Healing substrates with mobile, particle-filled microcapsules: designing a ‘repair and go’ system

We model the rolling motion of a fluid-driven, particle-filled microcapsule along a heterogeneous, adhesive substrate to determine how the release of the encapsulated nanoparticles can be harnessed to repair damage on the underlying surface. We integrate the lattice Boltzmann model for hydrodynamics and the lattice spring model for the micromechanics of elastic solids to capture the interactions between the elastic shell of the microcapsule and the surrounding fluids. A Brownian dynamics model is used to simulate the release of nanoparticles from the capsule and their diffusion into the surrounding solution. We focus on a substrate that contains a damaged region (e.g. a crack or eroded surface coating), which prevents the otherwise mobile capsule from rolling along the surface. We isolate conditions where nanoparticles released from the arrested capsule can repair the damage and thereby enable the capsules to again move along the substrate. Through these studies, we establish guidelines for designing particle-filled microcapsules that perform a ‘repair and go’ function and thus, can be utilized to repair damage in microchannels and microfluidic devices.     

Probing and repairing damaged surfaces with nanoparticle-containing microcapsules

Nanoparticles have useful properties, but it is often important that they only start working after they are placed in a desired location. The encapsulation of nanoparticles allows their function to be preserved until they are released at a specific time or location, and this has been exploited in the development of self-healing materials and in applications such as drug delivery. Encapsulation has also been used to stabilize and control the release of substances, including flavours, fragrances and pesticides. We recently proposed a new technique for the repair of surfaces called 'repair-and-go'. In this approach, a flexible microcapsule filled with a solution of nanoparticles rolls across a surface that has been damaged, stopping to repair any defects it encounters by releasing nanoparticles into them, then moving on to the next defect. Here, we experimentally demonstrate the repair-and-go approach using droplets of oil that are stabilized with a polymer surfactant and contain CdSe nanoparticles. We show that these microcapsules can find the cracks on a surface and selectively deliver the nanoparticle contents into the crack, before moving on to find the next crack. Although the microcapsules are too large to enter the cracks, their flexible walls allow them to probe and adhere temporarily to the interior of the cracks. The release of nanoparticles is made possible by the thin microcapsule wall (comparable to the diameter of the nanoparticles) and by the favourable (hydrophobic-hydrophobic) interactions between the nanoparticle and the cracked surface.     

Acoustic destruction of a microcapsule having a hard plastic shell

Acoustic destruction of a microcapsule having a hard plastic shell is discussed. In an ultrasonic drug delivery system, microcapsules having thin elastic shells release drugs that are contained therein when the shell is destroyed. In this paper, two subjects related to capsule destruction are discussed: the driving pulse duration for capsule destruction and the frequency dependence of capsule destruction. Optical observation of microcapsule destruction is performed with a high-speed video camera. In the case of capsule destruction by a pulse wave, the internal gas of the microcapsule cannot be ejected completely, and a portion of the internal gas remains inside the broken shell. It is found that capsule destruction by pulse waves depends on both the amplitude of the driving pressure and the pulse duration. The frequency dependence of microcapsule destruction also is investigated. In the case of capsule destruction by a low-amplitude acoustic wave, the destruction rate under the resonance condition is higher than under nonresonance conditions. By controlling the driving frequency, selective capsule destruction can be achieved.     

微胶囊埋植型自修复涂层研究进展

微胶囊埋植型自修复涂层是自修复复合材料的研究热点之一,它是模拟生物体损伤自愈合的原理,实现对涂层微裂纹、划痕等缺陷的自我修复,可有效提升涂层的耐蚀性能和使用寿命。从修复机理进行分类,微胶囊埋植型自修复涂层可分为腐蚀抑制型和反应修复型。腐蚀抑制型是通过微胶囊释放缓蚀剂延缓基体的腐蚀进程,反应修复型则是通过微胶囊释放的修复剂实现对涂层损坏处的修补。系统地回顾了微胶囊埋植型自修复涂层的典型研究成果和近期动态,详细阐述了自修复涂层的设计和修复机制,着重对不同涂层修复体系的优缺点和适用范围进行了讨论。最后,根据微胶囊埋植型自修复涂层的研究现状,提出了该领域存在的缺陷以及未来的发展方向。     

Pentagonal nanorods and nanoparticles with mismatched shell layers

Nano-scale rods and particles having the axes of fivefold symmetry, i.e., pentagonal nanorods and nanoparticles, are theoretically and experimentally investigated. Such objects possess elastic strains and mechanical stresses. In the present research a new mechanism of stress relaxation in nanorods and nanoparticles is considered. The mechanism is implemented by a formation of a surface layer with crystal lattice mismatch. The elastic fields and energies for nanorods and nanoparticles with the mismatched layers are calculated in the framework of the disclination model. The optimal mismatch parameter giving the maximal energy release is determined. The threshold radius as the minimal radius of nanorods or nanoparticles for which the formation of the layer is energetically favorable, is found. The threshold radius is approximately 10 nm for nanoparticle and 100 nm for nanorod of typical FCC metal.     

