Imaging of viscoelastic soft matter with small indentation using higher eigenmodes in single-eigenmode amplitude-modulation atomic force microscopy

In this short paper we explore the use of higher eigenmodes in single-eigenmode amplitude-modulation atomic force microscopy (AFM) for the small-indentation imaging of soft viscoelastic materials. In viscoelastic materials, whose response depends on the deformation rate, the tip–sample forces generated as a result of sample deformation increase as the tip velocity increases. Since the eigenfrequencies in a cantilever increase with eigenmode order, and since higher oscillation frequencies lead to higher tip velocities for a given amplitude (in viscoelastic materials), the sample indentation can in some cases be reduced by using higher eigenmodes of the cantilever. This effect competes with the lower sensitivity of higher eigenmodes, due to their larger force constant, which for elastic materials leads to greater indentation for similar amplitudes, compared with lower eigenmodes. We offer a short theoretical discussion of the key underlying concepts, along with numerical simulations and experiments to illustrate a simple recipe for imaging soft viscoelastic matter with reduced indentation.

Indentation of a floating elastic sheet: geometry versus applied tension

The localized loading of an elastic sheet floating on a liquid bath occurs at scales from a frog sitting on a lily pad to a volcano supported by the Earth’s tectonic plates. The load is supported by a combination of the stresses within the sheet (which may include applied tensions from, for example, surface tension) and the hydrostatic pressure in the liquid. At the same time, the sheet deforms, and may wrinkle, because of the load. We study this problem in terms of the (relatively weak) applied tension and the indentation depth. For small indentation depths, we find that the force–indentation curve is linear with a stiffness that we characterize in terms of the applied tension and bending stiffness of the sheet. At larger indentations, the force–indentation curve becomes nonlinear and the sheet is subject to a wrinkling instability. We study this wrinkling instability close to the buckling threshold and calculate both the number of wrinkles at onset and the indentation depth at onset, comparing our theoretical results with experiments. Finally, we contrast our results with those previously reported for very thin, highly bendable membranes.

Nano-Indentation for Characterizing Mechanical Properties of Soft Materials

We have attempted to apply the computer-based finite element analysis (FEA) method to accurately measure the mechanical properties (e.g., hardness and elasticity) of a soft material by an indentation test. First, an axisymmetric model has been developed using commercially available FEA code ANSYS. The FEA model consisted of a thin Al-film resting on Si-substrate. A spherical indenter has been used to indent the Al-film, which traveled a predefined depth during the loading and unloading cycles. First, numerical simulations were conducted to get the force vs. displacement plot, which was later used to determine the modulus of elasticity and hardness of Al-film. The effects of substrate modulus and indentation depth were thoroughly investigated to determine the modulus and hardness of Al-film. The effect of friction, considered at the interface of indenter and Al-film, was found to offer minimum impact for relatively small indentation depth. The induced force on the Al-film by the indenter has been found to be higher with increasing indentation depth when friction was considered. However the contact stiffness, represented by the slope of the unloading curve, has been found almost the same with and without considering friction. The variation of substrate modulus has been found to be ineffective to capture the Al-film modulus for relatively small indentation depth. However for higher indentation depth, the substrate modulus has been found to offer profound effect to capture the film modulus. The hardness of the Al-film has also been found to be relatively unaffected with variation of substrate modulus. However, the hardness of the Al-film has been found to be higher with friction for relatively high indentation depth. Results obtained from this preliminary research are important to continue further investigation and to characterize the mechanical properties of other soft-materials, e.g., biofilms to minimize its detrimental effects and utilize its favorable aspects in industrial and biomedical applications.

A Study on the Effect of Plate Stiffeners of Double Bottom During Ship Grounding With Large Contact Surface

