Deformation of an elastic substrate by a three-phase contact line

Young's classic analysis of the equilibrium of a three-phase contact line ignores the out-of-plane component of the liquid-vapor surface tension. While it is expected that this unresolved force is balanced by the elastic response of the solid, a definitive analysis has remained elusive because of an apparent divergence of stress at the contact line. While a number of theories have been presented to cut off the divergence, none of them have provided reasonable agreement with experimental data. We measure surface and bulk deformation of a thin elastic film near a three-phase contact line using fluorescence confocal microscopy. The out-of-plane deformation is well fit by a linear elastic theory incorporating an out-of-plane restoring force due to the surface tension of the solid substrate. This theory predicts that the deformation profile near the contact line is scale-free and independent of the substrate elastic modulus.     

Tribological properties of silicate materials on nano and microscale

We studied the friction properties of four model silicate materials at the nanoscale and microscale. From nanotribology, we characterized the tribological properties at single asperity contact scale and from microtribology, we characterized the tribological properties at multi asperity contact scale. First, for each material we measured chemical composition by XPS, Young's modulus by acoustical microscopy and roughness σ by atomic force microscopy (AFM). Second, we measured the nanofriction coefficients with an AFM and the microfriction coefficients with a ball probe tribometer, for three hardnesses of the ball probe. We identified one friction mechanism at the nanoscale (sliding friction) and two friction mechanisms at the microscale (sliding friction and yielding friction). Comparison of the nano and microfriction coefficients at the same sliding friction regime shown, that the tribological properties of these materials didn’t depend on roughness.     

Effects of post-annealing on the structural and nanomechanical properties of Ga-doped ZnO thin films deposited on glass substrate by rf-magnetron sputtering

In this study, the effects of post-annealing on the structure, surface morphology and nanomechanical properties of ZnO thin films doped with a nominal concentration of 3 at.% Ga (ZnO:Ga) are investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) and nanoindentation techniques. The ZnO:Ga thin films were deposited on the glass substrates at room temperature by radio frequency magnetron sputtering. Results revealed that the as-deposited ZnO:Ga thin films were polycrystalline albeit the low deposition temperature. Post-annealing carried out at 300, 400 and 500 °C, respectively, has resulted in progressive increase in both the average grain size and the surface roughness of the ZnO:Ga thin film, in addition to the improved thin films crystallinity. Moreover, the hardness and Young's modulus of ZnO:Ga thin films are measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option. The hardness and Young's modulus of ZnO:Ga thin films increased as the annealing temperature increased from 300 to 500 °C, with the best results being obtained at 500 °C.     

Evaluation of the contact resonance frequencies in atomic force microscopy as a method for surface characterisation (invited)

The combination of ultrasound with atomic force microscopy (AFM) opens the high lateral resolution of scanning probe techniques in the nanometer range to ultrasonics. One possible method is to observe the resonance frequencies of the AFM sensors under different tip-sample interaction conditions. AFM sensors can be regarded as small flexible beams. Their lowest flexural and torsional resonance frequencies are usually found to be in a range between several kHz and several MHz depending on their exact geometrical shape. When the sensor tip is in a repulsive elastic contact with a sample surface, the local indentation modulus can be determined by the contact resonance technique. Contact resonances in the ultrasonic frequency range can also be used to improve the image contrast in other dynamic techniques as, for example, in the so-called piezo-mode. Here, an alternating electric field is applied between a conducting cantilever and a piezoelectric sample. Via the inverse piezoelectric effect, the sample surface is set into vibration. This excitation is localised around the contact area formed by the sensor tip and the sample surface. We show applications of the contact resonance technique to piezoelectric ceramics.     

Nanomechanical characterization of amorphous hydrogenated carbon thin films

Amorphous hydrogenated carbon (a-C:H) thin films deposited on a silicon substrate under various mixtures of methane-hydrogen gas by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD) was investigated. Microstructure, surface morphology and mechanical characterizations of the a-C:H films were analyzed using Raman spectroscopy, atomic force microscopy (AFM) and nanoindentation technique, respectively. The results indicated there was an increase of the hydrogen content, the ratio of the D-peak to the G-peak (I_D/I_G) increased but the surface roughness of the films was reduced. Both hardness and Young's modulus increased as the hydrogen content was increased. In addition, the contact stress-strain analysis is reported. The results confirmed that the mechanical properties of the amorphous hydrogenated carbon thin films improved using a higher H₂ content in the source gas.     

