The mechanism for the formation of double-layer vertically aligned carbon nanotube arrays(VACNTs) through single-step CVD growth is investigated. The evolution of the structures and defect concentration of the VACNTs ...The mechanism for the formation of double-layer vertically aligned carbon nanotube arrays(VACNTs) through single-step CVD growth is investigated. The evolution of the structures and defect concentration of the VACNTs are tracked by scanning electron microscopy(SEM) and Raman spectroscopy. During the growth, the catalyst particles are stayed constantly on the substrate. The precipitation of the second CNT layer happens at around 30 min as proved by SEM.During the growth of the first layer, catalyst nanoparticles are deactivated with the accumulation of amorphous carbon coatings on their surfaces, which leads to the termination of the growth of the first layer CNTs. Then, the catalyst particles are reactivated by the hydrogen in the gas flow, leading to the precipitation of the second CNT layer. The growth of the second CNT layer lifts the amorphous carbon coatings on catalyst particles and substrates. The release of mechanical energy by CNTs provides big enough energy to lift up amorphous carbon flakes on catalyst particles and substrates which finally stay at the interfaces of the two layers simulated by finite element analysis. This study sheds light on the termination mechanism of CNTs during CVD process.展开更多
Volumetric elastic modulus （VEM） is an important parameter in biophysics and biomechanics of plants for in particular understanding cell growth. This paper proposes a new relation that can be used for precisely dete...Volumetric elastic modulus （VEM） is an important parameter in biophysics and biomechanics of plants for in particular understanding cell growth. This paper proposes a new relation that can be used for precisely determining VEM. With the aid of this relation, it shows that the exponential approximation of the pressure-volume relationship adopted in most of the literatures in this field may lead to serious errors on VEM.展开更多
Nano-particle capture is a key process in filtration, separation, and biomedical applications. Here we explored the mechanisms of soft particle capture using nanofiber networks. We identified possible states of the ca...Nano-particle capture is a key process in filtration, separation, and biomedical applications. Here we explored the mechanisms of soft particle capture using nanofiber networks. We identified possible states of the capture process, which are defined by their structural and material parameters. By performing numerical analysis, we provided a phase diagram in the parametric space of the network structure and interracial adhesion. The work provides a conceptual model for rational design of synthetic materials in related applications that focus on the protection against or removal of virus, as well as other soft particles.展开更多
Structural superlubricity(SSL)refers to a state where the friction and wear between two directly contacted solid surfaces are virtually zero.The realization of microscale SSL in 2012 rapidly explored SSL technologies ...Structural superlubricity(SSL)refers to a state where the friction and wear between two directly contacted solid surfaces are virtually zero.The realization of microscale SSL in 2012 rapidly explored SSL technologies which hold great potential in the development of reliable and energy⁃efficient micro devices.A key to a successful superlubric device is to control the movements of the superlubric slider.To solve this challenge,here two general principles are shown to guide and control the motion of the slider,i.e.,by minimization of interfacial energy and minimization of electrostatic energy.When the shapes of the slider and substrate are designed appropriately,the excess interfacial energy of the contact⁃pair provides restoring and constraining forces to the slider.Similarly,tunable driving and constraining forces are enabled by the electric fields induced by the electrodes buried in the substrate.These concepts are demonstrated on the design of a superlubric resonator whose natural frequency of the lateral translational mode is well⁃defined and unfavorable rotation is constrained.The above design principles should be applicable to superlubric devices in general and help the development of future applications of structural superlubricity.展开更多
One-dimensional nanostructured materials are often used as beams in many applications such as ultrahigh-frequency resonators and ultrasensitive sensors.Compared with usual macroscopic beams,nanobeams have much higher ...One-dimensional nanostructured materials are often used as beams in many applications such as ultrahigh-frequency resonators and ultrasensitive sensors.Compared with usual macroscopic beams,nanobeams have much higher surface/volume ratios so that their surface energies may play a significant role.Besides,they often bear large deflections due to their typically large slenderness ratios and larger elastic ranges.There is,however,lack of a theory that takes into account of both the above two features owned by nanobeams.In this paper,we present such a theory and give applied examples to show that surface energy and large deflection may individually or jointly have notable effects.展开更多
