亲爱的大家，

这确实是一个很吸引人的话题。正如道格正确指出的那样，研究人员现在正在把不稳定的敌人变成朋友。不稳定性改变了结构的状态。如果最终状态仍然起作用，那么就利用了不稳定性。否则，不稳定就会破坏。

类橡胶聚合物在小应变下表现出软化行为，在极限应变附近无限硬化(此时大多数长链聚合物被完全拉伸)。硬化行为可以为任何不稳定的最终状态创造一个安全的避风港:它限制了聚合物的最大应变。

对于介电弹性体，不稳定性以机电不稳定性的形式发生。在电压作用下，电介质弹性体变薄会产生更大的电场。在某种程度上，积极的反馈随之而来。在相同电压下，弹性体以较小的驱动应变(通常为< 30%), to a thin state with extremely large actuation strain (near the limiting strain, typical values for elastomers are in the order of 1000%). This extremely large strain almost always renders the elastomer to fail by dielectric breakdown, thereby crippling its function as an actuator. To turn this foe into a friend, one may either: Find a dielectric with an extremely large dielectric strength (> 1000 MV/m), or find a way to bring the limiting strain closer to the point of instability. The latter seems to be an easier way out.

A direct way is to apply pre-stretch on the elastomer. This brings the start point closer to the limit. Pelrine et. al., in his now classic 2000 Science Paper [1], has inadvertently exploited this fact to achieve an actuation strain of > 100%. A second way is to choose a polymer that has "shorter" chains, which limits its mechanical strain to a level where it allows the snap to survive dielectric breakdown. Ha et. al. has exploited this by designing polymer networks with short chains, interspersed within long chain polymers [2]. Both works have essentially made a friend out of instability, by allowing the polymer to safely snap into a large strain region, without dielectric breakdown taking place.

We crystallized these observations by using a model for dielectric elastomers [3]. Our model allows one to construct phase diagrams indicating regions of safe and unsafe snapping. By modifying the mechanical and electrical properties of the dielectric elastomer, one may achieve large actuations of above 500% by simply pre-stretching it. This provides a guide to materials selection and design to achieve large actuation strains.

But there is a caveat: Large actuation strains create large leakage currents. Experimental observations have shown that the current that leaks through a polymer dielectric increases exponentially with the applied field [4]. It is possible for a dielectric elastomer with an extremely good actuation performance, to be an energy-wasting device [5]. Excessive leakage currents may also put the dielectric at the verge of dielectric breakdown.

There exist layers of considerations in harnessing instabilities. An apparent foe may be turned into a friend, but it may in fact still be a foe. One certainly has to balance practical considerations with the desired function, and therein lies one exhilarating challenge for problems in instability.

[1] Pelrine, R.; Kornbluh, R.; Pei, Q.; Joseph, J. Science 287, 836–839 (2000).

[2] Ha, S. M.; Yuan, W.; Pei, Q. B.; Pelrine, R. Adv. Mater. 18, 887–891 (2006).