Pattern formation in stretching experiments of liquid crystal elastomers: stripe domains and polydomain textures

Liquid crystal elastomers (in particular nematic elastomers) are synthetic materials, composed by a polymeric network with embedded nematic mesogen molecules, that are the functional unit of liquid crystals. Due to the their constituents, nematic elastomers show both the properties of large deformation of rubber-like solids and the optical properties of liquid crystals. The rotation of the nematic mesogens can be induced with an external electric or magnetic field, since the rotational degree of freedom is strongly coupled with the deformation, the external field induces a macroscopic deformation of the sample. Vice-versa, it is possible to impose a deformation on a liquid crystal elastomer sample and observe a change in the refractive index of the material due to the rotation of the mesogens.

At rest, a nematic sample of liquid crystal elastomer shows a preferential direction along which all the mesogens are aligned. If we stretch the material in the direction orthogonal to that one, the nematic mesogens will rotate to align with the direction of highest stretch. Above a threshold stretch, the nematic material tends to shear to minimize the internal energy. In presence of lateral clamps the macroscopic shear of the sample is constrained, hence the sample develops a micro-structure with alternated areas of shear, resulting in a peculiar stripe pattern.

The mechanism that regulates the stripe micro-structure is well known, but very recently experimental observations showed the presence of a sub-stripes pattern made of very small spatial modulation of the orientation of the nematic mesogens, whose mechanism is not yet understood. The aim of this projectis to develop a mathematical and numerical tool to reproduce the sub-stripes pattern and understand which are the mechanisms of its formation.

Head of project

Dr. Mariarita de Luca, SISSA MathLab, mariarita.deluca (at) sissa.it