Physics and Applications of Active Complex Solids

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

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).

Publications

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Adams JM (2008) Mechanical response of smectic-C elastomers. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Adams JM (2009) Mechanical switching of ferroelectric rubber. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Adams JM (2008) Smectic-A elastomers with weak director anchoring. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Biggins J (2012) Elasticity of polydomain liquid crystal elastomers in Journal of the Mechanics and Physics of Solids

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Biggins J (2009) Textured deformations in liquid crystal elastomers in Liquid Crystals

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Biggins JS (2009) Supersoft elasticity in polydomain nematic elastomers. in Physical review letters

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Biggins JS (2009) Characterization of soft stripe-domain deformations in Sm-C and Sm-C* liquid-crystal elastomers. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Biggins JS (2008) Semisoft elastic response of nematic elastomers to complex deformations. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Corbett D (2015) Deep optical penetration dynamics in photobending. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Corbett D (2008) Nonlinear dynamics of optical absorption of intense beams in Physical Review A

 
Description The thrust of the fellowship grant was to do theory leading to effects useful for device development and to bring such theory closer to experimental groups. The main areas of theory directed to devices were:

1. Micro-opto-mechanical effects and devices.
(a) Opto-mechanical response of photo-elastomers was explored, including non-linear (non-Beer) processes for deep penetration of light to get volume actuation response for effective light-to-work conversion. The dynamics of light penetration when large conversion to excited states takes place (via in effect bleaching waves penetrating a solid) was established. This understanding then explained work from Freiburg and Zurich on the macroscopic dynamics of photo-elastomer conversion, with joint theory-experiment papers as envisaged in the application.
(b) A revolutionary effect was discovered - that with the application of light or heat, the Gaussian (intrinsic) curvature of initially flat sheets could be changed. The problem is well-known to map makers or wrappers of presents! How can you wrap a foot ball (a sphere, that is a shape with intrinsic curvature) with a flat sheet of paper without folds, creases, crumpling? It cannot be done. Liquid crystalline solid sheets with topological defects and/or gradients in their nematic fields were shown theoretically to react to heat or light by developing Gaussian curvature - a totally new departure. The domes and their inversion, as envisaged in the application were developed theoretically and published. Subsequent theory & experimental papers were also published jointly with groups in Eindhoven and Zaragossa that are interested in devices for microfluidics and health care.

2. The theory of the mechanics and electromechanics of especially ferro-electric chiral SmC* elastomeric solids was developed.
(a) In particular experiments on how shear can be mechanically reversed were modeled and the distribution of polarisation and internal space charge described, including where the route between start and end states is via textures that become charged.
(b) This work lead naturally into explaining mysterious experiments from Stuttgart where changes in the ferro-electric state are optically-driven. The photo-ferro-electric effect in liquid SmC* systems was successfully explained. It stimulated experiments and a search for possible devices for light harvesting via responsive photo-responsive SmC* in collaboration with groups in Ljubljana (Copic - theory and experiment while on leave at the Cavendish Lab) and Boulder (provision of samples by Dave Walba). This work involved the optical control of tilt and of the coupling between tilt and polarisation. Concrete calculations of a charge pump cycle based on a photo-ferro-electric working material were published. Our experiments in this area suggested an elastomeric SmC* would be better since it is more weakly conducting. [Experiments done at the Cavendish laboratory with Professor Copic, visiting from Ljubljana, Dr Kotar (pdra on the grant), and samples from Professor Walba, Boulder.]

3. A completely new effect was discovered theoretically that however fitted in well with the proposal. The soft elastic response of monodomain nematic elastomers is where elongation in response to an applied stretch is achieved by director rotation rather than a real distortion. The effect was thought to be nugatory in polydomains, which are much more practical to prepare for devices, because of the mismatch of the rotation requirement in neighbouring, connected domains. Our calculations however suggested special texturing in monodomains with an isotropic genesis could allow a super-soft elastic response. There was independent experimental confirmation of our paper by a Japanese group and soon afterwards an international workshop convened in Shanghai to discuss the new effect. We have received Chinese-supported manpower in Cambridge as a result. Subsequently we have looked at photo-versions of these systems and published new strategies for using them as the working materials in light harvesting machines (involving the inventions of 2-wheel and also solid state turbine devices - the latter work done under the grant but appearing in 2014).
Exploitation Route 1
(a) Photo-contraction of elastomers (monodomain, or better the polydomain systems mentioned above) can be envisaged in energy harvesting machines. We published papers on 3 types of light-to-work conversion schemes and machines using nematic elastomers as working materials. Bands made of isotropic genesis polydomain nematic photo-elastomers would be especially good candidates for the wheel and turbine devices since the stretch phase (like a compression part of a cycle) of the engine could be achieved at low energy cost, followed by the induction of high tension optically in the next part of the cycle by irradiation. [Materials in part 3 above.]

