecond vs 100 s) and very different quantities (volts/s vs molecules/s). Unlike pore-based hypotheses the AD hypothesis lacks explicit molecular transport mechanisms, and does not address the greatly delayed measured molecular uptake. We conclude that AD is an implausible candidate for explaining BPC.

physics.bio-ph↗

arXiv2018

Nanopore occlusion: A biophysical mechanism for bipolar cancellation in cell membranes

Extraordinarily large but short electric field pulses are reported by many experiments to cause bipolar cancellation (BPC). This unusual cell response occurs if a first pulse is followed by a second pulse with opposite polarity. Possibly universal, BPC presently lacks a mechanistic explanation. Multiple versions of the "standard model" of cell electroporation (EP) fail to account for BPC. Here we show, for the first time, how an extension of the standard model can account for a key experimental observation that essentially defines BPC: the amount of a tracer that enters a cell, and how tracer influx can be decreased by the second part of a bipolar pulse. The extended model can also account for the recovery of BPC wherein the extent of BPC is diminished if the spacing between the first and second pulses is increased. Our approach is reverse engineering, meaning that we identify and introduce an additional biophysical mechanism that allows pore transport to change. We hypothesize that occluding molecules from outside the membrane enter or relocate within a pore. Significantly, the additional mechanism is fundamental and general, involving a combination of partitioning and hindrance. Molecules near the membrane can enter pores to block transport of tracer molecules while still passing small ions (+/- 1) that govern electrical behavior. Accounting for such behavior requires an extension of the standard model.

q-bio.SC↗

arXiv2018

Quantifying the Trade-offs between Energy Consumption and Salt Removal Rate in Membrane-free Cation Intercalation Desalination

Electrochemical desalination devices that use redox-active cation intercalation electrodes show promise for desalination of salt-rich water resources with high water recovery and low energy consumption. While previous modeling and experiments used ion-exchange membranes to maximize charge efficiency, here a membrane-free alternative is evaluated to reduce capital cost by using a porous diaphragm to separate Na$_{1+x}$NiFe(CN)$_6$ electrodes. Two-dimensional porous-electrode modeling shows that, while charge efficiency losses are inherent to a diaphragm-based architecture, charge efficiency values approaching the anion transference number (61$\%$ for NaCl) are achievable for diaphragms with sufficiently low salt conductance. Closed-form equations are thereby derived that relate charge efficiency to the non-dimensional Pèclet and Damköhler numbers that enable the selection of current and flow velocity to produce a desired degree of desalination. Simulations using these conditions are used to quantify the tradeoffs between energy consumption and salt removal rate for diaphragm-based cells operated at a range of currents. The simulated distributions of reactions are shown to result from the local salt concentration variations within electrodes using diffusion-potential theory. We also simulate the cycling dynamics of various flow configurations and show that flow-through electrodes exceed the degree-of-desalination compared with flow-by and flow-behind configurations due to solution stagnation within electrodes.

physics.app-ph↗

arXiv2017

Broad pore lifetime distributions: A fundamental concept for cell electroporation

We describe a concept that has the potential to change how we think about the underlying mechanisms of cell membrane electroporation (EP). Prior experimental, theoretical and modeling have emphasized a single pore lifetime as adequate for particular conditions. Here we introduce a much more complex response: The rapid creation of many types of pore structures, of which some are traditional transient lipidic pores (TPs), but the great majority are complex pores (CPs) based on both lipids and other molecules or molecular segments. At the inner leaflet of the cell plasma membrane (PM) non-lipidic molecules come from the over-crowded cytoplasm. At the outer leaflet they originate from the extracellular medium and extracellular matrix. Some partially or fully insert into TPs during or shortly after TP formation, or bind to the membrane nearby. This process is complex, leading to mostly short-lived structures, with relatively few lasting for long times. We speculate that the characteristic pore lifetimes range from $\sim$100 ns to 1,000 s, based on implications from experiments. The frequency-of-occurrence probably falls off extremely rapidly with increasing lifetime, $τ_{CP}$, which implies that most are inaccessible to traditional experimental methods. It also suggests that unexpected behavior can occur early in pulsing, vanishing before post-pulse observations begin.

physics.bio-ph↗

arXiv2017

Uncovering the role of flow rate in redox-active polymer flow batteries: simulation of reaction distributions with simultaneous mixing in tanks