A displacement sensing nanoindentation study of tribo-mechanical properties of the Ni-Co system

An analysis of the tribo-mechanical properties of the Ni-Co system, at the submicrometric contact scale, is conducted using displacement sensing nanoindentation. In particular, the influence of contact depth and surface finishing methods on the hardness, H, and Young's modulus, E, of the materials is analysed. Mechanically and electrolitically polished samples were tested with a conospherical indenter using a range of loads between 0.05 and 10 mN. It is shown that the hardness of these materials depends on the surface finishing method and increases with decreasing contact depth, while the Young's modulus is relatively insensitive to contact depth. Furthermore, sample polycrystallinity leads to a large scattering of hardness values in Co-rich samples and of Young's modulus values in Ni-rich ones. The combined parametric ratio H/E, which can be related to the tribological behaviour of the material, was found to be higher in samples with Co content larger than 80 wt.%.     

Elasticity and nanomechanical response of Aspergillus niger spores using atomic force microscopy

The elasticity and nanomechanical response of Aspergillus niger spores determined using atomic force microscopy (AFM) and nanoindentation are discussed. The force-displacement curve of the spore surfaces shows that the average surface roughness of spores was approximately 33 nm and that the adhesion force ranged from 9 to 28 nN. The Young's modulus of the A. niger spores ranged from 0.1 to 21.4 GPa and the hardness ranged from 0.01 to 0.17 GPa. The critical buckling load of the spore membrane is 290 μN.     

Analysis of Single-Walled Carbon Nanotubes Using a Chemical Bond Element Model

提出了一种纳米尺度的有限元方法,碳纳米管中的碳-碳化学键被模拟为键单元．按照平衡关系,根据有限元理论,作用于每个碳原子上的作用力可以写成键单元的刚度矩阵与每个碳原子位移的乘积．在分子力学的基本假设下,键单元刚度矩阵的每个元素可以写为分子力学中力场常数的函数,这样建立起了宏观力学方法(有限元)与纳米尺度力学方法(分子力学)之间的联系．应用该方法模拟了扶椅型与锯齿型单壁碳纳米管的力学行为从而验证了该方法的有效性．分析结果说明单壁碳纳米管的弹性模量与管厚度的选取直接相关．此外,弹性模量对所选取的分子力学中的力场常数非常敏感,管的弹性模量显示出对半径的尺度依赖性,但是管长度对弹性模量的影响小到可以被忽略．     

Measurement of adhesion energies and Young's modulus in thin polymer films using a novel axi-symmetric peel test geometry

We present a novel method of probing adhesion energies of solids, particularly polymers. This method uses the axi-symmetric deformation of a thin spincast polymer membrane brought into contact with a flat substrate to probe the work of adhesion. The use of a thin membrane minimizes uncertainty in the radius of contact, while the use of spincast films provides very smooth surfaces by means of a very simple method. The experimental profile of the deformed membrane shows good agreement with the expected logarithmic profile. The experimental setup enables the measurement of Young's modulus and the solid-solid work of adhesion for thin films. The value obtained for Young's modulus of polystyrene (PS) was found to be in agreement with other conventional measurement techniques. In addition, measurement of the work of adhesion at the PS/silicon oxide interface was possible. The apparatus is well suited to studying the dependence of Young's modulus, work of adhesion and fracture energy on membrane thickness, temperature, pulling rate, and ageing of the interface, and can readily be modified to study biologically relevant samples.     

Immobilisation and characterisation of the demineralised, fully hydrated organic matrix of nacre - An atomic force microscopy study

Mimicry of the tough natural composite nacre in future bioengineering requires knowledge of the biomineralisation process. The insoluble organic matrix isolated from the shell of the gastropod Haliotis laevigata was characterised by protein chemistry, topographical and mechanical measurements. Demineralisation of nacre in dilute acetic acid or ethylenediaminetetraacetic acid revealed a set of soluble proteins and the insoluble matrix. The insoluble matrix contains a chitin core and firmly attached proteins, which could be removed by sodium dodecyl sulfate and glycerol indicating a hydrophobic interaction. Atomic force microscopy images of the native insoluble matrix showed a filamentous network with pores or holes, where the filaments showed globular attachments of different sizes, possibly the attached protein molecules. During direct observation of protein degradation imaged by atomic force microscopy the insoluble matrix gets smooth and flat indicating the removal of the attached proteins by proteases. We propose a model of protein coated chitin filaments for the insoluble matrix of nacre. Mechanical measurements by force mapping revealed a Young's modulus depending on the hydration state of the organic layers. The fully hydrated organic matrix has an elastic modulus below 1MPa comparable to some hydrogels.     

Elastic behavior of a two-dimensional crystal near melting

Using positional data from video microscopy, we determine the elastic moduli of two-dimensional colloidal crystals as a function of temperature. The moduli are extracted from the wave-vector-dependent normal-mode spring constants in the limit q-->0 and are compared to the renormalized Young's modulus of the Kosterlitz-Thouless-Halperin-Nelson-Young theory. An essential element of this theory is the universal prediction that Young's modulus must approach 16 pi at the melting temperature. This is indeed observed in our experiment.     

