Concentrated Gaussian curvature in curved creases of actuated spiral nematic solids

Liquid crystal elastomers/glasses are active materials that can have significant metric change upon stimulation. The local metric change is determined by its director pattern that describes the ordering direction and hence the direction of contraction. We study logarithmic spiral patterns on flat sheets that evolve into cones on deformation, with Gaussian curvature localized at tips. Such surfaces, Gaussian flat except at their tips, can be combined to give compound surfaces with GC concentrated in lines. We characterize all possible metric-compatible interfaces between two spiral patterns, specifically where the same metric change occurs on each side. They are classified as hyperbolic-type, elliptic-type, concentric spiral, and continuous-director interfaces. Upon the cone deformations and additional isometries, the actuated interfaces form creases bearing non-vanishing concentrated Gaussian curvature, which is formulated analytically for all cases and simulated numerically for some examples. Analytical calculations and the simulations agree qualitatively well. Furthermore, the relaxation of Gaussian-curved creases is discussed and cantilevers with Gaussian curvature-enhanced strength are proposed. Taken together, our results provide new insights in the study of curved creases, lines bearing Gaussian curvature, and their mechanics arising in actuated liquid crystal elastomers/glasses, and other related active systems.