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Contact mechanics of soft matter

The sliding contacts between soft matters are governed by an important adhesive contribution. Their full understanding is a great scientific challenge which I chose to get onto through the soft matter response to normal dynamic loading (dynamic JKR – mode I) and tangential loading (friction/sliding – mode II).
The major difficulty in the analysis of adhesive contact between viscoelastic solids is to separate the surface and bulk energy dissipations. We built an original “dynamic JKR” apparatus to analyze the energy dissipated (surface and volume) during a visco-elastic adhesive JKR contact between a flat surface and an elastomeric sphere submitted to an oscillating normal load (see video 2). The evolution of the adhesion hysteresis as a function of temperature and frequency shows results in accordance with bulk properties. The profiles of the near contact are assessed through image analysis of interferometric fringes and validate the previous discussion. The size of the dissipative zone was explored through different considerations: i) activation energy involved in the deformation and ii) independence of results of the height of the spherical cap in contact. It appears clearly that the dissipative zone is restricted to the very close vicinity of the contact, according to theoretical results published recently.

Video 2

This better understanding of adhesive viscoelastic contacts permits to consider both i) textured JKR contacts and ii) rough sliding contact of soft matter. In the first case, soft surface with pillars (range from µm to hundreds of µm) has been submitted to normal contacts with a smoothed sphere (see video 3). A static hysteresis has been found out when the contact between the pillars occur and a scale relationship has been pointed out.

Video 3

In the second case, a particular effort was provided to set up an image analysis in order to quantify the characteristics of the contact area as a function of temperature, sliding speed and bulk material properties (see video 4). Even though trends have already been, this work is still in progress.

Video 4

The study is conducted at both macro and micro scales.