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Bifurcation in most probable phase portraits for a bistable kinetic model with coupling Gaussian and non-Gaussian noises

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Applied Mathematics and Mechanics(English Edition) 2021年第12期42卷 1759-1770页

作者： Mengjiao HUA Yu WU Department of Engineering Mechanics Zhejiang UniversityHangzhou 310027China key laboratory of soft machines and smart devices of zhejiang province Zhejiang UniversityHangzhou 310027China soft Matter Research Center Zhejiang UniversityHangzhou 310027China State key laboratory of Fluid Power and Mechatronic Systems Zhejiang UniversityHangzhou 310027China

The phenomenon of stochastic bifurcation driven by the correlated non-Gaussian colored noise and the Gaussian white noise is investigated by the qualitative changes of steady states with the most probable phase portraits.To arrive at the Markovian approximation of the original non-Markovian stochastic process and derive the general approximate Fokker-Planck equation(FPE),we deal with the non-Gaussian colored noise and then adopt the unified colored noise approximation(UCNA).Subsequently,the theoretical equation concerning the most probable steady states is obtained by the maximum of the stationary probability density function(SPDF).The parameter of the uncorrelated additive noise intensity does enter the governing equation as a non-Markovian effect,which is in contrast to that of the uncorrelated Gaussian white noise case,where the parameter is absent from the governing equation,i.e.,the most probable steady states are mainly controlled by the uncorrelated multiplicative noise.Additionally,in comparison with the deterministic counterpart,some peculiar bifurcation behaviors with regard to the most probable steady states induced by the correlation time of non-Gaussian colored noise,the noise intensity,and the non-Gaussian noise deviation parameter are discussed.Moreover,the symmetry of the stochastic bifurcation diagrams is destroyed when the correlation between noises is concerned.Furthermore,the feasibility and accuracy of the analytical predictions are verified compared with those of the Monte Carlo(MC)simulations of the original system.

关键词： stochastic bifurcation non-Gaussian colored noise most probable steady state Fokker-Planck equation(FPE) Monte Carlo(MC)simulation

Functional hydrogel coatings

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National Science Review 2021年第2期8卷 151-169页

作者： Junjie Liu Shaoxing Qu Zhigang Suo Wei Yang Center for X-Mechanics Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province and Department of Engineering MechanicsZhejiang University State key laboratory of Fluid Power and Mechatronic System Zhejiang University Applied Mechanics and Structure Safety key laboratory of Sichuan province School of Mechanics and EngineeringSouthwest Jiaotong University John A.Paulson School of Engineering and Applied Sciences Kavli Institute for Bionano Science and TechnologyHarvard University

Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically,chemically and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently,functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity,biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness,toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings:(i) applications and functions enabled by hydrogel coatings,(ii) methods of coating various substrates with different functional hydrogels with tough adhesion, and(iii) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.

关键词： hydrogel coatings coating methods coating tests adhesion hydrogel applications

Robustly Tuning Bandgaps in Two-Dimensional soft Phononic Crystals with Criss-Crossed Elliptical Holes

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Acta Mechanica Solida Sinica 2018年第5期31卷 573-588页

作者： Nan Gao Yi-lan Huang Rong-hao Bao Wei-qiu Chen Department of Engineering Mechanics Zhejiang University Hangzhou 310027 China State key Lab of CAD & CG Zhejiang University Hangzhou 310058 China key laboratory of soft machines and smart devices of zhejiang province Zhejiang University Hangzhou 310027 China soft Matter Research Center Zhejiang University Hangzhou 310027 China

Tuning band gaps in soft materials by post-buckling deformation is becoming an appealing means to manipulate elastic waves. As one of the most favorable topologies, two- dimensional soft structures with circular holes have been extensively studied. Based on the contrarian thinking, this paper starts from the two-dimensional soft structures with criss-crossed elliptical holes, which is close to the post-buckling configuration of soft structures with circular holes, and then proposes to tune the band gaps through elongating or stretching rather than compressing. Influences of the loading magnitude and loading pattern （i.e., uniaxial and biaxial elongations） on the band gaps are studied via the nonlinear finite element simulations. Effects of the geometric parameters （the major-to-minor half-axis ratio and the porosity of the structure） are also discussed. It is shown that, compared with the traditional circular hole case, the band gaps of the unloaded structure with criss-crossed elliptical holes are much richer, and they could be reversely and continuously tuned by tensile loadings. In particular, the deformation is very robust and is insensitive to small geometric imperfections, which is always necessary for triggering the post-buckling deformations. The present work provides a useful reference to the manipula- tion of elastic waves in periodic structures as well as the design of soft phononic crystals/acoustic devices.

