Confinement Induced Splay-to-Bend Transition of Colloidal Rods

Abstract

We study the nematic phase of rodlike fd -virus particles confined to channels with wedgestructured walls. Using laser scanning confocal microscopy we observe a splay-to-bend transition at the single particle level as a function of the wedge opening angle. Lattice Boltzmann simulations reveal the underlying origin of the transition and its dependence on nematic elasticity and wedge geometry. Our combined work provides a simple method to obtain the splay-to-bend elasticity ratios and offers a way to control the position of defects through the confining boundary conditions. PACS numbers:Packing and confinement problems emerge in fields ranging from biology to engineering. In biological systems the organization of the cell is determined, among other things, by the packing of fibril-like particles (actin filaments, DNA) [1,2]. An example of the subtlety of packing phenomena is the plethora of liquid crystalline phases that can be found in arrangements of anisotropic particles by increasing concentration [3,4]. Confinement of liquid crystals adds to the complexity, since the interactions of the particles with the walls may lead to structures that compete with those formed in the bulk [5,6]. Many of the next generation liquid crystal display devices exploit this interplay by using structured or patterned surfaces as an essential element of their design [7,8]. Very recently, the ordering at sawtoothed structures has been studied theoretically within a Landau-De Gennes framework [9,10], with a focus on the wetting behaviour. In this Letter, in a combined experimental and theoretical effort, we show the rich phenomenology that emerges when confining a nematic liquid crystal to a microfluidic channel with a wedge structured wall. We seek to disentangle how the wedge geometry and elasticity of the fd -virus' nematic phase determine the adopted deformation in the wedge. We introduce a new method for estimating elastic constants suitable to colloidal and biological systems, as an alternative to previous methods using magnetic fields [11] or light scattering [12]. Specifically, we determine the transition from a splay to a bend director field, with increasing wedge angle.We use the fd -virus, which is an excellent model liquid crystal system for both static and dynamic behaviour [13][14][15][16]. The virus' contour length and diameter are 0.88 µm and 6.6 nm, respectively. These dimensions allow for the 3D determination of the position and orientation of individual particles by means of laser scanning confocal microscopy (LSCM). Thus we obtain detailed mechanistic insights on a single particle level of the director field. Moreover, due to the relatively large size of the particles, we can study effects of wall structures that are, when translated to the scale of thermotropic liquid crystals, very small and inaccessible. The particles were grown following standard protocols [17] and dispersed in 20 mM tris buffer at pH 8.15 with 100 mM NaCl and 15% EtOH. The ethanol was added to prevent the growth of bacteria. The virus concent...

Generalized Berreman's model of the elastic surface free energy of a nematic liquid crystal on a sawtoothed substrate

“…This problem has practical applications such as the design of zenithally bistable devices [4][5][6][7][8][9] or the trapping of colloidal particles in specified sites [10,11]. It is well known that the nematic director field, in the presence of a structured substrate, may be distorted, leading to an elastic contribution to the free energy.…”

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“…When in contact with structured surfaces, the interplay between bulk effects such as elasticity and surface effects such as anchoring leads to rich behaviour including complex wetting transitions 7,8 and the stabilisation of topological defects 9,10 . Such intricate surface effects can be exploited in novel microfluidic applications.…”

Lattice BGK kinetic model for high-speed compressible flows: Hydrodynamic and nonequilibrium behaviors

“…Therefore, how to exactly describe and capture the nonequilibrium effects is the key issue of physical modeling. For modeling such a system, as a multiscale approach, the lattice Boltzmann (LB) method [4][5][6], which contains system information beyond one level of description [7], has more intrinsic merits than the traditional hydrodynamic descriptions. The compelling reasons are as below.…”