Oscillation-based methods for actuation and manipulation of nano-objects

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in AIP Conference Proceedings 1882, 020056 (2017) and may be found at https://doi.org/10.1063/1.5001635.We discuss how oscillations can be used for fixation or manipulation of nano-objects or producing nano-drives. The underlying principles are scale-invariant and principally can be scaled down up to the molecular scale. The main underlying principle of fixation and actuation occurs to be symmetry breaking of an oscillating system. From this unifying standpoint, a series of actuation principles are discussed as dragging, ratchets, micro walking, friction-inertia actuators, oscillation tweezers, flagella motors for propulsion in liquids as well as some recently proposed actuation principles

Generalized rabinowicz’ criterion for adhesive wear for elliptic micro contacts

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in AIP Conference Proceedings 1909, 020178 (2017) and may be found at https://doi.org/10.1063/1.5013859.This paper is devoted to an old idea suggested in 1958 by E. Rabinowicz in his paper “The effect of size on the looseness of wear fragments”. Rabinowicz assumed a circular shape for two asperities coming into contact and being destroyed due to relative sliding. We generalize his analysis for the case of non-circular contacts, in particular those having elliptical shape and discuss the general case of arbitrary contact shape

Contacts With Negative Work of “Adhesion” and Superlubricity

Van der Waals forces between solids in vacuum are always attractive and are considered as the main source of adhesion. However, in the presence of an intermediate medium, they can also be repelling (Dzyaloshinskii et al., 1961) which means that the “work of adhesion” becomes negative. Similarly to the case of adhesion, the interaction range of these forces can be either comparable (or larger) than the minimum characteristic length scale of the contact problem or it can be negligible compared with all characteristic length scales. We call this latter case the “JKR-approximation,” as the JKR theory of adhesion (Johnson et al., 1971) is also valid in this limit. The repelling interaction can also be due to the presence (and squeezing out) of a thin fluid layer between solids as considered in Müser (2014). In the papers Popov and Hess (2018) and Heß and Popov (2019), it was shown that the contact of two oppositely charged surfaces at a constant voltage is equivalent to the adhesive contact with an effective van der Waals interaction. Similarly, the contact of the bodies with the same charge would be equivalent to repelling van der Waals forces with a negative work of adhesion. Further kinds of repelling forces may be solvation, structural, and hydration forces (Israelachvili, 2011). In the following, we speak about van der Waals forces, but they are thought as representative for a larger class of long range repelling forces. We argue that in the JKR approximation, the Hertz' solution of the contact problem with a repelling van der Waals interaction, remains practically unchanged. However, the contact area falls apart into the area of “weak (van der Waals) interaction” and “strong (rigid wall) interaction.” It is speculated that if the normal force is smaller than a critical value at which the core region of strong interaction disappears, a macroscopic superlubricity state of the contact may be observed.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Force-displacement relation in a tangential frictional contact with adhesion

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in AIP Conference Proceedings 2051, 020244 (2018) and may be found at https://doi.org/10.1063/1.5083487.We consider tangential contact between a rigid cylinder and elastic half-space in the presence of adhesion and Coulomb’s frictional force. In the limit of very small range of adhesive interaction, the main governing dimensionless parameters are identified and it is shown that the shape of the relation between the normalized force and normalized displacement is function of only one system parameter closely related to the Tabor parameter. However, the qualitative behavior is the same for arbitrary values of the Tabor parameter: the force monotonously increases from zero to the maximum value corresponding to the complete sliding. This behavior is qualitatively different from that known in the case of non-adhesive contact where—in the case of flat-ended cylindrical punch—the whole contact area remains in stick state until the displacement achieves some critical value, after which complete sliding starts

Comparative study of the two-phonon Raman bands of silicene and graphene

We present a computational study of the two-phonon Raman spectra of silicene and graphene within a density-functional non-orthogonal tight-binding model. Due to the presence of linear bands close to the Fermi energy in the electronic structure of both structures, the Raman scattering by phonons is resonant. We find that the Raman spectra exhibit a crossover behavior for laser excitation close to the \pi-plasmon energy. This phenomenon is explained by the disappearance of certain paths for resonant Raman scattering and the appearance of other paths beyond this energy. Besides that, the electronic joint density of states is divergent at this energy, which is reflected on the behavior of the Raman bands of the two structures in a qualitatively different way. Additionally, a number of Raman bands, originating from divergent phonon density of states at the M point and at points, inside the Brillouin zone, is also predicted. The calculated spectra for graphene are in excellent agreement with available experimental data. The obtained Raman bands can be used for structural characterization of silicene and graphene samples by Raman spectroscopy