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Über dieses Buch

Ever since the genesis of life, and throughout the course its further evolution, Nature has constantly been called upon to act as an engineer in solving technical problems. Organisms have evolved a variety of well-defined shapes and structures. Although often intricate and fragile, they can nonetheless deal with extreme mechanical loads. Some organisms live attached to a substrate; others can also move, fly, swim and dive. These abilities and many more are based on a variety of ingenious structural solutions. Understanding these is of major scientific interest, since it can give insights into the workings of Nature in evolutionary processes. Beyond that, we can discover the detailed chemical and physical properties of the materials which have evolved, can learn about their use as structural elements and their biological role and function. This knowledge is also highly relevant for technical applications by humans. Many of the greatest challenges for today's engineering science involve miniaturization. Insects and other small living creatures have solved many of the same problems during their evolution. Zoologists and morphologists have collected an immense amount of information about the structure of such living micromechanical systems. We have now reached a sophistication beyond the pure descriptive level. Today, advances in physics and chemistry enable us to measure the adhesion, friction, stress and wear of biological structures on the micro- and nanonewton scale. Furthermore, the chemical composition and properties of natural adhesives and lubricants are accessible to chemical analysis.

Inhaltsverzeichnis

Frontmatter

Basics and Physical Tools

Frontmatter

1. Introduction

Abstract
Micro- and nanotribology — considered as the mechanical interaction of moving bodies — is the science of friction, adhesion, lubrication and wear on the length scale of micrometers to nanometers and the force scale of millinewtons (mN) to nanonewtons (nN). This rather young field of (micro- and nano-) science was boosted by the advent of new analysis techniques, for example the Atomic Force Microscope (AFM), allowing insight into fundamental processes. In the microrange the data storage industry used tribology to solve problems concerning the head—disk—interface in magnetic disk drives. Due to miniaturization of the mechanical parts the forces at the interface decreased dramatically and have reached the nanorange for the case of IBM’s millepede, an AFM-based storage device [1]. More and more conventional mechanical devices, for instance motors [2], are becoming miniaturized. Former mesoscale applications like accelerometers in cars are being replaced by silicon-based mechanical microsystems (see [3] for an overview).
Matthias Scherge, Stanislav S. Gorb

2. Physical Principles of Micro- and Nanotribology

Abstract
In this chapter the physical principles are described mainly by presenting results of experiments performed with a model system comprising a silicon ball sliding on a flat silicon sample (referred to as the SILICON model system). Friction, lubrication and wear problems are discussed to obtain a comprehensive picture of tribology and adhesion. In addition, mechanical properties such as roughness, hardness, elasticity, viscoelasticity and contact theory are also introduced.
Matthias Scherge, Stanislav S. Gorb

Biological Friction Systems

Frontmatter

3. Biological Frictional and Adhesive Systems

Abstract
The biological world is part of the physical world and, therefore, the rules of mechanics also apply to living systems. The mechanics of living motion systems is the subject of an integrative discipline called biomechanics [39]. Living creatures move on land, in the air, or in water. There are complex motions inside their bodies to provide fluid circulation or to generate forces for locomotion. The resistance against motion mediated by surrounding media and by the mechanical contact with various substrates was an evolutionary factor, which contributed to the appearance of many devices adapted to reduce such resistance. On the other hand, one always needs friction to generate the force to move on a substrate or to overcome the drag caused by friction elsewhere. A living motion system becomes optimized when it is capable of minimizing friction at one end of the system while maximizing it at the other end [308]. In other words, a living device needs a combination of maximum friction required for acceleration, deceleration and maneuvering, combined with minimum friction in joints for economic energy expenditure. Adhesion phenomena can also contribute to the functionality of such a system.
Matthias Scherge, Stanislav S. Gorb

4. Frictional Devices of Insects

Abstract
Since our case studies (Part IV) have been performed on cricket (orthopteran) attachment pads, this chapter is written to give a short survey of insect surfaces, and the material structure of the outer layer of insect skin (cuticle), as well as to provide a short overview of the frictional surfaces described for the case of insects. Frictional devices have been reported on terminal segments of legs, head-arresting systems, mouth-part (galeal) linkage in some flies (Lepidoptera) [453], and armored membranes in butterflies and moths (Diptera) [454]. Copulation organs, the egg-laying apparatus (ovipositor), and intersegmental coupling are also often supplemented with frictional surfaces. The notion that insects are “animals of the surface” reflects the general significance of the microsculpture of the insect skin (integument) in the biology of this group of animals. Functions of cuticular protuberances are listed in Fig. 4.1. Highly specialized, jointless cuticular microstructures originating from a cell complex (trichoid complex of epidermal cells) are mainly sensory organs, but in some cases the original sensory function has been lost in evolution (scales). Some skin outgrowth (setae) may also have a defensive function, as in the dermestid beetle larva or the function of pheromone release. Cuticular microstructures originating from one cell (acanthae), or as a pattern from one epidermal cell (microtrichia), serve diverse mechanical function or form a coloration pattern.
Matthias Scherge, Stanislav S. Gorb

