The Turbulent Schmidt Number for Transient Contaminant Dispersion in a Large Ventilated Room Using a Realizable k-εModel

Buildings with large open spaces in which chemicals are handled are often exposed to the risk of explosions.Computational fluid dynamics is a useful and convenient way to investigate contaminant dispersion in such large spaces.The turbulent Schmidt number(Sc_(t))concept has typically been used in this regard,and most studies have adopted a default value.We studied the concentration distribution for sulfur hexafluoride(SF_(6))assuming different emission rates and considering the effect of Sc_(t).Then we examined the same problem for a light gas by assuming hydrogen gas(H_(2))as the contaminant.When SF_(6) was considered as the contaminant gas,a variation in the emission rate completely changed the concentration distribution.When the emission rate was low,the gravitational effect did not take place.For both low and high emission rates,an increase in S_(ct) accelerated the transport rate of SF_(6).In contrast,for H_(2) as the contaminant gas,a larger S_(ct) could induce a decrease in the H_(2) transport rate.

路易·艾黎职业教育思想助力职业院校健康永续发展

路易·艾黎上个世纪40年代曾在贫穷落后的甘肃山丹创办职业教育,提出"手脑并用、创造分析""理论联系实践""半工半读"的教育思想,其教育思想和方法延续至今,历久弥新,影响着后辈职业青年。培黎职业学院根植于其生前创办的甘肃省山丹培黎学校,承载着习总书记的殷殷重托,实现健康永续发展是培黎人的政治担当和奋斗目标。研究艾黎职业教育思想对西部欠发达地区职业教育发展有着深远的历史和现实意义。

Modulation mechanism of electron energy dissipation on superlubricity based on fluorinated 2D ZIFs

Electron energy dissipation is an important energy dissipation pathway that cannot be ignored in friction process.Two-dimensional zeolite imidazole frameworks(2D ZIFs)and fluorine doping strategies give 2D Zn-ZIF and 2D Co-ZIF unique electrical properties,making them ideal materials for studying electron energy dissipation mechanism.In this paper,based on the superlubricity modulation of 2D fluoridated ZIFs,the optimal tribological properties are obtained on the 2D F-Co-ZIF surface,with the friction coefficient as low as 0.0010.Electrical experiments,density functional theory(DFT)simulation,and fluorescence detection are used to explain the mechanism of fluorine doping regulation of tribological properties from the two stages,namely energy transfer and energy release.Specifically,the energy will transfer into the friction system through the generation of electron–hole pairs under an external excitation,and release by radiation and non-radiation energy dissipation channels.Fluorination reduces energy transfer by altering the electronic properties and band structures of ZIFs,and slows down the charge transfer by enhancing the shielding efficiency,thus slowing the non-radiative energy dissipation rate during the energy release stage.Our insights not only help us better understand the role of fluorine doping in improving tribological properties,but also provide a new way to further explore the electron energy dissipation pathway during friction.

Impacts of electrochemical disinfection on the viability and structure of the microbiome in secondary effluent water

Electrochemical disinfection(ECD)is a promising disinfection technique for wastewater reclamation;however,the impacts of ECD on the microbiome in secondary effluent wastewater remain unknown.In this study,Propidium monoazide-qPCR(PMA-qPCR)and the plate count method were used to evaluate the inactivation performance,and the PMA-16S rRNA gene sequences of living cells were targeted to study the microbiome.A discrepancy was found between PMA-qPCR and the plate count method in the evaluation of cell count,with increases of 1.5 to 2.2 orders of magnitude in the disinfection rate after 150 s of disinfection.However,the cell count recovered and occasionally exceeded original levels within 3 d after disinfection.Biodiversity was suppressed after ECD,but the microbiome after 150 s disinfection retained a higher level of evenness and stability in the community with a median Shannon index(>3.7).Pathogenic bacteria remained high in relative abundance even after 150 s of 25 V disinfection,but the biofilm-forming population was effectively suppressed by ECD.The co-occurrence network revealed a centralized and fragile network as disinfection persisted,demonstrating the destabilizing effects of ECD on the microbiome.Functional pathways for cell membrane synthesis and organic compound degradation were enriched after ECD.The reaction of the microbiome after ECD was similar to other disinfection techniques in terms of community structure.