关键词： soft phononic crystal Criss-crossed elliptical holes Band gap Tunability

Extension/Compression-Controlled Complete Band Gaps in 2D Chiral Square-Lattice-Like Structures

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Acta Mechanica Solida Sinica 2018年第1期31卷 51-65页

作者： Y. L. Huang N. Gao W.Q. Chen R.H. Bao Department of Engineering Mechanics Zhejiang University Hangzhou 310027 China State key Lab of CAD & CG Zhejiang University Hangzhou 310058 China key laboratory of soft machines and smart devices of zhejiang province Zhejiang University Hangzhou 310027 China soft Matter Research Center Zhejiang University Hangzhou 310027 China

Achieving tunable band gaps in a structure by external stimuli is of great importance in acoustic applications. This paper aims to investigate the tunability of band gaps in square-lattice-like elastic periodic structures that are usually not featured with notable band gaps. Endowed with chirality, the periodic structures here are able to undergo imperfection-insensitive large deformation under extension or compression. The influences of geometric parameters on band gaps are discussed via the nonlinear finite element method. It is shown that the band gaps in such structures with curved beams can be very rich and, more importantly, can be efficiently and robustly tuned by applying appropriate mechanical loadings without inducing buckling. As expected, geometry plays a more significant role than material nonlinearity does in the evolution of band gaps. The dynamic tunability of band gaps through mechanical loading is further studied. Results show that closing, opening, and shifting of band gaps can be realized by exerting real-time global extension or compression on the structure. The proposed periodic structure with well-designed chiral symmetry can be useful in the design of particular acoustic devices.

关键词： Band gap Tunability Square-lattice-like periodic structure Chiral symmetry,Large deformation

Influences of Environmental Motion Modes on the Efficiency of Ultrathin Flexible Piezoelec trie Energy Harvesters

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Acta Mechanica Solida Sinica 2019年第5期32卷 611-620页

作者： Chaofeng Lii Yangyang Zhang He Zhang Zhicheng Zhang Yisheng Chen College of Civil Engineering and Architecture Zhejiang University310058 HangzhouPeople Republic of China key laboratory of soft machines and smart devices of zhejiang province Zhejiang University310027 HangzhouPeople's Republic of China soft Matter Research Center Zhejiang University310027 HangzhouPeople's Republic of China

Harvesting energy from ambient vibration sources with ultrathin flexible piezoelectric energy harvesters(PEHs)for battery-free electronics has received attention in recent years.However,the excitation modes in the environment and human body motion are more complicated than the ideal harmonic modes employed in previous theoretical analyses,and their influence on the efficiency of PEHs has received little attention.In this study,these environmental excitation modes are classified into three types,i.e.,the triangular,sinusoidal,and square wave modes,with varied duty ratios.We derived theoretically the output power of flexible PEHs under these excitation modes and establish a simple scaling law,in which the normalized output power depends only on two combined normalized parameters,i.e.,the intrinsic system parameter and the excitation mode.Results reveal that the output power of PEHs changes dramatically for different excitation modes with varied duty ratios even when all the other parameters including excitation ampli tude,excitation frequency,elec trical parameter,and geometrical and material parameters of the PEHs are identical.This paper may provide a systematic understanding in the effect of exc计ation modes on the output power of flexible PEHs and promote the realization of energy harvesting from the complex environmental and human body motions.

