Modeling planar degenerate wetting and anchoring in nematic liquid crystals

Fournier, Jean‐Baptiste; Galatola, P.

doi:10.1209/epl/i2005-10253-5

Cited by 162 publications

(152 citation statements)

References 17 publications

Supporting

Mentioning

149

Contrasting

Order By: Relevance

“…Similar handle-shaped cores are expected to exist in all hyperbolic boojums at surfaces with strong surface anchoring (21) and are not related to the fact that the used colloids have handles. Furthermore, this splitting of cores of boojums into semicircular disclination loops is reminiscent of bulk point defects having a core structure in the form of handle-shaped disclination loops (21)(22)(23)(24)(25)(26)(27)(28). Because of decreased Q in the cores of disclination semiloops, which can be treated as being biaxial or isotropic in nature (3,18,29), one can define additional topologically nontrivial surfaces separating defect cores and the rest of the LC.…”

Section: Resultsmentioning

confidence: 99%

Nematic liquid crystal boojums with handles on colloidal handlebodies

Liu¹,

Senyuk

Tasinkevych

et al. 2013

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

111

View full text Add to dashboard Cite

Topological defects that form on surfaces of ordered media, dubbed boojums, are ubiquitous in superfluids, liquid crystals (LCs), Langmuir monolayers, and Bose-Einstein condensates. They determine supercurrents in superfluids, impinge on electrooptical switching in polymerdispersed LCs, and mediate chemical response at nematic-isotropic fluid interfaces, but the role of surface topology in the appearance, stability, and core structure of these defects remains poorly understood. Here, we demonstrate robust generation of boojums by controlling surface topology of colloidal particles that impose tangential boundary conditions for the alignment of LC molecules. To do this, we design handlebody-shaped polymer particles with different genus g. When introduced into a nematic LC, these particles distort the nematic molecular alignment field while obeying topological constraints and induce at least 2g − 2 boojums that allow for topological charge conservation. We characterize 3D textures of boojums using polarized nonlinear optical imaging of molecular alignment and explain our findings by invoking symmetry considerations and numerical modeling of experiment-matching director fields, order parameter variations, and nontrivial handle-shaped core structure of defects. Finally, we discuss how this interplay between the topologies of colloidal surfaces and boojums may lead to controlled self-assembly of colloidal particles in nematic and paranematic hosts, which, in turn, may enable reconfigurable topological composites.eing inspired by Lewis Carroll's poem The Hunting of the Snark, Mermin (1, 2) introduced a term "boojum" to name elusive at the time surface defects in superfluids. This term and the concept of boojums quickly penetrated different fields of physics and materials science, ranging from liquid crystals (LCs) to Langmuir monolayers, and to Bose-Einstein condensates (3-6). However, unlike in the case of their bulk topological counterparts (7-10), called "hedgehog" point defects, the appearance of boojums is rarely controlled at will. In LCs, boojums can spontaneously appear at their interfaces with isotropic media, such as water. They are commonly associated with the geometries of thin LC films on surfaces of isotropic fluids (11), LC droplets (3, 4, 12), and colloidal inclusions (13,14). For example, spherical surfaces with tangential boundary conditions for rod-like LC molecules, and director n describing their local average orientation, are known to induce two boojums per LC droplet or per colloidal inclusion in the LC host (3,4,13,14). However, despite the recent progress in generating and controlling bulk LC defects using director realignment with focused laser beams, chirality, and surface topology of colloidal particles with perpendicular surface anchoring introduced into LCs (7-10), similar control of boojums has not been demonstrated.In this work, we fabricate polymer microparticles with the topology of a handlebody of genus g ranging from one to five that are capable of imposing tangentially degenerate s...

show abstract

Section: Resultsmentioning

confidence: 99%

Nematic liquid crystal boojums with handles on colloidal handlebodies

Liu¹,

Senyuk

Tasinkevych

et al. 2013

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

111

View full text Add to dashboard Cite

show abstract

“…Equation 4 describes homoetropic or normal anchoring, where the preferred surface order is such that the local director is normal to the local surface. A somewhat more complicated expression is relevant to degenerate planar anchoring [13], where the preferred surface order favours a director in the tangent plane at the surface, which we write:…”

