Theory of Reciprocating Contact for Viscoelastic Solids

A theory of reciprocating contacts for linear viscoelastic materials is presented. Results are discussed for the case of a rigid sphere sinusoidally driven in sliding contact with a viscoelastic half-space. Depending on the size of the contact, the frequency and amplitude of the reciprocating motion, and on the relaxation time of the viscoelastic body, we establish that the contact behavior may range from the steady-state viscoelastic solution, in which traction forces always oppose the direction of the sliding rigid punch, to a more elaborate trend, never observed before, which is due to the strong interaction between different regions of the path covered during the reciprocating motion. Practical implications span a number of applications, ranging from seismic engineering to biotechnology.Comment: 8 pages, 5 figures, accepted for publication on Physical Review E, March 22, 201

Effect of sensitization on the electrochemical properties of nanostructured NiO

Screen-printed NiO electrodes were sensitized with 11 different dyes and the respective electrochemical properties were analyzed in a three-electrode cell with the techniques of cyclic voltammetry and electrochemical impedance spectroscopy. The dye sensitizers of NiO were organic molecules of different types (e.g., squaraines, coumarins, and derivatives of triphenyl-amines and erythrosine B), which were previously employed as sensitizers of the same oxide in dye-sensitized solar cells of p-type (p-DSCs). Depending on the nature of the sensitizer, diverse types of interactions occurred between the immobilized sensitizer and the screen-printed NiO electrode at rest and under polarization. The impedance data recorded at open circuit potential were interpreted in terms of two different equivalent circuits, depending on the eventual presence of the dye sensitizer on the mesoporous electrode. The fitting parameter of the charge transfer resistance through the electrode/electrolyte interface varied in accordance to the differences of the passivation action exerted by the various dyes against the electrochemical oxidation of NiO. Moreover, it has been observed that the resistive term RCT associated with the process of dark electron transfer between the dye and NiO substrate is strictly correlated to the overall efficiency of the photoconversion () of the corresponding p-DSC, which employs the same dye-sensitized electrode as photocathode

A numerical model for the deterministic analysis of adhesive rough contacts down to the nano-scale

AbstractA numerical model based on the Multi-Level Multi-Integration technique has been developed to study the adhesion between two surfaces. The model provides a self-consistent solution of surface separation and contact pressure throughout an arbitrary surface contact (including random surface roughness) with the adhesive interactions governed by the Lennard-Jones potential. Using this approach, the behaviour of rough surfaces can be assessed with a deterministic description of the surface, and contact stresses include valid adhesive interactions between all non-contacting surface nodes. The model is first compared to similar analyses from smooth surface models, where good agreement with published results is obtained. The model is then applied to randomly rough surfaces and shows both the significant impact of roughness on adhesive behaviour and how individual surface asperities influence the loading–unloading response of adhesive contacts. Lastly, the ability of the model to investigate nano-scale contacts is assessed through comparisons with atomistic simulations previously published elsewhere. It is clearly shown that our continuum mechanics-based model, in which an atomistic configuration is represented by a discretised continuum representation of the surface using a hard-sphere atomic model, is capable of reproducing many of the features identified through detailed atomistic simulations. The suitability of the presented model for studying adhesive contacts from the nano-scale to much larger, soft contacts, where adhesive forces can alter the contact mechanics, is demonstrated. The developed modelling tool and the algorithms implemented by the authors open the possibility to perform fast and accurate calculations using a deterministic description of the roughness for a wide variety of contact conditions

mixed mode crack propagation during needle penetration for surgical interventions

Abstract An accurate description of the penetration mechanics of flexible needles into target soft tissues is a complex task, including friction at the needle-tissue interface, large strains, non-predetermined penetration trajectories, fracture under mixed-mode loading and so on. In the present work, a finite element algorithm is employed to simulate the two-dimensional deep penetration of a flexible needle in a soft elastic material. The fracture process of the target material during penetration is described by means of a cohesive zone model, with a suitable mixed-mode criterion for determining the propagation direction of the crack. To illustrate the potential of the numerical algorithm, we have performed some simulations of the insertion of a flexible needle with an asymmetric tip, and the results are presented in terms of force-penetration curves as well as of the obtained penetration paths in the target tissue

A new finite element paradigm to solve contact problems with roughness

This article's main scope is the presentation of a computational method for the simulation of contact problems within the finite element method involving complex and rough surfaces. The approach relies on the MPJR (eMbedded Profile for Joint Roughness) interface finite element proposed in [arXiv:1805.07207], which is nominally flat but can embed at the nodal level any arbitrary height to reconstruct the displacement field due to contact in the presence of roughness. Here, the formulation is generalized to handle 3D surface height fields and any arbitrary nonlinear interface constitutive relation, including friction and adhesion. The methodology is herein validated with BEM solutions for linear elastic contact problems. Then, a selection of nonlinear contact problems prohibitive to be simulated by BEM and by standard contact algorithms in FEM are detailed, to highlight the promising aspects of the proposed method for tribology

Effect of particle size and surface charge on nanoparticles diffusion in the brain white matter

This project has received funding from the European Unions Horizon 2020 research and innovation programme under Grant Agreement No. 688279. Daniele Dini would like to acknowledge the support received from the EPSRC under the Established Career Fellowship Grant No. EP/N025954/1. Tian Yuan would also like to acknowledge financial support from CSC Imperial Scholarship. The authors declare that they have no conflict of interest.Peer reviewedPublisher PD

Validation of a rapid test to dose SO2 in vinegar

Sulfur dioxide is generally used in wine and vinegar production. It is employed to decrease the bacteria' growth, improve the wines' aroma (since it supports the extraction of polyphenols during maceration), protect the wines from chemical oxidation and the musts from chemical and enzymatic oxidation (blocking free radicals and oxidase enzymes such as tyrosinase and laccase). The composition and storage conditions (i.e., pH, temperature, and alcohol levels) affect oenological results. In various countries, competent authorities have imposed legal limits since it can have toxic effects on humans. It is crucial to dose SO2 levels to allow vinegar production and compliance with legal limits. The iodometric titration named "Ripper test" is the legal method used to dose it in vinegar. In this work, an automatized colorimetric test was validated using the international guidelines ISO/IEC (2017) to allow its use instead of the Ripper test. The test reliability was verified on white, red, and balsamic vinegar with low or high SO2 content. The automatized test showed linearity, precision, and reproducibility similar to the Ripper test, but the accuracy parameter was not respected for the vinegar with a low concentration of SO2. Therefore, the automatized colorimetric test can be helpful to dose SO2 in vinegar with high concentrations of SO2

Mathematical Optimisation of Magnetic Nanoparticle Diffusion in the Brain White Matter

European Unions Horizon 2020 research and innovation programme under Grant Agreement No. 688279. EPSRC Established Career Fellowship Grant No. EP/N025954/1. Children with Cancer UK Grant No. 16-224.Peer reviewedPublisher PD