Variation Of Surface Stress Research Articles

Sliding State Analysis of Fractal Rough Interface Based on the Finite Element Method.

Local high temperature, stress concentration, and abnormal friction coefficients will appear at the friction pair in a wet clutch as a result of surface topography real-time changing. In order to improve the reliability of clutch friction components and reduce the failure phenomenon, the three-dimensional fractal surface topography data of the actual rough surface asperities are scanned, extracted, and processed successively, and then the finite element model of the rotary sliding friction pair is established considering the micro surface topography. Based on the finite element model, the variation of surface stress and strain is analyzed, and the friction coefficient measured experimentally is taken as the model input. It is concluded that when the rough surface and smooth surface make contact with each other, the maximum stress and plastic strain appear at the inner radius edge. Therefore, this research has a guiding significance for the structural design and processing technology of the friction components in a vehicle clutch.

Competing Marangoni effects form a stagnant cap on the interface of a hydrogen bubble attached to a microelectrode

The formation of a stagnant cap on a bubble attached to a microelectrode is studied by means of numerical simulation including both thermo- and solutocapillary effects. Recently, strong Marangoni flow in an electrolyte around electrogenerated bubbles was observed by Yang et al. [1] and Massing et al. [2]. High local current density above the electrode led to Ohmic heating of the electrolyte near the bubble foot and resulted in thermocapillary convection. However, the experimentally observed Marangoni convection can be predicted better if a stagnant cap of surfactants on the top of the bubble is assumed. The present work provides evidence that supports this hypothesis and simulates the stagnant cap utilizing two methods. In the first method, a critical stagnation angle θs, which marks the border of the stagnant cap, is specified. At the top of the bubble above the stagnation θs the interface motion is suppressed whereas at the bottom of the bubble below the stagnation θs the thermocapillary effect dominates. In the more extensive second method, a transport equation for the surfactant concentration on the bubble interface is included. In this method, the thermo- and solutocapillary effects compete along the entire interface of the bubble. As a result, the top part of the bubble interface will stagnate. We quantify the rigidity of the bubble interface by a dimensionless number, the elasticity number. This elasticity number is the ratio of the solutocapillary stress due to surfactant variation to the thermocapillary stress due to temperature variation. The relevant temperature variation is due to Ohmic heating and is observed to scale with the square of the potential difference. As a consequence, the Marangoni velocity is also found to scale with the square of the potential difference.Additionally, the forces acting on the bubble before detachment are analysed. Special attention is given to the Marangoni force that is more dominant than reported previously because we included the force caused by the uneven pressure distribution along the bubble interface. The pressure distribution is uneven due to a secondary Marangoni vortex in the wedge between the bubble and the electrode. Furthermore, a general framework for Marangoni numbers is introduced to quantify the effect of each specific source of surface stress variation upon the spatial distribution of each variable governed by a convection-diffusion equation.

Inertia and diurnal oscillations of Ekman layers in atmosphere and ocean

A 1D model, including a time variation of eddy viscosity and mixed layer depth, is applied to study Ekman spirals. It simulates a weak velocity in the atmosphere but a jet in the upper oceanic mixed layer during daytime; and a strong velocity in the atmosphere but a weak, uniform velocity in the ocean at night. The mean spirals in both atmosphere and ocean are close to the average spirals at midday and midnight, they are not flat as suggested by previous studies but consistent with the observations of Polton et al (2013). Our results also show shorter length scale for magnitude decay than for rotation of mean velocity as observed in the ocean, which comes from the combined effects of the diurnal variation of PBL and the Coriolis force. The latter becomes more important away from the surface. In the upper oceanic mixed layer, the mean velocity mainly comes from the strong jets in the late afternoon and early evening. Near and below the depth of Ekman depth, the weak velocities change with time and cancel out each other if averaged timing is longer than the inertia period. It results in diminishing of magnitude of the mean velocity, but the amplitude of individual parcel oscillating can still be quite large near the Ekman depth. Meanwhile, the change of velocity angle from the surface is near or less than 90 degree. Hence, shorter length scale for magnitude decay than for rotation of the mean velocity is not controlled by viscosity alone. Meanwhile, the model does not need two viscosities as suggested previously.The results also show that either the diurnal variation of surface stress or eddy viscosity alone can create a diurnal oscillation of velocity in the ocean. The interactions between PBL force and the Coriolis force can create a weak instability in the atmosphere and ocean at 30° and 90°. This weak instability may explain the observed nocturnal LLJ near 30 °N on the lee of the Rocky Mountains and the intensification of mesoscale circulation simulated by Sun and Wu (1992).

