Surface Elastic Modulus Research Articles

Experimental study on the improvement of surface integrity of tungsten steel using acoustic levitation polishing

Surface integrity, including surface morphology, roughness, hardness and elastic modulus of tungsten steel, after the acoustic levitation polishing (ALP) process was investigated comprehensively. The mechanism of the ALP technology was first explored, and then numerical study revealed that a standing wave acoustic levitation can be formed in an in-house developed ALP system. Then experimental work was conducted to polish the tungsten steel using the mechanical polishing (MP) and further ALP processes, respectively. It is found that after the further ALP process the surface roughness decreases while some nano-dents can be found on the surface due to the cavitation bubbles induced by the ultrasonic vibration. The surface residual compressive stress is found to be decreased after the ALP process as compared with the MP process due to a cyclic loading of nano-particles on the target surface that reduces the energy stored in the deformed surface region and drives a residual stress relaxation. Similarly, the surface hardness and elastic modulus is found to be increased after the MP process with a work-hardening layer while after the further ALP process the cyclic loading of nano-particles could smooth the surface and refine the dislocation density, thereby reducing the surface hardness and elastic modulus.

Effect of annealing treatments on CeO2 grown on TiN and Si substrates by atomic layer deposition.

In this work, we investigate the effect of thermal treatment on CeO2 films fabricated by using atomic layer deposition (ALD) on titanium nitride (TiN) or on silicon (Si) substrates. In particular, we report on the structural, chemical and morphological properties of 25 nm thick ceria oxide with particular attention to the interface with the substrate. The annealing treatments have been performed in situ during the acquisition of X-Ray diffraction patterns to monitor the structural changes in the film. We find that ceria film is thermally stable up to annealing temperatures of 900 °C required for the complete crystallization. When ceria is deposited on TiN, the temperature has to be limited to 600 °C due to the thermal instability of the underlying TiN substrate with a broadening of the interface, while there are no changes detected inside the CeO2 films. As-deposited CeO2 films show a cubic fluorite polycrystalline structure with texturing. Further, after annealing at 900 °C an increase of grain dimensions and an enhanced preferential (200) orientation are evidenced. These findings are a strong indication that the texturing is an intrinsic property of the system more than a metastable condition due to the ALD deposition process. This result is interpreted in the light of the contributions of different energy components (surface energy and elastic modulus) which act dependently on the substrate properties, such as its nature and structure.

Coherent Timescales and Mechanical Structure of Multi-Cellular Aggregates

3D cellular aggregates are a broadly studied model system for biological processes including morphogenesis, carcinogenesis, malignant invasion, wound healing, and tissue engineering. Tissue-like aggregates are well known to exhibit liquid-like behavior in the long-times (hours), particularly manifesting a “surface tension” analogous to liquids. On the other hand, the mechanical behavior in the short times (seconds to minutes) is very different and has received less attention. The current work uses shape relaxation experiments to investigate the structural characteristics of aggregates in the short times. Two coherent timescales are observed, one on the order of seconds; the other, tens of seconds. Both are found to be persistent across all examined cell types (GBM, 3T3, Rat2, L-cells) despite drastic changes in other properties such as tissue surface tension (by two orders of magnitude) and adhesion strength. A precise mathematical theory previously developed is used to extract the viscoelastic properties of aggregates from the measured timescales. The analysis suggests that aggregates are a composite of an unusually “soft” interior (weak bulk elastic modulus) enclosed by a relatively stiffer envelope. Additionally, aggregate surface tension, elastic modulus, and viscosity observe proportionality. This result suggests that the underlying cellular properties are regulated in a precise manner. The root-cause for multi-cellular aggregation to preserve such timescales requires further investigation.

Design and preparation of sandwich-like polydimethylsiloxane (PDMS) sponges with super-low ice adhesion.