Electric field controlled superlubricity of fullerene-based host–guest assembly

Controlling friction by the electric field is a promising way to improve the tribological performance of a variety of movable mechanical systems.In this work,the assembly structure and microscale superlubricity of a host–guest assembly are effectively controlled by the electric field.With the help of the scanning tunneling microscopy(STM)technique,the host–guest assembly structures constructed by the co-assembly of fullerene derivative(Fluorene-C60)with macrocycles(4B2A and 3B2A)are explicitly characterized.Combined with density functional theory(DFT),the distinct different assembly behaviors of fullerene derivatives are revealed at different probe biases,which is attributed to the molecular polarity of the fullerene derivative.Through the control on the adsorption behavior,the friction coefficient of host–guest assembly is demonstrated to be controllable in the electric field by using atomic force microscopy(AFM).At positive probe bias,the friction coefficient of the host–guest assembly is significantly reduced and achieves superlubricity(μmin=0.0049).The efforts not only help us gain insight into the host–guest assembly mechanism controlled by the electric field,but also promote the further application of fullerene in micro-electro-mechanical systems(MEMS).

The genetic basis of the leafy seadragon's unique camouflage morphology and avenues for its efficient conservation derived from habitat modeling

The leafy seadragon certainly is among evolution’s most“beautiful and wonderful”species aptly named for its extraordinary camouflage mimicking its coastal seaweed habitat.However,limited information is known about the genetic basis of its phenotypes and conspicuous camouflage.Here,we revealed genomic signatures of rapid evolution and positive selection in core genes related to its camouflage,which allowed us to predict population dynamics for this species.Comparative genomic analysis revealed that seadragons have the smallest olfactory repertoires among all ray-finned fishes,suggesting adaptations to the highly specialized habitat.Other positively selected and rapidly evolving genes that serve in bone development and coloration are highly expressed in the leaf-like appendages,supporting a recent adaptive shift in camouflage appendage formation.Knock-out of bmp6 results in dysplastic intermuscular bones with a significantly reduced number in zebrafish,implying its important function in bone formation.Global climate change-induced loss of seagrass beds now severely threatens the continued existence of this enigmatic species.The leafy seadragon has a historically small population size likely due to its specific habitat requirements that further exacerbate its vulnerability to climate change.Therefore,taking climate change-induced range shifts into account while developing future protection strategies.

Achieving ultrafast superlubricity with layered double hydroxides

Layered double hydroxides(LDHs)have the potential to be superlubricated materials due to their strong adsorption effect and weak internal interaction.However,obtaining stable superlubricity during the ultrafast time(<10 s)is still a challenge.Here,we demonstrated macroscale superlubricity based on LDHs of multiple metal ions at high surface roughness,achieving superlow friction coefficients(0.006)and ultrafast wearing-in time(<7 s),which mainly originated from tribochemical reactions and the formation of nanostructured adsorption layers.Through cross-sectional analysis and density functional theory,we revealed the properties of the protective tribofilm to achieve ultrafast superlubricity.LDHs strongly adsorbed on the surface of the bearing steel,the sliding interface transformed into a heterogeneous interface between the polytetrafluoroethylene and LDH,leading to macroscale superlubricity.These findings demonstrate that tribochemical treatment of surfaces produces tribofilm that effectively reduces wearing-in time and promotes ultralow friction.

Adjustable superlubricity system using polyalkylene glycol with various acid aqueous solutions

Polyalkylene glycol(PAG)aqueous solutions have recently been demonstrated to exhibit an ultralow friction coefficient(COF,μ<0.01).However,the prolonged running-in period and low bearing capacity have limited its widespread application.In this study,we determined that the running-in period can be decreased by more than 75%when the pH value of the lubricant is controlled at 3 by introducing various acid solutions.Additionally,less time was required to realize stable superlubricity with inorganic acid at lower pH values.This was mainly attributed to the acceleration effect of hydrogen ions around the contact region.In case of PAG aqueous solution with organic acid,the wear loss between sliding solid surfaces was reduced,and thus the bearing pressure during the superlubricity period was significantly improved from approximately 30 to 160 MPa.Furthermore,the organic acid molecules were considered to form strong hydrogen bonds with PAG macromolecules and solid surfaces.This in turn strengthened the structure of the adsorption layers.The unique effect of different acids in aqueous polymer lubrication can potentially significantly aid in advancing the study of polymer tribology and broadening industrial applications.

