A new transport phenomenon in nanostructures: a mesoscopic analog of the Braess paradox encountered in road networks

ABSTRACT: The Braess paradox, known for traffic and other classical networks, lies in the fact that adding a new route to a congested network in an attempt to relieve congestion can degrade counterintuitively the overall network performance. Recently, we have extended the concept of the Braess paradox to semiconductor mesoscopic networks, whose transport properties are governed by quantum physics. In this paper, we demonstrate theoretically that, alike in classical systems, congestion plays a key role in the occurrence of a Braess paradox in mesoscopic networks.     

The τ-effective paradox: new measurements towards a resolution

New volume recovery measurements have been made on an epoxy glass for temperature down- and up-jumps from 1.0 to 4.0 K to investigate the τ-effective paradox. Due to the sensitivity of the measurements and to the fact that data points are obtained linearly in time, we are able to accurately measure the effective relaxation time at much smaller departures from equilibrium than those examined originally by Kovacs and subsequently by other researchers. The paradox is observed for the largest (2.0 and 4.0 K) up-jumps performed at the highest aging temperature. The magnitude of the expansion gap disappears as the departure from equilibrium approaches zero indicating a resolution of the paradox.     

Crystallization and relaxation dynamics of glass-forming liquids at the Kauzmann temperature

If the entropy extrapolation of supercooled liquids (SCL) suggested by Kauzmann was correct, then they would have the same entropy as their stable crystalline phase at a certain low temperature, below the laboratory glass transition (T_g), known as the Kauzmann temperature (T_K). Extrapolating even further, the liquid entropy would be null at a temperature above absolute zero, violating the Third Law of Thermodynamics and constituting a paradox. Several possibilities have been proposed over the past 70 years to solve this paradox with different degrees of success. Our objective here is to access liquid dynamics at deep supercoolings to test the so-called crystallization solution to the paradox. By comparing the relaxation and crystallization kinetics determined above T_g and extrapolated down to T_K, a possible solution would be that the crystallization time is shorter than the relaxation time, which would mean that a SCL cannot reach the T_K. In this case, the liquid stability limit or kinetic spinodal temperature (T_ks) should be higher than T_K. We tested two fragile glass-forming liquids (diopside and wollastonite) and two strong liquids (silica and germania). For the fragile substances, T_ks ? T_K, hence such a supercooled liquid cannot exist at T_K, and the entropy crisis is averted. On the other hand, the results for the strong liquids were inconclusive. We hope the findings of this work encourage researchers to further investigate the liquid dynamics of different strong glass-forming systems at deep supercoolings.     

煤岩气储层滑脱效应实验研究

岩石的孔隙度与岩石的渗透率成正相关。岩石的气透测渗率高于液测渗透率,且具有压力依赖性,该现象被称作滑脱效应或Klinkenberg效应。气测渗透率的压力依赖性是由于在计算渗透率时气体黏度取值不当所致,气体黏度在低压下随压力变化很大,但计算渗透率时却选用了定值。滑脱效应将使气体的黏度无法测量,从而出现测试悖论。气体分子每时每刻都在做不规则的热运动,会不停地与孔隙壁面发生碰撞,致使气体无法出现滑脱。岩石渗透率的气测值高于液测值,是测试介质的分子尺度与孔隙尺度对比的结果。     

Hydration of Nafion and Dowex in liquid and vapor environment: Schroeder's paradox and microstructure

Functioning of ion-exchange resins and ionomers strongly depends the state of hydration (equilibrium water uptake), which strongly affects their performance. The relation between water activity and uptake by such polymers (isotherm) has usually been measured by equilibration with water vapor of known activity. However, at the point of saturation (pure water) hydration by liquid water and water vapor differ, i.e., depend on the mode of equilibration. This effect, known as Schroeder's paradox, leads to an ambiguity in determination of the chemical potential and calculating the solvent transport in such systems. In order to verify the general nature of this phenomenon, we carried out a systematic examination of the paradox in a wide range of water activities using solutions of a polyelectrolyte (polyvinylsulfonic acid salts) as an osmotic stressor. This allowed measuring hydration isotherms in parallel vapor- and liquid-equilibration modes for two solid polymer electrolytes: Dowex (a cross-linked resin) and Nafion (an ionomer). The results indicate that for the studied polymers Schroeder's paradox extends to the whole range of the examined water activities. Furthermore, the difference between the isotherms could be related to the polymer microstructure, however, it suggested that a correction due to the difference between the surface and bulk microstructure is necessary for Nafion.     

