I am really sorry, but this is an area that has been "abandoned" and not updated for nearly 5 years... Hope to do better in the future. Some of the highlights are good though

The latest: Thermal and Photo-Actuators! The animation is just to show the extent of spontaneous thermal expansion of monodomain (uniaxially aligned) nematic elastomers, as it it heated above and then cooled back below its nematic-isotropic transition point. See some more serious papers on the subject, by our group and by H. Finkelmann, which also discuss possibilities of using these effects in thermo- and photo-mechanical actuators.

Very high amplitude (and high force exerted) make these artificial muscles very attractive! The plot on the right shows a typical variation of sample length L (along the nematic director), with respect to this length in the isotropic phase, L0: Several different materials show the same qualitative features, but rather different amplitudes (from 10-50% to 300% and more); transition temperatures could be made different too.

The work cycle of such an artificial muscle depends on the amplitude of motion, but also on the force the nematic rubber exerts -- or the load it can work under. The plots below show: (a) Extension l = L(T)/L0 on cooling cycles under increasing load, corresponding to stress of 0, 30, 60 and 90kPa; (b) The stress (force per 1 area) exerted by a clamped strip, heated from the room temperature. Symbols are the data points from different samples which were tearing at a stress of about 100kPa (circled); the red line is the plot of underlying effective strain e = [L-L0(T)]/L0 (values on the second y-axis)

     

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  • The latest: Damping in Liquid-Crystal Rubbers! Internal relaxation of director modes in nematic elastomers leads, among many others, to a dramatic enhancement in mechanical damping. The plots illustrate the point on the example of simple-shear deformation in two sample geometries - when the nematic director n is along the imposed shear (D)isplacement direction and the shear (V)orticity direction. In the second case, the "log-rolling", there is no internal director rotation and the storage modulus G' (in MPa) monotonously increases on cooling (trivially approaching the glass transition far below). In contrast, when in the (D) geometry the director is forced to rotate - we notice a dramatic decrease in the effective modulus G'. This is another representation of "soft elasticity"! The loss factor G''/G' reaches enormously high values.

    People think this might have a big impact on damping technology - from making quieter cars and refrigirators, to damage-control of jet engine turbines...

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  • The latest...  Let's return to Liquid Crystal Colloids. We know colloid particles wouldn't like to be homogeneously dispersed in a nematic matrix - they will aggregate to reduce the elastic energy. Well, in some cases we can make them aggregate into a really rigid cellular solid: pictures show PMMA particles in 5CB are densely aggregated on thin interfaces and look at the modulus!... {That's right, it is is not a mistake: 105 Pa}

    •          Recent papers: "Cellular solid behaviour of liquid crystal colloids"

    (Wilson Poon and the Edinburgh team are also interested in this effect)

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  • The latest... Piezoelectric effect in chiral liquid crystalline rubbers. Molecular theory, developed recently with M. Warner, uses a (we think - clever :-) concept of biasing the orientation of chiral monomers when the chain they are connected on is being stretched in some directions. Check out the Downloads page for recent papers on this, and other recent topics.

    • Imagine a piezoelectric rubber sheet - you stretch it or press on it and it responds with a charge!

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  • The latest... What happens if you apply an external field to a randomly disordered (polydomain) system? (A magnetic field to a spin glass, a stress to a polydomain nematic elastomer, etc...) Obviously, at high field the system will align, but there seems to be a phase transition at a critical value of field, between a "mathematically disordered" state and the one with small but finite long-range order: see the work with S. Fridrikh in recent abstracts.

    • The critical magnetic field may be too small in spin glasses, but it looks like the experiment on stretched nematic elastomers has seen this transition all along. The critical stress depends on nematic order parameter, the mean orientation order fits the S(h) plot above to an amazing accuracy...

    Now ask yourself - how long would it take to reach the equilibrium? All randomly disordered systems exhibit slow relaxation - but the polydomain (spin-glass like) nematic rubber is very slow... See some experimental results and a theoretical model of singular self-retardation in the recent paper with S.Clarke

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  • The latest... Light scattering of an randomly disordered (polydomain) nematic elastomer subjected to a tensile stress shows the details of texture evolution. The structure factor says that the transition from short to long-range correlations on increasing field takes places via reorientation rather than growth of domains. Amazingly, this is what the theory predicts in the first place, see a page of recent abstracts.

    • Stuart Clarke and I will do a lot more of this light scattering in the near future, as well as experiments on slow relaxation, mechanical transitions and even piezoelectricity. Peter Olmsted is also interested in the theory...

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  • The latest... Stripe domains in stretched liquid crystalline elastomers is an attempt of the material to follow the "soft elasticity" trajectory, that is a special set of deformations that do not cost any elastic energy (one of the unique features of nematic rubbers). The critical behaviour at both ends of this mechanical transition and topologically stable domain walls that localise under stress are very interesting too... [the polarised mic. image - courtesy of I.Kundler, Freiburg]

    • See a page of recent abstracts about liquid crystal elastomers - and check what Mark Warner and Giles Verwey have to say about it ...

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  • The latest... "Topological dereliction" - emulsion of nematic liquid crystal droplets, held apart by the energy barrier of topological constraints for S2/Z2 in a closed volume . Can you tell why these tightly squeezed droplets do not coalesce for several years?..

    • Liquid crystalline colloids have emerged as a prominent area of research and applications in the last 2-3 years; this also includes nematic or smectic (lamellar) systems filled with colloid particles, and their equilibrium and kinetic properties. See some recent abstracts on this subject.

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