The mitigation of ice on exposed surfaces is of great importance to many aspects of life. Ice accretion, however, is unavoidable as time elapses and temperature lowers sufficiently. One practical solution is to reduce the ice adhesion strength on a surface to as low as possible, by either decreasing the substrate elastic modulus, lowering surface energy or increasing the length of cracks at the ice-solid interface. Herein, we present a facile preparation of polydimethylsiloxane (PDMS) based sandwich-like sponges with super-low ice adhesion. The weight ratio of the PDMS prepolymer to the curing agent is tuned to a lower surface energy and elastic modulus. The introduction of PDMS sponge structures combined the advantages of both a reduced apparent elastic modulus and most importantly, the macroscopic crack initiators at the ice-solid interface, resulting in dramatic reduction of the ice adhesion strength. Our design of sandwich-like sponges achieved a low ice adhesion strength as low as 0.9 kPa for pure PDMS materials without any additives. The super-low ice adhesion strength remains constant after 25 icing and deicing cycles. We thus provide a new and low-cost approach to realize durable super-low ice adhesion surfaces.

Direct Measurement of Interparticle Forces of Titan Aerosol Analogs (“Tholin”) Using Atomic Force Microscopy

AbstractTo understand the origin of the dunes on Titan, it is important to investigate the material properties of Titan's organic sand particles on Titan. The organic sand may behave distinctively compared to the quartz/basaltic sand on terrestrial planets (Earth, Venus, and Mars) due to differences in interparticle forces. We measured the surface energy (through contact angle measurements) and elastic modulus (through Atomic Force Microscopy) of the Titan aerosol analog (tholin). We find that the surface energy of a tholin thin film is about 70.9 mN/m, and its elastic modulus is about 3.0 GPa (similar to hard polymers like PMMA and polystyrene). For two 20 μm diameter particles, the theoretical cohesion force is therefore 3.3 μN. We directly measured interparticle forces for relevant materials: tholin particles are 0.8 ± 0.6 μN, while the interparticle cohesion between walnut shell particles (a typical model materials for the Titan Wind Tunnel, TWT) is only 0.4 ± 0.1 μN. The interparticle cohesion forces are much larger for tholins and presumably Titan sand particles than materials used in the TWT. This suggests that we should increase the interparticle force in both analog experiments (TWT) and threshold models to correctly translate the results to real Titan conditions. The strong cohesion of tholins may also inform us how the small aerosol particles (∼1 μm) in Titan's atmosphere are transformed into large sand particles (∼200 μm). It may also support the cohesive sand formation mechanism suggested by Rubin and Hesp (2009), where only unidirectional wind is needed to form linear dunes on Titan.

Analyzing and Modelling the Corrosion Behavior of Ni/Al2O3, Ni/SiC, Ni/ZrO2 and Ni/Graphene Nanocomposite Coatings.

A study has been presented on the effects of intrinsic mechanical parameters, such as surface stress, surface elastic modulus, surface porosity, permeability and grain size on the corrosion failure of nanocomposite coatings. A set of mechano-electrochemical equations was developed by combining the popular Butler–Volmer and Duhem expressions to analyze the direct influence of mechanical parameters on the electrochemical reactions in nanocomposite coatings. Nanocomposite coatings of Ni with Al2O3, SiC, ZrO2 and Graphene nanoparticles were studied as examples. The predictions showed that the corrosion rate of the nanocoatings increased with increasing grain size due to increase in surface stress, surface porosity and permeability of nanocoatings. A detailed experimental study was performed in which the nanocomposite coatings were subjected to an accelerated corrosion testing. The experimental results helped to develop and validate the equations by qualitative comparison between the experimental and predicted results showing good agreement between the two.

AFM-detected apoptosis of hepatocellular carcinoma cells induced by American ginseng root water extract

American ginseng as a common and traditional herbal medicine has been used in cancer treatment for many years. However, the effect of American ginseng on the cancer cell response (i.e. apoptosis) has not been fully understood yet. Previous studies demonstrated that cellular apoptosis was associated with the changes of mechanical and morphological properties. Therefore, in this study, mechanical and morphological characterizations were carried out by both atomic force microscope (AFM) and inverted optical microscope to investigate the apoptosis of hepatocellular carcinoma (SMMC-7721) cells affected by American ginseng root water extract (AGRWE). The results showed that the cells treated with AGRWE exhibited significantly larger surface roughness, height and elastic modulus values than control group. Moreover, those parameters were upregulated under the higher concentration of AGRWE and longer culture time. Consequently, it indicates that the mechanical and morphological properties can be used as the apoptotic characteristics of SMMC-7721 cells. Also, the increased surface roughness and elastic modulus of cells under the AGRWE treatment have shown that the apoptosis of SMMC-7721 cells can be enhanced by AGRWE. This will provide an important implication for hepatocelluar carcinoma treatment and drug development.