Hierarchical self-assembled structure and frictional response of phthalocyanine molecules

Solid evidence is needed to demonstrate the effect of molecular orientation and structure on the frictional property of boundary lubricants.In this work,the frictional properties of phthalocyanine self-assembled monolayers(SAMs)with face-on(aromatic cores parallel to the substrate)and edge-on(aromatic cores stand on the substrate)orientations have been compared and the in situ structural variation of edge-on SAMs under frictional shear has been revealed by atomic force microscope(AFM).Face-on oriented SAMs show lower adhesion,lower friction,and stronger wear resistance,compared with edge-on oriented SAMs.Hierarchical structures of edge-on oriented SAMs have been revealed by frictional topography,which are consisted of nanoscale columns,micron-scale stripes,and centimeter-scale monolayer.The column structure deforms under increasing load force,leading to a stepwise friction force curve and a transition among three friction states(ordered friction,collapsed friction,and worn friction).The structural deformation depends on both the order degree and anisotropic stiffness of columns.Columns in phthalocyanine SAMs show a larger stiffness when shearing against molecular plane than shearing along the molecular plane.The presented study on the interfacial structure and frictional mechanism promisingly supports the designing of novel boundary lubricants and their application in engineering.

Molecular behaviors in thin film lubrication——Part one: Film formation for different polarities of molecules

There are three unsolved problems in thin film lubrication(TFL) since it was proposed 20 years ago, i.e., the determination of the type of molecules that can enter the contact region efficiently during sliding, the orientation of molecules in the contact region, and the effect of solid surfaces on the liquid molecular orientation in TFL. In order to answer the first two questions, an in situ measurement system comprising a self-designed Raman microscopy and relative optical interference intensity(ROⅡ) system was set up to study the molecular behaviors. A variety of binary mixtures were used as lubricants in the test, and the concentration distribution profile and orientation of the additive molecules in TFL were characterized. The molecular behavior was determined via a combination of shearing, confinement, and surface adsorption. Furthermore, the difference in molecular polarity resulted in different competing effect of surface adsorption and intermolecular interaction, the influence of which on molecular behavior was discussed. Polar additive molecules interacted with the steel surface and exhibited an enrichment effect in the Hertz contact region when added into a nonpolar base oil. No enrichment effect was observed for nonpolar molecules that were added into the nonpolar base oil and polar molecules added into polar base oil. The enrichment of additive molecules enhanced the film-forming ability of the lubricant and resulted in a reduction in the friction coefficient of up to 61%. The orderly arrangement of the additive molecules was another reason for the friction-reducing. A binary multilayer model was proposed to illuminate the molecular behavior in the TFL, and the model was supported by contrary experiment results in elastohydrodynamic lubrication. This research may aid in understanding the nanoscale lubrication mechanism in TFL and the development of novel liquid lubricants.

Superlubricity achieved with two-dimensional nano-additives to liquid lubricants

The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis.Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambient conditions,it is considered an effective solution to reduce unnecessary energy and material losses.However,certain practical problems such as low load-bearing pressure,dependence on hydrogen ions,and relatively long running-in processes still limit its widespread application.Two-dimensional(2D)nano-additives with ultrathin longitudinal dimensions can lower the shear resistance between sliding solid surfaces,and thus further optimize the applied conditions.In this review,the latest studies on 2D nano-additives with a combination of various water-based lubricants in the state of superlubricity are reported,typically including black phosphorus(BP),graphene oxide(GO),and layered double hydroxide.During the sliding process,composite lubricants effectively improved the load capacity(up to 600 MPa),reduced wear,and accelerated the running-in period(within 1,000 s)of the liquid superlubricity system.Both macromechanical experiments and microscopic tests are conducted to precisely analyze various interactions at the interfaces of the nano-additives and solid surfaces.These interactions can be described as tribochemical reactions,physical protection,and adsorption enhancement,and improved wear resistance.This review provides better guidance for applying 2D nanomaterials in liquid superlubricity systems.

