Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules
(a) Schematic illustration of isotropic shrinkage of a liquid capsule under hypertonic condition; the capsule shrinks until the osmotic pressure difference becomes negligible. (b) A series of optical microscope images showing isotropic shrinkage of liquid capsules consisting of 10 wt.% aqueous core of PVA with 100 mOsml−1 and ETPTA membrane, where the capsules are dispersed in aqueous solution with 460 mOsml−1. Scale bar, 100 μm. (Figures reprinted by permission from Macmillan Publishers Ltd: Nature ©2014.)
Colloidal crystals are promising structures for photonic applications requiring dynamic control over optical properties. However, for ease of processing and reconfigurability, the crystals should be encapsulated to form 'ink' capsules rather than confined in a thin film. Researchers from SEAS and KAIST, including Professors Manoharan and Weitz, demonstrate a class of encapsulated colloidal photonic structures whose optical properties can be controlled through osmotic pressure. The ordering and separation of the particles within the microfluidically created capsules can be tuned by changing the colloidal concentration through osmotic pressure-induced control of the size of the individual capsules, modulating photonic stop band. The rubber capsules exhibit a reversible change in the diffracted colour, depending on osmotic pressure, a property they call osmochromaticity. The high encapsulation efficiency and capsule uniformity of this microfluidic approach, combined with the highly reconfigurable shapes and the broad control over photonic properties, make this class of structures particularly suitable for photonic applications such as electronic inks and reflective displays.
(S.-H. Kim, J.-G. Park, T. M. Choi, V. N. Manoharan & D. A. Weitz, "Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules," Nature Communications 5: 3068 | doi:10.1038/ncomms4068)