Claudia Borri

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 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 was 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 for by fractal modeling.

Inkjet printed 2D-crystal based strain gauges on paper

Abstract We present an investigation of inkjet printed strain gauges based on two-dimensional (2D) materials. The technology leverages water-based and biocompatible inks to fabricate strain measurement devices on flexible substrates such as paper. We demonstrate that the device performance and sensitivity are strongly dependent on the printing parameter (i.e., drop-spacing, number of printing passes, etc.). We show that values of the Gauge Factor up to 125 can be obtained, with large sensitivity (>20%) even when small strains (0.3%) are applied. Furthermore, we provide preliminary examples of heterostructure-based strain sensors, enabled by the inkjet printing technology.

Topology simulation and contact mechanics of bifractal rough surfaces

A numerical method to generate bifractal surfaces due to a modification of the slope of the power spectral density function in the low- or high-frequency range is proposed. The method has been applied to simulate real surfaces of Ginkgo Biloba leaf scanned at two different magnifications by matching the corresponding experimental power spectral densities. Slight differences have been found in the statistical distributions of the asperity heights and curvatures for the lowest magnification that had marginal influence on the frictionless normal contact response of the surface. For highest magnification, however, the statistics of the simulated numerical surface were quite different from those of the real one, leading also to a significant difference in the normal contact results.

Adhesive behaviour of bonded paper layers: Mechanical testing and statistical modelling

In this study, an experimental methodology based on micromechanical testing inside a scanning electron microscope is proposed to characterise bonding of paper layers connected by wet pressing. The peeling force–displacement evolution law that characterises the delamination of micromechanical double cantilever beam specimens of paper tissue have been extracted from such peeling tests. It is observed that the force–displacement evolution curve achieves a steady-state value related to the effective adhesive energy of the interface. This behaviour is explained by examining the complex load transfer mechanism between the layers exerted by cellulose fibrils. A statistical approach is used for the computation of the effective adhesive energy. It is argued that the observed force–displacement evolution law may be satisfactory described by a stochastic model that depends on the distribution function of the fibrils strength, and on two geometrical distribution functions related to the in-plane and out-of-plane fibrils angles with respect to the undeformed interface configuration. Some applications of the proposed model are demonstrated on examples.

PERCOLATION PROPERTIES OF THE FREE VOLUME GENERATED BY TWO ROUGH SURFACES IN CONTACT

The mechanism of fluid leakage trough the free volume between rough surfaces in contact is relevant in physics and in many engineering applications. In the present study, the normal contact problem between randomly generated fractal rough surfaces is solved using the boundary element method. Then, an algorithm for the evaluation of the network involved in the percolation of fluid is proposed. Numerical results are synthetically collected in diagrams relating the free volume involved in the percolation to the dimensionless statistical parameters of the rough surface