Sébastien Neukirch
Institut Jean le Rond d'Alembert
Centre National de la Recherche Scientifique
Sorbonne Université, Campus Pierre et Marie Curie
Paris, Francetel: +33 1 44 27 72 61
e-mail: sebastien.neukirch (-atat-) upmc.fr
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Extraction of Silicone Uncrosslinked Chains at Air–Water–Polydimethylsiloxane Triple Lines
A. Hourlier-Fargette, J. Dervaux, A. Antkowiak, and S. Neukirch
Langmuir, vol. 34 (2018) 12244–12250
Abstract : Silicone elastomers such as polydimethylsiloxane (PDMS) are convenient materials routinely used in laboratories that combine ease of preparation, flexibility, transparency, and gas permeability. However, these elastomers are known to contain a small fraction of uncrosslinked low-molecular-weight oligomers, the effects of which are not completely understood, particularly when used in contact with liquids. Here, we show that triple lines involving air, water, and PDMS elastomers are responsible for the contamination of water–air interfaces by uncrosslinked silicone oligomers through a capillarity-induced extraction mechanism. We investigate both the case of static and moving contact lines and study various geometries ranging from partially immersed PDMS plates to water droplets or air bubbles deposited on PDMS plates, all involving air–water–elastomer triple lines. We demonstrate experimentally that the contamination timescale is strikingly shorter for moving contact lines than in the static case. Eventually, we propose a simple poroelastic model capturing the main features of contamination observed in experiments.
DOI: 10.1021/acs.langmuir.8b02128
download the journal version : PDF
Submitted (June 22nd, 2018)
Reports (received July 24th, 2018)
Reviewer: 1
Recommendation: Publish after minor revisions noted.
Comments: The paper addresses an interesting and important issue on relaxation of PDMS interface induced near air-water-PDMS triple lines. In particular, it shows the significant role played by poroelasticity near contact lines. The paper is very well written, and I enjoy reading it very much. Before I recommend it for publication, I would like to have authors to answer the following questions.
(1) Since the experiments of both sessile droplets and moving droplets are measured in dynamic processes, I wonder whether the values obtained by the tensiometer are equilibrium surface tension of the droplets. With regards to the sessile droplet experiments, for example, is measured surface tension location-dependent? Do the values change with where you put the probe on a droplet (Figure 3)?
(2) For estimating the t_{diff} (~10 ms) (page 15, Line 26), what value of diffusion constant D was used? Generally, to calculate the poroelastic diffusion constant, one needs to know porosity, viscous component of the gel and compressibility of the network. I think the paper needs to give the value of D (at least the order of magnitudes) since the authors later compare the diffusion in the network to the diffusion across interface. Recent papers by Zhigang Suo suggest some easy methods for measuring the diffusion coefficient.
(3) Related to question (2), why there is a difference between diffusion constants in the network and across the interface (page 17, Line 46)? According to poroelastic theory, should D be an intrinsic property purely depending on material parameters mentioned in (2)?
(4) For the model of sessile droplets, why the flux J at t>>t_{diff} is estimated as a constant (page 16, Line 9)? For t>>t_{diff}, I would imagine chemical potential be pretty much equal everywhere. What still drives the flow with a constant flux?
Reviewer 2:
Recommendation: Publish after minor revisions noted.
Comments: The authors examine the hypothesis that uncrosslinked PDMS chains may be to blame for the unexpectedly slow motion of droplets and bubbles traveling over PDMS surfaces in aqueous environments. They first show that both droplets and bubbles can switch abruptly from a slow to a significantly faster motion, which coincides with a substantial lowering of the surface tension. They propose that this threshold coincides with the uncrosslinked PDMS oligomers covering the whole of the droplet surface. They show that the surface tension of sessile droplets decreases much more slowly than for mobile ones, and they show empirically that the time until complete coverage appears to scale linearly with the droplet radius, and with the -2/3 power of the translational velocity. The authors compellingly demonstrate experimentally that oligomer extraction must happen at the triple line, then propose a scaling theory, based on a capillarity-induced extraction mechanism for the oligomers, which correctly retrieves the two scalings.
This work is exceptionally well conceived and executed, and every statement is carefully supported. The problem addressed by the paper is of great contemporary importance, since PDMS and similar materials are ubiquitous across chemistry, biology, and engineering. There are many areas of study where contamination by traces of surface-active substances can have disproportionately large impacts, which would immediately benefit from the insights of the present study. I can easily think of several very important follow-up studies that could build on this work. The presentation is clear and engaging, save for the occasional need for clarification or typo noted below.
Based on the comments above, the paper is certainly deserving of publication in Langmuir. I only have the minor comments and questions listed below.
1. P. 3, line 33: typo: I believe “an” should be removed before “unusual”
2. P. 5, line 30: Was the toluene bath agitated? Were the samples placed on the bottom, or were they raised from the bottom? I am wondering whether the time scale of one week, for the toluene bath, can in principle be reduced.
3. P. 5, line 34: Did the authors happen to measure the contact angle for the toluene-treated PDMS?
4. P. 6, line 10: presumably “platinium” should be “platinum”?
5. P. 9, line 30: I believe “experiment” should be plural
6. P. 10, lines 38, 42: I believe the article “the” is needed before “water”
7. P. 15, section title: I’m concerned the authors may be selling themselves short by naming their theory a “toy model”. I would not be opposed to a label like “Scaling theory”, although this is up to the authors.
8. P. 15, line 22: typo, “accucmulation”
9. P. 15, last line: typo, “mentionned”. This also occurs on P. 17.
10. P. 15, line 41: typo, “proportionnal “
11. P. 17, last paragraph: could the authors also estimate the diffusivity of PDMS along the interface?