Biphasic liquids with shape-shifting and bistable microdomains

Liquids comprising two coexisting phases can form a range of stable and metastable states, including wetting films, droplets and threads^1-3. Processes that permit rapid and reversible transformations between these morphologies, however, have been difficult to realize because physical properties required for rapid shape change (for example, low interfacial tension or viscosity) provide pathways for relaxation that result in short-lived states. Fully reversible formation of long-lived microdomain states would expand the palette of properties that can be accessed dynamically using biphasic liquids (for example, tunable optical metamaterials). Here we report the discovery of shape-shifting and bistable microdomains of a biphasic liquid system consisting of an isotropic oil and a liquid crystalline oil. The isotropic oil forms stable wetting films ('original' shape) between solid surfaces and an overlying liquid crystal phase, and, when exposed to a transient (<1 s) a.c. electric field at low frequency (10 Hz), transforms into long-lived (>24 h) spherical domains ('temporary' shape) stabilized by topological defects in the liquid crystal^1,4,5. Subsequent application of an a.c. electric field of high frequency (1 kHz) triggers solitons to form in the liquid crystal^6-8, creating kinetic pathways that lead to remarkably rapid (<3 s) coalescence of the dispersed spherical domains and recovery of the original shape (wetting film)^1,8,9. We show rapid and reversible switching between distinct optical states of the biphasic system, with each state persisting without continuous application of the field, thus providing a combination of optical properties long sought in thin liquid films^10-17. The fully reversible and long-lived emulsion formation reported here appears promising for materials synthesis, microchemical systems and tunable optical metamaterials (for example, to control visibility and transmittance of light through windows)^17-21.