Animals with widely varying body weight, such as flies, spiders, and geckos, can adhere to and move along vertical walls and even ceilings. This ability is caused by very efficient attachment mechanisms in which patterned surface structures interact with the profile of the substrate. An extensive microscopic study has shown a strong inverse scaling effect in these attachment devices. Whereas m dimensions of the terminal elements of the setae are sufficient for flies and beetles, geckos must resort to sub-m devices to ensure adhesion. This general trend is quantitatively explained by applying the principles of contact mechanics, according to which splitting up the contact into finer subcontacts increases adhesion. This principle is widely spread in design of natural adhesive systems and may also be transferred into practical applications.walking ͉ adhesion ͉ locomotion ͉ legs ͉ insects A ttachment structures have independently developed several times in animal evolution (1, 2). Setose or hairy systems of various animal groups, such as insects, spiders, and lizards contain surfaces covered by fine patterns of protuberances of different origin. These highly specialized structures are not restricted to one particular area of the leg and may be located on different derivatives of the tarsus and pretarsus (3). Even among insects, the protuberances belong to different types: representatives of the Coleoptera and Dermaptera have setae with sockets providing additional mobility of setae, whereas representatives of Diptera have setae without sockets (acanthae). Setae range in their length from several millimeters to a few micrometers (4).Despite Ͼ300 years of studies on hairy attachment systems, there is still a debate concerning the attachment mechanism of animals walking on smooth walls or ceilings. Different hypotheses have been proposed to explain the mechanism of attachment: sticking fluid, microsuckers, and electrostatic forces (5). Based on experimental data, some of these theories have been rejected, and adhesion has been attributed to a combination of molecular interactions and capillary attractive forces mediated by secretions (6) or purely van der Waals interactions (7). Because some animals produce secretory fluids (insects) (8-10) in the contact area, whereas others do not (spiders, geckos) (11, 12), one can expect different basic physical forces contributing to the overall adhesion. Recently, strong evidence has been presented (13) that the adhesion of gecko setae is caused by van der Waals interaction, rejecting mechanisms relying on capillary adhesion. Elements of contact mechanics have also been applied to this problem (13,14); it was predicted that arrays with smaller setae endings should result in greater adhesive strength. In the present study, we combine an extensive microscopical study ¶ of biological surface devices with the theory of contact mechanics based on molecular adhesion. We will show that the scaling of the surface protuberances, for animals differing in weight by 6 orders of magnitude, ca...

Scanning X-ray microdiffraction (microSXRD) combines the use of high-brilliance synchrotron sources with the latest achromatic X-ray focusing optics and fast large-area two-dimensional-detector technology. Using white beams or a combination of white and monochromatic beams, this technique allows for the orientation and strain/stress mapping of polycrystalline thin films with submicrometer spatial resolution. The technique is described in detail as applied to the study of thin aluminium and copper blanket films and lines following electromigration testing and/or thermal cycling experiments. It is shown that there are significant orientation and strain/stress variations between grains and inside individual grains. A polycrystalline film when investigated at the granular (micrometer) level shows a highly mechanically inhomogeneous medium that allows insight into its mesoscopic properties. If the microSXRD data are averaged over a macroscopic range, results show good agreement with direct macroscopic texture and stress measurements.

The resistivities of individual multiwalled pure and boron-doped carbon nanotubes have been measured in the temperature range from 25 to 300 °C. The connection patterns were formed by depositing two-terminal tungsten wires on a nanotube using focused-ion-beam lithography. A decrease of the resistivity with increasing temperature, i.e., a semiconductor-like behavior, was found for both B-doped and pure carbon nanotubes. B-doped nanotubes have a reduced room-temperature resistivity (7.4×10−7–7.7×10−6 Ωm) as compared to pure nanotubes (5.3×10−6–1.9×10−5 Ωm), making the resistivity of the doped tubes comparable to those along the basal plane of graphite. The activation energy derived from the resistivity versus temperature Arrhenius plots was found to be smaller for the B-doped (55–70 meV) than for the pure multiwalled nanotubes (190–290 meV).

The availability of high brilliance 3 rd generation synchrotron sources together with progress in achromatic focusing optics allow to add submicron spatial resolution to the conventional century-old X-ray diffraction technique. The new capabilities include the possibility to map in-situ, grain orientations, crystalline phase distribution and full strain/stress tensors at a very local level, by combining white and monochromatic X-ray microbeam diffraction. This is particularly relevant for high technology industry where the understanding of material properties at a microstructural level becomes increasingly important. After describing the latest advances in the submicron X-ray diffraction techniques at the ALS, we will give some examples of its application in material science for the measurement of strain/stress in metallic Work supported in part by the Department of Energy contract DE-AC03-76SF00515. August 2002 2 thin films and interconnects. Its use in the field of environmental science will also be discussed. KeywordsX-ray micro-diffraction, thin films, microtexture, strain/stress Contact Author Nobumichi Tamura, Lawrence Berkeley National Lab., MS 2-400, Berkeley, CA 94720, tel. (510) 486 6189, fax (510) 486 7696 e-mail: ntamura@lbl.gov Introduc tionMaterials properties such as strengthening, resistance to fatigue and failure intimately depend on their microstructural features such as grains, grain boundaries, inclusions, voids and other defects. However, at the so-called mesoscopic length scale (approximately between 0.1 and 10 microns) materials typically exhibit high inhomogeneity, and properties are extremely difficult to study both experimentally and theoretically. This length scale is situated between the atomic scale of atoms and individual dislocations, and the macroscopic scale of continuum mechanics.X-ray diffraction is a powerful technique, used for almost a century to measure grain orientation and strain, as well as for crystalline phase identification and structure refinement.Compared to electron microscopy, X-rays have the advantages of higher penetration depth (rendering possible the scanning of bulk and buried samples), do not require any particular sample preparation and can be used under a variety of different conditions (in air, liquid, 3 vacuum or gas, at different temperature and pressures). Its main drawback for the study of materials at the micron scale was until recently its poor spatial resolution.Today, the availability of high brilliance third generation synchrotron sources, combined with progress in X-ray focusing optics and fast 2D large area detector technology have made possible the development of Scanning X-ray Microdiffraction (µSXRD) techniques using either monochromatic or polychromatic focused beams of sizes ranging from a few microns to submicron [1][2][3][4][5][6][7][8]. The closest equivalents in the electron microscopy field are STEM (Scanning Transmission Electron Microscopy) and EBSD (Electron Back Scatter Diffraction).The spatial resolution of electron micr...

