Self-Regulating Surface Encapsulation of Liquid Crystals

Abstract

Liquid crystals (LCs) are complex fluids with both crystal-like long-range molecular ordering and liquid-like fluidity, leading to anisotropic properties (e.g., birefringence and elasticity) [1, 2]. This combination of properties provides the ability to change molecular orientations and thus report corresponding optical signals in response to a variety of physical and chemical cues, including surface modifications, pressure, and electric fields. In this work, we report a simple and versatile approach to produce LC droplets encapsulated by polymeric amphiphiles on substrates with controllable size and density upon thermal trigger. As shown in Fig. 1(a), the simplest system to observe the new approach is to place LCs between two glass substrates coated with poly (octadecyl methacrylate) (PODMA). At a room temperature (T0 = 25oC), the cells show a dark texture between crossed-polarizers (Fig. 1a) indicating a vertical orientation of LCs at substrates induced by the long aliphatic tails of PODMA. When the substrates were heated above 30oC and cooled back to T0, however, we observe the appearance of birefringent domains at substrates corresponding to radial configuration of LCs within the domains (Fig. 1b). The results suggest PODMA can play a role in thermally triggerable local encapsulation of LCs (and thus local reorientation of LCs). Specifically, with a use of polarizing optical and confocal fluorescence microscopy, we demonstrate the underlying mechanism of the thermo-optical effects to be related to thermal-induced diffusion of PODMA into LCs and their micelle formation. We found the size and density of locally encapsulated LC droplets to be controllable via heating temperature, rate of temperature changes, type of LCs, and length of aliphatic tails in PODMA, which also support our underlying mechanism. We envisage that the new thermo-optical phenomena will find applications in a variety of field, including sensors, photonics, surface modification, and structure patterning. In addition, it will be of interest to use the described mechanism to design the system that are triggered by various triggers beyond temperature.