45023-AC10
Laser- and Electric-Field-Induced Nucleation in Supersaturated Solutions and Supercooled Melts
Non-photochemical laser-induced nucleation (NPLIN) , a phenomenon in which intense laser pulses induce supersaturated solutions to nucleate, was discovered in our laboratory about 10 years ago, and has also been demonstrated to control polymorphism in some systems through “polarization switching”, in which the polymorph formed depends on the polarization state of the laser beam. We have hypothesized that NPLIN involves the electric-field induced alignment of molecules or groups of molecules in a pre-nucleating solute cluster, aiding the cluster to organize into a crystal-like entity, through the optical Kerr effect.
The liquid crystalline state exhibits optical properties that display a sensitivity to external electric and magnetic fields not seen in other fluids, owing to strong correlations between molecules. There is a substantial body of literature from the 1980s concerning the laser-induced molecular reorientation in isotropic, nematic, smectic and cholesteric phases, induced by the optical torque associated with the optical Kerr effect. These effects are responsible for “giant” optical nonlinearities in liquid crystal systems, and more specifically for the optical Freedericksz transition in nematic liquid crystals - a second-order transition in which the nematic director undergoes a change in direction in response to an applied optical field. More recently, there have been numerous reports of enhanced molecular reorientation in dye-doped nematic liquid crystals, owing to dye excited state – liquid crystal interactions.
NPLIN has been observed in supersaturated solutions of urea in water, ethanol and methanol, and as well as aqueous solutions of a variety of organic acids and amino acids. All of these cases have involved two-component solutions. We wanted to see whether NPLIN is a more general phenomenon in which light can induce a phase transition from a metastable isotropic state to an ordered state in a single-component system, for example, an isotropic à nematic transition in a single-component thermotropic liquid crystal. In this case, the metastable state is the supercooled isotropic state, rather than the supersaturated state in the case of a two-component system. Furthermore, we investigated whether the polarization of the incident light controls the ultimate order in the resulting nematic phase.
We chose to look for a non-photochemical laser-induced phase transition in a supercooled 4'-n-pentyl-4-cyanobiphenyl (5CB, also referred to as PCB and K15) liquid crystal, using linearly polarized 45-ps light pulses at a wavelength of 532 nm. The laser induced nucleation from the metastable supercooled isotropic phase to the nematic phase during slow cooling (0.001 ºC/min)and high light intensity (3.9 MW/cm2). The resulting nematic director tended to be aligned along the direction of the plane of polarization of the light. At the intensities used, there is no observable laser-induced reorientation of the director once the sample is in the nematic phase, nor any permanent laser-induced ordering when the sample is illuminated only in the stable isotropic phase during slow cooling. These experimental results are consistent with a mechanism based on optical Kerr alignment.