Section: Methodsmentioning

confidence: 99%

Large Colloids in Cholesteric Liquid Crystals

2015

View full text Add to dashboard Cite

We describe a coarse-grained Landau-de Gennes model of liquid crystals (LCs) including hydrodynamics based on the Beris-Edwards equations. The model is employed to study the impact of large colloids on the long range LC defect structure in the cholesteric LC blue phases. 'Large' here means that the particle size is comparable to the cholesteric pitch, the length scale on which the LC order undergoes a helical twist. We investigate the case of a single particle, with either normal or degenerate planar anchoring, placed initially in an equilibrium blue phase LC. It is found that in some cases, well defined steady disclination structure emerges at the particle surface, while in other cases no clear steady state is reached in the simulations, and disclination reorganisation appears to proliferate through the bulk LC. These systems are of potential interest in the context of using LCs to template self-assembly of colloid structure, e.g., for opto-electronic devices. Computationally, we demonstrate a parallel approach using mixed message-passing and threaded model on graphical processing units allows effective and efficient progress for this problem.

show abstract

“…The second term characterizes the elastic long-range free energy that arises from distortions of the nematic field (30). The last two terms describe degenerate-planar anchoring at the droplet and particle surface, respectively (31). The total free energy is calculated by using a finite-difference scheme on a uniform cubic grid, with a resolution of 7.15 nm, equal to the nematic coherence length of 5CB.…”

Section: Simulation and Experimental Detailsmentioning

confidence: 99%

Nanoparticle self-assembly at the interface of liquid crystal droplets

Rahimi

Roberts

Armas-Pérez

et al. 2015

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

105

View full text Add to dashboard Cite

Nanoparticles adsorbed at the interface of nematic liquid crystals are known to form ordered structures whose morphology depends on the orientation of the underlying nematic field. The origin of such structures is believed to result from an interplay between the liquid crystal orientation at the particles' surface, the orientation at the liquid crystal's air interface, and the bulk elasticity of the underlying liquid crystal. In this work, we consider nanoparticle assembly at the interface of nematic droplets. We present a systematic study of the free energy of nanoparticleladen droplets in terms of experiments and a Landau-de Gennes formalism. The results of that study indicate that, even for conditions under which particles interact only weakly at flat interfaces, particles aggregate at the poles of bipolar droplets and assemble into robust, quantized arrangements that can be mapped onto hexagonal lattices. The contributions of elasticity and interfacial energy corresponding to different arrangements are used to explain the resulting morphologies, and the predictions of the model are shown to be consistent with experimental observations. The findings presented here suggest that particle-laden liquid crystal droplets could provide a unique and versatile route toward building blocks for hierarchical materials assembly.growing body of theoretical and experimental work has sought to direct the assembly of molecules and nanoparticles at interfaces by exploiting the elastic forces that arise in liquid crystals (LCs) (1-5). Nematic LCs possess orientational order along a unit vector, the so-called nematic director. They also exhibit defects-regions of low order whose morphology and position depends on a delicate balance between elastic, enthalpic, and interfacial contributions to the free energy. The orientation of nematic LCs and any corresponding defects can be perturbed by introducing particles. The symmetry and structure of the director field around a particle also depends on the interaction between the LC and the particle, often referred to as anchoring. Particles with perpendicular (homeotropic) anchoring induce either dipolar or quadrupolar symmetry in the LC, leading to formation of point defects or Saturn-ring defects, respectively (6). Particles with planar anchoring induce quadrupolar symmetry, which is accompanied by two surface defects, generally referred to as boojums (6). Distortions of the nematic field cost elastic energy and therefore give rise to anisotropic, long-range interactions between particles. Indeed, particles in nematic LCs aggregate and "bind," thereby minimizing the volume of defects and the large free energy that is associated with their elastic strain. Equilibrium particle arrangements in nematic LCs depend strongly on the topology of the underlying defects. Homeotropic particles with point, dipolar defects form chains along the nematic director, whereas quadrupolar, Saturn-ring defects form kinked chains that are perpendicular to the nematic director (7). Particles with planar anchor...

show abstract

Modeling planar degenerate wetting and anchoring in nematic liquid crystals

Cited by 162 publications

References 17 publications

Nematic liquid crystal boojums with handles on colloidal handlebodies

Nematic liquid crystal boojums with handles on colloidal handlebodies

Large Colloids in Cholesteric Liquid Crystals

Nanoparticle self-assembly at the interface of liquid crystal droplets

Contact Info

Product

Resources

About