Contradictory Experimental Observations Research Articles

Quantifying electron transport in aggregated colloidal suspensions in the strong flow regime

Electron transport in complex fluids, biology, and soft matter is a valuable characteristic in processes ranging from redox reactions to electrochemical energy storage. These processes often employ conductor-insulator composites in which electron transport properties are fundamentally linked to the microstructure and dynamics of the conductive phase. While microstructure and dynamics are well recognized as key determinants of the electrical properties, a unified description of their effect has yet to be determined, especially under flowing conditions. In this work, the conductivity and shear viscosity are measured for conductive colloidal suspensions to build a unified description by exploiting both recent quantification of the effect of flow-induced dynamics on electron transport and well-established relationships between electrical properties, microstructure, and flow. These model suspensions consist of conductive carbon black (CB) particles dispersed in fluids of varying viscosities and dielectric constants. In a stable, well-characterized shear rate regime where all suspensions undergo self-similar agglomerate breakup, competing relationships between conductivity and shear rate were observed. To account for the role of variable agglomerate size, equivalent microstructural states were identified using a dimensionless fluid Mason number, [Formula: see text], which allowed for isolation of the role of dynamics on the flow-induced electron transport rate. At equivalent microstructural states, shear-enhanced particle-particle collisions are found to dominate the electron transport rate. This work rationalizes seemingly contradictory experimental observations in literature concerning the shear-dependent electrical properties of CB suspensions and can be extended to other flowing composite systems.

Ultrasensitive Response Explains the Benefit of Combination Chemotherapy Despite Drug Antagonism.

Most aggressive lymphomas are treated with combination chemotherapy, commonly as multiple cycles of concurrent drug administration. Concurrent administration is in theory optimal when combination therapies have synergistic (more than additive) drug interactions. We investigated pharmacodynamic interactions in the standard 4-drug "CHOP" regimen in peripheral T-cell lymphoma (PTCL) cell lines and found that CHOP consistently exhibits antagonism and not synergy. We tested whether staggered treatment schedules could improve tumor cell kill by avoiding antagonism, using in vitro models of concurrent or staggered treatments. Surprisingly, we observed that tumor cell kill is maximized by concurrent drug administration despite antagonistic drug-drug interactions. We propose that an ultrasensitive dose response, as described in radiology by the linear-quadratic (LQ) model, can reconcile these seemingly contradictory experimental observations. The LQ model describes the relationship between cell survival and dose, and in radiology has identified scenarios favoring hypofractionated radiotherapy-the administration of fewer large doses rather than multiple smaller doses. Specifically, hypofractionated treatment can be favored when cells require an accumulation of DNA damage, rather than a "single hit," to die. By adapting the LQ model to combination chemotherapy and accounting for tumor heterogeneity, we find that tumor cell kill is maximized by concurrent administration of multiple drugs, even when chemotherapies have antagonistic interactions. Thus, our study identifies a new mechanism by which combination chemotherapy can be clinically beneficial that is not contingent on positive drug-drug interactions.

Persistent Nonlinear Phase-Locking and Nonmonotonic Energy Dissipation in Micromechanical Resonators.

Many nonlinear systems are described by eigenmodes with amplitude-dependent frequencies, interacting strongly whenever the frequencies become commensurate at internal resonances. Fast energy exchange via the resonances holds the key to rich dynamical behavior, such as time-varying relaxation rates and signatures of nonergodicity in thermal equilibrium, revealed in the recent experimental and theoretical studies of micro- and nanomechanical resonators. However, a universal yet intuitive physical description for these diverse and sometimes contradictory experimental observations remains elusive. Here we experimentally reveal persistent nonlinear phase-locked states occurring at internal resonances and demonstrate that they are essential for understanding the transient dynamics of nonlinear systems with coupled eigenmodes. The measured dynamics of a fully observable micromechanical resonator system are quantitatively described by the lower-frequency mode entering, maintaining, and exiting a persistent phase-locked period-tripling state generated by the nonlinear driving force exerted by the higher-frequency mode. This model describes the observed phase-locked coherence times, the direction and magnitude of the energy exchange, and the resulting nonmonotonic mode energy evolution. Depending on the initial relative phase, the system selects distinct relaxation pathways, either entering or bypassing the locked state. The described persistent phase locking is not limited to particular frequency fractions or types of nonlinearities and may advance nonlinear resonator systems engineering across physical domains, including photonics as well as nanomechanics.