(b) Change in curvature is developed in response to light in order to avoid stretches that would develop if the sample remained flat. These are typically nematic photo-glasses. On removal of the light source, the Gaussian curvature must be eliminated (the sample returned to the flat state) in order to avoid stretches. This means that work can be done. Applications to health care devices (micro-fluidics components) have been pursued by collaborators in Holland (TU Eindhoven and Philips Eindhoven), and in Zaragossa, and we are in active exchange with an experimental group in the USAP (Wright Patterson AFB). The principle is that for instance fluids could be pumped by the changes in shape; the stimulus being light is very attractive since light is easy to deliver compared with heat or electricity, and is very fast.
Adaptive shapes for drag reduction are another possibility and discussions are being held with aerospace engineers (for instance with Professor Bhattacharya, a collaborator from Cal Tech, Engineering).
Dr Modes (pdra on the grant) and I proposed a grammar for combining the small set of fundamental texture elements of the nematic field in glass plates to achieve elastically consistent deformations. Putting these elements together allows one to develop a large range of 3-D shapes on illumination.
Dr. Modes and I proposed another reaction to light that avoids Gaussian curvature, but at the expense of changing the topology of a sheet. We calculated a field array of a nematic that on illumination would open up an array of micro to nano slits. Our collaborators in Holland and Spain made such an array an demonstrated the possibility of a nano sieve or filter based on this model.

2 We used our understanding derived of ferro-electric chiral SmC* systems to propose a 4th optical to mechanical work (electric) conversion device. A ferro-electric liquid SmC* is put through a cycle of light and dark, leading to the pumping of charge to higher potential. It is hoped that the publication of this design will prompt its exploration experimentally with elastomeric SmC* ferro-electrics where charge conduction will not be the problem we experienced in our experiments on liquid state analogues.

3 Continuation work with the Engineering Department, University of Cambridge, and with USAF collaborators has addressed the generally-unsolved problem of what topography actually results from dialling up curvature. The fruits of this investigation are being written up and have stimulated a new experimental programme in the USAF for adaptive aeronautics and for micro-actuation.
Sectors Aerospace, Defence and Marine,Energy,Healthcare,Pharmaceuticals and Medical Biotechnology

URL http://www.lcelastomer.org.uk/resource.html
 
Description Our prediction of how to create topography and Gaussian curvature from flat sheets by illumination or heating has been realised in micro-actuation. Miniature "lifters" that can lift thousands of times their own weight have been demonstrated by USAF research teams. One USAF team is putting 4-man years/year PDRA effort into development of micro-actuators based on our theory. Alternatively, systems that change their topology (develop slits of a width determined by heating or illumination) have been predicted by my group and realised by collaborators in Spain and Holland.
First Year Of Impact 2013
Sector Aerospace, Defence and Marine,Energy,Healthcare,Other
Impact Types Economic

 
Description Eindhoven 
Organisation Eindhoven University of Technology
Country Netherlands 
Sector Academic/University 
PI Contribution Theory of materials with new opto-mechanical properties directed to device applications.
Collaborator Contribution making of systems with topological and other defects/textures which respond to light and heat to: (a) develop Gaussian curvature (b) change topology Applications our collaborators envisage are (a) active components for microfluids (pumps, gates, valves) in health care products, and (b) micro and nano-sieves for filtration and selection.
Impact Publications - see publications outputs. Theoretical physics, chemistry, mechanics, optics
Start Year 2009
 
Description Ljubljana 
Organisation University of Ljubljana
Country Slovenia 
Sector Academic/University 
PI Contribution theory of photo-ferro-electric effect. theory and design of a solar energy harvester based on a SmC* ferro-electric liquid.
Collaborator Contribution Experiment on these liquids in the configuration agreed with group from theory. Building equipment for measurement at Cavendish Laboratory. 3 man months.
Impact Experimental results on magnitude and dynamics of effect.
Start Year 2013
 
Description Zaragossa 
Organisation University of Zaragoza
Country Spain 
Sector Academic/University 
PI Contribution theory of opto-mechanics directed at devices
Collaborator Contribution making of devices (a) curvature changes for microfluidics components (b) topology changing devices for optically-driven sieves and filters
Impact proto-type sieves and filters (nano and microscale) publications
Start Year 2009