关键词： Flexible piezoelec trie energy harves ter Environmental excitation modes Energy harvesting efficiency Theoretical predictions Scaling law

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

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Theoretical & Applied Mechanics Letters 2021年第6期11卷 375-379页

作者： Hongyu Luo Chenglong Li Chuanqian Shi Shuang Nie Jizhou Song Department of Engineering Mechanics Soft Matter Research Centerand Key Laboratory of Soft Machines and Smart Devices of Zhejiang ProvinceZhejiang UniversityHangzhou 310027China State key laboratory of Fluid Power and Mechatronic Systems Zhejiang UniversityHangzhou 310027China

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

关键词： Switchable dry adhesive Shape memory polymer Transfer printing

The effect of water quality on tip vortex cavitation inception

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Journal of Hydrodynamics 2017年第6期29卷 954-961页

作者：张凌新陈林娅彭晓星邵雪明 State key laboratory of Fluid Power and Mechatronic System Department of MechanicsZhejiang UniversityHangzhou 31002 7China key laboratory of soft machines and smart devices of zhejiang province Hangzhou 310027China National key laboratory on Ship Vibration and Noisel China Ship Scientific Research Center Wuxi 214082China

The inception of tip vortex cavitation is very sensitive to water quality. In order to quantify the effect of water quality on the inception of tip vortex cavitation, we develop a motion model to describe the migration and growth of nuclei in water. An analytical solution of migration of nuclei in a vortex flow is obtained so that the capture times of various nuclei can be given out directly. A criterion is built to determine the critical nucleus in a certain nuclei spectra distribution. Tensile strength of the critical nucleus is used to quantify the effect of water quality and correct the tip vortex cavitation inception number. Finally this change of cavitation inception number is compared with experimental results to validate our model.

关键词： Water quality cavitation inception capture time tensile strength

Molecular dynamics study on the dependence of thermal conductivity on size and strain in GaN nanofilms

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Chinese Physics B 2023年第6期32卷 454-458页

作者：唐莹刘俊坤于子皓孙李刚朱林利 College of Optics and Electronics Technology China Jiliang UniversityHangzhou 310018China key laboratory of soft machines and smart devices of zhejiang province School of Aeronautics and AstronauticsZhejiang UniversityHangzhou 310027China School of Science Harbin Institute of TechnologyShenzhen 518055China

The thermal conductivity of GaN nanofilm is simulated by using the molecular dynamics(MD)method to explore the influence of the nanofilm thickness and the pre-strain field under different temperatures.It is demonstrated that the thermal conductivity of GaN nanofilm increases with the increase of nanofilm thickness,while decreases with the increase of temperature.Meanwhile,the thermal conductivity of strained GaN nanofilms is weakened with increasing the tensile strain.The film thickness and environment temperature can affect the strain effect on the thermal conductivity of GaN nanofilms.In addition,the analysis of phonon properties of GaN nanofilm shows that the phonon dispersion and density of states of GaN nanofilms can be significantly modified by the film thickness and strain.The results in this work can provide the theoretical supports for regulating the thermal properties of GaN nanofilm through tailoring the geometric size and strain engineering.

关键词： molecular dynamics simulation GaN nanofilm thermal conductivity phonon properties size effect strain effect

Controllable Bending of Bi-hydrogel Strips with Differ ent ial Swelling

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Acta Mechanica Solida Sinica 2019年第5期32卷 652-662页

作者： Yi Wu Xingpeng Hao Rui Xiao Ji Lin Zi Liang Wu Jun Yin Jin Qian key laboratory of soft machines and smart devices of zhejiang province Department of Engineering MechanicsZhejiang UniversityHangzhou 310027China MOE key laboratory of Macromolecular Synthesis and Functionalization Department of Polymer Science and EngineeringZhejiang UniversityHangzhou 310027China State key laboratory of Fluid Power and Mechatronic Systems Key Laboratory of 3D Printing Process and Equipment of Zhejiang ProvinceSchool of Mechanical EngineeringZhejiang UniversityHangzhou 310028China

Bi-hydrogel strips consisting of a highly swelling layer and a non-swelling layer are investigated in this study,in which the differential swelling capabilities and mismatch strain in the composite structure drive the strips into a bending configuration when being exposed to solvents.A modified analytical model originated from Timoshenko theory on bi-material beam is proposed to predict the swelling-induced bending shape of the bi-hydrogel strips,with explicit relations of bending curvature and mid-span deflection versus mismatch strain.Different sets of experiments were performed to verify the analytical model and its solutions,in which the swellinginduced mismatch strain was systematically tuned by the selection of saline concentration.The broad agreements bet ween the t heore tical and experimen tai results indicate that the modified analytical model can be used to guide the design of morphing elements by modulating layer-bylayer mismatches in gel composites to produce precise and complex shape transformations for practical applications.

关键词： Bi-hydrogel strip Hydrogel swelling Mismatch strain Shape morphing Analytical model