Synergistic Adhesion and Shape Deformation in Nanowire-Structured Liquid Crystal Elastomers

Abstract

Nature provides many examples of the benefits of nanoscopic surface structures in areas of adhesion and antifouling. Herein, the design, fabrication, and characterization of liquid crystal elastomer (LCE) films are presented with nanowire surface structures that exhibit tunable stimuli-responsive deformations and enhanced adhesion properties. The LCE films are shown to curl toward the side with the nanowires when stimulated by heat or organic solvent vapors. In contrast, when a droplet of the same solvent is placed on the film, it curls away from the nanowire side due to nanowire-induced capillary forces that cause unequal swelling. This characteristic curling deformation is shown to be reversible and can be optimized to match curved substrates, maximizing adhesive shear forces. By using chemical modification, the LCE nanowire films can be given underwater superoleophobicity, enabling oil repellency under a range of harsh conditions. This is combined with the nanowire-induced frictional asymmetry and the reversible shape deformation to create an underwater droplet mixing robot, capable of performing chemical reactions in aqueous environments. These findings demonstrate the potential of nanowire-augmented LCE films for advanced applications in soft robotics, adaptive adhesion, and easy chemical modification, with implications for designing responsive materials that integrate mechanical flexibility with surface functionality.

Keywords: adhesion; liquid crystal elastomers; nanowire structures; stimuli‐responsive materials; superwettability.

Figure 1

Creation of nanowires on LCE films. a) Photographs of nano/micro‐wire array structures on gecko feet and filefish skin. The typical lengths of the arrays are 30–100 µm for gecko feet and 300–400 µm for fish skin. Photo credit: The gecko and fish images are royalty‐free images from Pixabay company. b) Molecular structure of the LC monomer and crosslinker RM257 used in this work, and the synthesis route for densely packed nanowire arrays on the surface of the LCE films. c) A representative photograph of an LCE nanowire film and the corresponding SEM images of the nanowires on the surface of the film.

Figure 2

Temperature‐induced adhesion and shape deformation of LCE nanowire films. a) Polarized light and bright field micrographs of individual LCE nanowires. The crossed double‐headed arrows indicate the orientation of the crossed polarizers. b) Polarized light micrographs and computer simulation showing the deformation of individual LCE nanowires upon an LC–isotropic phase transition. The black lines in the computer simulations represent the orientation of the LC moieties. c) The principal curvature of LCE nanowire films as a function of temperature. The insets show schematic illustrations of the LCE nanowire films at low and high temperatures. d) Computational simulation and photographs of the deformation of LCE nanowire films upon an LC–isotropic phase transition. The black lines in the computer simulations represent the orientation of the LC moieties. e) Adhesive forces of LCE films with and without nanowires on vertical glass surfaces as a function of applied pressure, which was held for 3 s before measuring the adhesive force. The inset shows a schematic of the experiment and a photo of a 10 mg LCE nanowire film (5 mm × 5 mm) holding an ≈2.3 g U.S. dime against a glass window after applying a 40 kPa pressure for 3 s. f) Adhesive forces of LCE films with and without nanowires on a curved glass surface with a curvature of ≈0.9 cm⁻¹ as a function of the temperature. A 40 kPa pressure was applied for 3 s before measuring the adhesive force. The insets show schematic illustrations of the curvature matching between the LCE nanowire films and curved glass surfaces. g,h) Plot and corresponding photographs showing the displacement of an LCE film with the nanowires removed from the left end, caused by cycling the temperature to induce reversible curling. No displacement was observed when all of the nanowires were removed. Error bars represent standard deviations from three independent measurements for each data point.

Figure 3

Chemical‐induced shape deformation of LCE films with nanowire structures. a) Schematic of the shape deformations of LCE nanowire films under the presence of an organic solvent vapor condition and after the application of an organic solvent droplet condition. b) Principal curvatures of LCE nanowire films as a function of time immersed in toluene vapor (24 ppm) and after the application of a toluene droplet (120 µL). The toluene vapor was removed at 20 min. c) Maximum adhesive forces and principal curvatures of LCE nanowire films as a function of applied toluene droplet volume. A 40 kPa pressure was applied for 3 s before measuring the adhesive force. d) Principal curvatures of LCE nanowire films as a function of the solvent polarity of 120 µL organic solvent droplets. Error bars represent standard deviations from three independent measurements for each data point.

Figure 4

Underwater superoleophobicity of LCE nanowire films induced by chemical modification. a) Molecular structures of AEAPTMS and 2‐CEA. b) Photographs and contact angle goniometer images showing the underwater oil wettability of LCE nanowire films before and after surface modification. c) Underwater adhesive force of a 5 µL dichloromethane droplet on chemically modified LCE nanowire films. d) Chemical reaction between colorless vanillin and decylamine producing yellow 4‐((decylimino)methyl)‐2‐methoxyphenol, driven by an LCE nanowire film‐based droplet mixer. Two 10 µL colorless n‐octanol oil droplets containing vanillin (left) and decylamine (right) were placed on an underwater superoleophobic substrate. IR light‐induced actuation of the LCE nanowire films (fixed at one end) mixed the droplets. e) ATR‐FTIR spectra showing the disappearance of the C═O peak at 1674 cm⁻¹ and the appearance of the C═N peak at 1645 cm⁻¹, confirming the reaction between the amine and aldehyde.