Noninvasive protein structural flexibility mapping by bimodal dynamic force microscopy

Mapping of the protein structural flexibility with sub-2-nm spatial resolution in liquid is achieved by combining bimodal excitation and frequency modulation force microscopy. The excitation of two cantilever eigenmodes in dynamic force microscopy enables the separation between topography and flexibility mapping. We have measured variations of the elastic modulus in a single antibody pentamer from 8 to 18 MPa when the probe is moved from the end of the protein arm to the central protrusion. Bimodal dynamic force microscopy enables us to perform the measurements under very small repulsive loads (30-40 pN).     

The structure and mechanical properties of thick rutile-TiO2 films using different coating treatments

The hardness and Young's modulus of thick rutile-TiO₂ films were determined using a continuous stiffness measurement (CSM) technique in this study. Pure rutile-TiO₂ nanopowders (TH₂O, TFeSO₄ and TCuSO₄) were prepared using a modified homogeneous-precipitation process at low temperature (MHPPLT) method. The TiO₂ films were prepared from sols using 3% (w/w) of the prepared-TiO₂ suspension solution coated onto silicon wafers. After dip-coating was completed, the coatings were further treated by natural air-drying, water-vapor exposure, and calcination, respectively. An ellipsometry with a monochromator was used to measure the thickness and refractive index of the TiO₂ films, and a scanning electron microscopy (SEM) to determine their morphology. Three coatings of TH₂O, TFeSO₄ and TCuSO₄ demonstrated their refractive indexes of around 1.60 under three treatments. Volumetric expansion and thickness of the coatings should influence their refractive index. Furthermore, the continuous stiffness measurement (CSM) technique was used to perform nanoindentation testing on the hardness and Young's modulus of prepared rutile-TiO₂ coatings. The mean hardness and Young's modulus of three coatings increased with preparation temperature. In addition, the TH₂O coatings demonstrated greater hardness and modulus than those of TFeSO₄ and TCuSO₄ coatings in the natural air-drying condition. Surface cracking observed on the calcinated TFeSO₄ should be the reason why an obvious decrease of the mean hardness and Young's modulus appeared. Finally, two mechanical properties and related nanoindentation depth of the coatings were discussed in detail.     

ART-induced biophysical and biochemical alterations of Jurkat cell membrane

Integrity of the cell membrane is a basic requirement for maintaining the biological characteristics of a cell. In this study, cell membrane as the target of drug action was investigated. CCK-8 assay suggested that Artesunate (ART) could significantly suppress the proliferation of Jurkat cells in a dose-dependent manner. Changes in the morphology and mechanics of Jurkat cells were studied by atomic force microscopy (AFM). These changes included decrease of Young's modulus (from 3.18±0.54 to 1.72±0.54kPa), increase in the fluctuation of surface components, increase in shrinkage, or even the appearance of pores. The Young's modulus change was according to the F-actin protein, not the Tubulin-β or integrin β1 protein. Meanwhile, the activities of plasma membrane Ca(2+)-Mg(2+)-ATPase and Na(+)-K(+)-ATPase were also repressed following ART exposure as well as membrane potential. Western blot was used to detect Caspase 3 and Cyclin D1 protein level. The Cyclin D1 was downregulated and Caspase 3 was activated. Hence, cellular membrane represented a plausible target for ART-induced injury.     

Measurement of the Elasticity of Biological Soft Tissue of Finite Thickness

Elasticity is of profound significance to evaluating the function of a biological soft tissue.When the elasticity of a tissue is macroscopically changed,it means that the biological function of the tissue is abnormal and some disease or injury may occur.In the present work,an elastometer is developed to measure the elasticity of biological soft tissues.The measurement is based on the indentation method and the force is measured by the bending of the cantilever.The force-indentation data of the soft tissue is experimentally measured by this elastometer and Young's modulus of the tissue is calculated using the Hertz-Sneddon model.For comparison,a numerical model for the indentation method is established using the finite element method.The difference between the actual modulus and the measured modulus is discussed.The effect of the thickness of the specimen on the measurement is investigated.Young's moduli of beef,porcine liver and porcine kidney are experimentally measured.The results indicate that our elastometer is effective in measuring Young's modulus of a soft tissue quantitatively.     

On Young's (1805) equation and Finn's (2006) ‘counterexample’

In 1805, Thomas Young considered the balance of forces acting on the contact line formed at the intersection of a liquid-fluid interface with a solid surface and introduced a macroscopic concept of “contact angle”. From Young's reasoning it follows that in equilibrium the contact angle is a material property of the liquid/fluid/solid system independent of a particular configuration. Two centuries later, Robert Finn considered a model problem which seems to suggest that there is a fundamental flaw in Young's force diagram and the reasoning behind it. In the present note, we show that the apparent paradox in Finn's model problem disappears once Young's original concepts are applied in a correct way.     