Superlubricity,or structural lubricity,is a state that has two contacting surfaces exhibiting no resistance to sliding.This effect has been theoretically described to be possible between two completely clean single cr...Superlubricity,or structural lubricity,is a state that has two contacting surfaces exhibiting no resistance to sliding.This effect has been theoretically described to be possible between two completely clean single crystalline solid surfaces.However,experimental observations of superlubricity were limited to nanoscale and under high vacuum or inert gas environments even after twenty years since the concept of superlubricity has been suggested in 1990.In the last two years,remarkable advances have been achieved in experimental observations of superlubricity ranging from micro-scale to centimeters and in ambient environment.This study aims to report a comprehensive understanding of the superlubricity phenomenon.展开更多
One of the promising approaches to achieving large scale superlubricity is the use of junctions between existing ultra-flat surface together with superlubric graphite mesas.Here we studied the frictional properties of...One of the promising approaches to achieving large scale superlubricity is the use of junctions between existing ultra-flat surface together with superlubric graphite mesas.Here we studied the frictional properties of microscale graphite mesa sliding on the diamond-like carbon,a commercially available material with a ultra-flat surface.The interface is composed of a single crystalline graphene and a diamond-like carbon surface with roughness less than I nm.Using an integrated approach,which includes Argon plasma irradiation of diamond-like carbon surfaces,X-ray photoelectron spectroscopy analysis and Langmuir adsorption modeling,we found that while the velocity dependence of friction follows a thermally activated sliding mechanism,its temperature dependence is due to the desorption of chemical groups upon heating.These observations indicate that the edges have a significant contribution to the friction.Our results highlight potential factors affecting this type of emerging friction junctions and provide a novel approach for tuning their friction properties through ion irradiation.展开更多
The past three decades have witnessed the explosion of nanoscience and technology, where notable research eftbrts have been made in synthesizing nanomaterials and controlling nanostructures of bulk materials. The unco...The past three decades have witnessed the explosion of nanoscience and technology, where notable research eftbrts have been made in synthesizing nanomaterials and controlling nanostructures of bulk materials. The uncovered mechanical behaviors of structures and materials with reduced sizes and dimensions pose open questions to the community of mechanicians, which expand the framework of continuum mechanics by advancing the theory, as well as modeling and experimental tools. Researchers in China have been actively involved into this exciting area, making remarkable contributions to the understanding of nanoscale mechanical processes, the development of multi-scale, multi-field modeling and experimental techniques to resolve the processing-microstructures-properties relationship of materials, and the interdisciplinary studies that broaden the subjects of mechanics. This article reviews selected progress made by this community, with the aim to clarify the key concepts, methods and applications of micro- and nano-mechanics, and to outline the perspectives in this fast-evolving field.展开更多
Smart textiles are attracting great interest.Particularly,air-conditioning textiles are highly desired for their merits in energy conservation and personal temperature/humidity management.Currently,air-conditioning te...Smart textiles are attracting great interest.Particularly,air-conditioning textiles are highly desired for their merits in energy conservation and personal temperature/humidity management.Currently,air-conditioning textiles can be fabricated by two strategies.One uses infrared-radiation-adaptive materials,and the other uses moisture-responsive actuators that can regulate temperature and humidity simultaneously.Here,the fabrication of a silk-yarn switch comprising electrospun highly aligned nanofibers is reported and its application in air-conditioning textiles is demonstrated.Silk yarn rotates in contact with liquid,and can be recovered by drying.The different responses and wetting behaviors of the switch to H_(2)O and C_(2)H_(6)O is investigated.It is argued that alignment and surface hydrophilicity of nanofibers play important roles in this term.To elaborate,actuating trait is mainly controlled by reduction of the surface free energy of aligned silk nanofibers,during the wetting process.As proof of concept,the application of the sweat-driven silk-yarn switch in regulating the temperature/humidity of the human body is demonstrated in this work.Considering the large production,versatile processibility,and good biocompatibility,silk actuator may have practical applications in designing smart switches(or valves)for intelligent textiles,artificial muscles,and other application scenarios.展开更多