Test Equipment

Frontmatter

5. Microscale Test Equipment

Abstract
Forces from newtons to millinewtons are commonly measured using strain gauge load cells. Load cells come with a Wheatstone bridge configuration, featuring a mV/V output or with an integrated amplifier providing common process output signals. Upon bending a deflection beam, areas with mechanical strain and compression develop that can be measured with mechanicalelectric transformers, e.g., foil-strain measurement stripes. These metallic stripes have an isolating carrier foil, which is glued to the backside of the beam. The force-dependent changes of the resistance are measured in a Wheatstone bridge, and in an amplifier the changes are transformed to a voltage or current signal for further processing. The use of load cells is limited to forces not smaller than a few millinewtons. If the forces become smaller, the signal-to-noise ratio does not allow a precise measurement.
Matthias Scherge, Stanislav S. Gorb

6. Nanoscale Probe Techniques

Abstract
The family of instruments which measure physical and chemical properties on the nanometer scale are called Scanning Probe Microscopes (SPM). In this section scanning probe microscopy is discussed in the context of its use in micro- and nanotribology [580]. Although the instruments — STM, AFM and their modifications — are commonly used on both length scales, distinctions between micro and nano have to be made, since the range of application differs significantly. Whereas on the microscale the instrument is mainly used to measure topography [581], friction [582], adhesion and mechanical properties, SPMs provide detailed information on structural properties [583–588], chemistry [589–592] and motion on the nanoscale [593–596]. The use of scanning probe techniques for biological applications [597, 598] was promoted by the simplification of preparation techniques and the handling of different sample types. Furthermore, a new interesting feature is the option to analyze living substrates in their native environment [599, 600]. The samples can be imaged in their hydrated state, eliminating shrinkage and thus degradation.
Matthias Scherge, Stanislav S. Gorb

7. Microscopy Techniques

Abstract
To understand the microtribological properties of biological material, information about the surface sculpture and material design are necessary. Additionally, many biological frictional and adhesive systems are supplemented by various secretions delivered in the contact area. Such secretions are products of specialized cells. The biological sciences profited enormously from the development of microscopy techniques. Light microscopy, together with histological and histochemical techniques, aided in understanding the structure and functions of tissues and cells. Scanning (SEM) and Transmission Electron Microscopy (TEM) have contributed to studies at the sub-cellular level of biological organization.
Matthias Scherge, Stanislav S. Gorb

Case Studies

Frontmatter

8. Samples, Sample Preparation and Tester Setup

Abstract
Biological motion systems are highly adapted micromechanical units. Due to the long evolution period these systems exhibit an optimum combination of mechanical (stability, elasticity) and tribological (friction, adhesion, wear) properties. Therefore, biological systems are excellent candidates as model systems for the design of new materials and for micromechanical applications.
Matthias Scherge, Stanislav S. Gorb

9. Case Study I: Indentation and Adhesion

Abstract
This study focusses on the attachment performance of the attachment pads. By means of force—distance curves the forces are evaluated quantitatively. Also, since contact between the flexible pad and the rigid surface is always accompanied by an indentation of the pad, indentation is discussed prior to the effects of adhesion.
Matthias Scherge, Stanislav S. Gorb

10. Case Study II: Friction

Abstract
For friction experiments, the interaction was measured between the attachment pad and the flat silicon surface. The leg containing the pad was attached to a sample holder that was oscillated in the tangential direction over a length of 30 µm by a piezo-transducer (x-piezo) at a constant velocity (30 µm/s). The silicon sample — a 5 mm by 5 mm piece cleaved from a silicon wafer — was attached to the double-leaf spring as shown in Fig. 10.1. For these experiments the interferometer-based tester was used because of its higher force resolution (see Sect. 5.1.4).
Matthias Scherge, Stanislav S. Gorb

11. Case Study III: Material Properties

Abstract
Since the tribological properties of a biological material are determined not only by the surface but also to a great extent by its inner structure, the mechanical response of the whole pad was investigated to determine its elastic and viscoelastic properties.
Matthias Scherge, Stanislav S. Gorb

12. Outlook

Abstract
This book was conceived as a first approach to discover the micromechanical properties of biological systems. The outcome of further studies could be used to foster new design philosophies for MEMS.
Matthias Scherge, Stanislav S. Gorb

Appendix

Frontmatter

A. Contact Models

Abstract
This appendix lists the friction force formulae derived by combining the Bow-den Tabor model (F f = τA c ) with the appropriate contact model. In addition, it lists the model parameters, which determine the applicability of the contact models.
Matthias Scherge, Stanislav S. Gorb

B. Capillary Theory

Abstract
In Sect. 2.4.3 a simple approximation (F c = 4πγR) was introduced to cal­culate the capillary force between the ball and the flat sample. Throughout the book cases were shown where this equation did not hold. This appendix, therefore, presents a detailed derivation of this equation and shows where simplifications are introduced.
Matthias Scherge, Stanislav S. Gorb

C. Glossary

Without Abstract
Matthias Scherge, Stanislav S. Gorb

D. List of Symbols

Without Abstract
Matthias Scherge, Stanislav S. Gorb

Backmatter

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