Vertical and torsional vibrations of a rigid circular disc on a transversely isotropic and layered half-space with imperfect interfaces

Rigid disc vibration over a layered half-space is essential to understand soil-structure interaction (SSI) between a foundation and the structure over it. In this article, we present a novel semi-analytical and very efficient method to calculate the SSI coefficients on the layered foundation due to both vertical and torsional vibrations. The material properties in each layer are transversely isotropic to take care of the possible different Young's moduli in different directions (vertical vs. horizontal). Furthermore, the interface between the layers is assumed to be general so that possible loose bonding between the layers (where the displacements and/or tractions are discontinuous across the interface) can be considered. The semi-analytical solution is based on the recently developed forward solution of the layered structure with imperfect interface under time-harmonic loadings within the circle on the surface. Since the present SSI problem is a mixed boundary-value problem, the method of superposition in terms of the powerful cylindrical system of vector functions combining with the integral least-square approach is proposed. To take care of multiple layers, the dual variable and position (DVP) is further introduced. The cylindrical system of vector functions has the advantage of separating the torsional vibration from the vertical vibration and yet expressing the two types of solutions uniformly. After validating the proposed semi-analytical method, numerical examples are presented to demonstrate the effect of material layering, interface imperfection, elastic anisotropy, and input frequency on the SSI coefficients, and on the surface displacement and stress variation. These new results should be also good benchmarks for future numerical methods.

Portable microsystem integrates multifunctional dielectrophoresis manipulations and a surface stress biosensor to detect red blood cells for hemolytic anemia.

Hemolytic anemia intensity has been suggested as a vital factor for the growth of certain clinical complications of sickle cell disease. However, there is no effective and rapid diagnostic method. As a powerful platform for bio-particles testing, biosensors integrated with microfluidics offer great potential for a new generation of portable point of care systems. In this paper, we describe a novel portable microsystem consisting of a multifunctional dielectrophoresis manipulations (MDM) device and a surface stress biosensor to separate and detect red blood cells (RBCs) for diagnosis of hemolytic anemia. The peripheral circuit to power the interdigitated electrode array of the MDM device and the surface stress biosensor test platform were integrated into a portable signal system. The MDM includes a preparing region, a focusing region, and a sorting region. Simulation and experimental results show the RBCs trajectories when they are subjected to the positive DEP force, allowing the successful sorting of living/dead RBCs. Separated RBCs are then transported to the biosensor and the capacitance values resulting from the variation of surface stress were measured. The diagnosis of hemolytic anemia can be realized by detecting RBCs and the portable microsystem provides the assessment to the hemolytic anemia patient.

Spatially Multiplexed Micro-Spectrophotometry in Bright Field Mode for Thin Film Characterization.

Thickness characterization of thin films is of primary importance in a variety of nanotechnology applications, either in the semiconductor industry, quality control in nanofabrication processes or engineering of nanoelectromechanical systems (NEMS) because small thickness variability can strongly compromise the device performance. Here, we present an alternative optical method in bright field mode called Spatially Multiplexed Micro-Spectrophotometry that allows rapid and non-destructive characterization of thin films over areas of mm2 and with 1 μm of lateral resolution. We demonstrate an accuracy of 0.1% in the thickness characterization through measurements performed on four microcantilevers that expand an area of 1.8 mm2 in one minute of analysis time. The measured thickness variation in the range of few tens of nm translates into a mechanical variability that produces an error of up to 2% in the response of the studied devices when they are used to measure surface stress variations.

Towards Understanding Why the Thin Membrane Transducer Deforms: Surface Stress-Induced Buckling

This study is directed towards a comprehensive exploration on the deformation mechanism of the thin membrane transducer (TMT) caused by surface stress variation. We stress that the biomolecular interaction has changed the magnitude of the surface stress; and when the surface stress exceeds a critical value the TMT will buckle and deform. Based upon Gurtin's theory of surface elasticity and principle of finite deformation, we abstract the TMT as a nanobeam with two clamped ends, and the close-formed governing equation set is derived accordingly. A computer code via the shooting method is developed to solve the presented two-point boundary value problem. In succession, the nanobeam deflection and critical parameters for buckling are quantitatively discussed. This investigation lays the theoretical foundation of TMTs; and it is also beneficial to gain deep insight into characterizing mechanical properties of nanomaterials and engineering nano-devices.

