Measuring adhesion, attraction, and repulsion between surfaces in liquids with an atomic-force microscope

Understanding the forces such as adhesion, attraction, and repulsion between surfaces and liquids is the key not only to understanding phenomena such as lubrication and indentation but also the key to understanding how best to operate an atomic-force microscope (AFM). In this paper, we examined the cases of an insulating tip on an insulating sample (silicon nitride tip on mica) and of a conducting tip on a conducting sample (tungsten carbide tip on a gold or platinum foil). The force-versus-distance curves for these two limiting systems were very different in different liquids. In ethanol, the curve is just what one would expect theoretically: a slightly attractive force before contact, a jump into contact, then a small pull-out force, about 0.2 nN for an insulating tip on the insulating surface and about 0.5 nN for two conducting surfaces. In pure water, the behavior is complex and variable. Pull-out forces vary from 0.2 to 1.5 nN for two insulating surfaces. For two conducting surfaces the force-versus-distance curves show large pull-out forces of order of 10 nN. These large forces are probably due to adsorption contamination layers on the metal surfaces that are not removed by the solvent action of the pure water. These forces, however, can be reduced to less than one-hundredth of the original value by adding ethanol to the water. This makes ethanol a useful liquid for routine imaging of macromolecules such as DNA, proteins, and polymers, that have been adsorbed to a substrate and that must be imaged at low force. In formamide, we observed a predicted repulsive interaction in the nontouching regime for insulating surfaces as predicted by Hartmann. In different concentrations of KCl aqueous solution, we observed again a repulsive interaction in the nontouching regime due to double-layer repulsion of charged surfaces across ionic solutions. The measured Debye length agrees well with the theoretical prediction. And last, the dependence of the pull-out force on the indentation in water has been investigated. The more the tip indents the sample surface in a force-versus-distance cycle, the larger the pull-out force will be. This shows also the usefulness of the AFM for investigations of micromechanical properties

Atomic Force Acoustic Microscopy to Measure Nanoscale Mechanical Properties of Cement Pastes

The measurement of elastic properties at the nanoscale is a prerequisite to building a foundation for nanomechanics applications. At present, nanoindentation is widely used to measure the properties of elasticity. Under this method, a sample is indented with a rigid probe and the resistant force of the indentation is measured. The reduced modulus measured on the basis of the resistant force and the indentation depth is then converted to the elastic modulus of the sample. However, its spatial resolution, the distance between two consecutive locations of measurement, is limited to about 5 pm because of the area of the indented tip. Ultrasonic atomic force microscopy is an alternative method of attaining spatial resolution at the nanometer level. It uses information based on the vibrations transferred from the piezoelectric actuator at the bottom of a sample to the cantilever contacting the top surface of the sample. The cantilever makes contact with a relatively small force; as a consequence, it decreases the contact area and improves the spatial resolution. The application of atomic force acoustic microscopy to a cementitious material is described, and the results of the measurement of the elastic modulus of a cement paste are presented.open1

The implementation and the performance analysis of the multi-channel software-based lock-in amplifier for the stiffness mapping with atomic force microscope (AFM)

In this paper the implementation of the surface stiffness mapping method with the dynamic measurement mode of atomic force microscopy (AFM) is presented. As the measurement of the higher harmonics of the cantilever’s torsional bending signal is performed, we are able to visualize non-homogeneities of the surface stiffness. In order to provide signal processing with the desired sensitivity and selectivity, the lock-in amplifier-based solution is necessary. Due to the presence of several useful frequencies in the signal, the utilization of several simultaneously processing channels is required. Therefore the eight-channel software-based device was implemented. As the developed solution must be synchronized with the AFM controller during the scanning procedure, the real-time processing regime of the software is essential. We present the results of mapping the surface stiffness and the performance tests results for different working conditions of the developed setup