Study on the nano machining process with a vibrating AFM tip on the polymer surface

The polymer has been proved to be nano machined by a vibrating tip in tapping mode of Atomic Force Microscope (AFM). The force between the tip and the surface is an important factor which determines success of the machining process. Controlling this force with high accuracy is the foundation of nanomachining in AFM tapping mode. To achieve a deeper understanding on this process, the tip is modeled as a driving oscillator with damping. Factors affecting the nano machining process are studied. The Hertz elastic contact theory is used to calculate the maximum contact pressure applied by the tip which is employed as a criterion to judge the deformation state of the sample. The simulation results show that: The driven amplitude can be used as a main parameter of controlling the machined depth. Sharper tips and harder cantilevers should be used for successful nanomachining with the vibrating tip. Under the same conditions, a larger tip radius will not only result in the machining error, but also lead to failure of the nanomachining process. The higher driving frequency will lead to a larger tapping force. However it cannot be used as a parameter to control the machined depth because of its narrow variation range. But it is a main error source for the nanomachining process in AFM tapping mode. Moreover, a larger Young's modulus polymer sample will induce a smaller machined depth, a larger maximum contact pressure and a bigger tapping force.     

Ultrasonic characterization of ceramic fibres at ambient and elevated temperatures

A non-destructive laser-generated ultrasonic inspection system has been developed to evaluate the elastic properties of ceramic fibres. The approach uses a pulsed Nd:YAG laser to excite ultrasonic signals in fibres. The signal is detected by a piezoelectric acoustic emission transducer to obtain the appropriate frequency response suitable for an elastically one-dimensional sample. By using a differential time-of-flight system, a very accurate measure of the velocity can be obtained in the fibre, with a total scatter of less than 0.5%. This approach has been used to investigate the Young's modulus of polycrystalline carbon and boron fibres as a function of stress. Both types of fibres were found to have a Young's modulus increase as greater applied loads were imposed. The carbon and boron fibres, along with silicon carbide fibres, were evaluated at elevated temperatures up to 700 °C. The carbon fibres were found to have an immediate decrease in the Young's modulus as the temperature was increased, due to oxidation of the carbon. The Young's modulus of the boron fibres decreased only at temperatures higher than 200 °C, probably the result of a microstructural transformation or relaxation. The silicon carbide fibres were found to have no significant change in the elastic properties up to 700 °C. The ultrasonic technique was also applied to polycrystalline alumina fibres and fibre tows between ambient temperature and 1200 °C in a specially designed furnace. Using this technique, it was possible to distinguish the changes in the elasticity of the alumina fibres as they were processed into -alumina. The change in the Young's modulus was readily apparent during phase transformations to -alumina. In addition, the ultrasonic velocity can be used to infer information concerning any coatings that were applied to the alumina fibres. This can be used to aid in the quantification of the coating thickness and uniformity. The application of the ultrasonic inspection system has demonstrated the ability to determine rapidly and non-destructively the elastic properties in ceramic fibres. The information gained from the measurements can be used as a quality assurance technique, or can be modified to be a real-time process control/process monitoring system.     

Calibration of a scanning Joule expansion microscope (SJEM)

Scanning thermal microscopy (SThM) is a scanning probe technique based on atomic force microscopy (AFM) enabling high-resolution topographical imaging together with visualization of the temperature distribution in the studied sample. For the thermal mapping, rather expensive, micro-fabricated cantilevers with integrated thermocouples have to be used. The spatial resolution is typically limited to 100 nm. Scanning Joule expansion microscopy (SJEM) uses an alternative approach to detect the temperature of the sample with a regular silicon cantilever and lock-in detection. By monitoring the thermal expansion of the sample (due to Joule heating), the local temperature can be monitored. The resolution of SJEM is comparable to that of contact AFM, which is an order of magnitude better than for SThM. Our research involves implementing a SJEM for the study of heating phenomena in mesoscopic structures prepared by electron beam lithography and lift-off techniques. In particular, we calibrated our SJEM in order to make quantitative temperature maps of the studied samples.     

Nanomechanics of microtubules

We have determined the mechanical anisotropy of a single microtubule by simultaneously measuring the Young's and the shear moduli in vitro. This was achieved by elastically deforming the microtubule deposited on a substrate tailored by electron-beam lithography with a tip of an atomic force microscope. The shear modulus is 2 orders of magnitude lower than the Young's, giving rise to a length-dependent flexural rigidity of microtubules. The temperature dependence of the microtubule's bending stiffness in the (5-40) degrees C range shows a strong variation upon cooling coming from the increasing interaction between the protofilaments.