Physics and Applications of Active Complex Solids

SummaryLiquid crystal elastomers (LCEs) combine the nano-scale order of nematic and smectic liquid crystals, with the weak solid character of rubber. Thus there is orientational and layering order in the liquid crystalline polymers that are crosslinked to form these special rubbers. The ordering arises from the rods that are incorporated chemically into the polymers. Rubber, like liquid crystals, is fluid-like at the molecular level. It is thus capable of huge extensions that are more like the flow of a liquid.LCEs are the same as classical rubber, except that they take a macroscopic shape that reflects the anisotropy of their underlying polymers. They can extend by 100s% when cooling through the thermal isotropic to nematic transition. They contract again on heating. Such changes can be equally induced by shining light onto dye-containing LCEs, with subsequent recovery in the dark. We called these light actuated materials photo-elastomers.We aim to develop these materials for device applications, for instance actuation by light to make new types of microfluidic devices. We envisage channels that can be blocked by a light-operated micro-valve, or fluid pumped along channels by a light-driven peristaltic pump. Gratings and reflectors could be optically written into LCE films.The coupling of nano-order to mechanical properties can also be from its rotation instead of by its (temporary) destruction. Strains applied perpendicular to the ordering direction can induce the order to rotate, with shape deformations occurring sympathetically. In cholesteric rubber there is a periodic modulation of the refractive index. One gets a photonic band solid, analogous to an electronic band solid that gives one semi-conductors. In rubber these are deformable photonic band structures, with the consequence one gets a mechanically shifting laser colour. An object of this proposal is to understand how to make these band solids more perfect for tunable laser devices.The layered liquid crystals (Smectics) yield rubbers with even richer behaviour. Their mechanical response can be rubbery in the 2-dimensions of their layers, and solid-like in the third. In their tilted, chiral form, they are soft solid ferro-electrics, that is they have a direction of spontaneous electric polarisation. They are unique as ferro-electrics in that they are liquid-like at the local level, rather than being very hard ceramics. Their polarisation direction can be induced to rotate in response to particular shears. Inversely, applying an electric field causes the polarisation direction to rotate and hence shears to develop. The former could be a sensor, the latter an actuator. We aim to exploit these materials as devices.We have written Liquid Crystal Elastomers (OUP, 2003) which has become the standard research monograph in the field. The first chapter (along with the contents and preface) is available on the web (http://www.tcm.phy.cam.ac.uk/~mw141/) and summarises for a non-specialist audience the effects we aim to explore for their materials and device potential. There are links on the same page to films of thermal actuation, and to benders (both thermo- photo-elastomeric).