Hydrogen-bonding layer protecting polyelectrolyte capsules and its mechanical property study with atomic force microscope

The hydrogen-bonding multilayered polyelectrolyte capsules with sizes around 6 microm were fabricated by layer-by-layer self-assembly method. The morphology of the obtained capsules was observed with Scanning Electron Microscope (SEM), Confocal Laser Scanning Microscope (CLSM) and Atomic Force Microscope (AFM), respectively. The elastic properties of the capsules were studied with AFM. The capsule was pressed by cantilever with different lengths, a glass bead glued at the end of the cantilever. The force curves were measured on the capsule in air. The Young's modulus of the capsule was obtained (E = 170 MPa for the loading). Results show that this model can predict the elastic deformation of the microcapsule. The accuracy of the elastic deformation of polymer capsule can be ensured using a cantilever of mediate stiffness. Our results show that the existence of the hydrogen-bonding layer makes the multilayered polyelectrolyte harder in comparison with the pure multilayered polyelectrolyte capsules.     

Morphological evolution driven by strain induced surface diffusion

We have developed a three-dimensional finite element method to simulate the morphological evolution of a strained surface via surface diffusion, with a view to understanding the self-assembly, shape transitions and stability of low-dimensional quantum structures. We model deposition of an elastic film on a large lattice mismatched substrate. The film surface evolves by surface diffusion, driven by a gradient of the surface chemical potential, which includes the elastic strain energy, elastic anisotropy, surface energy, surface energy anisotropy and the interaction between the film and the substrate. Our simulations reveal that surface energy anisotropy and elastic anisotropy have a strong effect on the morphological evolution and shape transitions of the self-assembled islands. Our simulation results show a good qualitative agreement with experimental results.     

Verification of numerical determination of carrying capacity of large rolling bearings with hardened raceway

For the purpose of the numerical analysis of the actual carrying capacity of rolling contacts in large rolling bearings with surface-hardened raceways, an elasto-plastic constitutive model was used which links the mechanics of material damage with the isotropic and kinematic hardening or softening.A damage material model, implemented into a commercial finite element program, allows us to monitor the elastic strain, plastic strain increase, stress changes and material damage growth, which are closely related to the number of load cycles. In this way, the location and the time of occurrence of bearing raceway damage can be determined along with the growth of damage up to the point when a microcrack is formed. In other words, low cycle life of rotational rolling connection can be assessed.The paper presents the material model, numerical analysis of the actual carrying capacity of the rolling contact in single-row ball bearings and the verification of the numerical material model with experimental results of low cycle carrying capacity.     

A particle‐based moving interface method (PMIM) for modeling the large deformation of boundaries in soft matter systems

The mechanics of the interaction between a fluid and a soft interface undergoing large deformations appear in many places, such as in biological systems or industrial processes. We present an Eulerian approach that describes the mechanics of an interface and its interactions with a surrounding fluid via the so‐called Navier boundary condition. The interface is modeled as a curvilinear surface with arbitrary mechanical properties across which discontinuities in pressure and tangential fluid velocity can be accounted for using a modified version of the extended finite element method. The coupling between the interface and the fluid is enforced through the use of Lagrange multipliers. The tracking and evolution of the interface are then handled in a Lagrangian step with the grid‐based particle method. We show that this method is ideal to describe large membrane deformations and Navier boundary conditions on the interface with velocity/pressure discontinuities. The validity of the model is assessed by evaluating the numerical convergence for a axisymmetrical flow past a spherical capsule with various surface properties. We show the effect of slip length on the shear flow past a two‐dimensional capsule and simulate the compression of an elastic membrane lying on a viscous fluid substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

The role of metal nanoparticles in remote release of encapsulated materials

Laser mediated remote release of encapsulated fluorescently labeled polymers from nanoengineered polyelectrolyte multilayer capsules containing gold sulfide core/gold shell nanoparticles in their walls is observed in real time on a single capsule level. We have developed a method for measuring the temperature increase and have quantitatively investigated the influence of absorption, size, and surface density of metal nanoparticles using an analytical model. Experimental measurements and numerical simulations agree with the model. The treatment presented in this work is of general nature, and it is applicable to any system where nanoparticles are used as absorbing centers. Potential biomedical applications are highlighted.     

