Direct Observation of Anisotropic Interparticle Forces in Nematic Colloids with Optical Tweezers

Makoto Yada, Jun Yamamoto, Hiroshi Yokoyama
2004 Physical Review Letters  
Interparticle forces in a nematic liquid-crystal colloid have been directly observed by the dual beam laser trapping method with pN sensitivity. We introduce two different types of spatial distributions of forces, detected between the particles accompanied by hyperbolic hedgehog defects. These force distributions lead to specific particle arrangements, which are both stabilized by the balance of the orientational stress field of nematics. On the basis of these results, we propose novel
more » ... pose novel artificial construction for multiparticle regular arrangements. Dispersions of particles in liquid crystals (LCs), referred to as liquid-crystal colloids or emulsions, have attracted considerable scientific and technological attention in recent years [1] [2] [3] [4] . Unlike conventional colloids and emulsions, the interparticle forces in the LC media become highly anisotropic and long ranged, making it possible to spontaneously develop a variety of ordered arrangements of particles. A single particle breaks the continuous rotational symmetry of a liquid crystal, and, when the surface anchoring is sufficiently strong, it behaves topologically as a core of an orientational defect yet with a much lower energy of formation because of its nonsingularity [5] [6] [7] . Various types of topological defects, such as a hyperbolic hedgehog, a Saturn ring, and boojums, have been recently reported in dispersions of water droplets in a nematic phase [1, 8, 9] . The particularly unusual multiparticle behavior is to form linear chains of water droplets having hyperbolic hedgehog defects, without direct contact between the droplets [1]. The hyperbolic hedgehog defect is created to one side of a droplet with a homeotropic anchoring surface (that is, a radial hedgehog defect) along the surrounding director field, in order to satisfy a total topological charge of zero. Thus induced distortion of the liquid crystalline order assumes a dipolar property, which leads to the chain formation owing to the balance of long-range dipolar attractive and short-range defectmediated repulsive interactions. In other systems of LC emulsions, there are some attractive reports about spontaneously formed three-dimensional defect structures, which are composed of water droplets and nematic [2] or cholesteric fluids [4, 10] . Liquid-crystal dispersions can be thus expected to show further new liquid-crystal structures and unique phenomena [11] , owing to the interparticle and/or particle-LC interactions. In this Letter, we report quantitative evaluation of anisotropic interactions between solid particles dispersed in a nematic fluid, particularly in various positional con-figurations of two particles in a two-dimensional plane. The interparticle forces of a pN order in each specified configuration were examined by using a laser trapping method [12, 13] : a micron-scale particle can be trapped on a focused laser beam owing to the radiation pressure generated by the difference of the refractive indices between the particle and the medium. We show two different types of anisotropic spatial distributions of trapping forces, interacting between two particles with hyperbolic hedgehog defects. One force distribution, which leads to a linear chain formation of the particles [1], was compared with a theoretical prediction for the dipolar attractive interaction [6] . On the other hand, the other distribution causes one stable particle arrangement, which is not a linear chain. On the basis of these two types of force distributions, we will finally propose and show examples of artificial construction of multiparticle regular arrangements in nematic colloids. As our solid particle/LC system, we prepared crosslinked polystyrene (PS) particles (radius a: 7:5 15 m, from Duke Science) and a nematic LC (ZLI-4792, N 96:5 C I, from Merck). The refractive indices of PS and ZLI-4792 at the measurement temperature, 70 C, are 1.59 and 1.49, respectively, where the value of the nematic is the weighted average of the ordinary and extraordinary refractive indices (1.47 and 1.52, respectively), evaluated with an Abbe refractometer (wavelength: 589 nm). The surface of PS particles was treated with the surfactant (FS150, from Dainippon Ink & Chemicals) to promote homeotropic orientation of LC molecules by immersing in its aqueous solution and then dried. The particles and ZLI-4792 were simply mixed and sandwiched with glass substrates having planar anchoring surfaces (from EHC Co., Ltd., Japan). The sample thickness was fixed at 50 m by using spacer beads. For the dual optical traps, we applied a pair of linearly polarized laser beams (wavelength: 1064 nm) with polarizing directions perpendicular to each other, which were P H Y S I C A L
doi:10.1103/physrevlett.92.185501 pmid:15169496 fatcat:ih7cftrlefa4hkpvhu7v3iqmem