Cardiff: Electromechanical Energy Conversion via Liquid Crystal Elastomers

Cardiff, Featured, liquid crystals, Solar energy, Sustainable Energies

Liquid crystal elastomers (LCEs) have captured the imagination of scientists around the world after they were first envisioned by P.G. de Gennes in 1975, synthesised successfully by H. Finkelmann and collaborators in 1981, and modelled theoretically by M. Warner and co-workers since 1988 [1, 2]. Thanks to their shape-shifting ability through large repeatable deformations under natural stimuli like temperature or light, they are the stuff of science-fiction.

Figure 1. All-atom molecular dynamics simulation of thermotropic LCE: The material extends along the director n as liquid crystals align in the nematic phase at low temperature and contracts in the isotropic phase at high temperatures [3]. 

The quest for responsive materials with the ability to mimic living systems or enable green energy production and conversion processes is a major challenge for modern materials design. Due to their rich physical responses in the presence of ambient stimuli and electric or magnetic fields (Fig. 1), LCEs are suitable for a wide range of applications in science, manufacturing, and medical research. Biodegradable, recyclable and reprocessable LCEs have also been achieved.

However, while their constituent ingredients, namely polymeric networks and liquid crystal (LC) mesogens, are well understood and widely utilised in major industries, such as rubber tyres and LC displays, the vast potential of LCEs remains largely untapped. This is in great part because, although experimental studies of LCEs have witnessed an unprecedented surge in recent years, their deeper understanding necessitates advanced mathematical tools, many of which are yet to be produced.

Figure 2. Schematics of voltage-driven or charge-driven parallel-plate capacitor with dielectric LCE.

For instance, charge pumps are convenient and economical devices that use capacitors to generate higher voltages from a lower voltage supplied by a source battery. The simplest capacitor consists of two parallel plate electrical conductors separated by air or by an insulating material known as the “dielectric” (Fig. 2). The plates are connected to two terminals, which can be wired into an electric circuit. When the performance of a capacitor changes by altering the distance between plates or the amount of plate surface area, a variable capacitor is obtained. Dielectrics made of rubber-like materials demonstrate great potential for generating low carbon renewable energy in emerging technologies. Moreover, in dielectric elastomers, pre-stretching increases the area of the dielectric and reduces the distance between plates, increasing the amount of charge that can be taken from the battery. 

LCEs are top candidates as dielectric responsive material and can be more efficient than rubber within a charge pump capacitor [6]. Generally, macroscopic deformations of nematic LCEs caused by applied forces induce director rotation. This rotation is not always uniform and can generate some interesting macroscopic effects. 

Figure 3. Finite element simulation of necking in a nematic LCE subject to uniaxial stress [6].

When subject to a large tensile force, LCEs may exhibit localised necking [5, 6]. This phenomenon occurs when there is a critical extension ratio, such that the force required to extend the material beyond this critical value changes from increasing to decreasing. In this case, the homogeneous deformation becomes unstable, and the material sample suddenly elongates more in a small region than in the rest of the sample. Locally, the material appears much narrower than before the critical stretch was reached (Fig. 3).

Figure 4. Finite element simulations of nematic LCEs developing shear stripes under uniaxial stress where stripe width increases with the sample initial length [6]. 

When the LCE is pre-stretched, bifurcation to a pattern of stripe domains may also occur where adjacent stripes deform by the same shear but in opposite directions (Fig. 4). The theoretical explanation of this phenomenon is that the energy depending on the macroscopic deformation only through the relative strain of the microstructure is minimised by these materials through a force-free state, or “soft elasticity”, where the microstructure consists of many homogeneously deformed parts [2, 6, 7]. 

These unusual behaviours and many others are specific to LCEs and must be taken into account within modern designs and industrial applications. 

REFERENCES

  1. de Gennes PG, Prost J. 1993. The Physics of Liquid Crystals, 2nd ed, Clarendon Press, Oxford.
  2. Warner M, Terentjev EM. 2003. Liquid Crystal Elastomers, Oxford University Press, Oxford, UK.
  3. Mahardika N, Raistrick T, Mihai LA, Wang H. 2024. All-atom molecular dynamics simulations of nematic liquid crystal elastomers, International Journal of Solids and Structures 291, 112717 (doi: 10.1016/j.ijsolstr.2024.112717).
  4. Mihai LA. 2023. A theoretical model for power generation via liquid crystal elastomers, Mathematics and Mechanics of Solids 29(6), 1198-1215 (doi: 10.1177/10812865231193735).
  5. Clarke SM, Terentjev EM, Kundler I, Finkelmann H. 1998. Texture evolution during the polydomain-monodomain transition in nematic elastomers, Macromolecules 31(15), 4862-4872 (doi: 10.1021/ma980195j).
  6. Poudel R, Sengul Y, Mihai LA. 2024. Deformation localisation in stretched liquid crystal elastomers, Mechanics of Soft Materials 6, 8 (doi: 10.1007/s42558-024-00063-2).
  7. Mihai LA. 2022. Stochastic Elasticity: A Nondeterministic Approach to the Nonlinear Field Theory, Springer Cham, Switzerland (doi: 10.1007/978-3-031-06692-4).