Structural Coloration in Water Droplets Containing Suspended Clay Mineral Nanosheets

Na-fluorohectorite (Na-FHt), a synthetic clay mineral which spontaneously forms nematic phases of single 1-nm-thick nanosheets when immersed into water, show structural coloration. Our previous work has demonstrated that two single nanosheets pinned together by Cs⁺ (so-called Double Layers, DBLs) produce bright and non-iridescent structure coloration. In the present study, we demonstrate experimentally that a single acoustic levitated water droplet (with diameter around 2mm) containing clay DBL nanosheets can easily and rapidly present structure coloration. The color changes during droplet evaporation as the clay DBL concentration increases. In addition, we observe droplets with similiar volume placed on non-wetting and wetting surfaces respectively. This study paves the way further for sustainable structural coloration design at the microscale.<br/>Key words: nematic gel, structure, optical properties, environmental confinement, droplets and film<br/><b>Acknowledgements:</b> This work was funded by Norwegian University of Science and Technology grant 81771176 (PHMB), Research Council of Norway through the grant 250619 (BP, JOF, PHMB) and 315135 (YY, PHMB, JOF), German Science Foundation grant SFB 840 (B3) (JB) and Research Council of Norway project number 280643 (Petromaks2Program) (JOF, KA). CNPq 442599/2019-6, 401581/2016-0, 303137/2016-9 (KA), FAPESP 18/21489-1, 2017/50129-0 (KA) and Petrobrás 0050.0101557.16.9 (KA).<br/><br/>References:<br/>[1] Fossum, Jon Otto. "Clay nanolayer encapsulation, evolving from origins of life to future technologies." <i>The European Physical Journal Special Topics</i> 229 (2020): 2863-2879.<br/>[2] Michels-Brito, Paulo H., et al. "Bright, noniridescent structural coloration from clay mineral nanosheet suspensions." <i>Science Advances</i> 8.4 (2022): eabl8147.