Effect of surface charge state on the surface stress of a microcantilever

The surface charge state at a liquid–solid interface is important to the variations in the physical/chemical properties of adsorbate film such as surface stress and the ensuing tip deflection of the microcantilever. The well-known Stoney’s equation, derived more than 100 years ago, conceals the film electrical properties with the replacement of substrate deformation induced by adsorptions of particles. This implicit expression provides a shortcut to circumvent the difficulty in identifying some film properties, however, it limits the capacity to ascertain the relation between surface stress variation and the surface charge state. In this paper, we present an analytical expression to quantify the cantilever deflection/surface stress and the film potential difference by combining the piezoelectric theory and Poisson–Boltzmann equation for electrolyte solution. This updated version indicates that the two linear correlations between surface stress and surface charge density or the bias voltage are not contradictory, but two aspects of one thing under different conditions. Based on Parsegian’s mesoscopic interaction potential, a multiscale prediction for the piezoelectric coefficient of double-stranded DNA (dsDNA) film is done, and the results show that the distinctive size effect with variations in salt concentration and nucleotide number provides us with an opportunity to obtain a more sensitive potential-actuated microcantilever sensor by careful control of packing conditions.

An in vivo study of electrical charge distribution on the bacterial cell wall by atomic force microscopy in vibrating force mode.

We report an in vivo electromechanical atomic force microscopy (AFM) study of charge distribution on the cell wall of Gram+ Rhodococcus wratislaviensis bacteria, naturally adherent to a glass substrate, under physiological conditions. The method presented in this paper relies on a detailed study of AFM approach/retract curves giving the variation of the interaction force versus distance between the tip and the sample. In addition to classical height and mechanical (as stiffness) data, mapping of local electrical properties, such as bacterial surface charge, was proved to be feasible at a spatial resolution better than a few tens of nanometers. This innovative method relies on the measurement of the cantilever's surface stress through its deflection far from (>10 nm) the repulsive contact zone: the variations of surface stress come from the modification of electrical surface charge of the cantilever (as in classical electrocapillary measurements) likely stemming from its charging during contact of both the tip and the sample electrical double layers. This method offers an important improvement in local electrical and electrochemical measurements at the solid/liquid interface, particularly in high-molarity electrolytes when compared to techniques focused on the direct use of electrostatic force. It thus opens a new way to directly investigate in situ biological electrical surface processes involved in numerous practical applications and fundamental problems such as bacterial adhesion, biofilm formation, microbial fuel cells, etc.

Surface stress variation as a function of applied compressive stress and temperature in microscale silicon

Surface stress has been shown to affect the mechanical properties of materials at or below the microscale. Surface-stress-induced dislocation activity at such length scales has been shown to be a major factor affecting the mechanical behavior of materials. Defect generation as a function of applied stress at the microscale has previously been measured experimentally and predicted using simulations. However, the change in surface stress in a material in response to externally applied stress as a function of temperature has not been explored experimentally. Such an investigation is presented in this work for the case of microscale silicon samples. In-situ nondestructive measurements of the applied compressive stress and the corresponding microscale surface stress were performed from room temperature to 100 °C. The applied stress was controlled by a nanomechanical loading system. Micro-Raman spectroscopy was used to measure the surface stress in-situ as the samples deformed under the applied uniaxial compressive stress. The surface stress was found to be lower than the applied stress at all temperatures. The difference between the surface stress and the applied stress became higher at higher temperatures indicating that surface relaxation was induced by the temperature increase. Based on the measured values and observed trends, an exponential Gaussian function is proposed to describe the stress as a function of surface depth.

Experimental investigation on variation of machined residual stresses by turning and grinding of hardened AISI 1053 steel

Residual stresses in machined surfaces are of great importance to the service life of a component under various loading conditions. In many cases, the material damage initiates from the weakest spot with the least compressive stress in the component surfaces. This situation leads to the consideration of residual stress moving beyond the traditional thinking of single or average values to the inclusion of variation of stress values on different measurement points. In this paper, we experimentally investigated the surface and in-depth residual stresses in hardened AISI 1053 steels machined using hard turning and surface grinding processes. Cubic boron nitride (CBN) cutting tools were used in both processes. The effects of depth of cut and number of passes were also studied. It was found that both processes produce a significant amount of compressive stress on the machined surfaces, as well as steep stress gradients underneath the surfaces. Compared with hard turning, surface grinding produces higher magnitudes of average compressive residual stresses, but it also generates up to 14 times higher scattering of residual stresses, indicated by the standard deviation of the residual stress measurements. As a result, the benefits of a highly compressive average residual stress will be offset by highly scattered individual measurements. The stochastic nature of abrasive grit distribution and orientations in grinding wheels is believed to be the contributing factor for the significant scattering. Meanwhile, for hard turning, the variation of surface and in-depth residual stresses greatly increases, up to 3.8 times, with a larger depth of cut and the use of multiple passes; however, this trend is less significant for surface grinding.