Proposal of two parameters to evaluate in-situ apparent toughness of interphase in fiber reinforced ceramic matrix composites: Three-dimensional finite element simulations

Interphase structure characters such as single-layer interfacial thickness or number of multilayers may have a great influence on mechanical properties of continuous fibers reinforced ceramic matrix composites (CFRCMCs). However, the determination of the optimal interphase is still unclear for the complex microstructures of CFRCMCs. In this study, a novel Finite Element Method (FEM) -based numerical method considering mean scalar stiffness degradation and mean damage dissipation energy are proposed to quantitatively assess the effect of above structure characters on in-situ toughness of interphase. The proposed method has successfully applied on single pyrolytic carbon (PyC) layer and alternating silicon carbide/pyrolytic carbon (SiC-PyC) n multilayer systems. The results suggest that thicker PyC interphase will prone cause brittle fracture with lower toughness, whilst more layers with the same total thickness will improve fracture toughness for (SiC-PyC) n system. On the contrary, more layers with the same sublayer thickness will decrease in situ apparent fracture toughness. The proposed method explains well the previous contradictory experimental observations.

Water dopant control of structural stability and charge recombination of perovskite solar cells: A first-principles study

Humidity is a key factor in affecting the long-term stability of perovskite solar cells. However, how water interacts with the perovskite film and influences the individual energy conversion process in perovskite solar cells remains virtually unexplored. By using ab initio molecular dynamics and nonadiabatic molecular dynamics, we reveal that the MAX-terminated surface is more susceptible to water erosion. The intrusion of water brings in severe lattice distortion, whose degree follows the law of MAPbI3 > MAPbBr3 > MAPbCl3, thereby leading to attenuation in optical absorbance. Unexpectedly, the carrier lifetimes of hydrated MAX-terminated systems are prolonged, while the PbX-terminated surfaces are largely unaffected. This is due to the dielectric shielding effect of invading water molecules, hindering electron-hole recombination. Our results shed light on a comprehensive understanding of the complex effect of humidity on the perovskite performance, rationalize the contradictory experimental observations, and provide novel insights for further optimization of renewable energy devices.

Tumor Cell Glycolysis-At the Crossroad of Epithelial-Mesenchymal Transition and Autophagy.

Upregulation of glycolysis, induction of epithelial–mesenchymal transition (EMT) and macroautophagy (hereafter autophagy), are phenotypic changes that occur in tumor cells, in response to similar stimuli, either tumor cell-autonomous or from the tumor microenvironment. Available evidence, herein reviewed, suggests that glycolysis can play a causative role in the induction of EMT and autophagy in tumor cells. Thus, glycolysis has been shown to induce EMT and either induce or inhibit autophagy. Glycolysis-induced autophagy occurs both in the presence (glucose starvation) or absence (glucose sufficiency) of metabolic stress. In order to explain these, in part, contradictory experimental observations, we propose that in the presence of stimuli, tumor cells respond by upregulating glycolysis, which will then induce EMT and inhibit autophagy. In the presence of stimuli and glucose starvation, upregulated glycolysis leads to adenosine monophosphate-activated protein kinase (AMPK) activation and autophagy induction. In the presence of stimuli and glucose sufficiency, upregulated glycolytic enzymes (e.g., aldolase or glyceraldehyde 3-phosphate dehydrogenase) or decreased levels of glycolytic metabolites (e.g., dihydroxyacetone phosphate) may mimic a situation of metabolic stress (herein referred to as “pseudostarvation”), leading, directly or indirectly, to AMPK activation and autophagy induction. We also discuss possible mechanisms, whereby glycolysis can induce a mixed mesenchymal/autophagic phenotype in tumor cells. Subsequently, we address unresolved problems in this field and possible therapeutic consequences.

Dissect the role of particle size through collision-attachment simulations for colloidal fouling of RO/NF membranes

Colloidal size affects the whole process of particle transport and membrane filtration. However, its compound effect on fouling remains controversial. In the present study, we adopt a collision-attachment approach to systematically investigate the role of colloidal size on fouling. Our study highlights the critical importance of four competing mechanisms: reduced specific cake resistance and enhanced foulant-membrane interaction of larger particles tend to mitigate flux decline, while the simultaneously increased hydrodynamic drag and reduced particle back-transport tend to promote fouling. The net effect of particle size on fouling is governed by the competition among these mechanisms. When strong foulant-membrane repulsion prevails, we show enhanced flux stability for larger particles as a result of a greatly increased energy barrier to resist particle deposition. Nevertheless, this trend could be reversed for weak foulant-membrane interaction. Our study reconciles the contradictory experimental observations of the effect of particle size on colloidal fouling and provide important insights for effective fouling mitigation.

