"Electrostatically-induced instabilities in elastomers"
Electrostatically-induced instabilities in elastomers
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Elastomeric materials, which are often colloquially referred to as rubbers, have been known for a very long time. In the beginning, elastomeric materials were mainly used in mechanical applications, but in recent years they found usage in various electrical designs, such as a dielectric layer in deformable capacitors or soft actuators. Even though elastomeric materials have many excellent properties, there exist also downsides. Elastomer films bonded to a substrate are prone to failure due to the formation of instabilities under a DC electric field. Elastomeric material forms two kinds of instabilities, wrinkles and creases. Wrinkles are smooth sinusoidal surface corrugations and creases are deep, localized dents in the surface. There exist theories to predict the occurrence of these instabilities, but these theories often consider the wrinkles and creases separately. In this thesis, I quantify the formed instabilities of different elastomers with different material properties and compare them to a single unified model. This model is based on the energy function of the system. The Young's modulus of the elastomers ranged from ~ 3 kPa to ~ 120 kPa. I varied the thickness of the elastomer films from ~ 10 µm up to ~ 60 µm. The theory and the results show that the critical electrical field and the wavelength of the instabilities are determined by the surface energy, the shear modulus and the thickness of the elastomer film. Furthermore, I measure the changes of the elastomer film capacitance when the instabilities occur, and also at higher post-threshold electric fields. The capacitance change is greater for thicker elastomer films. From the change of the capacitance, I estimate the post-threshold amplitudes of the wrinkles and creases, which has not been done before for such systems.