Tribological properties of novel palygorskite nanoplatelets used as oil-based lubricant additives

Layered palygorskite(PAL),commonly called attapulgite,is a natural inorganic clay mineral composed of magnesium silicate.In this study,an aqueous miscible organic solvent treatment method is adopted to prepare molybdenum-dotted palygorskite(Amo-PMo)nanoplatelets,which greatly improved the specific surface area of PAL and the dispersion effect in an oil-based lubricant system.Their layered structure and size were confirmed using transmission electron microscopy(TEM)and atomic force microscopy.Following a tribological test lubricated with three additives(PAL,organic molybdenum(SN-Mo),and Amo-PMo),it was found that the sample of 0.5 wt%Amo-PMo exhibited the best tribological properties with a coefficient of friction of 0.09.Moreover,the resulting wear scar diameter and wear volume of the sliding ball surface were 63%and 49.6%of those lubricated with base oil,respectively.Its excellent lubricating performance and self-repairing ability were mainly attributed to the generated MoS2 adsorbed on the contact surfaces during the tribochemical reaction,thereby effectively preventing the direct collision between asperities on sliding solid surfaces.Thus,as-prepared Amo-PMo nanoplatelets show great potential as oil-based lubricant additives,and this study enriches the existing application of PAL in industry.

Tribological behavior of layered double hydroxides with various chemical compositions and morphologies as grease additives

The layered double hydroxide(LDH)is a kind of natural mineral,which can also be manually prepared.It has been practically applied in various fields due to its unique crystal structure and diversity of composition,size,and morphology.In this work,LDHs with different chemical compositions(Co^(2+),Mg^(2+),Zn^(2+),and Ni^(2+))and topographical features(flower‐like,spherical,and plate‐like)were successfully prepared by controlling the reaction conditions.Then,they were mechanically dispersed into base grease and their tribological properties were evaluated by a ball‐on‐disk tester under a contact pressure of 2.47 GPa.It was found that the variation of morphology,instead of chemical composition,had great influence on the tribological performance.The“flower‐like”LDH sample with high specific surface area(139 m^(2)/g)was demonstrated to show the best performance.With 1 wt%additive,the wear volume was only about 0.2%of that lubricated by base grease.The tribofilm with unique microscopic structure and uniform composition was derived from tribochemical reaction between LDH additives and sliding solid surfaces,effectively improving tribological properties of the lubrication system.This work provided the guidance for optimizing lubricant additives and held great potential in future applications.

Morphological residual convolutional neural network(M-RCNN)for intelligent recognition of wear particles from artificial joints

Finding the correct category of wear particles is important to understand the tribological behavior.However,manual identification is tedious and time-consuming.We here propose an automatic morphological residual convolutional neural network(M-RCNN),exploiting the residual knowledge and morphological priors between various particle types.We also employ data augmentation to prevent performance deterioration caused by the extremely imbalanced problem of class distribution.Experimental results indicate that our morphological priors are distinguishable and beneficial to largely boosting overall performance.M-RCNN demonstrates a much higher accuracy(0.940)than the deep residual network(0.845)and support vector machine(0.821).This work provides an effective solution for automatically identifying wear particles and can be a powerful tool to further analyze the failure mechanisms of artificial joints.

Dynamic Model and Motion Characteristics of an Underwater Glider with Manta-inspired Wings

An underwater glider with bionic wings controlled by two operating modes is proposed to perform a variety of marine exploration tasks.The system composition of the vehicle and the structural design of wings inspired by manta ray are presented.The bionic wings can keep outstretched or realize oscillating motions according to the operating modes of the vehicle.A universal dynamic model of the vehicle was derived from multibody theory.Gliding,sailing and steering motions were simulated based on the dynamic model to illustrate the dynamic behaviors of the vehicle under different types of propulsion techniques.The results obtained through simulated calculation are basically consistent with the experimental data,which indicate that the developed dynamic model is applicable to describe the motion characteristics of the vehicle.Experiments were conducted in coastal area to analyze the propulsive characteristics of the bionic wings and sea trials involving multifarious motions were carried out,the applicability of the vehicle in marine environment was verified.

Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

Polyacrylamide(PAAm)hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries:sphere-on-flat and flat-pin-on-flat.Irrespective of the contact geometry,the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of frictionμ(COF)compared to the crosslinked surfaces,even achieving superlubricity(μ<0.01).In general,a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds.However,after normalizing the friction force by the contact area,the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds,indicating that hydrogel friction is unaffected by the contact geometry at these speeds.At high sliding speeds,the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration.This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact,which presumably enhances the water supply in the contact,promotes rehydration,and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact.

