Institut Jean le Rond d'Alembert
Centre National de la Recherche Scientifique
Université Pierre et Marie Curie
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Role of uncrosslinked chains in droplets dynamics on silicone elastomers
A. Hourlier-Fargette, A. Antkowiak, Antoine Chateauminois, and S. Neukirch
Soft Matter, 13 (2017) 3484-3491
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 an homogeneous oil film.
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Submitted (March, 3rd)
Reports (received March, 20th, 2017)
Comments to the Author
The authors have conducted a very nice study on the role of free chains in elastomer in droplet dynamics. Although the effects of free chains in elastomer on wetting, adhesion and friction have been raised previously at conferences and published literature, the carefully conducted study of this paper is very timely and a welcome contribution. It is interesting that long ago, Tabor indicated similar roles played by the free polytetrafluroethelene chains in certain surface related behaviors of Teflon. Overall, I liked this paper and recommend its publication in Soft Matter. I'll only ask the authors to make some comments, if possible, on what would they expect if the chains were not crosslinked but grafted on a surface. This question relates to a marked dependence of the wetting hysteresis and the molecular weight of the grafted chains as reported in Langmuir, Vol. 14, No. 11, 1998 (figure 17).
Comments to the Author
I enjoyed reading this paper by Hourlier-Fargette et al. It describes a rather surprising change in wetting dynamics on a soft elastomer due to a small quantity of uncrosslinked material in the elastomer. I think it will be of interest to a wide variety range of researchers - in particular from the large wetting and microfluidics communities.
I don't have any major comments - the results are clearly and presented, and rather convincing. Just a few notes below:
1) The parameter S is used to mean two different things (wetting parameter and contact radius)
2) I was wondering if there is any physical insight that can be shed on the observation that gamma_i-gamma_f is a constant?
3) In figure 7 (a,b), I think the colors are the wrong way round.
4) The authors say that before the droplet reaches the second regime, there is an incomplete coverage of oligomers on the droplet surface. However, they say that the dynamics of the film doesn't affect the speed of the droplet. Instead it's the size of the wetting ridge that does this. Can they offer an alternative viewpoint in terms of the size of the wetting ridge (vs time) that would give more insight into the crossover between the two different regimes?
5) At one point, 'theoretical frame' should be 'theoretical framework'.