Role of uncrosslinked chains in droplets dynamics on silicone elastomers

TL;DR

This study reveals that uncrosslinked chains in silicone elastomers influence droplet wetting dynamics, causing a transition between two speed regimes linked to surface tension changes due to oligomer extraction.

Contribution

It uncovers the role of uncrosslinked oligomers in droplet dynamics on silicone elastomers and visualizes their extraction and impact on surface tension.

Findings

Two successive speed regimes observed in droplet spreading.

Uncrosslinked oligomers are extracted by water droplets, forming a homogeneous oil film.

Surface tension abrupt change correlates with oligomer coverage on droplets.

Abstract

We report an unexpected behavior in wetting dynamics on soft silicone substrates: the dynamics of aqueous droplets deposited on vertical plates of such elastomers exhibits two successive speed regimes. This macroscopic observation is found to be closely related to microscopic phenomena occurring at the scale of the polymer network: we show that uncrosslinked chains found in most widely used commercial silicone elastomers are responsible for this surprising behavior. A direct visualization of the uncrosslinked oligomers collected by water droplets is performed, evidencing that a capillarity-induced phase separation occurs: uncrosslinked oligomers are extracted from the silicone elastomer network by the water-glycerol mixture droplet. The sharp speed change is shown to coincide with an abrupt transition in surface tension of the droplets, when a critical surface concentration in uncrosslinked oligomer chains is reached. We infer that a droplet shifts to a second regime with a faster speed when it is completely covered with a homogeneous oil film.