Nanoscale surface engineered living cells with extended substrate spectrum

We report on cell surface engineering of living microorganisms by using Layer-by-Layer (LbL) technology to extend the substrate spectrum. The yeast Arxula adeninivorans LS3 (Arxula) was employed as a model organism and biological template. By using LbL technology, Arxula cells were encapsulated by polyelectrolyte and enzyme layers. The biological activity of the Arxula was retained after the encapsulation process. The polymeric capsule surrounding the Arxula provides a stable interface for surface engineering of living cells. LbL of polyelectrolytes followed by an enzyme layer of lactate oxidase were assembled. The outer enzyme layer provides an additional biological function for Arxula to convert the unfavourable substrate lactate into the favourable substrate pyruvate, thus extending the substrate spectrum of the organism. Moreover, capsule stability and enzyme conjugate stability of the surface engineered Arxula were studied.     

A localization analysis of a non-uniform damage lattice in presence of strength gradient

The failure of a non-uniform axial damage chain under uniform tension is studied both with discrete damage mechanics (DDM) and continuum damage mechanics (CDM). It is shown that a micomechanics-based nonlocal CDM model may be built from a DDM formulation, that may include material heterogeneities. DDM is based on a microstructured model consisting in multiples elastic-damage springs, whose elastic yield threshold is variable and depends on the position along the chain. We aim to develop a nonlocal CDM model as a relevant continuous formulation of the lattice DDM system. To do this, we rely upon a continualisation procedure applied to the difference formulation of the lattice problem, which gives us a nonlocal propagating damage model. The boundary conditions of the nonlocal CDM problem are equivalent to a finite length damage cohesive law. Analytical and numerical results show a strong proximity of the discrete and enriched continuous approaches for this heterogeneous bar problem, as well as the effectiveness of the nonlocal damage model to capture the softening localization phenomenon in heterogeneous quasi-brittle fields.     

Susceptor-assisted microwave annealing for activation of arsenic dopants in silicon

Microwave annealing of arsenic-doped silicon was employed to achieve nearly complete dopant activation and repair of damage caused by ion implantation. Analysis of Rutherford backscattering spectra suggested that volumetric heating from microwaves can repair ion-implantation damage. Secondary ion mass spectroscopy depth profiling revealed that even with high damage due to implanted arsenic, microwave annealing achieves repair of lattice damage, and electrical activation of dopants without allowing any significant dopant diffusion into the silicon substrate. Surface temperatures greater than 700 °C were achieved within ~ 100 s with assisted microwave heating, marking this as a quick annealing technique when compared to un-assisted annealing. This temperature was sufficient for solid phase epitaxial growth in Si. The temperature profile recorded by a thermocouple-calibarated IR pyrometer was explained based upon the type of losses the sample undergoes while heating. The mechanism for susceptor-assisted microwave heating was dominated by dipole polarization losses in the initial stages of anneal and by Ohmic conduction losses at higher temperatures. Cross-section transmission electron microscopy, along with ion channeling spectra indicated that the silicon lattice regained nearly all of its crystallinity during the microwave anneal. Hall measurement and sheet resistance characterization were used to assess the extent of dopant activation.     

A novel boundary-domain element method of initial stress, finite deformation and discrete cracks in multilayered anisotropic elastic solids

We introduce a novel boundary-domain element method of initial stress, finite deformation (due to large rotation) and discrete cracks in multilayered anisotropic elastic solids. Because the special Green’s function that satisfies the interfacial continuity and surface boundary conditions is employed, the numerical discretization is reduced to be along one side of the cracks and over the subdomains of finite deformation. Two examples are presented. First, the process of interfacial delamination is simulated around a growing through-thickness crack in a pre-stretched film bonded to a flexible substrate. It is shown that the progression of delamination damage is stable but the initiation of delamination crack can be a snap-back instability. This simultaneous damage and fracture process is approached by the cohesive zone model. Second, the postbuckling of a circular delaminated and pre-compressed film is simulated on a flexible substrate. It is shown that the compliance of substrate can play a significant role on the critical behavior of buckling. If the substrate is more compliant or stiffer than the film, the instability initiates as a subcritical hard or a supercritical soft bifurcation. The critical magnitude of pre-strain for the initiation of buckling increases with substrate stiffness. Also, the transition of buckling from the first to the second mode is captured in the simulation.     

Failure analysis method of concrete arch dam based on elastic strain energy criterion

The arch dam is a type of massive water-retaining structure made of concrete. The overall failure mechanism and corresponding analysis criterion are key issues of concern in the dam engineering field. In this paper, the energy evolution of arch dams in the failure process is studied first, which can be decomposed as energy dissipation accompanied by concrete damage and elastic strain energy absorption and release during elastic deformation. An evaluation criterion for failure analysis of concrete arch dams is then established based on elastic strain energy. An orthotropic damage constitutive model for dam concrete is then proposed along with its numerical simulation method, which is established for structural failure analysis. Numerical simulations show that the elastic strain energy in elements increases with increasing overload safety coefficient and finally converges to the concrete material surface energy, at which time the locally plastic damage area develops rapidly and finally leads to cracking failure of structures. The proposed failure analysis criterion for concrete dams under integrated loads is suitable for analyzing dam instability failure, which has great operability and value in engineering applications in the future.     