设计约束与创新思维

约束与创新是影响工业设计专业学生实际工作能力的两个重要因素。本文从分析我国工业设计教育现状入手,揭示了设计约束与创新思维的对立统一关系,并结合自身教学改革提出了在教育和训练学生的过程中解决这一矛盾的有效途径。     

The Efficacy of Compounds with Different Polarities as Antioxidants in Emulsions with Omega-3 Lipids

According to the so-called polar paradox hypothesis, the efficacy of an antioxidant in emulsions is highly affected by its polarity and thereby location in the different phases. However, other factors also affect the efficacy of antioxidants in multiphase systems. The aim of this study was to evaluate the efficacy of antioxidants [ascorbic acid, ascorbyl palmitate, ascorbyl CLA and CLA (conjugated linoleic acid)] with different polarities in two different emulsion systems: o/w emulsion (5% oil) and w/o emulsion (98% oil) stabilized with citrem and PGPR, respectively. The efficacy of the antioxidants was compared to their partitioning in an o/w emulsion system and to results obtained from different antioxidant assays: iron reducing power, chelating activity and radical scavenging activity. For the w/o emulsions the efficacy of the antioxidants followed the polar paradox hypothesis: ascorbyl palmitate = ascorbyl CLA > ascorbic acid ≥ CLA > reference. For the o/w emulsion the antioxidative effects were not in accordance with the polar paradox. In the beginning of the storage, ascorbyl palmitate and ascorbic acid were most efficient, however in the end they acted as prooxidants. Ascorbyl CLA was located at the interface but was inactive as an antioxidant. This may be due to impurities or interaction with citrem.     

塑料薄壁件注射成型工艺参数优化

针对传统遗传算法在处理薄壁件注射成型工艺参数优化问题时存在的全局搜索和局部搜索之间的矛盾以及局部搜索能力不足等问题,提出了优化组合遗传算法,实例验证了该算法的可行性。     

Antioxidant effect of mono‐ and dihydroxyphenols in sunflower oil with different levels of naturally present tocopherols

Antioxidant properties of mono‐ and dihydroxyphenolic acids and their alkyl esters were examined, with emphasis on the relationship between their molecular structure and antioxidant activity. Test media with different tocopherol level were used for determining the oxidative stability: original refined sunflower oil (total tocopherols 149.0 mg/kg), partially tocopherol‐stripped sunflower oil (total tocopherols 8.7 mg/kg) and distilled fatty acid methyl esters (FAME) as a tocopherol‐free medium. The chemical reaction of tocopherols with diazomethane tested for the purpose to eliminate their antioxidant activity failed due to the negligible degree of methylation of hydroxyl group in the tocopherol molecule. Caffeic acid and protocatechuic acid (3,4‐dihydroxyphenolic acids) and their alkyl esters were found to be more active antioxidants than monohydroxyphenolic acid (p‐hydroxybenzoic acid), 2,5‐dihydroxyphenolic acid (gentisic acid), 3‐methoxy‐4‐hydroxyphenolic acids (vanillic and ferulic acids) and their corresponding alkyl esters. Naturally present tocopherols in refined sunflower oil proved to have a synergistic effect on gentisic acid but not on its alkyl esters. In contrast, tocopherols showed an antagonistic effect on alkyl esters of caffeic acid, because their protection factors decreased with increasing level of tocopherols in the test medium. Moreover, the antioxidant activity of these alkyl esters decreased with increasing length of their alkyl chain in conformity with the polar paradox hypothesis. Practical applications: Tocopherols as naturally present antioxidants influence considerably the antioxidant activity of other antioxidants added to plant oils used as a test medium. Distilled fatty acid methyl esters prepared from refined sunflower oil may serve as an optimal tocopherol‐free test medium. Some alkyl esters of phenolic acids were evaluated to be applicable as natural more lipophilic antioxidants in comparison with phenolic acids.     