When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the CassieBaxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a cr...When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the CassieBaxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a critical value. It has been assumed that the reverse transition(Wenzel-to-Cassie-Baxter wetting state) cannot happen spontaneously after the pressure has been removed.In this paper, we report a new wetting-state transition. When external pressure is exerted on a droplet in the Cassie-Baxter wetting state on textured surfaces with high micropillars to trigger the breakdown of this wetting state, the droplet penetrates the micropillars but does not touch the base of the surface to trigger the occurrence of the Wenzel wetting state. We have named this state the suspended penetration wetting state. Spontaneous recovery from the suspended penetration wetting state to the initial Cassie-Baxter wetting state is achieved when the pressure is removed. Based on the experimental results, we built models to establish the penetration depth that the suspended penetration wetting state could achieve and to understand the energy barrier that influences the equilibrium position of the liquid surface. These results deepen our understanding of wetting states on rough surfaces subjected to external disturbances and shed new light on the design of superhydrophobic materials with a robust wetting stability.展开更多
The presence of a capillary bridge between solid surfaces is ubiquitous under ambient conditions.Usually,it leads to a continuous decrease of friction as a function of bridge height.Here,using molecular dynamics we sh...The presence of a capillary bridge between solid surfaces is ubiquitous under ambient conditions.Usually,it leads to a continuous decrease of friction as a function of bridge height.Here,using molecular dynamics we show that for a capillary bridge with a small radius confined between two hydrophilic elastic solid surfaces,the friction oscillates greatly when decreasing the bridge height.The underlying mechanism is revealed to be a periodic ordered-disordered transition at the liquid–solid interfaces.This transition is caused by the balance between the surface tension of the liquid–vapor interface and the elasticity of the surface.This balance introduces a critical size below which the friction oscillates.Based on the mechanism revealed,a parameter-free analytical model for the oscillating friction was derived and found to be in excellent agreement with the simulation results.Our results describe an interesting frictional phenomenon at the nanoscale,which is most prominent for layered materials.展开更多
基金supported by NSFC(51422204,51372132)National Basic Research Program of China(2013CB934200)+2 种基金SRFDP(20120002120038)TNLIST Cross-discipline FoundationBNLMS Cross-discipline Foundation
文摘The mechanism for the formation of double-layer vertically aligned carbon nanotube arrays(VACNTs) through single-step CVD growth is investigated. The evolution of the structures and defect concentration of the VACNTs are tracked by scanning electron microscopy(SEM) and Raman spectroscopy. During the growth, the catalyst particles are stayed constantly on the substrate. The precipitation of the second CNT layer happens at around 30 min as proved by SEM.During the growth of the first layer, catalyst nanoparticles are deactivated with the accumulation of amorphous carbon coatings on their surfaces, which leads to the termination of the growth of the first layer CNTs. Then, the catalyst particles are reactivated by the hydrogen in the gas flow, leading to the precipitation of the second CNT layer. The growth of the second CNT layer lifts the amorphous carbon coatings on catalyst particles and substrates. The release of mechanical energy by CNTs provides big enough energy to lift up amorphous carbon flakes on catalyst particles and substrates which finally stay at the interfaces of the two layers simulated by finite element analysis. This study sheds light on the termination mechanism of CNTs during CVD process.
基金supported by the National Natural Science Foundation of China(10772100)
文摘Volumetric elastic modulus （VEM） is an important parameter in biophysics and biomechanics of plants for in particular understanding cell growth. This paper proposes a new relation that can be used for precisely determining VEM. With the aid of this relation, it shows that the exponential approximation of the pressure-volume relationship adopted in most of the literatures in this field may lead to serious errors on VEM.
基金supported by the Boeing Company,the National Natural Science Foundation of China (11222217 and 11002079)Tsinghua University Initiative Scientific Research Program (2011Z02174)the Tsinghua National Laboratory for Information Science and Technology of China
文摘Nano-particle capture is a key process in filtration, separation, and biomedical applications. Here we explored the mechanisms of soft particle capture using nanofiber networks. We identified possible states of the capture process, which are defined by their structural and material parameters. By performing numerical analysis, we provided a phase diagram in the parametric space of the network structure and interracial adhesion. The work provides a conceptual model for rational design of synthetic materials in related applications that focus on the protection against or removal of virus, as well as other soft particles.
