Abstract
Liquid crystal elastomers (LCEs) hold a major promise as a versatile material platform for smart soft coatings since their orientational order can be predesigned to program a desired dynamic profile. In this work, we introduce temperature-responsive dynamic coatings based on LCEs with arrays of singular defects-disclinations that run parallel to the surface. The disclinations form in response to antagonistic patterns of the molecular orientation at the top and bottom surfaces, imposed by the plasmonic mask photoalignment. Upon heating, an initially flat LCE coating develops linear microchannels located above each disclination. The stimulus that causes a non-flat profile of LCE coatings upon heating is the activation force induced by the gradients of molecular orientation around disclinations. To describe the formation of microchannels and their thermal response, we adopt a Frank–Oseen model of disclinations in a patterned director field and propose a linear elasticity theory to connect the complex spatially varying molecular orientation to the displacements of the LCE. The thermo-responsive surface profiles predicted by the theory and by the finite element modeling are in good agreement with the experimental data; in particular, higher gradients of molecular orientation produce a stronger modulation of the coating profile. The elastic theory and the finite element simulations allow us to estimate the material parameter that characterizes the elastomer coating's response to the thermal activation. The disclination-containing LCEs show potential as soft dynamic coatings with a predesigned responsive surface profile.
Highlights
Soft coatings with stimuli-responsive topographies attract considerable research interest, triggered by a wide range of potential applications.1–5 Surface topography defines many useful properties of a coating, including optical reflection, wettability, friction, adhesion, and an ability to control adjacent media such as biological cell cultures.6–9 Tremendous progress has been achieved in understanding and designing static coatings
The thermoresponsive surface profiles predicted by the theory and by the finite element modeling are in good agreement with the experimental data; in particular, higher gradients of molecular orientation produce a stronger modulation of the coating profile
Even more exciting opportunities are offered by the director that changes in space, n^(r) = const: In particular, thin Liquid crystal elastomers (LCEs) films with in-plane director patterns, when free of any substrates, develop thermally activated 3D shape changes with non-trivial mean and Gaussian curvatures,4,17–21 while director deformations across the LCE films trigger oscillations22 and locomotion23 when activated by light
Summary
Soft coatings with stimuli-responsive topographies attract considerable research interest, triggered by a wide range of potential applications. Surface topography defines many useful properties of a coating, including optical reflection, wettability, friction, adhesion, and an ability to control adjacent media such as biological cell cultures. Tremendous progress has been achieved in understanding and designing static coatings. Liquid crystal elastomers (LCEs) show several advantages over other materials, thanks to the coupling of their rubber-like elasticity to the intrinsic orientational order.10–13 This coupling enables a broad platform for the design of elastic response of LCE coatings to various external cues that modify the orientational order. The orientational order is specified by its amplitude, called the scalar order parameter S that describes how well the molecular entities are aligned along the preferred direction, and its phase, scitation.org/journal/jap called the director n^ (jn^j 1⁄4 1, n^ ; Àn^) that is the local preferred direction of orientation.14 Modification of both S and n^ produces an elastic response of an LCE.
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