New actuation modes of dielectric elastomer composite devices

For a usual Dielectric Elastomer (DE) actuator, the attraction between the electrodes owing to Maxwell forces entails a reduction of thickness with an expansion of in-plane area. However, the overall response also depends on the local effects induced by polarization, which can be macroscopically accounted for assuming the dielectric constant as a function of the strain. In this case, actuators can exhibit the remarkable theoretical property of CONTRACTING and THICKENING under an electric stimulation. In this work, we show that such counterintuitive behaviours can be achieved by tuning the electric input on tailored hierarchical dielectric composites whose phases obey an ideal dielectric behaviour. Our study concerns laminates for which we highlight the main design parameters, i.e. the shear modulus-- and dielectric constant-ratios as well as the lamination angle, to achieve the new modes. The effect of the layout in limiting, suppressing or promoting electromechanical instability under voltage control is also presented; in some cases, where the instability is suppressed, the maximum in-plane stretch orthogonal to the laminae can increase considerably with respect to a homogeneous actuator. On the basis of our results, hierarchical DE composite materials can be conceived, whose counterintuitive properties can be taken advantage of in the design of innovative actuators, sensors and energy harvesters.