基金National Natural Science Foundation of China(Grant Nos.11572173,11890671,51961145304 and 11921002)the National Key Basic Research Program of China(Grant No.2013CB934200)+3 种基金the Cyrus Tang Foundation(Grant No.202003)the Beijing Municipal Science&Technology Commission(Grant No.Z151100003315008)the Tsinghua University Initiative Scientific Research(Grant Nos.2014Z01007 and 2012Z01015)the State Key Laboratory of Tribology Tsinghua University Initiative Scientific Research(Grant No.SKLT2019D02).
文摘Structural superlubricity(SSL)refers to a state where the friction and wear between two directly contacted solid surfaces are virtually zero.The realization of microscale SSL in 2012 rapidly explored SSL technologies which hold great potential in the development of reliable and energy⁃efficient micro devices.A key to a successful superlubric device is to control the movements of the superlubric slider.To solve this challenge,here two general principles are shown to guide and control the motion of the slider,i.e.,by minimization of interfacial energy and minimization of electrostatic energy.When the shapes of the slider and substrate are designed appropriately,the excess interfacial energy of the contact⁃pair provides restoring and constraining forces to the slider.Similarly,tunable driving and constraining forces are enabled by the electric fields induced by the electrodes buried in the substrate.These concepts are demonstrated on the design of a superlubric resonator whose natural frequency of the lateral translational mode is well⁃defined and unfavorable rotation is constrained.The above design principles should be applicable to superlubric devices in general and help the development of future applications of structural superlubricity.
基金Project supported by the National Natural Science Foundation of China(Nos.10832005,10732080,and 10572071)NSFC/RGC(No.50518003)+2 种基金the Ministry of Science and Technology(grant No.2008AA03Z302)the Ministry of Education(grant No.20070003053)the National Basic Research Program of China(grant Nos.2007CB936803 and2004CB619304)
文摘One-dimensional nanostructured materials are often used as beams in many applications such as ultrahigh-frequency resonators and ultrasensitive sensors.Compared with usual macroscopic beams,nanobeams have much higher surface/volume ratios so that their surface energies may play a significant role.Besides,they often bear large deflections due to their typically large slenderness ratios and larger elastic ranges.There is,however,lack of a theory that takes into account of both the above two features owned by nanobeams.In this paper,we present such a theory and give applied examples to show that surface energy and large deflection may individually or jointly have notable effects.
基金supports from the National Natural Science Foundation of China(NSFC)(Grant No.10832005)the the National Key Basic Research(973)Program of China(Grants Nos.2007CB936803 and 2013CB934200)are acknowledged.
文摘Superlubricity,or structural lubricity,is a state that has two contacting surfaces exhibiting no resistance to sliding.This effect has been theoretically described to be possible between two completely clean single crystalline solid surfaces.However,experimental observations of superlubricity were limited to nanoscale and under high vacuum or inert gas environments even after twenty years since the concept of superlubricity has been suggested in 1990.In the last two years,remarkable advances have been achieved in experimental observations of superlubricity ranging from micro-scale to centimeters and in ambient environment.This study aims to report a comprehensive understanding of the superlubricity phenomenon.
基金Wengen Ouyang acknowledges the financial support from a fellowship program for outstanding postdoctoral researchers from China and India in Israeli Universites.Ming Ma wishes to acknowledge the financial support by Thousand Young Talents Program and the NSFC grant Nos.11632009,11772168,and 11890673Quanshui Zheng wishes to acknowledge the financial support by the NSFC grant No.11890671.
文摘One of the promising approaches to achieving large scale superlubricity is the use of junctions between existing ultra-flat surface together with superlubric graphite mesas.Here we studied the frictional properties of microscale graphite mesa sliding on the diamond-like carbon,a commercially available material with a ultra-flat surface.The interface is composed of a single crystalline graphene and a diamond-like carbon surface with roughness less than I nm.Using an integrated approach,which includes Argon plasma irradiation of diamond-like carbon surfaces,X-ray photoelectron spectroscopy analysis and Langmuir adsorption modeling,we found that while the velocity dependence of friction follows a thermally activated sliding mechanism,its temperature dependence is due to the desorption of chemical groups upon heating.These observations indicate that the edges have a significant contribution to the friction.Our results highlight potential factors affecting this type of emerging friction junctions and provide a novel approach for tuning their friction properties through ion irradiation.