Redox flow batteries (RFBs) are potential solutions for grid-scale energy storage, and deeper understanding of the effect of flow rate on RFB performance is needed to develop efficient, low-cost designs. In this study we highlight the importance of modeling tanks, which can limit the charge/discharge capacity of redox-active polymer (RAP) based RFBs. The losses due to tank mixing dominate over the polarization-induced capacity losses that arise due to resistive processes in the reactor. A porous electrode model is used to separate these effects by predicting the time variation of active species concentration in electrodes and tanks. A simple transient model based on species conservation laws developed in this study reveals that charge utilization and polarization are affected by two dimensionless numbers quantifying (1) flow rate relative to stoichiometric flow and (2) size of flow battery tanks relative to the reactor. The RFB's utilization is shown to increase monotonically with flow rate, reaching 90% of the theoretical value only when flow rate exceeds twenty-fold of the stoichiometric value. We also identify polarization due to irreversibilities inherent to RFB architecture as a result of tank mixing and current distribution internal to the reactor, and this polarization dominates over that resulting from ohmic resistances particularly when cycling RFBs at low flow rates and currents. These findings are summarized in a map of utilization and polarization that can be used to select adequate flow rate for a given tank size.

physics.chem-ph↗

arXiv2016

Theoretical evaluation of electrochemical cell architectures using cation intercalation electrodes for desalination

Water scarcity is a dilemma facing much of the global population. Cation intercalation desalination (CID) cells, which use intercalation host compounds (IHCs) in combination with ion-exchange membranes (IEMs), could aid in addressing this challenge by treating saline water sources. Originally, the performance of such cells was predicted utilizing continuous flow of saline water through porous IHC electrodes. Here, we use two-dimensional porous-electrode theory with concentrated solution transport to evaluate the performance of various cell architectures where flow occurs through open flow channels (OFCs) when two IHC electrodes comprised of nickel hexacyanoferrate (NiHCF) are used to store Na+ ions. We show that, when two OFCs are used, cation exchange membranes (CEMs) are adjoined at flow-channel/electrode interfaces, and an anion exchange membrane (AEM) is arranged between flow channels, salt removal increases relative to the original design with flow-through (FT) electrodes. The IEM stacking sequence within such a membrane flow-by (MFB) cell is the fundamental repeat unit for electrodialysis (ED) stacks using many IEMs (CEM/AEM/.../CEM/AEM/CEM) with many diluate streams. Accordingly, we simulate the performance of such ED stacks using NiHCF IHCs, and we predict that salt adsorption capacity (per unit NiHCF mass) is amplified by twenty-fold relative to MFB and FT cells, while simultaneously decreasing 0.7 M NaCl feed water to 0.2-0.3 M within diluate streams. The generality of these findings is further supported by simulations using Na0.44MnO2 IHC instead of NiHCF. Thus, we propose the use of cation IHCs as alternatives to the gas-evolution reactions used in conventional ED.

physics.chem-ph↗

arXiv2015

Design of bi-tortuous, anisotropic graphite anodes for fast ion-transport in Li-ion batteries

Thick Li-ion battery electrodes with high ion transport rates could enable batteries that cost less and that have higher gravimetric and volumetric energy density, because they require fewer inactive cell-components. Finding ways to increase ion transport rates in thick electrodes would be especially valuable for electrodes made with graphite platelets, which have been shown to have tortuosities in the thru-plane direction about 3 times higher than in the in-plane direction. Here, we predict that bi-tortuous electrode structures (containing electrolyte-filled macro-pores embedded in micro-porous graphite) can enhance ion transport and can achieve double the discharge capacity compared to an unstructured electrode at the same average porosity. We introduce a new two-dimensional version of porous-electrode theory with anisotropic ion transport to investigate these effects and to interpret the mechanisms by which performance enhancements arise. From this analysis we determine criteria for the design of bi-tortuous graphite anodes, including the particular volume fraction of macro-pores that maximizes discharge capacity (approximately 20 vol.%) and a threshold spacing interval (half the electrode's thickness) below which only marginal enhancement in discharge capacity is obtained. We also report the sensitivity of performance with respect to cycling rate, electrode thickness, and average porosity/electroactive-material loading.

cond-mat.mtrl-sci↗

arXiv2014

Variable-cell method for stress-controlled jamming of athermal, frictionless grains

A new method is introduced to simulate jamming of polyhedral grains under controlled stress that incorporates global degrees of freedom through the metric tensor of a periodic cell containing grains. Jamming under hydrostatic/isotropic stress and athermal conditions leads to a precise definition of the ideal jamming point at zero shear stress. The structures of tetrahedra jammed hydrostatically exhibit less translational order and lower jamming-point density than previously described `maximally random jammed' hard tetrahedra. Under the same conditions, cubes jam with negligible nematic order. Grains with octahedral symmetry jam in the large-system limit with an abundance of face-face contacts in the absence of nematic order. For sufficiently large face-face contact number, percolating clusters form that span the entire simulation box. The response of hydrostatically jammed tetrahedra and cubes to shear-stress perturbation is also demonstrated with the variable-cell method.