Piezoresistive measurement of Swine H1N1 Hemagglutinin peptide binding with microcantilever arrays

Effective detection of Swine H1N1 Hemagglutinin peptide is crucial as it could be used as a positive control to screen for highly infectious flu strains such as Swine-Origin Influenza A (H1N1). Piezoresistive microcantilever arrays present a pathway towards highly sensitive and label-free detection of biomolecules by transducing the antigen-antibody binding into change in resistivity via induced surface stress variation. We demonstrate a mechanical transduction of Swine H1N1 Hemagglutinin peptide binding and suggest the employed technique may offer a potential platform for detection of the H1N1 virus, which could be clinically used to diagnose and provide subsequent relief.

2D numerical simulation of submerged hydraulic jumps

Hydraulic jumps are usually used to dissipate energy in hydraulic engineering. In this paper, the turbulent submerged hydraulic jumps are simulated by solving the unsteady Reynolds averaged Navier–Stokes equations along with the continuity equation and the standard k–ɛ equations for turbulence modeling. The Lagrangian moving grid method is employed for the simulation of the free surface. In the developed model, kinematic free-surface boundary condition is solved simultaneously with the momentum and continuity equations, so that the water elevation can be obtained along with velocity and pressure fields as part of the solution. Computational results are presented for Froude numbers ranging from 3.2 to 8.2 and submergence factors ranging from 0.24 to 0.85. Comparisons with experimental measurements show that numerical model can simulate the velocity field, variation of free surface, maximum velocity, Reynolds shear and normal stresses at various stations with reasonable accuracy.

High Sensitivity Microbridge for Molecular Sensing Applications

This paper presents the design and simulation of a SiO2 microbridge which measures the differential surface stress induced by the adsorption of molecules. Materials with smaller Young's modulus would be appropriate to measure the small variation of surface stress on the microbridge and therefore SiO2 is chosen as the material for microbridge. The SiO2 microbridge could be a potential application for the glucose detection if the enzyme glucose oxidase was immobilized on a top surface. The microbridge can be constructed on Silicon on insulator (SOI) substrate using the bulk micromachining technology. The surface stress causes displacement which is measured using the piezoresistors placed on top surface of the microbridge. The piezoresistors are arranged in a Wheatstone bridge configuration to measure the change resistance in terms of voltage. The two piezoresistors are placed at the edges where they experience the maximum stress. The obtained sensitivity, ΔR/R of SiO2 microbridge is 6.15x10-4 which is four times higher than that of existing work [1]. The mechanical behavior of the SiO2 microbridge and the electrical response of the piezoresistors are analyzed using the Finite Element Method (FEM). The dimensions for the microbridge are optimized by considering both the sensitivity and nonlinearity with respect to the deflection. The placement location of the piezoresistors and the effect of geometrical parameters of the piezoresistors on the sensitivity SiO2 microbridge are studied.

Self-aligned process for the development of surface stress capacitive biosensor arrays

A new fabrication process and first experimental results of a surface stress based capacitive biosensor array are presented. Flexible membranes and a fixed electrode on the substrate constitute the capacitive sensors. Probe molecules are immobilized on the membrane surface and the surface stress variations during biological interactions force the membrane to deflect and effectively change the capacitance between the flexible membrane and the fixed substrate. The array consists of 60 sensors and thus is suitable for parallel sensing. The process is characterized by the self-alignment of the sensitive flexible membranes and the use of silicon fusion bonding to fabricate the complete device. First experimental results on biosensing indicate that the sensors are able to detect the hybridization of beta-thalassemia oligonucleotides.

