David Le Bolloc’h

Contributor : P. Davidson Collaborators : L.J. Michot, I. Bihannic, S. Maddi (LEM, Nancy ); S. S. Funari (HASYLAB, Hambourg); C. Baravian ( LEMTA, Nancy); P. Levitz (LPMC, Polytechnique Palaiseau)

We study here liquid-crystalline suspensions of mineral rod- or plate-like nanoparticles in a solvent. Natural clays are good candidates for this purpose because many natural types of clay can be fully exfoliated in water, leading to suspensions of charged nano-sheets of 1 nm thickness and 50-1000 nm diameter. However, these suspensions also have a strong tendency to form gels at low volume fractions ( 1 %). Even though these gels are birefringent, they are not fluid and cannot be aligned in electric or magnetic fields. Therefore, the sol/gel transition completely precludes the liquid-crystalline properties of these materials, thus preventing so far the use of natural clays for any display application, in spite of the efforts of many scientists worldwide. Recently, in collaboration with geochemists and other physicists, we have discovered that the fluid suspensions of a natural clay, called nontronite, clearly display liquid-crystalline properties. We could observe the 1st order isotropic liquid / nematic transition at thermodynamic equilibrium (Figure 1). In this system, the transition takes place at a concentration smaller than that of the sol/gel transition. Then, the phase fluidity and anisotropy, which are the typical properties of liquid crystals, can be exploited: for example, this makes it possible to align all the clay nanoparticles in suspension in the same direction with the help of a small magnetic field, and then to reorient them at will (Figure 2). The discovery of a nematic phase opens the way to the production of new composite materials, and, in principle, to cheaper display devices. Moreover, the spontaneous orientation of clay platelets determines, to a large extent, the flow properties of these colloidal suspensions. This can prove dramatic as in landslide triggering or can be used for the rational design of engineering complex fluids. Actually, these alignment properties, although most often hidden by the sol/gel transition, are quite commonly found in the realm of clays.

Reference : Liquid-crystalline aqueous clay suspensions, L.J. Michot, I. Bihannic, S. Maddi, S.S. Funari, C.Baravian, P. Levitz, P. Davidson, Proc. Nat. Acad. Sci. USA, 103, 16101 (2006).