Viscometer using drag force measurements

A robust and precise viscometer using the forces exerted by a laminar flow inside a small duct is presented: the force is measured on a long cylindrical sensor dipped into the flow. Two devices of respective volumes 1.4 and 0.031?ml have been realized, demonstrating that the technique is usable with small fluid volumes. Several Newtonian and non-Newtonian fluids have been tested at shear rates ranging from 0.3 to 10?s(-1) for the first device and from 85 to 2550?s(-1) for the second one. For Newtonian fluids, of viscosities ranging from 10(-3) to 0.1?Pa?s, the linear response of the device has been verified and a 90% agreement with the values provided by commercial rheometers is obtained. For non-Newtonian polymer solutions, the variation of the force with the flow velocity allows one to determine the dependence of the viscosity on the shear rate. Two shear thinning polymer solutions with a power law behavior at intermediate shear rates have been investigated and their rheological parameters have been determined.     

Contractile force measurements of cardiac myocytes using a micro-manipulation system

In order to develop a cell based robot, we present a micro-mechanical force measurement system for the biological muscle actuators, which utilize glucose as a power source. The proposed measurement system is composed of a micro-manipulator, a force transducer with a glass probe, a signal processor, an inverted microscope and video recording system. Using this measurement system, the contractile force and frequency of the cardiac myocytes were measured in real time and the magnitudes of the contractile force of each cardiac myocyte under different conditions were compared. From the quantitative experimental results, we could estimate that the force of cardiac myocytes is about 20–40 μN, and show that there are differences between the control cells and the micro-patterned cells.     

Application of evanescent wave optics to the determination of absolute distance in surface force measurements using the atomic force microscope

A combined scanning near field optical/atomic force microscope (AFM) is used to obtain surface force measurements between a near field sensing tip and a tapered optical fibre surface, whilst simultaneously detecting the intensity of the evanescent field emanating from the fibre. The tapered optical fibre acts as a compliant sample to demonstrate the possible use of the near field intensity measurement system in determining 'real' surface separations from normal AFM surface force measurements at sub-nanometer resolution between deformable surfaces.     

Viscosity measurements of liquids under pressure by using the quartz crystal resonators

A quartz crystal viscometer has been developed for measuring viscosity in liquids under pressure. It employs an AT-cut quartz crystal resonator of fundamental frequency 3 MHz inserted in a variable-volume vessel designed for working up to 80 MPa. Viscosity is determined by two methods from resonance frequency and bandwidth measurements along up to eight different overtones. The resonance frequency allows an absolute measurement of the viscosity but leads to an accuracy limited to 5% whereas the bandwidth technique which works in a relative way provides an accuracy of 2%. The techniques were tested by carrying out measurements in two pure compounds: heptane and toluene. Measurement results demonstrate the feasibility of the technique in this viscosity range. The apparatus was also used to determine the viscosity of n-decane with dissolved methane. The results obtained with these mixtures reveal the applicability of the apparatus for reservoir fluids study.     

Mechanical property measurements of nanoscale structures using an atomic force microscope

This paper describes nanometer-scale bending tests of fixed single-crystal silicon (Si) and silicon dioxide (SiO2) nanobeams using an atomic force microscope (AFM). The technique is used to evaluate elastic modulus of the beam materials and bending strength of the beams. Nanometer-scale Si beams with widths ranging from 200 to 800 nm were fabricated on a Si diaphragm using field-enhanced anodization using an AFM followed by anisotropic wet etching. Subsequent thermal oxidation of Si beams was carried out to create SiO2 beams. Results from the bending tests indicate that elastic modulus values are comparable to bulk values. However, the bending strength appears to be higher for these nanoscale structures than for large-scale specimens. Observations of the fracture surface and calculations of the crack length from Griffith's theory appear to indicate that the maximum peak-to-valley distance on the beam top surfaces influence the values of the observed bending strengths.     

