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Electro-creasing instability in deformed polymers: experiment and theory
Electro-creasing instability in deformed polymers: experiment and theory
Qiming Wang, Mukarram Tahir, Lin Zhang, and Xuanhe Zhao*
Soft Matter, In Press
Abstract: Subjected to an electric field, a substrate-bonded polymer film develops a biaxial compressive stress parallel to the film. Once the electric field reaches a critical value, the initially flat surface of the polymer locally folds against itself to form a pattern of creases. We show that mechanical deformation of the polymer significantly affects the electro-creasing instability. Biaxially pre-stretching the polymer film before bonding to the substrate greatly increases the critical field for the instability, because the pre-stretch gives a biaxial tensile stress that counteracts the electric-field-induced compressive stress. We develop a theoretical model to predict the critical field by comparing the potential energy of the film at flat and creased states. The theoretical prediction matches consistently with the experimental results. The theory also explains why biaxially pre-stretching a dielectric-elastomer film greatly enhances the measured breakdown field of the film.
* To whom correspondence should be addressed. E-mail: xz69@duke.edu
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Nice work.
Hi Qiming,
Congratulation for your nice work!
It is very interesting to investigate the critical electric field and breakdown fields of pre-stretched dielectric films bonded on substrates.
I have noticed in your paper that the breakdown electric fields of dielectric films were measured by two experimental setups, current approach (Fig.2) and previous approach (Fig.6). From the results shown in Fig.7 (a), a good agreement was achieved between the previous test and the corresponding theoretical results. But the current test deviated from the previous experimental results. I am wondering what factors cause the difference between the previous and current test? (air bubbles?) In current experimental setup demonstrated in Fig.2, could the constrained dielectric film without subjecting to electric field (the polymer outside the copper rod) affect the measurement of the breakdown field? In addition, have you considered the deformation of the metal electrode if it is very thin compared to the dielectric film？
Thanks
Lianhua
Re Lianhua's comments
Hi Lianhua,
Thank you so much for your comments on our paper. Let me try to response to your three questions.
First, you are absolutely right. In the previous test steup (Fig. 6), A pair of metal electrodes was clamped around a biaxially pre-stretched film of a dielectric elastomer. The air bubbles are easily trapped on the interface, which allows the creasing instability. The measured breakdown field are probably the critical fields of creasing instability. The consistency between the experimental results and the theory demonstrates our hypothesis. But in the current test setup (Fig. 2), because the breakdown occurs in a very small area of the film under the spherical tip of the electrode rod, the effect of air bubbles is eliminated. Besides, the top electrode rod is also fixed, and therefore the pull-in instability is eliminated. The measured breakdown fieds are possible the intrinsic breakdown fields of polymer, which should be higher than the creasing critical fields.
Second, this is a good point. In our experiment (Fig. 2), however, the breakdown of the polymer always occurs in a very small area of the film under the spherical tip of the top electrode.
Third, the critical fields of creasing instability can be affected if the metal electrode is thin enough. But in our experiment, the film with substrate is bonded on a thick copper electrode with conductive epoxy. The deformation of the metal electrode is elimenated.
Best,
Qiming