基金supported by the National Natural Science Foundation of China (Grant No. 11472150)
文摘The past three decades have witnessed the explosion of nanoscience and technology, where notable research eftbrts have been made in synthesizing nanomaterials and controlling nanostructures of bulk materials. The uncovered mechanical behaviors of structures and materials with reduced sizes and dimensions pose open questions to the community of mechanicians, which expand the framework of continuum mechanics by advancing the theory, as well as modeling and experimental tools. Researchers in China have been actively involved into this exciting area, making remarkable contributions to the understanding of nanoscale mechanical processes, the development of multi-scale, multi-field modeling and experimental techniques to resolve the processing-microstructures-properties relationship of materials, and the interdisciplinary studies that broaden the subjects of mechanics. This article reviews selected progress made by this community, with the aim to clarify the key concepts, methods and applications of micro- and nano-mechanics, and to outline the perspectives in this fast-evolving field.
基金This work was supported by the NSF of China(51672153,51422204,21975141)the National Key Basic Research and Development Program(No.2016YFA0200103)the National Program for Support of Top-notch Young Professionals.
文摘Smart textiles are attracting great interest.Particularly,air-conditioning textiles are highly desired for their merits in energy conservation and personal temperature/humidity management.Currently,air-conditioning textiles can be fabricated by two strategies.One uses infrared-radiation-adaptive materials,and the other uses moisture-responsive actuators that can regulate temperature and humidity simultaneously.Here,the fabrication of a silk-yarn switch comprising electrospun highly aligned nanofibers is reported and its application in air-conditioning textiles is demonstrated.Silk yarn rotates in contact with liquid,and can be recovered by drying.The different responses and wetting behaviors of the switch to H_(2)O and C_(2)H_(6)O is investigated.It is argued that alignment and surface hydrophilicity of nanofibers play important roles in this term.To elaborate,actuating trait is mainly controlled by reduction of the surface free energy of aligned silk nanofibers,during the wetting process.As proof of concept,the application of the sweat-driven silk-yarn switch in regulating the temperature/humidity of the human body is demonstrated in this work.Considering the large production,versatile processibility,and good biocompatibility,silk actuator may have practical applications in designing smart switches(or valves)for intelligent textiles,artificial muscles,and other application scenarios.
基金supported by the National Natural Science Foundation of China (Grant Nos.11632009,and 11872227)。
文摘When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the CassieBaxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a critical value. It has been assumed that the reverse transition(Wenzel-to-Cassie-Baxter wetting state) cannot happen spontaneously after the pressure has been removed.In this paper, we report a new wetting-state transition. When external pressure is exerted on a droplet in the Cassie-Baxter wetting state on textured surfaces with high micropillars to trigger the breakdown of this wetting state, the droplet penetrates the micropillars but does not touch the base of the surface to trigger the occurrence of the Wenzel wetting state. We have named this state the suspended penetration wetting state. Spontaneous recovery from the suspended penetration wetting state to the initial Cassie-Baxter wetting state is achieved when the pressure is removed. Based on the experimental results, we built models to establish the penetration depth that the suspended penetration wetting state could achieve and to understand the energy barrier that influences the equilibrium position of the liquid surface. These results deepen our understanding of wetting states on rough surfaces subjected to external disturbances and shed new light on the design of superhydrophobic materials with a robust wetting stability.
文摘The presence of a capillary bridge between solid surfaces is ubiquitous under ambient conditions.Usually,it leads to a continuous decrease of friction as a function of bridge height.Here,using molecular dynamics we show that for a capillary bridge with a small radius confined between two hydrophilic elastic solid surfaces,the friction oscillates greatly when decreasing the bridge height.The underlying mechanism is revealed to be a periodic ordered-disordered transition at the liquid–solid interfaces.This transition is caused by the balance between the surface tension of the liquid–vapor interface and the elasticity of the surface.This balance introduces a critical size below which the friction oscillates.Based on the mechanism revealed,a parameter-free analytical model for the oscillating friction was derived and found to be in excellent agreement with the simulation results.Our results describe an interesting frictional phenomenon at the nanoscale,which is most prominent for layered materials.
基金This work was supported by the National Natural Science Foundation of China (Nos.21203107,51422204,and 51372132) and the National Basic Research Program of China (No.2013CB228506).
