Mechanics of Soft Active Materials (SAMs)

I have recently given seminars on Mechanics of Soft Active Materials (SAMs) at several universities, using this set of slides (pdf, 1.4 MB).  I also attach the slides as ppt; please feel free to use anyway you want.  Here is an abstract of the seminars, followed by a list of papers published by my group on the topic.  Each paper has initiated on iMechanica a thread of discussion, to which I'll link.  I'll give a talk at the ASME Congress in Seattle, in Session 10-12-4 Instability in Solids, 9:45 am - 11:15 am, Thursday, 15 November 2007.  

Abstract.  Soft materials can be made active in that they can greatly change shape and volume in response to stimuli, including mechanical stresses, electric fields, and trace amount of enzymes. For example, an elastomer may strain more than 100% under an electric field. As another example, a gel in contact with a solvent may imbibe a large quantity of small molecules and swell thousand times its initial volume. The amount of swelling may be changed abruptly by small changes in the environment. These soft active materials (SAMs) are being developed in diverse technologies, including muscle-like actuators, drug delivery, and tissue engineering.

My group has recently started a project to formulate field theories for SAMs subject to mechanical, electrical, and chemical loads. The theories address commonly asked questions. How do stress, electric field, and chemical potential interact? What is the interplay of large deformation, polarization, and mass transport? Why do abrupt changes, or instabilities, occur? How do viscoelasticity and diffusion affect the durability and performance of devices? How does one model the effect of enzymes on the swelling of a gel?

In this talk I'll focus on our work on elastic dielectrics. We show that the notion of Maxwell stress, which is widely used in the literature, has no general theoretical basis.  However, for a very special class of materials, which we call ideal dielectric elastomers, our theory recovers the Maxwell stress. 

As an illustration of the theory, we analyze the electromechanical instability of dielectric elastomer actuators.  We find that the free energy functions of dielectric elastomers are often non-convex, leading to coexistent states.  Our calculation shows that stability of the actuators is markedly enhanced by pre-stresses, agreeing with existing experimental observations.

Time permitting, I’ll also outline our work on polymeric gels.

A list of papers on SAMs (in order of dates of completion):

• Zhigang Suo, Xuanhe Zhao and William H. Greene, A nonlinear field theory of deformable dielectrics. Journal of the Mechanics and Physics of Solids. http://dx.doi.org/10.1016/j.jmps.2007.05.021 (Preprint and discussion on iMechanica).
• Xuanhe Zhao, Wei Hong and Zhigang Suo, Electromechanical coexistent states and hysteresis in dielectric elastomers. Physical Review B 76, 134113 (2007). (Preprint and discussion on iMechanica).
• Jinxiong Zhou, Wei Hong, Xuanhe Zhao, Zhiqian Zhang and Zhigang Suo, Propagation of instability in dielectric elastomers. International Journal of Solids and Structures. http://dx.doi.org/10.1016/j.ijsolstr.2007.09.031. (Preprint and discussion on iMechanica).
• Xuanhe Zhao, Z. Suo, Method to analyze electromechanical stability of dielectric elastomers. Applied Physics Letters 91, 061921 (2007). (Preprint and discussion on iMechanica).
• Xuanhe Zhao, Wei Hong and Zhigang Suo, Stretching and polarizing a dielectric gel immersed in a solvent. (Preprint and discussion on iMechanica).
• Wei Hong, Xuanhe Zhao, Jinxiong Zhou and Zhigang Suo, A theory of coupled diffusion and large deformation in polymeric gels. (Preprint and discussion on iMechanica).