Long-range interaction on nano-particles embedded in liquid crystals
and theory of liquid crystals/magnetic nano-particle gels
Sergei L. Lopatnikov* and Alexander H.-D. Cheng
University of Delaware, Department of Civil & Environmental Engineering,
Newark, DE 19716 USA
This is an abstract
for a presentation given at the
Foresight Conference on Molecular Nanotechnology.
There will be a link from here to the full article when it is
available on the web.
In this paper we consider the interaction of non-magnetic and magnetic nano-particle with the liquid crystal director field, and among themselves, and the theory of a class of innovative materials built as a ferro-fluid on the base on liquid-crystal solvent that can manipulate liquid crystal behavior. Because the liquid crystal director field is dependent on the boundary conditions, the character of chemical bounding of its molecules with the surface of nano-particles (as well as the magnetic field in the presence of magnetic nano-particles) defines the orientation of liquid crystal molecules (the director field) [Lopatnikov & Namiot, 1978].
Using far-field approximation for the liquid crystal director field, we show that the major term in multipole decomposition of field can be expressed in terms of "spinor charge" of nano-particle with potential that depends on the symmetry of crystal and decreases with distance as R^(-2) for nematics. Depending on the affinity with liquid crystal, particles with one sign of the spinor charge will move in the direction of larger stresses, and the other sign in opposite direction. It opens the possibility to use liquid crystals for chemical separation of nano-particles.
If we consider two nano-particles immersed in liquid crystal, the long-range interaction between them will be observed because perturbation of director field caused by one molecule impacts on the director field of the other. It provides specific attraction force between nano-particles up to the distance of several diameters of nano-particles.
Spinor character of effective charges of nano-particle in liquid crystals defines the most energy efficient relative orientation of nano-particles. We shall demonstrate that the interaction among nano-particles provides them with the opportunity to "recognize" chemical properties of other particles up to the distance of hundreds of angstroms under room temperature. This can have interesting applications in cell membranes biophysics.
It is clear that because liquid crystals are sensitive to electric and magnetic fields, systems consisting of liquid crystals and electrically and magnetically charged nano-particles can demonstrate singular behavior in external field. One of the most interesting systems is the liquid crystals/magnetic nano-particles gels (LQMNPG). We present the phenomenological theory of these materials that allows us to answer the following questions: Can the long range interaction of nano-particles in liquid crystals provide spontaneous magnetization of gel? If we suspend nano-particles in solvent initially in liquid state, then apply external magnetic field and accomplish the phase transition to the liquid crystal phase, and then turn the magnetic field off, will the magnetization of LQMNPG remain? Will the direction of spontaneous or "frozen" magnetization of LQMNPG change if the director field is under the influence of external electric field? Will the optical properties of LQMNPG change into organized phase under applied external magnetic and/or electric fields? The answers to these questions are all "yes".
A few practical applications of LQMNPG will be discussed.
S.L. Lopatnikov and V. Namiot, "On the nature of interaction of giant molecules dissolved in liquid crystal," Soviet Biophysical Journal, No.12, p. 1110, 1978.
S.L. Lopatnikov and V. Namiot, "Interaction of giant molecules dissolved in liquid crystal," Soviet J. of Experimental and Theoretical Physics (Sov. Phys. ZETP), No. 8, p. 361. 1978.
S.L. Lopatnikov and V. Namiot, "On new possible method of the investigation of giant molecules," Soviet J. Applied Physics Letters, (Sov. Phys ZTP Lett.), No. 15, p. 894, 1978.
Sergei L. Lopatnikov
Distinguished Visiting Professor, Department of Civil & Environmental Engineering
University of Delaware
Newark, DE 19716 USA
Formerly, Department of Chemistry, Moscow State University, Russia.