Image encoding with unconventional appearance through direct ink writing of a cholesteric liquid crystal oligomer ink

We demonstrate a method combining direct ink writing of a chiral nematic liquid crystal oligomer ink with a photolithography step in a procedure that gives the user great flexibility in the design of striking polymer optics. The printable chiral nematic ink is first made by oligomerizing a reactive nematic liquid crystal monomer with a reactive chiral dopant. This ink can be readily printed using direct ink writing and quickly forms the cholesteric phase after deposition. Increasing the lateral nozzle speed forces the helical chiral nematic alignment into a slanted configuration, which has deviating optical properties compared to conventional planar chiral nematic liquid crystal reflectors—when inspected from the surface normal, these slanted photonics do not display circular polarization dependence, a feature characteristic for chiral nematics. The direction of this slant is determined by the printing direction, and thus highly customizable. For instance, when printing in a single direction, the peak reflected wavelength λ_max is only seen at ca. 50° angle of incidence. Writing a back-and-forth pattern turns the object into an “inverse cholesteric” in two dimensions, where the longest reflected wavelength is seen from 50° in either direction. More intricate print path designs lead to more complex appearances. With a two-step photo-crosslinking procedure using masks, two-dimensional designs can be imprinted in direct ink written coatings, resulting in highly complex but programmable reflection patterns.