Coercive force measurements in nondestructive testing

The paper considers applications of coercive force meters with external electromagnets to nondestructive measurements of strength, plastic, and viscous properties of rolled high-carbon and low-alloyed steels, characterization of annealed high-carbon, predominantly chromium-doped, steels, testing of the quality of quenched and tempered steels, measurements of the depth and hardness of surface hardened layers on metallic components, and their use in sorting out different steels.     

Calibration of lateral force measurements in atomic force microscopy with a piezoresistive force sensor

We present here a method to calibrate the lateral force in the atomic force microscope. This method makes use of an accurately calibrated force sensor composed of a tipless piezoresistive cantilever and corresponding signal amplifying and processing electronics. Two ways of force loading with different loading points were compared by scanning the top and side edges of the piezoresistive cantilever. Conversion factors between the lateral force and photodiode signal using three types of atomic force microscope cantilevers with rectangular geometries (normal spring constants from 0.092 to 1.24 N/m and lateral stiffness from 10.34 to 101.06 N/m) were measured in experiments using the proposed method. When used properly, this method calibrates the conversion factors that are accurate to +/-12.4% or better. This standard has less error than the commonly used method based on the cantilever's beam mechanics. Methods such of this allow accurate and direct conversion between lateral forces and photodiode signals without any knowledge of the cantilevers and the laser measuring system.     

Note: Quantitative (artifact-free) surface potential measurements using Kelvin force microscopy

The measurement of local surface potentials by Kelvin force microscopy (KFM) can be sensitive to external perturbations which lead to artifacts such as strong dependences of experimental results (typically in a ～1 V range) with KFM internal parameters (cantilever excitation frequency and/or the projection phase of the KFM feedback-loop). We analyze and demonstrate a correction of such effects on a KFM implementation in ambient air. Artifact-free KFM measurements, i.e., truly quantitative surface potential measurements, are obtained with a ～30 mV accuracy.     

Harnessing bifurcations in tapping-mode atomic force microscopy to calibrate time-varying tip-sample force measurements

Torsional harmonic cantilevers allow measurement of time-varying tip-sample forces in tapping-mode atomic force microscopy. Accuracy of these force measurements is important for quantitative nanomechanical measurements. Here we demonstrate a method to convert the torsional deflection signals into a calibrated force wave form with the use of nonlinear dynamical response of the tapping cantilever. Specifically the transitions between steady oscillation regimes are used to calibrate the torsional deflection signals.     

Improving the precision of Hall effect measurements using a single-crystal copper probe

The circuitry and components of a Hall measurement kit were replaced with single-crystal copper (SCC) wires and parts prepared by a novel wire fabrication process. This process preserved the grain-free structure of SCC grown by the Czochralski method. The new kit was used to determine, with greatly improved precision, the electrical coefficients such as carrier density and mobility, establish the reproducibility of the measured values, and define the semiconductor type. The observed reduction in electrical signal losses and distortion has been attributed to grain boundary elimination.     

Comparison of friction measurements using the atomic force microscope and the surface forces apparatus: the issue of scale

Results are presented of lateral force measurements using the atomic force microscope (AFM) and the surface forces apparatus (SFA). Two different probes are used in the AFM measurements; a sharp silicon nitride tip (radius R20 nm) and a glass ball (R15 m). The lateral force is measured between the (silicon nitride or glass) probe and a mica surface which has been coated by a thin lubricant film. In the SFA, a thin lubricant film separates two molecularly smooth mica surfaces (R1 cm) which are slid relative to each other. Perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) were used as the lubricant films. In the SFA where the contact diameter is largest, the PFPE film shows much lower friction than PDMS. As the size of the probe decreases, the difference in the measured friction decreases. For sharp AFM tips, no clear distinction between the tribological properties of the films can be made. Hence, the measured coefficient of friction varies according to the length scale probed, at least for small dimensions.     

Offset reduction in Hall effect measurements using a nonswitching van der Pauw technique

A nonswitching van der Pauw technique, which uses two electrically isolated alternating current sources operating at two different frequencies and two lock-in amplifiers, is suggested for Hall effect measurements. Parasitic offset voltage, typical for this type of measurements, is reduced by averaging two sets of data accumulated simultaneously. Application of the technique is particularly useful when the offset changes on a time scale comparable to the measurement cycle.     

