(20g) A New Mechanism to Realize Reversibly Tunable Structural Color Using Binary Liquids

Structural color is widely observed in nature from flower petals to bird feathers. It attracts a lot of interests in man-made biomimetic materials because it is non-fading, environmentally friendly and tunable across full color range. Typically, structure color is realized by assembling solid building blocks (spherical particles) of a few hundred nanometers (comparable size to the wavelength of visible light). Here, a completely new mechanism is proposed based solely on binary liquid structures aided by the colloidal self-assembly. Particles under 30 nm in diameter are used to form colloidal gel matrix with bicontinuous structures that exhibits tunable color. The colloidal self-assembly provides the solid matrix that can hold the binary solvent. And the formation of the binary solvent structures in this matrix gives rise to the tunable color that can be thermo-reversibly controlled. Temperature perturbation as small as 0.3˚C is enough to alter the color from purple to yellow.

Contrast matching experiment with small-angle neutron scattering (SANS) identifies particles to be within one of the continuous domains that is rich in one component of the solvents due to the highly charged (hydrophilic) particle surface. The particle domain size is characterized to be 2 μm by ultra-small angle neutron scattering (USANS) and the particles are closely packed with an inter-particle distance of ~30 nm. Neither domain size nor inter-particle distance changes with temperature when the sample exhibits different colors. The possibility of particle crystallization is ruled out by small angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Due to the difference in contrast, static light scattering (SLS) reveals temperature-responsive solvent structures that are not visible by either neutron or X-ray scattering. The binary solvent, rather than solid particles, is responsible to the tunable structure color under different temperatures. To the best of our knowledge, this mechanism has never been reported before. Within this system, not only the color is tunable, the range of the temperature that the sample change color can also be customized at will.