At the Advanced Light Source in Berkeley we have instrumented a beam line that is devoted exclusively to x-ray micro diffraction problems. By micro diffraction we mean those classes of problems in Physics and Materials Science that require x-ray beam sizes in the sub-micron range. The instrument is for instance, capable of probing a sub-micron size volume inside micron sized aluminum metal grains buried under a silicon dioxide insulating layer. The resulting Laue pattern is collected on a large area CCD detector and automatically indexed to yield the grain orientation and deviatoric (distortional) strain tensor of this sub-micron volume. A four-crystal monochromator is then inserted into the beam, which allows monochromatic light to illuminate the same part of the sample. Measurement of the diffracted photon energy allows for the determination of d spacings. The combination of white and monochromatic beam measurements allow for the determination of the total strain/stress tensor (6 components) inside each sub-micron sized illuminated volume of the sample. Keywords X-ray micro-diffraction, electromigration, x-ray focusing, Contact Author Alastair MacDowell, Lawrence Berkeley National Lab., MS 2-400, Berkeley, Ca 94720, tel. 510 486 4276, fax 510 486 7696 e-mail:aamacdowell@lbl.gov Introduction X-ray diffraction is a technique that has been used for about a century for elucidating the structure of materials on the macroscopic scale (0.1-10mm). With the increasing need from industry to develop materials of high mechanical performance, a good understanding of their properties at the mesoscopic scale (0.1-10µm) has become critical since many of these properties are dependant on the behavior of structural entities at this scale (grain boundaries, inclusions, intrinsic intra-and inter-granular stress distribution). There is a significant amount of complexity when dealing with the mutual interactions of a large number of mesoscale grains especially when dislocations are involved. The situation is influenced by such parameters as mechanical stress, temperature, and contact with other materials. The limited amount of experimental data is due to the lack of a suitable technique. This has long prevented modeling material behavior at this length scale, which in turn has prevented progress in developing a systematic link of material properties from the macroscopic to the microscopic. With the recent availability of bright third generation synchrotron sources and progress in xray focusing optics, it is now practical to develop x-ray diffraction and apply it on the micron scale length to measure the mesoscopic properties of materials.

Articles you may be interested inDetermination of elastic constants of a fiber-textured gold film by combining synchrotron x-ray diffraction and in situ tensile testing J. Appl. Phys. 98, 093511 (2005); 10.1063/1.2126154 Effect of tensile offset angles on micro/nanoscale tensile testing Rev. Sci. Instrum. 76, 033904 (2005); 10.1063/1.1865732Measuring thin film and multilayer elastic constants by coupling in situ tensile testing with x-ray diffractionThe mechanical properties of metallic thin films on the nanoscale acquire increasingly more importance as applications in microelectromechanical systems/NEMS as well as microelectronics have reached this size scale. Here, we present a synchrotron x-ray diffraction technique by which it is possible to characterize the evolution of mechanical stress in a metallic film thinner than 100 nm at measurement times shorter than 60 s per data point. This high data acquisition rate is achieved because no relative motions or tilting of specimen, x-ray source and detector ͑a large-area charge coupled device camera͒ are required. The technique comprises an initial ''sin 2 '' measurement to establish the absolute stress values followed by periodic ''sin 2 '' measurements during straining to determine stress increments. We describe an experimental setup established at the synchrotron radiation source ANKA ͑Karlsruhe, Germany͒ which is specifically suited for monitoring the stress evolution during in situ tensile tests on thin films deposited on compliant polymeric substrates. First stress-strain curves, measured in this way, for a 40-nm-thick gold film are presented.

We have used scanning white beam x-ray microdiffraction to study microstructural evolution during an in-situ electromigration experiment on a passivated Al(Cu) test line. The data show plastic deformation and grain rotations occurring under the influence of electromigration, seen as broadening, movement, and splitting of reflections diffracted from individual metal grains. We believe this deformation is due to localized shear stresses that arise due to the inhomogeneous transfer and deposition of metal along the line. Deviatoric stress measurements show changes in the components of stress within the line, including relaxation of stress when current is removed.

Electromigration occurs when a high current density drives atomic motion from the cathode to the anode end of a conductor, such as a metal interconnect line in an integrated circuit. While electromigration eventually causes macroscopic damage, in the form of voids and hillocks, the earliest stage of the process when the stress in individual micron-sized grains is still building up is largely unexplored. Using synchrotron-based x-ray microdiffraction during an in-situ electromigration experiment, we have discovered an early prefailure mode of plastic deformation involving preferential dislocation generation and motion and the formation of a subgrain structure within individual grains of a passivated Al (Cu) interconnect. This behavior occurs long before macroscopic damage (hillocks and voids) is observed.