Shear horizontal wave dispersion in nanolayers with surface effects and determination of surface elastic constants

In this work, the shear horizontal (SH) wave dispersion in two dissimilar nanolayers is investigated by using the surface elasticity theory in which the surface effects are featured by surface elastic constants. It is found that the SH wave dispersion shows distinct dependence on the nanolayer thickness as well as the surface elastic constants. The larger the surface elastic modulus and/or the smaller the thickness, the higher the phase velocity. In particular, as the wave frequency approaches zero, the analytical relation between the phase velocity in the first mode dispersion and the surface elastic constants is deduced. Thereby, a facile method is suggested to determine the surface elastic constants from the phase velocity of SH waves scattered in nanolayers.

The effects of liquid crystal-based composite substrates on cell functional responses of human umbilical cord-derived mesenchymal stem cells by mechano-regulatory process.

Physical properties of extracellular matrix, including elasticity and microstructure, have been considered as important factors inducing stem cell differentiation. This study developed a novel type of liquid crystal-based matrix by combining the elastic property of polyurethane with viscoelastic liquid crystal to generate a soft elastic response resembling physical microenvironment of stem cell niche, and explored the mechano-driving cell behaviors. Addition of varying liquid crystal concentration (10 wt%, 30 wt% and 50 wt%) had great effects on surface morphology and elastic modulus of liquid crystal/ polyurethane composite substrates. Changes in microstructure and elastic modulus of the substrates could cause intense cell responses that influenced cell properties, including proliferation, adhesion, and osteogenic differentiation. Human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) cultured on both liquid crystal-10/polyurethane and liquid crystal-30/polyurethane substrates exhibited higher viability, more actin filament, and larger spreading area while liquid crystal-50/polyurethane substrates seemed not to favor cell attachment and spreading. Alkaline phosphatase activity and calcium deposition were significantly improved with hUC-MSCs on both liquid crystal-10/ polyurethane and liquid crystal-30/ polyurethane substrates, and the maximal alkaline phosphatase activity was observed in liquid crystal-10/ polyurethane while the lowest in liquid crystal-50/ polyurethane. Osteopontin was upregulated to a high level in both liquid crystal-10/ polyurethane and liquid crystal-30/ polyurethane groups after 14 days culturing; the maximal expression of osteocalcin and related transcription factor 2 were found in liquid crystal-10/ polyurethane group on day 21. Our findings revealed that hUC-MSCs could intensely sense the bioactive patterns and soft-matter feature of liquid crystal domains and subsequently modulated cell behaviors, which may prove useful in the development of new class of biomaterials for applications in tissue engineering and regenerative medicine.

Extended Hertz Theory of Contact Mechanics for Case-Hardened Steels With Implications for Bearing Fatigue Life

The analytical expressions currently available for Hertzian contact stresses are applicable only for homogeneous materials and not for case-hardened bearing steels, which have inhomogeneous microstructure and graded elastic properties in the subsurface region. Therefore, this article attempts to determine subsurface stress fields in ball bearings for graded materials with different ball and raceway geometries in contact. Finite element models were developed to simulate ball-on-raceway elliptical contact and ball-on-plate axisymmetric contact, to study the effects of elastic modulus variation with depth due to case hardening. Ball bearings with low, moderate, and heavy load conditions are considered. The peak contact pressure for case-hardened steel is always more than that of through-hardened steel under identical geometry and loading conditions. Using equivalent contact pressure approach, effective elastic modulus is determined for case-carburized steels, which will enable the use of Hertz equations for different gradations in elastic modulus of raceway material. Nonlinear regression tools are used to predict effective elastic modulus as a weighted sum of surface and core elastic moduli of raceway material and design parameters of ball–raceway contact area. Mesh convergence study and validation of equivalent contact pressure approach are also provided. Implications of subsurface stress variation due to case hardening on bearing fatigue life are discussed.

