Controlling Gaussian and mean curvatures at microscale by sublimation and condensation of smectic liquid crystals

Abstract

Soft materials with layered structure such as membranes, block copolymers and smectics exhibit intriguing morphologies with nontrivial curvatures. Here, we report restructuring the Gaussian and mean curvatures of smectic A films with free surface in the process of sintering, that is, reshaping at elevated temperatures. The pattern of alternating patches of negative, zero and positive mean curvature of the air–smectic interface has a profound effect on the rate of sublimation. As a result of sublimation, condensation and restructuring, initially equilibrium smectic films with negative and zero Gaussian curvature are transformed into structures with pronounced positive Gaussian curvature of layers packing, which are rare in the samples obtained by cooling from the isotropic melt. The observed relationship between the curvatures, bulk elastic behaviour and interfacial geometries in sintering of smectic liquid crystals might pave the way for new approaches to control soft morphologies at micron and submicron scales.

Curvatures of smectic liquid crystals and their applications

“…Because the elastic energy is concentrated around the focal surfaces, the smectic layers want to have smaller areas. Therefore, focal surfaces are degenerated into two lines which are a paired hyperbola and ellipse (the ellipse is perpendicular to the plane including hyperbola), or even smaller, a point (Figure 3 shrinkage of the focal surfaces into a point results in a concentric packing of layers with spherical shapes, which is rarely found during the formation of FCDs with free surfaces but appears in the sublimable smectics that will be discussed in Section 3 [68].…”

“…Recently, it was reported that the sublimation and subsequent recondensation of semi-fluorinated LC materials can occur in the smectic phase of synthetic semifluorinated molecules, labeled Y001 to Y006, composed of a rigid biphenyl group with semi-fluorinated alkyl tails (Figure 6(a)) [67,68]. These molecules show the smectic A phase in a broad temperature range upon cooling from isotropic liquid, and maintain LC structures even after they go to the smectic E phase and into the soft crystal or crystal phase at room temperature (Table 1), which is very useful behavior for investigating the nanometer-scale morphology of smectic phases [67].…”

“…These molecules show the smectic A phase in a broad temperature range upon cooling from isotropic liquid, and maintain LC structures even after they go to the smectic E phase and into the soft crystal or crystal phase at room temperature (Table 1), which is very useful behavior for investigating the nanometer-scale morphology of smectic phases [67]. Similar to other smectics, TFCDs are generated at smectic A phase on the free surface due to the conflicting anchoring conditions, resulting in geometrical depression and cusp-like structures accommodated by the formation of TFCDs, which can be easily observed to show Maltese cross-patterns by polarized optical microscopy (POM) [50,67,68]. The main concept of this work is that the sublimation of semi-fluorinated LC molecules can lead to peeling off of TFCD layers.…”

“…Recently, thermal treatment of smectic materials at the smectic LC phase temperature was shown to change the mean and Gaussian curvatures of smectic structures, in which negative to positive Gaussian curvature transformation was achieved via gradual peeling off and recondensation of smectic layers [67]. Based on this phenomenon, three dimensional bent layered structures have emerged at the nanoscale, having quite different morphologies compared with original LC textures [67,68]. This kind of self-assembling behavior of soft matter systems under thermally non-equilibrium conditions could open a route to create such small feature structures as well as more exotic curvatures.…”

Fast Fabrication of Sub-200-nm Nanogrooves Using Liquid Crystal Material

“…[22][23][24] Here, we developed highly periodic nanogrooves with a sub-200-nm feature size over large areas using a smectic LC material that can sublime and recondense to form secondary structures. 25,26 Unlike previously reported self-assembling materials, our platform takes advantage of the ability to achieve fast fabrication of sub-200-nm patterns in less than 30 min and obtain robust structures to mold other materials. To realize this goal, first, long-range (centimeter-scale) ordered smectic LC domains were prepared using micron-scale topographic patterns made of silicon.…”