cond-mat.soft↗

arXiv2012

Conduction in jammed systems of tetrahedra

Control of transport processes in composite microstructures is critical to the development of high performance functional materials for a variety of energy storage applications. The fundamental process of conduction and its control through the manipulation of granular composite attributes (e.g., grain shape) are the subject of this work. We show that athermally jammed packings of tetrahedra with ultra-short range order exhibit fundamentally different pathways for conduction than those in dense sphere packings. Highly resistive granular constrictions and few face-face contacts between grains result in short-range distortions from the mean temperature field. As a consequence, 'granular' or differential effective medium theory predicts the conductivity of this media within 10% at the jamming point; in contrast, strong enhancement of transport near interparticle contacts in packed-sphere composites results in conductivity divergence at the jamming onset. The results are expected to be particularly relevant to the development of nanomaterials, where nanoparticle building blocks can exhibit a variety of faceted shapes.

cond-mat.soft↗

arXiv2012

Models for Metal Hydride Particle Shape, Packing, and Heat Transfer

A multiphysics modeling approach for heat conduction in metal hydride powders is presented, including particle shape distribution, size distribution, granular packing structure, and effective thermal conductivity. A statistical geometric model is presented that replicates features of particle size and shape distributions observed experimentally that result from cyclic hydride decreptitation. The quasi-static dense packing of a sample set of these particles is simulated via energy-based structural optimization methods. These particles jam (i.e., solidify) at a density (solid volume fraction) of 0.665+/-0.015 - higher than prior experimental estimates. Effective thermal conductivity of the jammed system is simulated and found to follow the behavior predicted by granular effective medium theory. Finally, a theory is presented that links the properties of bi-porous cohesive powders to the present systems based on recent experimental observations of jammed packings of fine powder. This theory produces quantitative experimental agreement with metal hydride powders of various compositions.

cond-mat.soft↗

arXiv2011

Isostaticity of Constraints in Jammed Systems of Soft Frictionless Platonic Solids

The average number of constraints per particle $< C_{total} >$ in mechanically stable systems of Platonic solids (except cubes) approaches the isostatic limit at the jamming point ($< C_{total} > \rightarrow 12$), though average number of contacts are hypostatic. By introducing angular alignment metrics to classify the degree of constraint imposed by each contact, constraints are shown to arise as a direct result of local orientational order reflected in edge-face and face-face alignment angle distributions. With approximately one face-face contact per particle at jamming chain-like face-face clusters with finite extent form in these systems.

cond-mat.soft↗

arXiv2010

Athermal jamming of soft frictionless Platonic solids

A mechanically-based structural optimization method is utilized to explore the phenomena of jamming for assemblies of frictionless Platonic solids. Systems of these regular convex polyhedra exhibit mechanically stable phases with density substantially less than optimal for a given shape, revealing that thermal motion is necessary to access high density phases. We confirm that the large system jamming threshold of $0.623 \pm 0.003$ for tetrahedra is consistent with experiments on tetrahedral dice. Also, the extremely short-ranged translational correlations of packed tetrahedra observed in experiments are confirmed here, in contrast with those of thermally simulated glasses. Though highly ordered phases are observed to form for small numbers of cubes and dodecahedra, the short correlation length scale suppresses ordering in large systems, resulting in packings that are mechanically consistent with `orientationally disordered' contacts (point-face and edge-edge contacts). Mild nematic ordering is observed for large systems of cubes, whereas angular correlations for the remaining shapes are ultra short-ranged. Power-law scaling exponents for energy with respect to distance from the jamming threshold exhibit a clear dependence on the `highest order' percolating contact topology. These nominal exponents are 6, 4, and 2 for configurations having percolating point-face (or edge-edge), edge-face, and face-face contacts, respectively. These Platonic solids exhibit hypostatic behavior, with average jamming contact number between the isostatic value for spheres and that of asymmetric particles. These shapes violate the isostatic conjecture, displaying contact number that decreases monotonically with sphericity. The common symmetry of dual polyhedra results in local translational structural similarity. (Note abstract abridged due to length constraint)

cond-mat.soft↗

arXiv2010

Dopant-vacancy binding effects in Li-doped magnesium hydride

We use a combination of ab initio calculations and statistical mechanics to investigate the substitution of Li+ for Mg2+ in magnesium hydride (MgH2) accompanied by the formation of hydrogen vacancies with positive charge (with respect to the original ion at the site). We show here that the binding energy between dopants and vacancy defects leads to a significant fraction of trapped vacancies and therefore a dramatic reduction of the number of free vacancies available for diffusion. The concentration of free vacancies initially increases with dopant concentration, but reaches a maximum at around 1 mol% Li doping and slowly decreases with further doping. At the optimal level of doping, the corresponding concentration of free vacancies is much higher than the equilibrium concentrations of charged and neutral vacancies in pure MgH2 at typical hydrogen storage conditions. We also show that Li-doped MgH2 is thermodynamically metastable with respect to phase separation into pure magnesium and lithium hydrides at any significant Li concentration, even after considering the stabilization provided by dopant-vacancy interactions and configurational entropic effects. Our results suggest that lithium doping may enhance hydrogen diffusion hydride, but only to a limited extent determined by an optimal dopant concentration and conditioned to the stability of the doped phase.

cond-mat.mtrl-sci↗