Microstructural Evolution of NiCoCrAlHfYSi and NiCoCrAlTaY Coatings Deposited by AC-HVAF and APS

The chemical composition of NiCoCrAlHfYSi with a suitable particle size, deposited using an activated combustion-high velocity air fuel (AC-HVAF) spray, is a potentially promising process because dense, continuous and pure alumina can be formed on the surface of the MCrAlY metallic coatings after isothermal oxidation exposure. The NiCoCrAlHfYSi (Amdry386) and NiCoCrAlTaY (Amdry997) coatings were produced using AC-HVAF and APS, respectively. Isothermal oxidation was subsequently conducted at 1050 °C in air for 200 h. This paper compares the characteristics of four coated samples, including the surface roughness, elastic modulus, hardness, oxide content, microstructural characteristics and phase evolution of thermally grown oxides (TGO). The growth of both the TGO and alumina scales in the TGO of the HVAF386 coating was relatively rapid. The θ- to α-alumina phase transformation was strongly determined by the Hf and Si dopants in the HVAF386 coating. Finally, the extent of grain refinement and deformation storage energy in the HVAF997 coatings were determined to be significantly crucial for the θ- to α-alumina phase transformation.

Comparison on the effects of water-borne and dietary-borne accumulated ZnO nanoparticles on Daphnia magna

Nanoparticles (NPs) can exert toxicity to aquatic animals through water and dietary exposure. However, little is known about the contribution of the two exposure pathways to the total toxicity of NPs to aquatic animals. In the present study, we fed Daphnia magna with ZnO NPs-accumulated-algae or directly exposed them to ZnO NPs suspension at the same Zn concentration. The body Zn concentrations were the same in both treatments, but the surface elastic modulus in the water-borne group was lower than that in the dietary-borne group, indicating more severe surface damage in the water-borne treatment. Total egg numbers of the first brood in both Zn treatments were significantly lower than the control, but they were not significantly different between the two treatments. However, the first brood in the water-borne treatment was distinctly delayed than that in the dietary-borne group. The dietary-borne treatment suppressed the expression of Hem and SOD genes. The water-borne treatment significantly suppressed the expression of Pro, SOD, VTG, but increased the GST expression. In particular, the expression of Pro, a gene functions in modulation of cell multiplication, was lower in the water-borne treatment, which might be related to the suppression of body growth in D. magna. The expression of SOD in the water-borne treatment decreased by about four times in comparison with the dietary-borne treatment, and the SOD enzyme activity of the water-borne treatment was significantly lower than that of the dietary-borne treatment. These results indicate that the two exposure routes have different targets and responding toxic mechanisms.

Regioselective Synthesis of Eight-Armed Cyclosiloxane Amphiphile for Functional 2D and 3D Assembly Motifs.

A crystalline tetramethylcyclotetrasiloxane (TMCS)-derived amphiphile was regioselectively synthesized with eight peripheral hydrophilic amide groups and hydrophobic dodecyl chains by Pt(0)-catalyzed hydrosilylation and amidation reactions. The as-synthesized materials showed ordered lamellar structure formation in the powder form. It also exhibits superior two-dimensional (2D) monolayer formation properties at the air-water interface with unexpectedly high collapse surface pressure and elastic modulus. The monolayers act as two-dimensional building blocks with finely controllable thickness on a several nanometer scale irrespective of the substrate type and properties. The amphiphile forms nanofibers spontaneously by good-poor solvent strategies, which contributes to porous three-dimensional (3D) structures possessing superhydrophobic surface wettability.

