Cellulose nanocrystals (CNCs) are anisotropic and chiral colloidal nanorods able to self-assemble spontaneously into a cholesteric phase above a critical concentration. When the suspension is allowed to dry, it forms a solid film that retains the cholesteric order, enabling the reflection of specific wavelengths, typically in the visible range. Due to their negative diamagnetic anisotropy, CNCs also tend to align perpendicular to an external magnetic field. Experimentally, the cholesteric phase can be aligned under moderate magnetic fields (µ₀H ≈ 0.5 T), enabling an angular control of the helical direction of the domains.
In this talk, I will present a simple and robust method to spatially control the angular response of CNC films by casting them in close vicinity to commercially available templated magnets (Polymagnets^®). The spatially modulated magnetic field applied upon casting results in the local alignment of the cholesteric structure inside the film in the direction dictated by the field pattern, yielding a replica of the magnetic pattern with an illusion of depth. Angular-resolved optical spectroscopy and k-space optical microscopy analysis both converged toward a quantified illusion of depth in qualitative agreement with the profile expected from the magnetic field mapping.

Example of a flat photonic cellulose nanocrystal film with an imprinted pattern providing an illusion of depth. The bottom left corner of the film is shown in black and white to emphasize the 3D effect.

Silvia Vignolini

Cambridge Univeristy

Appearances