The Normobaric Oxygen Paradox—Hyperoxic Hypoxic Paradox: A Novel Expedient Strategy in Hematopoiesis Clinical Issues

Hypoxia, even at non-lethal levels, is one of the most stressful events for all aerobic organisms as it significantly affects a wide spectrum of physiological functions and energy production. Aerobic organisms activate countless molecular responses directed to respond at cellular, tissue, organ, and whole-body levels to cope with oxygen shortage allowing survival, including enhanced neo-angiogenesis and systemic oxygen delivery. The benefits of hypoxia may be evoked without its detrimental consequences by exploiting the so-called normobaric oxygen paradox. The intermittent shift between hyperoxic-normoxic exposure, in addition to being safe and feasible, has been shown to enhance erythropoietin production and raise hemoglobin levels with numerous different potential applications in many fields of therapy as a new strategy for surgical preconditioning aimed at frail patients and prevention of postoperative anemia. This narrative review summarizes the physiological processes behind the proposed normobaric oxygen paradox, focusing on the latest scientific evidence and the potential applications for this strategy. Future possibilities for hyperoxic-normoxic exposure therapy include implementation as a synergistic strategy to improve a patient’s pre-surgical condition, a stimulating treatment in critically ill patients, preconditioning of athletes during physical preparation, and, in combination with surgery and conventional chemotherapy, to improve patients’ outcomes and quality of life.     

大卫 鲁思

雕塑是一种存在于立体空间之中的形态，由于玻璃具有透明性，人们才有可能观察到该形态的内部空间。在近些年里，我一直都对岩石般的玻璃块的内外部空间所扮演的角色及其内部的自相矛盾特别感兴趣。在过去的15年里，我所做的压铸作品之中的隐喻就是欲通过玻璃的内部空间来表达人类的内心生活。     

Core@double-shell structured fillers for increasing dielectric constant and suppressing dielectric loss of PVDF-based composite films

High dielectric constant polymer dielectrics have attracted a great deal of attention in flexible electronics. However, it appears to be a paradox for polymer dielectrics that the enhancement of their dielectric constant often comes along with the increase of dielectric loss. Hence, we reported core@double shell structured filler/poly(vinylidene fluoride) (PVDF) composites to overcome this paradox. The hybrid filler with BaTiO₃ (BT) as the core, conductive carbon as the inner shell, and insulating polydopamine (PDA) as the outer shell was synthesized. As a result, the BT@C@PDA/PVDF composites at the filler content of 11 vol% exhibit an outstanding dielectric performance with a dielectric constant of 45 and a dielectric loss of 0.053 at 10³ Hz. This phenomenon can be attributed to the increased interfacial polarization induced by the inner carbon shell and the conductive paths blockade caused by the outside PDA shell inside the BT@C@PDA/PVDF composites. This work reveals that rational design of core@double shell structured hybrid fillers maybe a promising way to optimize the overall dielectric performance of the PVDF-based composites.     

On the activation energies of the forward and reverse water-gas shift reaction

Published experimental results demonstrate that the reverse water-gas shift reaction is surprisingly fast over several varied catalysts (metallic and oxidic). The observed values of activation energy of the reaction are shown here to be inconsistent with values calculated from the standard enthalpy change and the activation energies of the forward reaction. Possible explanations of this paradox are discussed in terms of the limits to the application of the “principle of microscopic reversibility” to catalytic systems.     

Mechanism Study on the Severe Foaming of Rhamnolipid in Fermentation

Although the biosurfactant rhamnolipid has been previously characterized as having low foam ability, its fermentation is largely impeded by severe foaming. Hence, the investigation of this paradox is critically important for improving the mass production of rhamnolipid. Unexpectedly, the hydrophobic cell, instead of rhamnolipid, has been claimed to explain such severe foaming in rhamnolipid fermentation. This study tried to systematically investigate the severe foaming in fermentation, aiming to propose an effective strategy for foam control. The overflowing foam sustained a super high stability in terms of half‐time for over 30 min. The major product of rhamnolipid largely contributed to the severe foaming in the fermentation process whereas other products like cells elicited much more limited effects. Furthermore, the foam stability of the fermentation broth increased with rhamnolipid concentration and noticeably increased with agitation speed. In the classic Bikerman foam test system without stirring, rhamnolipid showed foam stability as low as Tween 20 which is well known for its poor foam stability. However, in a stirring Bikerman system, rhamnolipid exhibited a foam stability almost as high as sodium dodecyl sulfate (SDS) at 10 g/L and even surpassed SDS at a higher concentration of 20 g/L. Hence, the extraordinarily increased foam stability of rhamnolipid with both agitation and concentration could explain the severe foaming at its late‐stage fermentation when rhamnolipid‐rich solution is mechanically agitated.     

