Reverse-Mode Operation Switchable Nematic Emulsions G. Di Profio, F. P. Nicoletta,* G. De Filpo, and G. Chidichimo Dipartimento di Chimica, Universita` degli Studi della Calabria, 87036 Rende (CS), Italy Received July 31, 2001. In Final Form: December 21, 2001 Switchable nematic emulsions are dispersions of micron-sized liquid crystal droplets in a monomer matrix, which can be switched from an opaque OFF state to a transparent ON state by application of very low electric fields. In this paper, we present our preliminary results concerning the behavior of switchable nematic emulsions operating in a reverse mode, that is, droplets characterized by a transparent OFF state that can be electrically turned into an opaque ON state. Radial droplets of liquid crystal with a negative dielectric anisotropy have been prepared with a well-defined transparent state by choosing a monomer mixture that matched the average refractive index of the liquid crystal droplets. The application of an electric field determines the reorientation of liquid crystal directors through an elastic distortion and the achievement of a more scattering state. The effect of temperature on the electro-optical response is also reported. I. Introduction Emulsions are dispersions of liquid droplets of one immiscible fluid in another one. They are of considerable technological importance with applications in widespread fields such as foods, drugs, paints, and coatings. 1 Emul- sions are generally metastable systems because of at- tractive interactions among drops. They tend to coalesce into larger ones, and a net separation between the two fluids can appear. To prevent coalescence, surface-active molecules are added in emulsions. Surfactants provide strong short-range repulsions (Coulombic, steric, or other repulsive interactions), which counterbalance the attrac- tive forces and give a long-term stability. Nematic emulsions are formed by nematic liquid crystal droplets dispersed in a continuous liquid solvent. They have been the object of both experimental and theoretical studies. 2-11 The kinetic stability of nematic emulsions is strongly affected by viscosity and surface tension of both fluids. The use of matrixes with low viscosity values generally results in metastable droplets. The presence of anisotropic properties in one fluid changes the nature of interactions between the droplets and the host matrix. In fact, liquid crystal directors can assume different align- ments (for example, tangent and normal) at the droplet surface, giving rise to different structures (bipolar and radial configurations, respectively). 2,3 The distortion of the nematic director field in droplets can induce interac- tions among droplets, 3,10,11 and several different configu- rations can be observed in the presence of either additives or external fields. 12-15 Bipolar droplets are characterized by a director field parallel to the interfaces with two point defects at opposite ends of the droplets and a cylindrical symmetry. The application of an electric field involves the reorientation of liquid crystal directors along a direction parallel to the external field (if the dielectric anisotropy of mesogens is positive) through a simple rotation of droplet directors with no additional elastic deformation. Radial droplets are characterized by a director field perpendicular to the droplet walls with a central point defect and a spherical symmetry. Such a configuration is not easily found in nature in agreement with the Friedel- Creagh-Kmetz rule, which predicts the incompatibility between long-term stability and radial configuration in emulsions. As a consequence, the simplest way to achieve a normal alignment in liquid crystal droplets is the addition of small amounts of surfactants. The application of an external electric field determines the reorientation of liquid crystal directors through an additional elastic deformation and the achievement of the axial configu- ration. To induce reorientation, the electric energy must overcome the elastic free energy difference between the ON and OFF states. The existence of this energy difference in radial droplets gives reorientation fields larger and relaxation times faster (as they scale inversely with the square of reorientation fields) than those in bipolar emulsions. Recently, nematic emulsions have attracted high inter- est since nematic droplets dispersed in a highly viscous monomer matrix can be used as cheap, fluid, switchable electro-optical devices. 8,9,16 Films are prepared by cooling the initial isotropic mixture of monomers and liquid crystal at a controlled rate in order to give micron-sized bipolar droplets. They can be switched from an opaque state to * To whom correspondence should be addressed. E-mail: fiore.nicoletta@unical.it. (1) Becher, P. Emulsions, Theory and Practise, 2nd ed.; Reinhold: New York, 1965. (2) Meyer, R. B. Phys. Rev. Lett. 1969, 22, 918. (3) Dubois-Violette, E.; Parodi, O. J. Phys. (France) 1969, 30, C4- 57. (4) Candau, S.; Le Roy, P.; Debeauvais, F. Mol. Cryst. Liq. Cryst. 1973, 23, 283. 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Cryst. 2000, 27, 1029. 3034 Langmuir 2002, 18, 3034-3038 10.1021/la011217p CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002