2 post doc positions in Italy --- Regulating adhesion using viscoelastic oscillating contact --- some solutions

We are hiring 2 post-docs soon in Italy at the Department of Mechanics DMMM of the Politecnico di BARI.  The subject is here described.

Shui et al (Nat.Comm.(2020), 11(1), 1583) have recently shown that applying high-frequency vibrations, we can increase the mean adhesion between viscoelastic solids. This is due to the fact that oscillating contact area leads to an effect of increased apparent surface energy during the retraction phase which can be described by the well known empirical Gent and Schultz law (GS). However, Shui et al solution surprisingly appears not to depend on GS constants, which would imply perhaps no amplification. Yi et al (Advanced Intelligent Systems, (2024), 2400394) have made similar experiments, and proposed a simpler fitting model, which seems to work however with widely different GS constant when changing the sphere radius. Here, we solve the JKR dynamic adhesion problem for a sphere oscillating on a substrate by imposing an harmonic oscillation of the contact area, which permits to obtain a very simple solution by simply averaging the resulting cycle of indentation. We find that the solution is close to a JKR form for the mean indentation vs mean force, which we find in a simple approximation. Although there is saturation in the amplification when the contact radius shrinks to zero and the problem becomes that of impacts at large amplitudes of vibrations, experiments show that saturation occurs first, probably due to other saturations, e.g. in the GS law. We discuss also the influence of resonances. We find reasonable agreement with experiments conducted on PDMS.

Please see here one preprint explaining our work so far. 

However this technique is limited by the roughness of countersurface, probably similarly to how roughness kills adhesion in quasi static conditions. Hence, we generated a model using an extension of the Fuller-Tabor model of asperities, see here.    To overcome roughness, it would be necessary to separate the contact in a fibrillar structure or at least a patterned surface, but at that point it is not clear whether the viscoelastic amplification mechanism still works --- in the limit of nanoscopic fibrils, we arrive at cohesive adhesion mechanisms at the cohesion tension of the material as in Gecko, which is very high, and not further amplifiable by vibrations.

I wonder if you have suggestions on the project --- interest to collaborate.   

Also, I just received a 2 year project on the related issue of the fibrillar/patterned surface, do you have candidates for two POST-DOC positions to recommend?

The candidates should be proficient in solid mechanics and in mathematical asymptotic expansion methods for non linear class of problems, possibly also expert in non linear FEM using cohesive models.

For any detail, please contact me at mciava@poliba.it

Regards,

Prof. Michele Ciavarella

Professor of Mechanical Design.