Contact lines on silicone elastomers promote contamination of water-air interfaces

Résumé: Silicone elastomers are used in contact with aqueous liquids in a large range of applications. Due to numerous advantages such as its flexibility, optical transparency, or gas permeability, polydimethylsiloxane (PDMS) is widely spread in rapid prototyping for microfluidics or elastocapillarity experiments. However, silicone elastomers are known to contain a small fraction of uncrosslinked low-molecular-weight oligomers, the effects of which are not completely understood. We first highlight that processes occurring at scale of the polymer network can lead to unexpected macroscopic behaviors: a water droplet deposited on a PDMS inclined plane exhibits two successive speed regimes. We analyze this phenomenon experimentally, demonstrating that this suprising behavior is due to the water droplet extracting uncrosslinked oligomers from the silicone elastomer through a capillarity-induced phase separation at the triple line. We then demonstrate that such a capillarity-induced extraction of uncrosslinked chains occurs not only for moving droplets but also in the case of sessile droplets or triple lines, and finally study the dynamics of the contamination of water-air interfaces using surface tension measurements in the case of sessile triple lines and speed measurements in the case of moving droplets. This work reveals how minute amounts of contaminants have dramatic effects on the wetting dynamics, highlights the importance of being careful about uncrosslinked chains in elastomers in contact with liquids, but also provides a simple test to evaluate the presence of uncrosslinked oligomers in an elastomer sample.

Les personnes souhaitant rencontrer Aurélie Hourlier-Fargette sont priées de prendre contact avec Wiebke Drenckhan , tél. 03 88 41 40 43 ou mail (Wiebke.Drenckhan@ics-cnrs.unistra.fr)