Drag on particles in a nematic suspension by a moving nematic-isotropic interface

Abstract

We report the first clear demonstration of drag on colloidal particles by a moving nematic-isotropic interface. The balance of forces explains our observation of periodic, strip-like structures that are produced by the movement of these particles.PACS numbers: 61.30.Jf, 64.70.Md, Colloidal dispersions of small particles in nematic liquid crystals are a novel, interesting type of soft matter. The difference from ordinary colloids arises from the orientational ordering of the liquid crystal molecules and the resulting structure in the colloid. Topological defects [1,2,3,4] and additional long-range forces between the colloidal particles [5] are immediate consequences of this ordering. The nematic-induced interparticle interaction brings a new range of effects to the system: supermolecular structures [6,7,8,9], cellular structures [10,11], and even a soft solid [12] can be observed. Colloidal dispersions in liquid crystals also have a wide variety of potential applications [13].A range of problems similar to those of polymer dispersed liquid crystals also arise in nematic colloidal dispersions. The nematic ordering makes it difficult to suspend small particles in a liquid crystal host. Particles often segregate into agglomerates distributed nonuniformly in the cell. The resulting spatial distribution of the particles is difficult to control. Our research explores the factors that affect the spatial distribution of these particles and indicates ways to control the complex morphology of these systems.In this paper we report the first demonstration of drag on colloidal particles by a moving nematic-isotropic (NI) interface. We calculate a critical radius above which the particles cannot be captured by the moving interface. We predict that this critical radius is sensitive to the viscous properties of the host liquid crystal, the value of the anchoring coefficient of the liquid crystal on the particle surface, and the velocity of the moving interface. Most important, we can move particles of specified radius and can control the spatial distribution of these particles in the cell.In order to understand how the particles are moved by the nematic-isotropic transition front we used particles of different size as well as particles made of different materials. In the first part of our experiments we used nearly monodisperse spheres of silica (R = 0.005, 0.5, and 1µm). To prove our predictions based on initial experimental observations and to demonstrate the controllability of the spatial distribution of particles in anisitropic colloidal suspensions, we used large polymer particles, R = 8µm [17]. In all cases particles were dispersed at concentrations of φ = 1 − 5wt %, in the liquid crystal 5CB at room temperature (25 0 C) (the isotropic-nematic transition temperature of pure 5CB is T NI ≈ 35 0 C). The sample was subjected to ultrasound in order to uniformly disperse the particles in 5CB. Some of the preparations were made at higher temperatures, in the nematic or isotropic state of the liquid crystal.The homogeneous mixtur...

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“…Trapping and dragging colloids by moving isotropic-LC interface offers another interesting mechanism of transport 163 . The moving isotropic-LC interface can be controlled thermally or by photoinduced trans-cis isomerization 164 .…”

Drag of Micro-Particles by an Extended Nematic-Isotropic Interface

“…Micro-particles can be effectively positioned and dragged by the boundary between the liquid phases of different symmetry. We found drag effect on colloidal particles by a moving interface between isotropic and nematic phase of liquid crystal (LC) [10]. This effect is mainly due to the surface tension effects and long-range distortions of the director around the colloidal particle when it is in the nematic phase.…”

“…In the Reference [10] it was suggested that particles are dragged by a flat interface between the nematic and isotropic phases. However, as was shown by Oswald et al [11], the moving interface has rather complicated, extended, shape of a meniscus of nematic phase extended into isotropic phase.…”