Magnetic Assembly of Colloids
In magnetic field, superparamagnetic colloids form chain-like structures with periodical interparticle, which strongly diffract the visible light. The responsive photonic structure undergoes an instant assembly and deassembly by applying and removing the magnetic field. The balanced interaction of magnetic attraction and electrostatic repulsion determines the lattice spacing and the optical properties. By changing the external field strength, one can tune the diffraction wavelength over the entire visible spectrum.
"Orientational Printing" is an unusual photonic printing technique base on the orientational tuning of photonic structures. In the printing process, the “θ - contrast” pattern is produced by magnetic alignment, orientational tuning and lithographical photopolymerization. Labels printed with two mirror-symmetric photonic orientations show switchable color distribution when the angle of incident light changes or samples are tilted.
"Invisible Photonic Printings" were prepared on a siloxane-contained photonic paper by creating contrast of crosslinking or hydrophobization degree with lithographical methods. When the invisible prints are soaked in water for several minutes, the small difference in reflection wavelength (λ) between the pattern and the background is significantly amplified due to their different swelling speed, which leads to a visible image in the end. It is fully reversible and accessible to show and hide the patterns through soaking and drying. Highlighted by Chemistry World (RSC)
Photonic Sensing, Display Unit and Optic Switch
"Magnetic Display Unit" Nonmagnetic polymer colloids are assembled into colloidal photonic crystals in ferrofluid by applying an external magnetic field based on the dipole-dipole interactions of “magnetic holes”, and the photonic crystal deassembles immediately when the magnetic field is removed. In this work, increasing the size of building blocks still produces colorful photonic crystals due to their 2nd order diffraction. With a larger size building block, the magnetic response between polymer colloids is greatly enhanced, so that an instant and reversible assembly/deassembly can be realized in a much weaker magnetic field and lower ferrofluid concentration.