Physical theory of biological noise buffering by multicomponent phase separation

Maintaining homeostasis is a fundamental characteristic of living systems. In cells, this is contributed to by the assembly of biochemically distinct organelles, many of which are not membrane bound but form by the physical process of liquid-liquid phase separation (LLPS). By analogy with LLPS in binary solutions, cellular LLPS was hypothesized to contribute to homeostasis by facilitating "concentration buffering," which renders the local protein concentration within the organelle robust to global variations in the average cellular concentration (e.g., due to expression noise). Interestingly, concentration buffering was experimentally measured in vivo in a simple organelle with a single solute, while it was observed not to be obeyed in one with several solutes. Here, we formulate theoretically and solve analytically a physical model of LLPS in a ternary solution of two solutes (ϕ and ψ) that interact both homotypically (ϕ-ϕ attractions) and heterotypically (ϕ-ψ attractions). Our physical theory predicts how the coexisting concentrations in LLPS are related to expression noise and thus, generalizes the concept of concentration buffering to multicomponent systems. This allows us to reconcile the seemingly contradictory experimental observations. Furthermore, we predict that incremental changes of the homotypic and heterotypic interactions among the molecules that undergo LLPS, such as those that are caused by mutations in the genes encoding the proteins, may increase the efficiency of concentration buffering of a given system. Thus, we hypothesize that evolution may optimize concentration buffering as an efficient mechanism to maintain LLPS homeostasis and suggest experimental approaches to test this in different systems.

Heart-Lung Interactions During Mechanical Ventilation: Analysis via a Cardiopulmonary Simulation Model.

Heart-lung interaction mechanisms are generally not well understood. Mechanical ventilation, for example, accentuates such interactions and could compromise cardiac activity. Thereby, assessment of ventilation-induced changes in cardiac function is considered an unmet clinical need. We believe that mathematical models of the human cardiopulmonary system can provide invaluable insights into such cardiorespiratory interactions. In this article, we aim to use a mathematical model to explain heart-lung interaction phenomena and provide physiologic hypotheses to certain contradictory experimental observations during mechanical ventilation. To accomplish this task, we highlight three model components that play a crucial role in heart-lung interactions: 1) pericardial membrane, 2) interventricular septum, and 3) pulmonary circulation that enables pulmonary capillary compression due to lung inflation. Evaluation of the model’s response under simulated ventilation scenarios shows good agreement with experimental data from the literature. A sensitivity analysis is also presented to evaluate the relative impact of the model’s highlighted components on the cyclic ventilation-induced changes in cardiac function.

Network and cellular mechanisms underlying heterogeneous excitatory/inhibitory balanced states.

Recent work has explored spatiotemporal relationships between excitatory (E) and inhibitory (I) signaling within neural networks, and the effect of these relationships on network activity patterns. Data from these studies have indicated that excitation and inhibition are maintained at a similar level across long time periods and that excitatory and inhibitory currents may be tightly synchronized. Disruption of this balance-leading to an aberrant E/I ratio-is implicated in various brain pathologies. However, a thorough characterization of the relationship between E and I currents in experimental settings is largely impossible, due to their tight regulation at multiple cellular and network levels. Here, we use biophysical neural network models to investigate the emergence and properties of balanced states by heterogeneous mechanisms. Our results show that a network can homeostatically regulate the E/I ratio through interactions among multiple cellular and network factors, including average firing rates, synaptic weights and average neural depolarization levels in excitatory/inhibitory populations. Complex and competing interactions between firing rates and depolarization levels allow these factors to alternately dominate network dynamics in different synaptic weight regimes. This leads to the emergence of distinct mechanisms responsible for determining a balanced state and its dynamical correlate. Our analysis provides a comprehensive picture of how E/I ratio changes when manipulating specific network properties, and identifies the mechanisms regulating E/I balance. These results provide a framework to explain the diverse, and in some cases, contradictory experimental observations on the E/I state in different brain states and conditions.

The role of the chemical composition on the photoluminescence properties of N-doped carbon nanoparticles

Carbon nanoparticles (CNPs) have aroused a great interest due to its strong blue fluorescence emission. However, the blue fluorescence limits its applications in several cases; therefore, the synthesis of CNPs with multicolor emissions becomes mandatory. A widely used strategy of modulation of CNP photoluminescence is doping the carbon network with nitrogen atoms. Despite the great number of works carried out with N-doped CNPs, some contradictory experimental observations have been reported. These contradictions seem to be due to structural differences resulting of the different synthesis routes. Therefore, it is difficult to correlate the Photoluminescence properties with the nanoparticle structure. To address this issue, we study the Photoluminescence properties of CNPs with different chemical composition and sizes. Nanoparticles were synthesized by an acidic treatment for cutting carbonaceous materials previously reported and five different precursors. Using this methodology, we obtained five different materials which allow correlating the Photoluminescence spectrum with chemical composition and nanoparticle sizes.