The applicability of the first-order grinding law to particles having a distribution of strengths

The applicability of the first-order law of grinding kinetics is widely observed. On the other hand, tests of compressive strengths of particles of a given size show a distribution of strengths, so that some particles appear weak and others strong, which would not lead to first-order kinetics of grinding. It is proposed that this paradox is partially resolved by considering the distribution of compressive strengths to arise from orientation of particles, so that a given particle would have an apparent distribution of strenghts if it could be fractured in different orientations.     

The swelling of rubber in liquid and vapour (schroeder's paradox)

Polymeric substances usually swell less when placed in the vapour of a liquid in a closed vessel than they do in the liquid itself. This is known as ‘Schroeder's Paradox’, and many attempts have been made to explain it. With lightly vulcanised rubber and carbon tetrachloride the paradoxical behaviour is caused by small differences in temperature (of order 0.1 °c) which exist under normal conditions between a liquid and its vapour in a closed vessel; the vapour is normally the warmer of the two. Strict temperature control abolishes the paradox.     

The motion of electrolytic gas bubbles near electrodes

Several reports in the literature mention oscillatory bubble motions close to, or in contact with electrodes. Such behaviour is obviously related to a time-variable force, but it was not apparent what the origin of this variable force was. Recent advances in understanding of the surfactant behaviour of gases, and in particular the gases liberated during electrolysis, have cast a new light on the underlying mechanism of these oscillations. Other unexpected, but not necessarily oscillatory bubble behaviour is discussed, and the role of electrolytic gas acting as a surfactant illuminates these non-oscillatory motions as well. Finally, the paradox of bubbles remaining attached to electrodes with zero contact angle is resolved.     

Peel Mechanics

Published treatments of peel mechanics are shown to yield inconsistent relations for the dependence of peel force upon the angle of peel. The paradox is resolved by limiting the stress analyses to small bending deformations of the detaching strip in the still-attached region. This condition holds when the moment arm of the applied peel force is much larger than the length of the high-stress region in the bond, which must therefore be considered a prerequisite for use of the published bond stress distributions.

Failure of some experimental results to conform to the theoretical dependence of peel force upon peel angle are ascribed to inelastic deformation or stretching of the detaching strip.     

The Preference of Silicon Carbide for Growth in the Metastable Cubic Form

A paradox is discussed concerning the growth of SiC polytypes from the vapor or the melt, based on recent ab initio quantum calculations of the relative energies of several polytypes in bulk. Why does the cubic (3C) structure grow in preference to all others, although the calculations indicate it is not the stable phase at any temperature? This can be explained from the calculations, with some further approximations, as due to the constrained equilibrium when adding one atomic double layer at a time to the growing crystal in the hexagonal direction without allowing rearrangement of the lower layers. The differing roles of donor and acceptor impurities are also discussed, with donors being found to favor the cubic structure.     

基于膜射流乳化技术的TiO2大孔陶瓷制备方法研究

A novel method to prepare macroporous TiO2 ceramic, based on membrane emulsification was reported.To solve the paradox between the instability of nonaqueous emulsion and long emulsification time required by themembrane emulsification, a two-stage ceramic membrane jet-flow emulsification .was. proposed. Discussion wasconducted on the evolution of droplet size with time, which followed the Ostwalcl npemng theory. And a monodispersed nonaqueous emulsion with an average droplet size of 1.6μm could beprepared. Using the emulsion, as atemplate, TiO2 ceramics with an average pore size ot 1.1 μm were obtaineed. Tne material could be prospectivelyused for preparation of catalysts, adsorbents, and membranes.