Wetting of soft substrates

With Mehdi Banaha, Tadashi Kajiya, François Lequeux, Laurent Limat, Tetsu Narita, Laurent Royon, …

Hydrogels, sort of polymer sponges swollen with water, are frequently encountered in the food, health and beauty industries, for their astonishing and tunable properties (from very soft to hard, brittle or elastic, sensitive to temperature or acidity, …). We are particularly interested in the surface properties of these gels, and in particular their wettability by liquids. We would like to understand the strong dependency of static and dynamic wetting on the elasticity of the material, and on its swelling ratio. To this end we perform controlled wetting and de-wetting experiments on gels.

The video above shows the evolution of a water drop deposited onto an Agar-based hydrogel (0.7% Agar). The optical distortion of a grid set up behind the gel helps in visualising not only the drop, but also the deformations of the gel. It can be used to calculate the surface slope. The drop does not spread, but remains in a partial wetting state with constant contact area for about 90s. Then it starts receding, and disappears about 135s after deposition. This is due to the fact that the liquid diffuses into the gel, which swells. How the duration of the process, and the finite advancing and receding contact angles, depend on the gel concentration is one question we would like to answer. (Banaha et al 2009)

Inflating a drop on a visco-elastic gel leads to a continuous spreading only for low or high flow rates. At intermediate flow rates, we observe a stick-slip behaviour of the inflating drop (see video above). We showed that the typical deformation rate of the substrate in this regime corresponds to the rheological cross-over between viscous and elastic behaviours of the substrate. (Kajiya et al 2013)

The PhD Thesis of Mehdi Banaha (2009) describes some of our experiments in more detail.

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Last modified: 31 Jan 2016