沥青混凝土级配曲线的数值设计方法与对有效弹性模量影响的研究

沥青混凝土有效性能的预测和评估是其在道路工程与大坝防渗等应用中的关键问题.论文利用细观随机建模技术和有限元方法研究了级配曲线对沥青混凝土有效弹性模量的影响.首先,基于改进的蒙特卡洛法分别实现了服从工程筛分级配曲线或给定函数级配曲线的沥青混凝土细观随机模型的高效生成;然后,利用建立的模型分析了骨料的含量、弹性模量和试样尺寸对沥青混凝土的有效弹性模量的影响,并与实验结果进行对比,验证了所建模型的可靠性;从而,预测了在AC-13级配范围内级配曲线的凹凸程度,即粗细骨料的组成比例对沥青混凝土有效弹性模量的影响.结果表明,级配曲线的凹凸程度对其有效弹性模量影响存在较大差异,级配曲线的下凹(粗骨料占比增大)会导致有效弹性模量迅速增大,级配曲线的上凸(细骨料占比增大)也会引起有效弹性模量略有上升;另外提出的随机建模方法可实现对级配曲线,即对粗细骨料组成比例的定制设计.

Output feedback control for semi-Markovian jump systems with time-varying delay

This study investigates the problem of output feedback control for semi-Markovian jump systems(S-MJLSs)with time-varying delay.Due to the relaxed conditions on the stochastic process,the S-MJLSs are with time-varying transition rates and can describe a broader class of dynamical systems than the traditional Markovian jump linear systems.Specially,first,by using a novel free-matrix-based integral inequality(FMBII)including well-known integral inequalities as special cases,the H∞performance analysis and mode-dependent nonrational synthesis conditions for the underlying S-MJLSs are developed,respectively.Secondly,the conditions obtained in this paper are formulated in terms of linear matrix inequalities(LMIs),which can be efficiently solved via standard numerical software.Finally,numerical examples are presented to demonstrate the effectiveness and advantages of the theoretical results.

Macroscale superlubricity under ultrahigh contact pressure in the presence of layered double hydroxide nanosheets

It is difficult to achieve macroscale superlubricity under high contact pressures and high normal loads.Layered double hydroxide(LDH)nanoadditives were introduced into an ionic liquid alcohol solution(IL(as))with contact pressures up to 1.044 GPa,which resulted in a friction coefficient(COF)of 0.004 and a robust superlubricity state lasting for 2 h.Compared with the LDH particles(LDH-Ps)with ca.90-nm widths and 18-nm thickness,micron-scale LDH nanosheet(LDH-N)additives with ca.1.5-pm width and 6-nm thickness increased the load-bearing capacity by approximately three times during superlubricity.The lubricant film thickness and the ultrathin longitudinal dimension of the LDH-N additives did not influence the continuity of the fluid film on the contact surface.These improvements resulted from the protective adsorption layer and ion distribution formed on the contact interface,as revealed by detailed surface analyses and simulation studies.In particular,the sliding energy barrier and Bader charge calculation revealed that weak shear sliding between the nanosheet and the solid surface formed easily and the anions in the liquid adsorbed on the solid surface exhibited electrostatic repulsion forces,which generated stable tribological properties synergistically.This research provides a novel method for obtaining macroscale superlubricity for practical industrial applications.

Influence of functional groups on the self-assembly of liquid crystals

Functional groups in the molecule play an important role in the molecular o rganization process.To reveal the influence of functional groups on the self-assembly at interface,herein,the self-assembly structures of three liquid crystal molecules,which only differ in the functional groups,are explicitly characterized by using scanning tunneling microscopy(STM).The high-resolution STM images demonstrate the difference between the supramolecular assembly structures of three liquid crystal molecules,which attribute to the hydrogen bonding interaction andπ-πstacking interaction between different functional groups.The density functional theory(DFT)results also confirm the influence of these functional groups on the self-assemblies.The effort on the self-assembly of liquid crystal molecules at interface could enhance the understanding of the supramolecular assembly mechanism and benefit the further application of liquid crystals.