The effect of plate stiffeners of double bottom during ship grounding with large contact surface was analyzed in this paper, by numerical simulation technique. It is always taken for granted that the effect of the stiffeners on the plate can be neglected, but they should not be dealt with in such a simple way. The objective ship is a double bottom tanker, and four different finite element models are made. Five numerical simulations are designed for each model, with different indentations from 10 percent to 50 percent of the double bottom height. With the simulation results, two sets of comparisons are made. The purpose of the comparison is to demonstrate how much role the stiffeners play in the structural performance during the grounding, and whether the smeared thickness method can predict the role of the stiffeners adequately. It is found that the vertical and horizontal resistances of the cases with stiffeners are all larger than those cases without stiffeners quite a while, which can prove that the stiffeners do have obvious influences on the resistance of the double bottom structure. The difference for horizontal resistance is more obvious than that for vertical resistance. Besides, the value of resistance in the model with stiffeners can be nearly double of that in the corresponding case in the model without stiffeners, especially for those cases with small indentation, which means the stiffeners play an important role in the structure deformation in mild grounding course. Furthermore, it is also indicated that the effect of smeared thickness can not predict the role of the stiffeners adequately, due to the perpendicular disposal of the stiffeners on the plate is of crucial importance in the structural deformation mode. A conclusion is drawn that the influence of the stiffeners on the plate should be taken into consideration and the deformation mechanism of the stiffeners should be included in the simplified analytical method in future.

Effective indenter radius and frame compliance in instrumented indentation testing using a spherical indenter

We introduce a novel method to correct for imperfect indenter geometry and frame compliance in instrumented indentation testing with a spherical indenter. Effective radii were measured directly from residual indentation marks at various contact depths (ratio of contact depth to indenter radius between 0.1 and 0.9) and were determined as a function of contact depth. Frame compliance was found to depend on contact depth especially at small indentation depths, which is successfully explained using the concept of an extended frame boundary. Improved representative stress-strain values as well as hardness and elastic modulus were obtained over the entire contact depth.

Density Measurement of Powder Metallurgy Compacts by Means of Small Indentation

In this study, the density of powder metallurgy compacts was measured by means of small indentation. Zinc (Zn) and aluminum (Al) tablets were fabricated by cold compaction of powders: different tablet densities were obtained by changing the packing pressure. Instrumented indentation tests were carried out by means of flat cylindrical indenters having diameters of 1mm and 2mm. After a calibration procedure, the slope of the indentation curve or the indentation load can be used for the density evaluation.

Determining the Radial Modulus of DNA Measured by VPSFM

Mechanical properties of DNA, for example the elastic modulus, are of vital importance for its biological function. Previously, the modulus is mainly obtained by bending, stretching and twisting DNA using various techniques and tools. By applying vibrating mode scanning polarization force microscopy (VPSFM), deformations of DNA under ultra-small indentation forces can be measured and so the radial modulus can be computed. In this paper, modeling of the VPSFM measuring system is presented. The system is modeled as a spring-mass-damper oscillator under various force fields, such as van der Waals force, attractive electrical force and repulsive interactions between the tip and sample. The electrical polarization force is described by using uniformly charged line model and the DNA is considered to be a simple elastic rod. By numerically integrating the equation of tip motion, the contact force and the radial modulus of DNA under different deformation can be calculated. We found that in measuring radial modulus of DNA, the existence of substrate cannot be neglected, especially when the relative large deformation is reached.

Location-Controlled Large-Grains in Near-Agglomeration Excimer-Laser Crystallized Silicon Films

ABSTRACTLarge grains in thin silicon films were grown by controlling the location of unmolten islands, which are left after near-complete melting of the film during excimer laser crystallization. As the initially amorphous film was first transformed in small grain polycrystalline silicon, these islands contain seeds for crystal growth. To get a single large grain, either the number of seeds was reduced to one or a single one was selected from the seeds by a ‘grain filter’. Former was achieved by making a small indentation in the isolating layer underlying the silicon film so that seeds remain embedded in the indentation. Latter was achieved by making a small diameter hole in the underlying isolating layer, which was filled with amorphous silicon. The lateral growth is preceded by a vertical growth phase during which a single grain is filtered from the initial set of seeds present at the bottom of the hole. In the experiment described, highest yield was achieved for samples in which the melt-depth to hole- diameter ratio was largest.

Quantitative evaluation of adhesion of diamond coatings

This paper presents a model to evaluate quantitatively the adhesion of diamond coating according to indentation tests. It is found that small indentation load causes round spallation of the coating, no matter what the shape of the indenter. An exponential sink-in deformation of the coating under the indentation is proposed [y = −a × exp(−bx)]. The deformation stress at the spallation edge is considered the coating adhesion. Using an experimentally observed relation of the indentation load versus the film spallation radius, we evaluate the adhesion of a diamond coating on copper to be about 1.921–1.956 GPa, which is in agreement with thermal quench results. The validity of this model is also verified by its self-consistence.