Shape and chirality transitions in off-axis twist nematic elastomer ribbons

Abstract

Using both experiments and finite element simulations, we explore the shape evolution of off-axis twist nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90° left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample midplane. Both experiments and theoretical studies show that when the director at midplane is parallel to either the ribbon's long or short axes, ribbons form either helicoids or spirals depending on aspect ratio and temperature. Simulation studies show that if the director at midplane is off-axis, ribbons never form helicoids, instead evolving to distorted spiral shapes. Experimental studies for two samples with off-axis geometry show agreement with this prediction. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulations predict off-axis samples also change their macroscopic chirality at fixed temperature, depending on the angular offset. These results provide insight into the mechanisms driving shape evolution and macroscopic chirality, enabling engineering design of these materials for future applications.

Programmed morphing of liquid crystal networks

“…Therefore, a long ribbon bends along its long axis while suppressing the bending along the short axis [20]. In addition, some out-of-plane twisting deformation can be expected, due to the chirality of the twisted nematic alignment (vide infra) [21], and this has also been observed in practice [22]. These predictions do not take into account any outside influences.…”

“…5). There is also a small influence of the twist handedness on the formation of the coils [21], which will be discussed below.…”

“…However, the effect is very weak and can be cancelled out by having q deviate a few degrees from 0. In this case, achiral bending can be obtained [21]. UV-driven coiling actuators have also been prepared by adding azobenzene-based mesogens to the liquid crystal networks.…”

Modeling Defects, Shape Evolution, and Programmed Auto-Origami in Liquid Crystal Elastomers

“…For instance, a flat, straight LCE ribbon with director twist through the thickness can form either helicoid or spiral structures, depending on sample properties including director twist geometry and sample aspect ratio; remarkably, these chiral structures undergo transitions from right-to left-handed with temperature (Sawa et al, 2011(Sawa et al, , 2013. A larger sample with multiple twisted domains arrayed in stripes gives rise to actuation via accordion folds, while a checkerboard pattern of twisted domains produces a buckled texture .…”

Toying with science

“…Experimenting with chiral structures in macramé later inspired me to study chiral symmetrybreaking in random copolymers, 2 lipid membranes, 3 and liquid crystal elastomers. 4,5 Many other art activities taught valuable lessons in symmetry and geometry. Drawing with an Etch-a-Sketch TM made me a citizen of Flatland, 6 and I spent many happy hours navigating a stylus over a Cartesian plane with knobs controlling x and y positions.…”