Lattice strain and lattice expansion of nanoparticles of MgAl2O4 as a function of particle size

Reduction in the physical dimension of a crystalline solid lead to regular changes in the lattice constants. The lattice expansion or contraction in nanoparticles depend on a number of factors like nature of atoms in the interior, surface atoms, surface surroundings, dangling bonds, and oxygen concentration on the surface. XRD patterns of nanoparticles of MgAl2O4 having average particle size 7 nm, 9 nm, and 19 nm show that lattice parameters undergo contraction or repulsion along different planes. The lattice contraction along with lattice expansion is possible since the morphology of the particle is not spherical which is evident from the SEM micrographs. Lattice expansion is observed mainly in nanocrystals having covalent or ionic bonds. The existence of ionic bonds is evident from the IR spectrum. Along planes where there is no deficiency of oxygen, contraction is observed while where there is deficiency of oxygen lattice expansion is observed. As the particle size increases due to heating most of the planes show lattice contraction and on further heating due to the excess oxygen accumulation in interstitial positions lattice expansion is observed along most of the planes. The reason for the observation of lattice strain or lattice expansion may be due to the electrostatic attraction or repulsion between ions along different planes.     

Barnacles resist removal by crack trapping

We study the mechanics of pull-off of a barnacle adhering to a thin elastic layer which is bonded to a rigid substrate. We address the case of barnacles having acorn shell geometry and hard, calcarious base plates. Pull-off is initiated by the propagation of an interface edge crack between the base plate and the layer. We compute the energy release rate of this crack as it grows along the interface using a finite element method. We also develop an approximate analytical model to interpret our numerical results and to give a closed-form expression for the energy release rate. Our result shows that the resistance of barnacles to interfacial failure arises from a crack-trapping mechanism.     

单体固化特性对光热敏微胶囊显影密度的影响

利用界面聚合技术制备了以不同单体为囊芯的新型感光材料光热敏微胶囊.借助于傅里叶红外光谱及原子力显微技术对不同单体的双键交联固化及固化产物的表面形貌进行了分析,利用热显影打印技术检测了光热敏微胶囊显影密度随曝光时间的变化.实验结果表明,不同单体的双键交联固化速度主要与单体官能度有关,并且单体固化产物的表面形貌与单体的双键...     

Mechanical alignment of quasi-one-dimensional nanoparticles

Randomly oriented rod or rope-like nanoparticles on the surface of an elastomeric substrate are aligned along one direction simply by stretching the substrate. The technique is demonstrated here using single-walled carbon nanotube ropes, and the degree of alignment is assessed by polarized Raman spectroscopy. The alignment is preserved after the particles are removed from the substrate surface, showing that the aligned nanoparticles can be stamped in patterns onto another surface.     

Equilibrium surface roughness of a strained epitaxial film due to surface diffusion induced by interface misfit dislocations

In recent years, developments in the microelectronics industry have led to extensive studies of the growth and characterization of thin solid films and their implementation in electronic and opto-electronic devices. A goal is to produce thin films with minimal bulk and surface defects. For those systems produced by epitaxial growth of a film on a substrate that has a slightly different lattice parameter, the stress associated with the elastic mismatch strain needed to satisfy the constraint of epitaxy provides a driving force for nucleation and growth of undesirable defects in the film material or on its surface. Among the most common defects are interface misfit dislocations, arranged more or less periodically on the film-substrate interface, which partially relax the elastic mismatch strain in the film. It has been observed that, for some material systems, surface roughness or waviness arises which correlates spatially with the positions of interface misfit dislocations. It is suggested here that the waviness along the surface may be a result of surface diffusion which is driven by a gradient in the chemical potential of the material along the surface. The chemical potential gradient arises from the nonuniform strain field of the interface misfit dislocations, as well as from the unrelaxed elastic mismatch strain. The focus here is on the development of a relatively simple model of this system which leads to an estimate of the magnitude and profile of surface waviness under conditions of thermodynamic equilibrium, i.e., after the material responds to the chemical potential gradient by seeking out a new configuration for which stresses are redistributed and the chemical potential is again uniform. The condition of uniform chemical potential for the final shape leads to an integro-differential equation for the equilibrium surface shape which is solved numerically. For representative values of system parameters, estimates of equilibrium surface roughness are obtained which can vary from less than one percent of film thickness to a significant fraction of film thickness. Although transient aspects of the process are not studied here, the characteristic time for achieving an equilibrium configuration is estimated.