Cantilever Torsion Research Articles

Investigation of thin film shear modulus from spatially resolved measurement of Love wave velocity

The investigation of transversely polarized surface acoustic waves (SAWs) is strongly hindered by their missing out-of-plane oscillation component because commonly applied measurement techniques are based on access to that component. Nevertheless, these waves are very interesting for accurate extraction of thin film elastic properties as well as for SAW devices and sensor applications. In this contribution we report a new development of spatially resolved SAW analysis. The scanning acoustic force microscope (SAFM) has been extended in its sensitivity to in-plane polarized waves. A nonlinear mechanism could be found that couples the in-plane surface oscillations to torsional cantilever vibrations. This mechanism is used to transfer the rf SAW signals to kHz frequencies and to measure the SAW phase velocity. The new technique has been applied to study the Love wave propagation in the layered system fused silica on ST-cut quartz. The phase velocity could be derived from measurements of the phase shift along a propagation distance in the micron range. The obtained velocities show a clear spatial variation at elastic changes within the investigated surface. This is used to investigate the shear modulus of the layer material by numerically solving the inverse problem of SAW propagation.

Torsion cantilever as magnetic torque sensor

A macroscopic cantilever for capacitive torque magnetometry has been developed and tested. It is based on torsion arms in order to obtain better damping against external vibrations than with ordinary cantilevers of similar size but anchored on one side. Microfabricated out of silicon-on-insulator wafers by deep reactive ion etching, the sensor consists of a long (14 mm) and thin (40 μm) cantilever with two capacitive plates, anchored at its center to a rigid frame by two torsion bars having a rectangular cross section (80×40 μm2). By comparing the theoretical and experimental resonance frequencies in three different oscillation modes, we show that the elastic properties of the torque sensor can be evaluated with good accuracy. Calibrations performed with a piece of Fe2O3 audiotape and a cylindrical NdFeB magnet yield a torque sensitivity better than 5×10−13 N m under optimized conditions. This device can also be used as a sensitive Gauss meter to detect magnetic fields down to 10 nT.

Microelectromechanical scanning probe instruments for array architectures

A compact (150 μm×150 μm), electrostatic actuator for out-of-plane (z) actuation of a probe tip has been designed, fabricated, and tested. Static deflections of ±0.7 μm out of the plane of the substrate have been achieved. The device consists of a single crystal silicon tip on a torsional cantilever with out-of-plane interdigitated electrode capacitors. This cantilever and electrode design allows a wide range of stiffnesses, actuator force, and frequency response. Significant improvements in performance (larger tip displacement and increased sense capacitance) and a higher density of devices per unit area can be obtained as the minimum feature size decreases. Applications such as information storage, molecular manipulation, and nanolithography require high density, parallel arrays for reasonable throughput. To demonstrate the suitability of this device for use in such arrays, a 12×12 array of microelectromechanical probe tips with integrated actuators and capacitive sensors for scanning probe microscopy has been fabricated. The size of each array element is about 150 μm by 150 μm with a tip-to-tip spacing in the array of 200 μm. Given these dimensions, the packing density of the devices is about 2500 units/cm2.

Lateral Force Modulation Atomic Force Microscopy of Langmuir-Blodgett Film in Water

The mechanical properties of a Langmuir-Blodgett (LB) film in water were investigated using an atomic force microscope with lateral force modulation. During the force curve measurement, the dynamic friction force is measured by observing the amplitude and phase of the cantilever torsion vibration, as the sample is deformed by a tip with gradually increasing normal force. In contrast to conventional friction force microscopy, the friction measurement is performed without x–y scanning, and therefore is in real time. This method was applied to a LB film consisting of an arachidic acid (AA) single layer on top of a single layer of partially fluorinated ether of carboxylic acid (PFECA), and interesting results were obtained. In particular, the film was found to be stable for a short period (<120 min) and the structure and ratio of the friction forces between the AA and PFECA areas are identical with those observed in air. Moreover, a friction force transition was observed at a normal force of around 14 nN on loading and an almost reversible decrease was observed on unloading. This result indicates penetration of the AA film by the tip and recovery of the penetrated hole on unloading.

Analysis of Subsurface Imaging and Effect of Contact Elasticity in the Ultrasonic Force Microscope

We examined, both theoretically and experimentally, the characteristics of subsurface imaging with nanometer resolution and the effect of contact elasticity in the ultrasonic force microscope (UFM). In particular, the effect of the surface energy and effective elasticity on the maximum tip-sample force and the shift of the averaged tip-sample distance were examined. Furthermore, kink formation in the cantilever deflection (z a) against the ultrasonic frequency vibration (UFV) amplitude (a) characteristics was predicted. This model was used to explain experimental observations in UFM, such as the features of the measured z a(a) curve and the damping of the cantilever torsion vibration by the UFV. Moreover, the previously reported lateral ultrasonic force microscope image of subsurface features was explained by the response of subsurface edge dislocation to a large instantaneous force enhanced by the UFV.

Biomechanical analysis of internal fixation techniques for proximal interphalangeal joint arthrodesis

Although numerous fixation techniques have been developed for performing small joint arthrodesis, no previous study of the biomechanical properties of these constructs has been published. The strength of specimens of arthrodesis of the proximal interphalangeal joint performed with cadaver material was determined for three-point anteroposterior bending, axial torsion, and lateral cantilever bending stress. Crossed Kirschner wires, intraosseous wiring, and figure of eight tension-band wiring were studied. Figure of eight tension-band wiring demonstrates superior strength in anteroposterior bending and torsion. For lateral bending stress, no significant difference exists among the techniques studied.

Slow Crack Growth in Cement Composites

The resistance to crack growth in mortar, concrete and fiber reinforced concrete was studied by testing double torsion and double cantilever speciments. R-curves (plots of strain energy release rate vs. crack extension) were obtained from the test data. To include the inelastic strain energy absorbed during the crack extension, the definition of the strain energy release rate was modified. It was found that R-curves for mortar and concrete were similar for both double torsion and double cantilever speciments. R-curves were sensitive to the volume and size of aggregates and to the presence of fibers. It appears that R-curves can be a useful specimen-geometry-independent parameter for chracterizing the fracture behavior of cementitious composites. However, the effects of differing crack-opening displacement on the strain energy release rate must be considered; especially for fiber reinforced concrete.