Recent Techniques for the Detection of β-Thalassemia: A Review

β-thalassemias are very heterogeneous in nature, with respect to the molecular basis of the disease. This type of disorder is caused by extensive deletions occurring within the β-globin gene locus, present on chromosome number 11. Due to point mutations, stop codons in the β-globin mRNA is introduced, thus curbing its normal function. This paper reviews conventional and latest β-thalassemia detection techniques. Amongst the conventional techniques, the DNA-based piezoelectric biosensors were based on the observation of distinguishable frequency shifts, resulting from the hybridisation between an oligonucleotide probe which was immobilized on a gold electrode of a quartz crystal and the complementary strand; present in solution. Q-primer real time PCR (Q-PCR) system utilises the 5’>3’ exonuclease activity of a DNA polymerase, which cleaves the nucleotides and results in fluorescence. Amongst the latest detection methods, the capacitive micro-membrane and surface stress based biosensors require flexible structures, microcantilevers or membranes, where one of their surfaces is functionalized with the probe biomolecules. The interaction with the appropriate target molecules induces surface stress variations and finally changes in the deflection of the structure. In bead based biosensors, the natural immobilization of the beads into the polyacrylamide gel pads allows the beads to acquire unique spatial addresses that is recorded via image, and thus helps in avoiding labeling or color encoding steps. These techniques are more efficient and overcome the disadvantages of conventional techniques that were costly, less sensitive and complex too.

Mixed layer modeling in the East Pacific warm pool during 2002

Two vertical mixing models (the modified dynamic instability model of Price et al.; PWP, and K-Profile Parameterizaton; KPP) are used to analyze intraseasonal sea surface temperature (SST) variability in the northeast tropical Pacific near the Costa Rica Dome during boreal summer of 2002. Anomalies in surface latent heat flux and shortwave radiation are the root cause of the three intraseasonal SST oscillations of order 1°C amplitude that occur during this time, although surface stress variations have a significant impact on the third event. A slab ocean model that uses observed monthly varying mixed layer depths and accounts for penetrating shortwave radiation appears to well-simulate the first two SST oscillations, but not the third. The third oscillation is associated with small mixed layer depths (<5 m) forced by, and acting with, weak surface stresses and a stabilizing heat flux that cause a transient spike in SST of 2°C. Intraseasonal variations in freshwater flux due to precipitation and diurnal flux variability do not significantly impact these intraseasonal oscillations. These results suggest that a slab ocean coupled to an atmospheric general circulation model, as used in previous studies of east Pacific intraseasonal variability, may not be entirely adequate to realistically simulate SST variations. Further, while most of the results from the PWP and KPP models are similar, some important differences that emerge are discussed.

Self-Aligned Process for the Development of Surface Stress Capacitive Biosensor Arrays

The fabrication process of a surface stress based capacitive biosensor array is presented. Flexible membranes and a fixed electrode on the substrate constitute the capacitive biosensors. Probe molecules are immobilized on the membrane surface and the surface stress variations during biological interactions force the membrane to deflect and effectively change the capacitance between the flexible membrane and the fixed substrate. The array consists of 60 sensors and thus is suitable for parallel sensing. Through the presented fabrication process, which is described in detail, sensitive membranes can be created. The process simplicity and the increased sensitivity of the biosensors are related to the use of boron implantation and the consequent self-aligned creation of the conductive membranes after the silicon fusion bonding technique.

Detection of DNA mutations using a capacitive micro-membrane array

The detection of DNA hybridization using capacitive readout and a biosensor array of ultrathin Si membranes is presented. The biosensor exploits the ability of the ultrathin membranes to deflect upon surface stress variations caused by biological interactions. Probe DNA molecules are immobilized on the membrane surface and the surface stress variations during hybridization with their complementary strands force the membrane to deflect and effectively change the capacitance between the flexible membrane and the fixed substrate. The sensor array comprises 256 such sensing sites thus allowing the concurrent sensing of multiple DNA mutations. The biosensor and its performance for the detection of complementary DNA strands are demonstrated using beta-thalassemia oligonucleotides. The experimental results show that the presented sensors are able to detect DNA hybridization and to discriminate single nucleotide mismatches.

Surface Stress-Charge Response of a (111)-Textured Gold Electrode under Conditions of Weak Ion Adsorption

We report a cantilever bending investigation into the variation of surface stress, f, with surface charge density, q, for (111)-textured thin films of gold in aqueous NaF and HClO 4. The graphs of f(q) are highly linear, and the surface stress-charge coefficients, d f/d q, are -1.95 V for 7 mM NaF and -2.0 V for 10 mM HClO 4 near the potential of zero charge. These values exceed some previously published experimental data by a factor of 2, but they agree with recent ab initio calculations of the surface stress-charge response of gold in vacuum.