Colloidal Particles in Thin Nematic Wetting Films

Abstract

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.

Phase-field model for elastocapillary flows of liquid crystals

“…The finite-element approximation allows for complex geometries and one can further equip the model with adaptive mesh refinement or moving mesh capabilities. Our model is well-suited for further development to tackle more complex problems such as particle interactions under coupled elastocapillary forces at the interface of an LC [e.g., 9,18,75].…”

“…To model the behaviour of elastocapillary phenomena involving a nematic LC, there have been mainly three types of approaches. First, many existing studies prescribe various strong simplifications, such as a fixed interfacial shape [18], a class of candidate shapes [19] or the absence of defects in the bulk of the LC [20]. It is clear, however, that these assumptions rely on decoupling elasticity and capillarity and apply only in limiting situations.…”

Design and Preparation of Nematic Colloidal Particles

“…Most of the studies of LC colloids deal with solid spherical particles. ,,− ,− ,− Typically, to obtain solid spherical colloids with homeotropic anchoring at the surface, silica or glass colloids are used additionally treated with an aqueous or alcohol solution (0.05 wt %) of N , N -dimethyl- N -octadecyl-3-aminopropyl-trimethoxysilyl chloride (DMOAP). ,− , Different polymer-based solid spherical particles, like melamine resin particles, intrinsically provide tangential boundary conditions for LCs . Silica particles treated with 3-methylaminopropyl trimethoxysilane (MAP) also provide tangential boundary conditions. ,, …”

“…Colloidal and liquid crystalline systems are classic examples of soft condensed matter, exhibiting structural similarities with the forms of self-organization found in both living organisms and in solid-state materials. , Nematic liquid crystals (LCs) are fluids having anisotropic properties stemming from the spontaneous, long-range orientational order of their constituent anisotropic molecules . Their utility in electro-optic devices, such as LC displays, arises from their large birefringence and strong coupling to external fields and confining surfaces. , The use of nematic LCs as fluid hosts for colloidal dispersions enriches colloidal behavior due to anisotropic molecular interactions at colloidal surfaces and particle-induced defects and elastic distortions of the LC order. − Molecular interactions of rod-like LC molecules at the interface of another material result in energetically preferred alignment directions of the molecules at that interface. − Known as surface anchoring, this anisotropic interaction at LC surfaces defines surface boundary conditions and plays a key role in electro-optic applications involving LCs and in nematic colloidal self-assembly. − Surface anchoring , is one of the most important properties of LCs because of its relevance in practical applications. For instance, chemically and topographically patterned surfaces , have provided promising avenues for LC applications in displays, chemical sensing, biodetection, colloidal self-assembly, − …”

Elastocapillary Driven Assembly of Particles at Free-Standing Smectic-A Films

“…Studies of the dynamics of rodlike particles at the free surface of thin hybrid nematic films reveal complementary roles for elasticity and capillarity on the assembly of cylindrical particles. For particles trapped in thin nematic films, long-range interactions mediated by elastocapillarity occur via the formation and interactions of novel defect configurations. , While these recent works focus on capillarity at nematic interfaces, little is known about capillary effects at more complex LC phases such as smectic films. An overview of experimental and theoretical progress on understanding colloidal interactions in liquid crystals, such as nematic and smectic phases, can be found in refs and .…”