Compact cantilever force probe for plasma pressure measurements

A simple, compact cantilever force probe (CFP) has been developed for plasma pressure measurements. It is based on the pull-in phenomenon well known in microelectromechanical-system electrostatic actuators. The probe consists of a thin (25 mum) titanium foil cantilever (38 mm of length and 14 mm of width) and a fixed electrode separated by a 0.75 mm gap. The probe is shielded by brass box and enclosed into boron nitride housing with a 9 mm diameter window for exposing part of cantilever surface to the plasma. When the voltage is applied between the cantilever and the electrode, an attractive electrostatic force is counterbalanced by cantilever restoring spring force. At some threshold (pull-in) voltage the system becomes unstable and the cantilever abruptly pulls toward the fixed electrode until breakdown occurs between them. The threshold voltage is sensitive to an additional externally applied force, while a simple detection of breakdown occurrence can be used to measure that threshold voltage value. The sensitivity to externally applied forces obtained during calibration is 0.28 V/microN (17.8 VPa for pressure). However, the resolution of the measurements is +/-0.014 mN (+/-0.22 Pa) due to the statistical scattering in measured pull-in voltages. The diagnostic temporal resolution is approximately 10 ms, being determined by the dynamics of pull-in process. The probe has been tested in the tokamak ISTTOK edge plasma, and a plasma force of approximately 0.07 mN (plasma pressure approximately 1.1 Pa) has been obtained near the leading edge of the limiter. This value is in a reasonable agreement with the estimations using local plasma parameters measured by electrical probes. The use of the described CFP is limited by a heat flux of Q approximately 10(6) W/m(2) due to uncontrollable rise of the cantilever temperature (DeltaT approximately 20 degrees C) during CFP response time.     

Differential force microscope for long time-scale biophysical measurements

Force microscopy techniques including optical trapping, magnetic tweezers, and atomic force microscopy (AFM) have facilitated quantification of forces and distances on the molecular scale. However, sensitivity and stability limitations have prevented the application of these techniques to biophysical systems that generate large forces over long times, such as actin filament networks. Growth of actin networks drives cellular shape change and generates nano-Newtons of force over time scales of minutes to hours, and consequently network growth properties have been difficult to study. Here, we present an AFM-based differential force microscope with integrated epifluorescence imaging in which two adjacent cantilevers on the same rigid support are used to provide increased measurement stability. We demonstrate 14 nm displacement control over measurement times of 3 hours and apply the instrument to quantify actin network growth in vitro under controlled loads. By measuring both network length and total network fluorescence simultaneously, we show that the average cross-sectional density of the growing network remains constant under static loads. The differential force microscope presented here provides a sensitive method for quantifying force and displacement with long time-scale stability that is useful for measurements of slow biophysical processes in whole cells or in reconstituted molecular systems in vitro.     

Tip-to-sample distance dependence of an electrostatic force in KFM measurements

In most scanning probe methods like an atomic force microscopy, a cantilever is mechanically vibrated in order to obtain topographies. Therefore, a tip-to-sample distance periodically changes during the scanning. Since the electrostatic force, which is a long-range force, is used for potential feedback in Kelvin probe force microscopy (KFM), such mechanical vibration leads to the fluctuation of the electrostatic force between the tip and the sample. In this study, firstly, we performed two-dimensional simulations of the electric fields between surfaces of the tip and the sample and evaluated the tip-to-sample distance dependence of the electrostatic force. Secondly, we experimentally confirmed the existence of the fluctuation of the electrostatic force and the tip-to-sample distance dependence of the electrostatic force was evaluated. Both the simulations and the experiments on the tip-to-sample distance dependence showed the importance of considering the tip sidewall effect in the KFM potential determination.     

