7,944 research outputs found
Topological characterization of antireflective and hydrophobic rough surfaces: are random process theory and fractal modeling applicable?
The random process theory (RPT) has been widely applied to predict the joint
probability distribution functions (PDFs) of asperity heights and curvatures of
rough surfaces. A check of the predictions of RPT against the actual statistics
of numerically generated random fractal surfaces and of real rough surfaces has
been only partially undertaken. The present experimental and numerical study
provides a deep critical comparison on this matter, providing some insight into
the capabilities and limitations in applying RPT and fractal modeling to
antireflective and hydrophobic rough surfaces, two important types of textured
surfaces. A multi-resolution experimental campaign by using a confocal
profilometer with different lenses is carried out and a comprehensive software
for the statistical description of rough surfaces is developed. It is found
that the topology of the analyzed textured surfaces cannot be fully described
according to RPT and fractal modeling. The following complexities emerge: (i)
the presence of cut-offs or bi-fractality in the power-law power-spectral
density (PSD) functions; (ii) a more pronounced shift of the PSD by changing
resolution as compared to what expected from fractal modeling; (iii) inaccuracy
of the RPT in describing the joint PDFs of asperity heights and curvatures of
textured surfaces; (iv) lack of resolution-invariance of joint PDFs of textured
surfaces in case of special surface treatments, not accounted by fractal
modeling.Comment: 21 pages, 13 figure
On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion
Surface roughness has a huge impact on many important phenomena. The most
important property of rough surfaces is the surface roughness power spectrum
C(q). We present surface roughness power spectra of many surfaces of practical
importance, obtained from the surface height profile measured using optical
methods and the Atomic Force Microscope. We show how the power spectrum
determines the contact area between two solids. We also present applications to
sealing, rubber friction and adhesion for rough surfaces, where the power
spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some
figures are in Postscript Level
Loading-unloading hysteresis loop of randomly rough adhesive contacts
In this paper we investigate the loading and unloading behavior of soft
solids in adhesive contact with randomly rough profiles. The roughness is
assumed to be described by a self-affine fractal on a limited range of
wave-vectors. A spectral method is exploited to generate such randomly rough
surfaces. The results are statistically averaged, and the calculated contact
area and applied load are shown as a function of the penetration, for loading
and unloading conditions. We found that the combination of adhesion forces and
roughness leads to a hysteresis loading-unloading loop. This shows that energy
can be lost simply as a consequence of roughness and van der Waals forces, as
in this case a large number of local energy minima exist and the system may be
trapped in metastable states. We numerically quantify the hysteretic loss and
assess the influence of the surface statistical properties and the energy of
adhesion on the hysteresis process.Comment: 8 pages, 9 figures, published on Physical Review E, Volume 92, Issue
6, 8 December 2015, Article number 06240
Light scattering from self-affine fractal silver surfaces with nanoscale cutoff: Far-field and near-field calculations
We study the light scattered from randomly rough, one-dimensional self-affine
fractal silver surfaces with nanoscale lower cutoff, illuminated by s- or
p-polarized Gaussian beams a few microns wide. By means of rigorous numerical
calculations based on the Green theorem integral equation formulation, we
obtain both the far- and near-field scattered intensities. The influence of
diminishing the fractal lower scale cutoff (from below a hundred, down to a few
nanometers) is analyzed in the case of both single realizations and ensemble
average magnitudes. For s polarization, variations are small in the far field,
being only significant in the higher spatial frequency components of evanescent
character in the near field. In the case of p polarization, however, the
nanoscale cutoff has remarkable effects stemming from the roughness-induced
excitation of surface-plasmon polaritons. In the far field, the effect is
noticed both in the speckle pattern variation and in the decrease of the total
reflected energy upon ensemble averaging, due to increased absorption. In the
near field, more efficient excitation of localized optical modes is achieved
with smaller cutoff, which in turn leads to huge surface electric field
enhancements.Comment: REVTeX 4, 10 page
A multiscale Molecular Dynamics approach to Contact Mechanics
The friction and adhesion between elastic bodies are strongly influenced by
the roughness of the surfaces in contact. Here we develop a multiscale
molecular dynamics approach to contact mechanics, which can be used also when
the surfaces have roughness on many different length-scales, e.g., for self
affine fractal surfaces. As an illustration we consider the contact between
randomly rough surfaces, and show that the contact area varies linearly with
the load for small load. We also analyze the contact morphology and the
pressure distribution at different magnification, both with and without
adhesion. The calculations are compared with analytical contact mechanics
models based on continuum mechanics.Comment: Format Revtex4, two columns, 13 pages, 19 pictures. Submitted for
publication in the European Physical Journal E. Third revision with minimal
changes: Corrected a few mistypin
Earthquake statistics and fractal faults
We introduce a Self-affine Asperity Model (SAM) for the seismicity that
mimics the fault friction by means of two fractional Brownian profiles (fBm)
that slide one over the other. An earthquake occurs when there is an overlap of
the two profiles representing the two fault faces and its energy is assumed
proportional to the overlap surface. The SAM exhibits the Gutenberg-Richter law
with an exponent related to the roughness index of the profiles. Apart
from being analytically treatable, the model exhibits a non-trivial clustering
in the spatio-temporal distribution of epicenters that strongly resembles the
experimentally observed one. A generalized and more realistic version of the
model exhibits the Omori scaling for the distribution of the aftershocks. The
SAM lies in a different perspective with respect to usual models for
seismicity. In this case, in fact, the critical behaviour is not Self-Organized
but stems from the fractal geometry of the faults, which, on its turn, is
supposed to arise as a consequence of geological processes on very long time
scales with respect to the seismic dynamics. The explicit introduction of the
fault geometry, as an active element of this complex phenomenology, represents
the real novelty of our approach.Comment: 40 pages (Tex file plus 8 postscript figures), LaTeX, submitted to
Phys. Rev.
Multi-scale roughness transfer in cold metal rolling
We report on a comparative Atomic Force Microscope (AFM) multi-scale roughness analysis of cold rolled Al alloy and steel roll, in order to characterize the roughness transfer from the steel roll to the workpiece in cold strip rolling processes. More than three orders of length-scale magnitudes were investigated from 100 microns to 50 nanometers on both types of surfaces. The analysis reveals that both types of surfaces are anisotropic self-affine surfaces. Transverse and longitudinal height profiles exhibit a different roughness exponent (Hurst exponent) z֊=0.93±0.03 and zʈ=0.5±0.05 Different length-scale cut-offs are obtained in each direction lsup=50mm, lsupՆ100mm. Height and slope distributions are also computed to complement this study. The above mentionned self-affine characteresitics are found to be very similar for the roll and the strip surfaces, which suggest that roughness transfer takes place from the macroscopic (100 µm) to the very small scale (50 nm)
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