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Prof Mark Warner FRS
Fellow of Corpus Christi College
Office: 505 Mott Bld
Phone: +44(0)1223 3 37380
Email: mw141 @ cam.ac.uk
Personal web site
TCM Group, Cavendish Laboratory
19 JJ Thomson Avenue,
Cambridge, CB3 0HE UK.
Co-director, isaacphysics Project
Novel solids having the directional order of liquid crystals (LCs). Such solids derive from
- rubber: they are weak, suffer huge shape changes at constant volume (hyper-elasticity), and have liquid-like molecular mobility -- their directors n(r) are also mobile and respond to imposed shape change or to polarised light.
- glass: they have high moduli and immobile directors convected with imposed deformations.
- Changing the molecular order by heating or illumination gives reversible macroscopic elongations/contractions along the director of ~4% for glasses and ~400% for rubbers. Such macro-changes mirror the changes in molecular shapes in response to anisotropy. We are exploring uses in actuation, micromechanics and energy recovery, especially light-driven examples. Order is changed optically when rod-like dye guests bend on absorbing a photon and disrupt the orientational order of their rod hosts. Polydomain LC solids are sensitive to light polarisation since it selects out a direction for absorption and thus mechanical response.
- Nematic order can be rotated instead of reduced. The natural direction of molecular elongation is along n and, when rotated, causes a macroscopic shape change (elongations, contractions and shears) without a rise in the free energy since the distribution of chain shapes is rotated unaltered. We dubbed this "soft elasticity". Experimentally it is verified to occur, with some very small cost know as semi-softness, over huge strains comparable of course to the spontaneous distortion. Lamination (stripe formation) of the director occurs during distortions, such a route also enabling polydomains to demonstrate a remarkable super-softness we predicted against all expectations. Smectic order, with its rigid layering embedded in the soft rubber matrix, renders this mechanics still more subtle, giving rise to 2-D rubbery response. A survey of these effects is in chapter 1 of our book on LC elastomers.
- Topological defects and other textures in the 2-D director field give mechanical response to light and heat with changes in Gaussian curvature (topographic changes) or changes in topology. Examples we predicted include the formation of "anti-cones" where curvature is concentrated at a point, or a lattice of defects of topological change +1 and -1 which keeps a flat sheet flat, but with an array of slits that open and close in response to light - an optically-driven nano filter that we realised with collaborators.
In Plain English
In the Hookean, elastic classification of the states of matter of the 1670s, gases take the volume (V) and shape (S) of their container, liquids require energy to change V but S is free, and solids require energy to have V and S changed.
Even "oddities" fit into these categories; glass has well-defined V and S, as does rubber though S is weak while V is strong. Liquid crystals will flow and are liquids, despite being anisotropic about a (mobile) director.
But Hooke's scheme is not complete: Rubber is a weak solid because it is molecularly mobile like a liquid. Combination with liquid crystals gives liquid crystal elastomers -- weak, rubbery solids with mobile anisotropy. They respond to some imposed S-changes by rotating their their anisotropy of molecular elongation. They then macroscopically extend without distorting their molecular distribution. This subset of all shape changes have no accompanying rise in energy. Hooke would have called these shape changes liquid-like. We call them "soft elasticity". They dominate, and make very rich, the mechanics of these new materials.
One can switch off the anisotropy and underlying elongation of liquid crystal elastomers (and of their strong, glassy relatives) by heating or by light, if dye molecules are present. There is a macroscopic shape change of up to 400% that is reversed on cooling or darkness. This too is unknown in mechanics. Light is a particularly beautiful way to drive mechanics. It can be delivered remotely, quickly, and the mechanics is polarisation sensitive. Defects in the pattern of anisotropy, especially in glassy LC solids, lead to changes in topography and even in topology of solid sheets.
We are also exploring exploitation of these new, rule-breaking phenomena for actuation, energy recovery, medical devices, chiral separation, . . .
- Materials with programmable shapes Physics Today 69 32 (2016)
- Understanding the chain fountain Proc. Royal Soc. A - Math. Phy. 470 20130689 (2014)
- Optomechanical elastomeric engine. Phys. Rev. E 88 040501 (2013)
- Photodynamics of stress in clamped nematic elastomers. Phys. Rev. E 87 062503 (2013)
- Photoferroelectric solar to electrical conversion Appl. Phys. Lett. 102 043902 (2013)