Nonlinear analysis of functionally graded nanoscale beams incorporating the surface energy and microstructure effects

A nonclassical size–dependent beam model is developed to study the geometrically nonlinear response of functionally graded (FG) nanoscale beams including the combined effect of surface elasticity, surface residual stress, microstructure and size-dependency. To this, the Gurtin–Murdoch surface elasticity theory in combination with the modified couple stress theory is employed to capture the simultaneous effects of surface energy and microstructure size phenomenon. Euler–Bernoulli and Timoshenko beam models considering the von Kármán geometric nonlinearity are adopted. The material properties of the bulk and surface of the FG beam are assumed to be changed continuously through the thickness according to a power law scheme and Mori–Tanaka homogenization technique. The geometrically nonlinear size–dependent governing equations and corresponding nonclassical boundary conditions are exactly obtained through Hamilton's principle. The generalized differential quadrature method (GDQM) is employed to discretize the nonlinear nonclassical governing differential equations and the nonclassical boundary conditions for various end supports. A good agreement between the results of the present model and those available in the literature is obtained. Selected numerical results show explicitly that the material length scale parameter, surface elastic modulus, surface residual stress, gradient index and material distribution have a significant influence on the geometrically nonlinear bending response of FG nanobeams.

Self-Assembly of Escin Molecules at the Air-Water Interface as Studied by Molecular Dynamics.

Escin belongs to a large class of natural biosurfactants, called saponins, that are present in more than 500 plant species. Saponins are applied in the pharmaceutical, cosmetics, and food and beverage industries due to their variously expressed bioactivity and surface activity. In particular, escin adsorption layers at the air-water interface exhibit an unusually high surface elastic modulus (>1100 mN/m) and a high surface viscosity (ca. 130 N·s/m). The molecular origin of these unusual surface rheological properties is still unclear. We performed classical atomistic dynamics simulations of adsorbed neutral and ionized escin molecules to clarify their orientation and interactions on the water surface. The orientation and position of the escin molecules with respect to the interface, the intermolecular interactions, and the kinetics of molecular aggregation into surface clusters are characterized in detail. Significant differences in the behavior of the neutral and the charged escin molecules are observed. The neutral escin rapidly assembles in a compact and stable surface cluster. This process is explained by the action of long-range attraction between the hydrophobic aglycones, combined with intermediate dipole-dipole attraction and short-range hydrogen bonds between the sugar residues in escin molecules. The same interactions are expected to control the viscoelastic properties of escin adsorption layers.

Longitudinal vibrations of aluminum nanobeams by applying elastic moduli of bulk and surface: molecular dynamics simulation and continuum model

The elastic moduli of the bulk and surface as well as the equivalent mass density are applied to the continuum model of a nanobeam to predict the free longitudinal vibrations of aluminum (Al) nanobeams, especially in cases in which applying the classical theories results in significant errors—i.e. the cross section area of the nanobeams is lower than 4 × 4 nm2. To this end, the bulk elastic modulus and the surface elastic modulus of the Al nanobeam are extracted using surface elasticity theory, a bulk-surface model and molecular dynamics (MD) simulation. These elastic moduli are extracted by several strain values and the results show that the surface elastic modulus is very sensitive to the strain value, so that when the strain value increases from 0.001 to 0.02, the surface elastic modulus decreases significantly while the bulk elastic modulus shows fewer changes. Moreover, the size dependency of the mass density in the continuum nanobeam models is investigated in detail. Also, the natural frequencies of the Al nanobeams are obtained by MD simulation and they are compared with those obtained by the continuum model. Comparisons of the results show that the extracted bulk and surface elastic moduli are acceptable, and that the modified continuum model can predict the dynamic behavior of the Al nanobeam.