Investigating the effect of clamping force on the fatigue life of bolted plates using volumetric approach

In this paper, the effects of bolt clamping force on the fatigue life for bolted plates made from Al7075-T6 have been studied on the values of notch strength reduction factor obtained by volumetric approach. To attain stress distribution around the notch (hole) which is required for volumetric approach, nonlinear finite element simulations were carried out. To estimate the fatigue life, the available smooth S-N curve of Al7075-T6 and the notch strength reduction factor obtained from volumetric method were used. The estimated fatigue life was compared with the available experimental test results. The investigation shows that there is a good agreement between the life predicted by the volumetric approach and the experimental results for various specimens with different amount of clamping forces. Volumetric approach and experimental results showed that the fatigue life of bolted plates improves because of the compressive stresses created around the plate hole due to clamping force.     

Local phase measurements of light in a one-dimensional photonic crystal

For the first time the local optical phase evolution in and around a small, one-dimensional photonic crystal has been visualized with a heterodyne interferometric photon scanning tunnelling microscope. The measurements show an exponential decay of the optical intensity inside the crystal, which consists of a periodic array of subwavelength air rods fabricated in a conventional ridge waveguide. In addition it is found that the introduction of the air rods has a counterintuitive effect on the phase development inside the structure. The heterodyne detection scheme allows the detection of low-intensity scattered waves. In the vicinity of the scattering air rods phase singularities are found with a topological charge of plus or minus one.     

High temperature and force measurements using a type II cascaded regenerated fiber Bragg grating (RFBG)

Abstract

A Type II-IR (infrared) grating cascaded with a regenerated fiber Bragg grating (RFBG) is described for force measurements at high temperature. The sensor was fabricated by fusion splicing a Type II grating with a Type I seed grating that was subjected to isothermal annealing in order to obtain the RFBG. Due to the different responses to temperature and force, these parameters were determined by measuring the reflection peak of the Type II grating and RFBG. The experimental results indicate that the sensor may be used for simultaneous measurement of temperature and force from 250?°C to 500?°C and 0.003?N to 0.797?N.     

An atomic force microscope-based investigation of vertical transport through GaAs/GaAlAs/InAlAs/GaAs step-barrier heterostructures

Study of vertical transport through heterostructures consisting of single, double, or multiple quantum barriers is of both fundamental and technological interest. While extensive data regarding electron transport is available for single- and double-barrier structures, relatively less information is available for transport through step-barrier structures. In this paper, we present results from a study of room temperature vertical transport through a GaAs/GaAlAs/InAlAs/GaAs multistep-barrier heterostructure. A typical atomic force microscope has been adapted to perform transport measurements, thus allowing precise control of the physical location of the region of measurement. I-V measurements reveal negative differential resistance (NDR) peaks, thus confirming the formation of resonant states in a triangular well created when a voltage bias is applied across the step barrier. I-V curves have also been calculated by numerically solving the Schr?dinger wave equation for this step-barrier structure. Comparison between the measured and calculated I-V curves shows reasonable agreement in the number of NDR peaks. However, discrepancies exist between the measured and calculated values for the voltages at which these NDR peaks occur. Some possible reasons for these discrepancies are discussed in this work.     

High-speed force load in force measurement in liquid using scanning probe microscope

This article presents an inversion-based iterative feedforward-feedback (II-FF/FB) approach to achieve high-speed force load in force measurement of soft materials in liquid using scanning probe microscope (SPM). SPM force measurement under liquid environment is needed to interrogate a wide range of soft materials, particularly live biological samples. Moreover, when dynamic evolution of the sample occurs during the measurement, and/or measuring the rate-dependent viscoelasticity of the sample, the force measurement also needs to be acquired at high-speed. Precision force load in liquid, however, is challenged by adverse effects including the thermal drift effect, the reduction of the signal to noise ratio, the distributive hydrodynamic force effect, and the hysteresis and vibrational dynamics effects of the piezoelectric actuators (for positioning the probe relative to the sample), particularly during high-speed measurement. Thus, the main contribution of the article is the development of the II-FF/FB approach to tackle these challenges. The proposed method is illustrated through an experimental implementation to the force-curve measurement of a poly (dimethylsiloxane) sample in liquid at high-speed.