Tribology and Lubrication Technology April 2012 : Page 9‘In the case of crystalline surfaces, kinks and antikinks run quite undisturbed across the entire surface. This is not true for quasicrystalline surfaces because they can become stuck at quasicrystalline surfaces.’ As the two layers interacted with each other, the researchers expected that the particles in both layers will move simultaneously in a jerky fash-ion and then move back into their original positions after the layers sepa-rate. Instead, the two atomic layers be-haved differently. As shown in Figure 1, some parti-cles moved while others did not. The blue spheres in Figure 1 slide together to form a compression zone, while the green particles remained in place on the surface. Areas where the particles moved closer together are known as kinks. In other cases, particles moved far-ther apart as one layer passed over the other. This is a situation where an ex-pansion zone was formed, which is known as an antikink. Bechinger explains, “Kinks are re-gions where the particles are closer than on average (compression zone), while the opposite is true for anti-kinks. Such compression and expan-sion zones can move much more easily across a surface than if all the particles would keep the same distance while sliding across the surface.” The formation of kinks and anti-kinks is due to energetic reasons. Bechinger adds, “It turns out that the energy required for formation of com-pression/expansion zones is smaller than the energy gain when sliding the system with these local deformations across the surface.” The use of the light crystalline sur-face provided the researchers with two important advantages in studying fric-tion at the atomic scale. Bechinger says, “First, we can image this inter-face, which is normally hidden (e.g., in a pin-on-disk device or an AFM tip). Second, we can continuously vary the length scales and the symmetry of the substrate potential just by slightly Figure 1 | As two microscopic surfaces interact with each other, some particles move while others remain in place. The particles (shown in blue) slide together to form a compression zone known as a kink, while the particles (shown in green) remain in place on the surface. (Courtesy of the Universitat of Stuttgart and the Max Planck Institute for Intelligent Systems) changing the number and angle of in-cidence of the laser beams forming the interference pattern.” Besides studying crystalline layers, the researchers also prepared an opti-cal quasicrystal surface. Bechinger says, “Quasicrystals are perfectly or-dered materials that lack periodicity. It is known from macroscopic experi-ments (e.g., pin-on-disk studies) that quasicrystals have a friction coefficient about one order of magnitude smaller than comparable crystals.” The lack of periodicity in quasi-crystals affects the formation and movement of kinks and antikinks across the surface. Bechinger says, “In the case of crystalline surfaces, kinks and antikinks run quite undisturbed across the entire surface. This is not true for quasicrystalline surfaces be-cause they can become stuck at quasi-crystalline surfaces.” The observation of kinks and anti-kinks confirms theoretical predictions, which have previously claimed their existence on the atomic level. Bech-inger says, “In addition, our experi-ments demonstrate how important they are for the understanding of fric-tion at small-length scales.” Future work will involve learning more about how friction depends on the size of the contact and how the added presence of vibrations affects the process. Bechinger says, “Due to the complex mechanism responsible for friction, we expect a nontrivial de-pendence of the friction force on the contact size. We intend to study vibra-tions because they are typically pres-ent in macroscopic systems once you start sliding them.” Additional information can be found in a recent article 2 or by contact-ing Bechinger at c.bechinger@physik. uni-stuttgart.de. REfEREncES 1. Canter, N. (2010), “Size Does Matter for Nanoscale Friction,” TLT, 66 (8), pp. 10-11. 2. Bohlein, T., Mikhael, J. and Bech-inger, C. (2012), “Observation of Kinks and Antikinks in Colloidal Monolayers,” Nature Materials , 11 (2), pp. 126-130. on a 172-page digital guide available at www.stle.org. Searchable by topic or author. 9 Publication List Using a screen reader? Click Here |
