Tunable nano-wrinkling of chiral surfaces: Structure and diffraction optics

Area of Science:

• Materials Science
• Biophysics
• Optics

Background:

• Periodic surface nano-wrinkling is prevalent in biological liquid crystalline materials.
• These structures are responsible for dynamic structural colors in nature, responding to environmental changes.
• Understanding nano-wrinkling mechanisms is key for biomimetic material design.

Purpose of the Study:

• Investigate the formation of surface undulations in cellulosic cholesteric liquid crystals (CCLCs).
• Analyze the role of anisotropic interfacial tension, hydration-induced swelling, and capillarity.
• Determine the relationship between nano-wrinkling, pitch gradients, and optical properties.

Main Methods:

• Applied generalized shape equation for anisotropic interfaces with Cahn-Hoffman capillarity vector and Rapini-Papoular anchoring energy.
• Modeled nano-wrinkling in plant-based plywood free surfaces with water-induced cholesteric pitch gradients.
• Utilized scaling laws to derive amplitude-pitch relationships and finite difference time domain (FDTD) simulations for optical response analysis.

Main Results:

• Derived explicit relations between nano-wrinkling amplitude, anchoring strength, and spatially varying pitch.
• Demonstrated that CCLC surfaces with spatially varying pitch reflect light at higher wavelengths compared to constant pitch surfaces.
• Showcased that hydration-induced pitch gradients control structural color changes.

Conclusions:

• The study provides a foundational understanding of structural color phenomena driven by nano-wrinkling in biological materials.
• Findings offer insights for designing advanced optical sensor devices with tunable color responses.
• Hydration-induced pitch gradients are identified as a key mechanism for controlling optical properties in CCLC materials.