Influence of self-affine roughness on the friction coefficient of rubber at high sliding velocity

In this work we investigate the influence of self-affine roughness on the friction coefficient of a rubber body onto a solid surface at high speeds. The roughness is characterized by the rms amplitude w, the correlation length ξ, and the roughness exponent H. It is shown that the friction coefficient decreases with increasing correlation length ξ and increasing roughness exponent H for sufficiently large correlation lengths. However, for small correlation lengths the opposite behavior takes place because the system is within the strong roughness limit or equivalently average local surface slopes larger than 1. Moreover, direct plots of the friction coefficient as a function of the roughness exponent H indicate that as the correlation length ξ decreases, a maximum of the friction coefficient develops. The latter is followed by a continous increment of the friction coefficient with increasing H and decreasing ξ.

Contact angle influence on the pull-in voltage of microswitches in the presence of capillary and quantum vacuum effects

Capillary condensation between the electrodes of microswitches influences the effective pull-in voltage in a manner that depends on the contact angle of the capillary meniscus and the presence of plate surface roughness. Indeed, surface roughening is shown to have a stronger influence on the pull-in potential for relatively small contact angles with respect to that of a flat surface when capillary condensation takes place. For long wavelength roughness ratios w/ξ«1 with w the rms roughness amplitude and ξ the in-plane correlation length, the pull-in voltage increases with increasing theoretical contact angle θ0 for flat surfaces. With decreasing correlation length ξ (increasing roughness), the pull-in potential decreases faster for smaller contact angles θ0 In addition, with decreasing roughness exponent H (0<H<1), which characterizes short wavelength roughness fluctuation at short length scales (<ξ), the pull-in potential shows a steeper decrease with decreasing correlation length ξ. Finally, with increasing relative humidity, the sensitivity of the pull-in voltage at small correlation lengths attenuates significantly with increasing contact angle θ0.

Influence of self-affine roughness on Parsons-Zobel plots for electrical double layers

In this paper we investigate the dependence of Parsons-Zobel plots on characteristic self-affine roughness parameters of the metal electrode in electrical double layers. Among the roughness amplitude w, the correlation length ξ, and roughness exponent H, the latter appears to have the most prominent effect especially for values in the range H<0.5. In addition, with decreasing compact layer thickness the influence of roughness leads to stronger nonlinear behavior of the plots for relatively large electrode potentials. Finally, it is shown that dynamic changes of the electrode roughness (for example by growth on metal films) should be carefully quantified with respect to their influence on the Parson-Zobel plots and related double-layer systems.

Influence of random roughness on cantilever resonance frequency

In this paper we investigate the influence of random roughness on the oscillation frequency of cantilevers coated with thin film overlayers. First the theory expressions for the roughness-induced frequency shift are derived using the cantilever equation of motion. Subsequently it is shown that the roughness induced shift depends on the particular roughness parameters, assuming the general case of self-affine rough surfaces for the overlayer film, the material properties of the overlayer film, and the dimensions mainly of the bare cantilever. Indeed it is shown that the roughness influence becomes significant for relatively thin cantilevers (≤1 µm), and increased local surface slopes (>0.5) within the limits of applicability of the proposed formalism. The results of this study can be used in high precision frequency sensing applications in the field of micro/nanomechanics.

Graphene-on-silicon near-field thermophotovoltaic cell

A graphene layer on top of a dielectric can dramatically influence ability of the material to radiative heat transfer. This property of graphene is used to improve the performance and reduce costs of near-field thermophotovoltaic cells. Instead of low bandgap semiconductors it is proposed to use graphene-on-silicon Schottky photovoltaic cells. One layer of graphene absorbs around 90% of incoming radiation and increases the heat transfer. This is due to excitation of plasmons in graphene, which are automatically tuned in resonance with the emitted light in the mid infrared range. The absorbed radiation excites electron-hole pairs in graphene, which are separated by the surface field induced by the Schottky barrier. For a quasi-monochromatic source the generated power is one order of magnitude larger and efficiency is on the same level as for semiconductor photovoltaic cells.Comment: 6 pages, 3 figures, to be published in Phys. Rev. Applie

Random surface roughness influence on gas damped nanoresonators

The author investigates quantitatively the influence of random surface roughness on the quality factor Q of nanoresonators due to noise by impinging gas molecules. The roughness is characterized by the amplitude w, the correlation length ξ, and the roughness exponent H that describes fine roughness details at short wavelengths. Surface roughening (decreasing H and increasing ratio w/ξ) leads to lower Q, which translates to lower sensitivity to external perturbations, and a higher limit to mass sensitivity. The influence of the exponent H is shown to be important as that of w/ξ, indicating the necessity for precise control of the surface morphology.

Allan variance of frequency fluctuations due to momentum exchange and thermomechanical noises

We investigate the Allan variance of nanoresonators with random rough surfaces under the simultaneous influence of thermomechanical and momentum exchange noises. Random roughness is observed in various surface engineering processes, and it is characterized by the roughness amplitude w, the lateral correlation length ξ, and the roughness exponent 0<H<1. The roughness influence becomes significant for measurement time τA so that ωoτA~1, with ωo the fundamental resonance frequency. The Allan variance increases significantly with increasing roughness (decreasing H and/or increasing ratio w/ξ) if the quality factor due to gas collisions is smaller than the intrinsic quality factor associated with thermomechanical noise.

Influence of random roughness on cantilever curvature sensitivity

In this work we explore the influence of random surface roughness on the cantilever sensitivity to respond to curvature changes induced by changes in surface stress. The roughness is characterized by the out-of-plane roughness amplitude w, the lateral correlation length x, and the roughness or Hurst exponent H (0<H<1). The cantilever sensitivity is found to decrease with increasing roughness (decreasing H and/or increasing ratio w/x) or equivalently increasing local surface slope. Finally, analytic expressions of the cantilever sensitivity as a function of the parameters w, x, and H are derived in order to allow direct implementation in sensing systems.Comment: 10 pages, 3 figure

Repulsive Casimir forces between solid materials with high refractive index intervening liquids

In order to explore repulsive Casimir/van der Waals forces between solid materials with liquid as the intervening medium, we analyze dielectric data for a wide range of materials as for example PTFE, polystyrene, silica and more than twenty liquids. Although significant variation in the dielectric data from different sources exist, we provide a scheme based on measured static dielectric constants, refractive indices, and applying Kramers Kronig (KK) consistency to dielectric data to create accurate dielectric functions at imaginary frequencies. The latter is necessary for more accurate force calculations via the Lifshitz theory allowing reliable predictions of repulsive Casimir forces.Comment: 20 pages, 7 figures, 1 tabl

Adhesion of elastic films on mound rough surfaces

In this work, we have investigated the adhesive behaviour of elastic films in contact with solid substrates, which are bounded by mound surface roughness. This type of roughness is described by the rms roughness amplitude w, the average mound separation Lambda, and the system correlation length zeta. It is shown that both lateral roughness parameters Lambda and zeta strongly influence adhesive characteristics. Indeed, with increasing elastic film modulus E, film adhesion is only possible for sufficiently large mound separations Lambda. Moreover, the critical elastic modulus E-c (for which spontaneous film decohesion takes place for E > E-c) is shown to increase fast with increasing mound separation Lambda when Lambda less than or equal to zeta, while as a function of the system correlation length it increases relatively fast when zeta less than or equal to Lambda. (C) 2003 Elsevier Science B.V. All rights reserved