Sébastien Neukirch
Institut Jean le Rond d'Alembert
Centre National de la Recherche Scientifique
Sorbonne Université, Campus Pierre et Marie Curie
Paris, France

tel: +33 1 44 27 72 61
e-mail: sebastien.neukirch (-atat-) upmc.fr

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Auxiliary soft beam for the amplification of the elasto-capillary coiling: Towards stretchable electronics

P. Grandgeorge, A. Antkowiak, and S. Neukirch

Advances in Colloid and Interface Science, vol. 255 (2018) 2-9

Abstract : A flexible fiber carrying a liquid drop may coil inside the drop thereby creating a drop-on-fiber system with an ultra-extensible behavior. During compression, the excess fiber is spooled inside the droplet and capillary forces keep the system taut. During subsequent elongation, the fiber is gradually released and if a large number of spools is uncoiled a high stretchability is achieved. This mechanical behaviour is of interest for stretchable connectors but information, may it be electronic or photonic, usually travels through stiff functional materials. These high Young’s moduli, leading to large bending rigidity, prevent in-drop coiling. Here we overcome this limitation by attaching a beam of soft elastomer to the functional fiber, thereby creating a composite system which exhibits in-drop coiling and carries information while being ultra-extensible. We present a simple model to explain the underlying mechanics of the addition of the soft beam and we show how it favors in-drop coiling. We illustrate the method with a two-centimeter long micronic PEDOT:PSS conductive fiber joined to a PVS soft beam, showing that the system conveys electricity throughout a 1900% elongation.

DOI: 10.1016/j.cis.2017.08.011

download the journal version : PDF

hal-01451716
Submitted (Jan 25th, 2017)
Reports (received August, 2017)

Reviewer 1:
 The paper describes a very interesting strategy to produce self-coiling threads that may be used for stretchable electronics applications. The principle is inspired by a previous work from the group. It consists in depositing some droplets of a wetting liquid along an initially stretched fiber. As the fiber is progressively released, capillary forces tend to attract the fiber inside the droplets, leading to coils. These capillary forces provide a permanent tension to the fiber. In the absence of gravity the “necklace” would appear as a straight thread that can be stretched by a factor 20 by applying a constant load (equal to the capillary force). A certain range of parameters is nevertheless necessary to obtain such unique self-coiling properties. Rather thick fibers made with very soft materials would be ideal. Unfortunately, standard conductive materials turn out to be too rigid. The authors propose an ingenious strategy where a thin conductive fiber of stiff material is associated with a thick and soft partner. The authors describe in detail how the material properties of both fibers have to be tuned to obtain the self-coiling effect.

I found the paper interesting from both engineering and fundamental points of view and I recommend the publication of this work after minor adjustments. I found the coiling conditions very well described except in section 5 where the derivation of equations 13-15 is abrupt. The factors f(k) and g(k) are for instance quite obscure. I think this is a point that the authors can easily improve.

From an engineering point of view, it would be very interesting to show how reversible the process is. Does the system resist to successive coiling-uncoiling operations?

Minor details:
- Abstract: I found both sentences “During... achieved” and “This mechanical... coiling” too long. A phrasing with shorter sentences is welcome.
- Introduction: Same remark with the sentence “The mechanics... 15]”
- Materials and method line 68: I think this is “Polylactic” acid.
I guess it would also be worth commenting in this section that the PEDOT:PSS is conductive.
- Section 3, page 3, last line left column: I think this is “menisci”.
- page 4 to left: I guess it is \mathcal{V} in dV/dL = 0.
- section 5: The conductive fiber seems to separate from the auxiliary fiber once in the droplet. I wonder if it not possible to embed the conductive fiber inside the auxiliary one while the PVS is curing.

Reviewer 2:
 This paper reports a combined theoretical study of how the elasto-capillary windlass (recently demonstrated by these same authors) can be used to coil relatively stiff fibers, as might be of use in stretchable electronic applications. The problem with using the simple windlass mechanism is that to coil a thin element of a (stiff) metal requires fibers that are too thin to be readily made. The solution is to combine them with a very soft, larger beam in parallel: this increases the surface tension force that drives coiling without adding considerably to the bending stiffness.

Generally the paper is well written and extremely interesting scientifically. I therefore support its publication, though I have a few minor comments that the authors should consider first:

There are a few minor errors in the English: e.g. “explicate” instead of “explain” in the abstract, “straigth” (rather than “straight”) and “surfaces forces” (rather than “surface forces”), both at the beginning of section 3.

The different surface energies in the problem are only discussed in detail in section 3. However, they were already used around line 144 when the Young angle was defined and discussed.

The definition of k is given early on (around line 126), but is not really mentioned in the results equation (14). I think it would be good to include a reminder of its definition and meaning around eqn (14).

At the end of section 5, the authors state that there is a systematic factor of 2 error in the prefactor delimiting “coilable with auxiliary beam” from “uncoil able”. However, they do not discuss any possible reasons for this.

Finally, the authors are clear that they do not believe there to be any short-circuits in their device. However, in figure 10, it is clear that there are some dips in the end-end resistance of the device for both the single and double PEDOT fibers - with a little imagination it seems that these dips occur in the same places for each case. Could this not be from a temporary short circuiting? (What would be the change in resistance if the effective length of the fiber were to change by the perimeter of the drop?)