(Invited) Understanding the Solid-Electrolyte and Electrode Interface via First-Principles Prediction of Potentials and Space-Charge Layers in All-Solid-State Batteries

Solid-state electronic heterojunctions at the solid-electrolyte and electrode interfaces play a critical role in determining ionic transport, and therefore the performance of solid-state batteries and fuel cells. The formation of a “space-charge layer” at the solid electrolyte and electrode interface is often cited as a barrier for ion transport, but a comprehensive first-principles modeling approach is still missing. For solid-state-batteries (SSB), predicting the space-charge-layer formation at the solid-electrolyte-and-electrode interface is even more challenging, as ion insertion and/or reaction with the electrode alters the material and thus band alignments at the interfaces. In this talk, we established a theoretical framework to predict the interface potential profiles from thermodynamic driving forces [1]. We first assumed the electrochemical potential for Li+ ions reached a constant at the open circuit equilibrium condition, then derived the relationship among the electrostatic potential, the lithium chemical potential, Fermi level, ionization potential, and the work function. This relationship yielded quantitative profiles of the electrostatic potential and electronic energy level alignments across the entire battery. This model complemented direct microscopic and macroscopic simulations by rigorously and simultaneously determining the potential drop, electrostatic dipole, and space-charge layer at the interface. The application of this model to the Li/LiPON/LixCoO2 system led to the important discovery that the space-charge layer varies with the state of charge (SOC, i.e. Li concentration in LixCoO2). More specifically we predicted that Li+ transferred from LIPON to Li0.5CoO2, but transferred from Li1.0CoO2 to LiPON, this result unified the seemingly contradictory experimental observations. We also predicted that electron transfer caused the interface dipole that can accelerate or impede Li+ flow during the discharge process at the in LixCoO2/LiPON interface, depending on the SOC. This will have a profound impact on the overpotentials and the overall cycling efficiency. An important design rule was suggested that increasing solid electrolytes’ valence band or engineering the interlayer at the interface can overcome the additional barrier caused by the interface dipole. This modeling framework is general for other solid-state energy storage devices with ionic and mixed conductor interfaces. [1] Michael W. Swift and Yue Qi, Phys. Rev. Lett. 122, 167701 (2019)

Modeling the Origin of the Interface Resistance in Solid-State Batteries

The major bottleneck for the all-solid-state batteries lies at the high interfacial resistance, which is due to two main factors, physical contact and chemical effect, both will be evaluated via modeling in this talk. First, we devise a model, incorporating atomic electrochemical potentials, thermodynamic stability of bulk electrolyte phases, and point defect formation energies to yield quantitative profiles of the electrostatic potential, lithium chemical potential, and electronic energy levels. This model complements direct microscopic and macroscopic simulations by rigorously and simultaneously determining the potential drop, electrostatic dipole, and space-charge layer at the interface. The application of this model to the Li/LiPON/LixCoO2 system leads to the important discovery that the space-charge layer varies with the state of charge (i.e. Li concentration in LixCoO2). This new physics insight unifies the seemingly contradictory experimental observations and leads to new device design rules to promote interfacial ion transport in future SSBs. Secondly, we combined contact mechanics with 1D Newman battery model to capture the effect of imperfect electrode/electrolyte interfacial contacts, which can be formed during cell fabrication and worsened due to cycling. Constant current discharging processes at different rates and contact areas were simulated to correlate the capacity drop with the contact area loss. Furthermore, the model suggested how much pressures should be applied to recover the capacity drop due to contact area loss.

Unified first-principles theory of thermal properties of insulators

The conventional first-principles theory for the thermal and thermodynamic properties of insulators is based on the perturbative treatment of the anharmonicity of crystal bonds. While this theory has been a successful predictive tool for strongly bonded solids such as diamond and silicon, here we show that it fails dramatically for strongly anharmonic (weakly bonded) materials, and that the conventional quasiparticle picture breaks down at relatively low temperatures. To address this failure, we present a unified first-principles theory of the thermodynamic and thermal properties of insulators that captures multiple thermal properties within the same framework across the full range of anharmonicity from strongly bonded to weakly bonded insulators. This theory features a new phonon renormalization approach derived from many-body physics that creates well-defined quasiparticles even at relatively high temperatures, and it accurately captures the effects of strongly anharmonic bonds on phonons and thermal transport. Using a prototypical strongly anharmonic material, sodium chloride (NaCl), as an example, we demonstrate that our new first-principles framework simultaneously captures the apparently contradictory experimental observations of large thermal expansion and low thermal conductivity of NaCl on the one hand, and anomalously weak temperature dependence of phonon modes on the other, while the conventional theory fails in all three cases. We demonstrate that four-phonon scattering due to higher-order anharmonicity significantly lowers the thermal conductivity of NaCl and is required for a proper comparison to experiment. Furthermore, we show that our renormalization framework, along with four-phonon scattering, also successfully predicts the measured phonon frequencies and thermal properties of a weakly anharmonic material, diamond, indicating universal applicability for thermal properties of insulators. Our work gives new insights into the physics of heat flow in solids, and presents a computationally efficient and rigorous framework that captures the thermal and thermodynamic properties of both weakly and strongly bonded insulators simultaneously.

Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose the ‘Surface Distortion’ as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.

Disproportionate entrance length in superfluid flows and the puzzle of counterflow instabilities

A turbulent wake retains a memory of upstream disturbances which fades over a longer distance for lower viscosity. For a zero-viscosity superfluid, numerical simulations find a subtle memory effect that explains a number of contradictory experimental observations.

A unified theory of calcium alternans in ventricular myocytes.

Intracellular calcium (Ca2+) alternans is a dynamical phenomenon in ventricular myocytes, which is linked to the genesis of lethal arrhythmias. Iterated map models of intracellular Ca2+ cycling dynamics in ventricular myocytes under periodic pacing have been developed to study the mechanisms of Ca2+ alternans. Two mechanisms of Ca2+ alternans have been demonstrated in these models: one relies mainly on fractional sarcoplasmic reticulum Ca2+ release and uptake, and the other on refractoriness and other properties of Ca2+ sparks. Each of the two mechanisms can partially explain the experimental observations, but both have their inconsistencies with the experimental results. Here we developed an iterated map model that is composed of two coupled iterated maps, which unifies the two mechanisms into a single cohesive mathematical framework. The unified theory can consistently explain the seemingly contradictory experimental observations and shows that the two mechanisms work synergistically to promote Ca2+ alternans. Predictions of the theory were examined in a physiologically-detailed spatial Ca2+ cycling model of ventricular myocytes.

Do “Hot” Charge-Transfer Excitons Promote Free Carrier Generation in Organic Photovoltaics?

In organic photovoltaics, the role of hot charge-transfer (CT) excitons on free carrier generation is currently under intense debate. In this paper, we carry out first-principles time-dependent density functional theory calculations to examine hot CT dissociation in polymer/fullerene heterojunctions. We reveal that whether or not hot CT states promote charge separation depends on excitation spectral range and crystallinity of the donor and acceptor phases. We find that while the crystallinity of the donor phase underlies the energy dependence of CT exciton dissociation, the crystallinity of the acceptor determines charge separation efficiency. We propose a theory that can reconcile contradictory experimental observations and provide insight into hot CT dissociation. Crucially, the timescale of hot CT dissociation is found to be comparable to the timescale of its relaxation to the lowest-lying CT state, which is localized in all interfacial models considered here.

The E1–E2 center in gallium arsenide is the divacancy

Based on defect energy levels computed from first-principles calculations, it is shown the E1–E2 center in irradiated GaAs cannot be due to an isolated arsenic vacancy. The only simple intrinsic defect with levels compatible with E1 and E2 is the divacancy. The arsenic monovacancy is reassigned to the E3 center in irradiated GaAs. These new assignments are shown to reconcile a number of seemingly contradictory experimental observations.

What Limits the Chain Growth from Flat Surfaces in Surface‐Initiated ATRP: Propagation, Termination or Both?

Surface‐initiated controlled radical polymerization, such as ATRP, has been proven to be a powerful method for preparation of well‐controlled functional grafted polymers. There are thousands of experimental works published, by varying polymers, surfaces, and applications. In comparison, theoretical developments are very lacking. Many fundamental questions still remain to be answered. These questions include, but not limited to: What determine the surface initiator efficiency? How many chains per square nanometer can be fully grown? What is the maximum thickness can polymer grow to? If chains are simultaneously grown from surface and solution, which has higher molecular weight? Answers to these questions are most helpful for innovation and further development in this important area. In this work, we employed recently developed theories to explain experimentally observed kinetic profiles of polymer layer thickness growth. What limits the growth of surface chains? Is it the monomer starvation in grafting layer that slow down propagation? Or is it the termination of radicals that stop the chain growth? It was found that no existing models could fully explain the often contradictory experimental observations. It is our hope that this work will provoke further discussions and inspire more effort in resolving the fundamental issues of this area.