Dynamics of ordered colloidal particle monolayers at nematic liquid crystal interfaces

Wei, Wei-Shao; Gharbi, Mohamed Amine; Lohr, Matthew; Still, Tim; Gratale, Matthew; Lubensky, T. C.; Stebe, Kathleen J.; Yodh, Arjun G.

doi:10.1039/c6sm00295a

Cited by 9 publications

(5 citation statements)

References 65 publications

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“…Similar to electrically charged particles, like-sign topological defects, in general, repel each other, while defects of opposite sign attract. However, this can be additionally affected by the geometry and surface properties of * anupams@ethz.ch the environment [17,18], and the presence of an external stimulus [19][20][21][22][23]. Emergence of topological defects in a field, and the resulting interactions between them, have been well characterized [24].…”

Section: Introductionmentioning

confidence: 99%

Cross-talk between topological defects in different fields revealed by nematic microfluidics

Giomi

Kos

Ravnik

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Topological defects are singularities in material fields that play a vital role across a range of systems: from cosmic microwave background polarization to superconductors and biological materials. Although topological defects and their mutual interactions have been extensively studied, little is known about the interplay between defects in different fields-especially when they coevolve-within the same physical system. Here, using nematic microfluidics, we study the cross-talk of topological defects in two different material fields-the velocity field and the molecular orientational field. Specifically, we generate hydrodynamic stagnation points of different topological charges at the center of starshaped microfluidic junctions, which then interact with emergent topological defects in the orientational field of the nematic director. We combine experiments and analytical and numerical calculations to show that a hydrodynamic singularity of a given topological charge can nucleate a nematic defect of equal topological charge and corroborate this by creating −1, −2, and −3 topological defects in four-, six-, and eight-arm junctions. Our work is an attempt toward understanding materials that are governed by distinctly multifield topology, where disparate topology-carrying fields are coupled and concertedly determine the material properties and response.multifield topology | nematic liquid crystals | topological defects | microfluidics | cross-interactions D efects are ubiquitous in nature and are at the heart of numerous physical mechanisms, including melting in 2D crystals (1), cosmic strings (2), and other topological defects in the early universe (3). Vortices are possibly the most common examples of defects in flowing media (4, 5). In a typical hydrodynamic vortex, the fluid velocity, v, rotates by 2π along any closed loop around the vortex core and has an undefined direction at the core. More generally, topological defects are singular points or lines in a distinct scalar, vector, or tensor field that can be characterized by topological invariants, including winding number (or index) for 2D, and topological charge for 3D variations of the fields (6, 7). Topological defects have been long known to mediate key processes in a wide range of settings, including knotted flow field stream lines (8), defects in light fields (9), knotted defect lines in complex fluids (10), defects in type 2 superconductors (11), spontaneous flow in active fluids (12-15), and even, conduction properties of electron nematics (16).The interaction between topological defects is governed by the defect topology and the underlying energetics. Similar to electrically charged particles, like-sign topological defects, in general, repel each other, whereas defects of opposite sign attract. However, this interaction can be additionally affected by the geometry and surface properties of the environment (17, 18) and the presence of an external stimulus (19-23). Emergence of topological defects in a field and the resulting interactions between them hav...

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Section: Introductionmentioning

confidence: 99%

Cross-talk between topological defects in different fields revealed by nematic microfluidics

Giomi

Kos

Ravnik

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…Problem B concerns the drag force on a particle straddling an Iso-N interface. As mentioned in the introduction, a few experiments have touched upon this topic [24,28,29] but no prior theoretical or computational work seems to have been reported so far. Fig.…”

Section: Problem B: Drag Forcementioning

confidence: 99%

“…At the single particle level, Jeridi et al [25,26] reported capillary-induced giant elastic dipoles in thin nematic wetting films, whereas the influence of interfacial curvature was addressed by Gharbi et al [27] by placing particles on NLC shells. More dynamical aspects such as the Brownian diffusion of micro-and nano-particles at the nematic-aqueous phase interface were first investigated by Abras et al [28], whereas the vibrational phonon modes of 2D crystalline packings of particles at the air-NLC interface were discussed by Wei et al [29].…”

Section: Introductionmentioning

confidence: 99%

Particle trapped at the isotropic-nematic liquid crystal interface: Elastocapillary phenomena and drag forces

et al. 2022

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We present numerical simulations of a particle trapped at the isotropic-nematic liquid crystal (Iso-N) interface. We use our recent model, based on a phase-field approach (see Qiu et al., Phys. Rev. E, 103, 022706 (2021)), to couple the capillary forces acting on the interface with the elastic stresses in the nematic phase along with topological defects. A range of floating configurations are first investigated as a function of the contact angle and various anchoring conditions at the fluid interface. The results show that the response of the system is driven by the existence of an anchoring conflict at the contact line. Substantial particle displacements and/or interfacial deformations may occur in this case even for moderate anchoring strengths. These findings highlight the coupling between elastic and capillary forces.In a second part, we compute drag forces exerted on a particle that moves along the Iso-N interface for several contact angles and a moderate Ericksen number. Because

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“…In simple fluids, the capillary forces compete mostly with electrostatic interactions . When inclusions are attached at complex fluid interfaces such as a bilayer membrane ,,, or a liquid crystal interface, − other long-range interactions related to bulk distortions of the surrounding fluid appear. For instance, a nematic liquid crystal phase is characterized by a preferred orientationthe director n(r )of its long rigid molecules.…”

Section: Introductionmentioning

confidence: 99%

Colloidal Particles in Thin Nematic Wetting Films

et al. 2016

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We experimentally and theoretically study the variety of elastic deformations that appear when colloidal inclusions are embedded in thin wetting films of a nematic liquid crystal with hybrid anchoring conditions. In the thickest films, the elastic dipoles formed by particles and their accompanying defects share features with the patterns commonly observed in liquid crystal cells. When the film gets thinner than the particles size, however, the capillary effects strongly modify the appearance of the elastic dipoles and the birefringence patterns. The influence of the film thickness and particles sizes on the patterns has been explored. The main experimental features and the transitions observed at large scale-with respect to the inclusions' size-are explained with a simple two-dimensional Ansatz, combining capillarity and nematic elasticity. In a second step, we discuss the origin of the variety of observed textures. Developing a three-dimensional Landau-de Gennes model at the scale of the particles, we show that the presence of free interfaces and the beads confinement yield metastable configurations that are quenched during the film spreading or the beads trapping at interfaces.

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Dynamics of ordered colloidal particle monolayers at nematic liquid crystal interfaces

Cited by 9 publications

References 65 publications

Cross-talk between topological defects in different fields revealed by nematic microfluidics

Cross-talk between topological defects in different fields revealed by nematic microfluidics

Particle trapped at the isotropic-nematic liquid crystal interface: Elastocapillary phenomena and drag forces

Colloidal Particles in Thin Nematic Wetting Films

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