ANALYSIS OF THE STRESS CHARACTERISTICS OF CFG PILE COMPOSITE FOUNDATION UNDER IRREGULARITY CONDITION

By the excitation load function corresponding to the irregularity management standard the vertical load of the train is simulated. Based on the finite difference software FLAC3D the three-dimensional dynamic coupling finite difference model of track-embankment-pile-soil composite foundation is established. Focuses on the analysis of the dynamic response characteristics of the embankment, pile and soil foundation caused by the change of foundation, pile and cushion elastic modulus and cushion thickness. Results show: by the excitation load, the centre pile dynamic stresses are maximum, piles dynamic stress away from the centre side pile decreases gradually. The dynamic response of the pile and soil caused by subgrade surface elastic modulus variation has a little effect with more obvious by the cushion effect. With the increase of elastic modulus and thickness of cushion, the dynamic interaction between pile and cushion is increased while the dynamic interaction between soil and cushion is weakened. Therefore, the bearing capacity of the pile is fully utilized. With the increase of the elastic modulus of the pile, the dynamic stress of pile top increases correspondingly, but the dynamic stress increases gradually, and the pile bears most of the load, thus effectively reduce the dynamic load of the foundation soil.

Preparation and evaluation of amphiphilic polymer as fouling-release coating in marine environment

Polystyrene–polybutadiene–polystyrene (SBS), vinyl fluoride silicone (BD-FT-LSR) and polyethylene glycol were chosen as the reactors to synthesize amphiphilic polymer which was used to prepare a new type of fouling-release coating. To evaluate the fouling-release performance and explore the mechanism of the coating, artificial and real-sea environment tests were carried out in the experiments, respectively. IR and 1H NMR were utilized to characterize the structure of the polymer. The effect of fouling-release agent was explored by considering changes in the parameters of surface contact angle, XPS, elastic modulus, and TEM. Results of real-sea tests show that the new amphiphilic fouling-release coating has a very good antifouling performance and can be re-cleaned after 6 months of testing in the real-sea environment. These can be explained through hydrophilic PEG migrating to the surface to form a comb-type structure which reduces the adhesion strength of fouling organisms that can then be self-cleaned by flow from the surface of coatings.

Mechanical behavior and chloride penetration of high strength concrete under freeze-thaw attack

This paper proposes an experimental study to investigate the freeze-thaw (FT) effects on mechanical behavior and chloride diffusion in high strength concrete. Sets of high strength concrete cubes are cyclically frozen in range from +18°C to −20°C. The mechanical property and the chloride penetration tests are performed on the specimens after 0, 5, 10…95, and 100 FT cycles. FT effects on surface scaling damage, compressive strength, elastic modulus, stress-strain relationship and chloride resistance of high strength concrete are clarified. Also, a simple model is discussed and verified to predict the chloride concentration of high strength concrete attacked by FT cycles. Results show that the behavior of high strength concrete depends on the number of FT cycles. Visible damages appear in concrete after about 40 FT cycles. Before 25–60 FT cycles, no significant difference exists in FT-caused weight loss between normal strength concrete and high strength concrete. Beyond this, the FT-caused weight loss in normal strength concrete becomes faster than that in high strength concrete. FT damage decreases the compressive strength, the elastic modulus and the chloride resistance of concrete, but increases its ductility. In the present test, the compressive strength and elastic modulus decrease about 32% and 24% after 100 FT cycles, respectively. The surface chloride concentration and chloride diffusion coefficient increase to about twice of the control values.

Negative Thermal Expansion of Ultrathin Metal Nanowires: A Computational Study.

Most materials expand upon heating because the coefficient of thermal expansion (CTE), the fundamental property of materials characterizing the mechanical response of the materials to heating, is positive. There have been some reports of materials that exhibit negative thermal expansion (NTE), but most of these have been in complex alloys, where NTE originates from the transverse vibrations of the materials. Here, we show using molecular dynamics simulations that some single crystal monatomic FCC metal nanowires can exhibit NTE along the length direction due to a novel thermomechanical coupling. We develop an analytic model for the CTE in nanowires that is a function of the surface stress, elastic modulus, and nanowire size. The model suggests that the CTE of nanowires can be reduced due to elastic softening of the materials and also due to surface stress. For the nanowires, the model predicts that the CTE reduction can lead to NTE if the nanowire Young's modulus is sufficiently reduced while the nanowire surface stress remains sufficiently large, which is in excellent agreement with the molecular dynamics simulation results. Overall, we find a "smaller is smaller" trend for the CTE of nanowires, leading to this unexpected, surface-stress-driven mechanism for NTE in nanoscale materials.