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Liquid Crystal Conic Flowers

Daniel A. Beller, Mohamed A. Gharbi, Apiradee Honglawan, Kathleen J. Stebe, Shu Yang, and Randall D. Kamien. Phys. Rev. x 3, 041026 (2013).

Figure 1)  System A: Smectic flower texture around a single colloid inclusion: a) Bright field microscopy. b) Polarized microscopy. c) A bright field image of a flower texture organized around a colloidal dimer.  
Figure 2 - overlay image on far right )  System B: Schematic representation illustrating layered bending of the smectic layers.

Focal conic domains (FCDs) in smectic-A liquid crystals have the ability to direct the assembly of micro- and nanomaterials. FCDs can arrange themselves in a fan-like texture comprised of focal curve pairs, the hyperbolae of which intersect at a single point, and can be used to form things like microlens arrays. In contrast to the fan-like texture, the patterns studied by the authors take on a flower-like appearance, created by using curved interfaces to confine smectic LCs into the desired pattern.

The researchers, working at the University of Pennsylvania, present two systems exhibiting this flower-like pattern. In system A, a large colloidal inclusion was placed in the LC, causing the FCDs to arrange themselves radially around the particle (Fig 1). In system B, patches of SiO2 nanoparticles on the surface were used to promote degenerate planar anchoring of the smectic layers, causing the layers to bend (Fig 2). In both systems, the flower texture was observed, with thickness decreasing as distance from the center increased. In system A, the researchers show that it’s not the LC anchoring to the colloid that causes the flower texture, but the colloid’s wetting chemistry that deforms the LC-air interface. In system B, the flower texture was produced by the same geometry present in system A, but due to the bend of the LC layers, the geometry is upside down. The researchers found the flower texture in both systems to be the result of the outward tilt of the normal vector of the homeotropic interface.

These patterns of self-organization could be controlled by manipulating the eccentricity of the FCDs, which varied with the curvature of the homeotropic interface. The resulting orientation mismatch between the hybrid aligning surfaces of a smectic thin film produced changes in the overall texture of the system. The authors suggest future research can focus on self assembly and using arrangements of smectic-a liquid crystals to guide the assembly of other materials, including colloids and nanoparticles.

Read the full article at APS Physical Review X

- Michael Lane

 


Induction of Defect Loops in Nematic Fields

Smalykuh, Ivan I, Angel Martinez, Miha Ravnik, Bruce Lucero, Rayshan Visvanathan and Slobodan Zumer. Nature Materials, 2014

Fig 2a) Images taken from a scanning electron micrograph showing a single knot of T(3,2) i.e. 3 loops, 2 revolutions. Fig 2b) Scanning electron micrograph of a 4x4 array of knots T(5,3). Above both images are the corresponding 3D models.

Designing and assembling three-dimensional (3D) structures of low-symmetry colloidal particles is challenged by the lack of systems and techniques that allow for controlling their spatial arrangements. When considering the nanoscale confinement and mesoscale self-assembly of nanoparticles in liquid crystal, the types and the arrangements of the spatial defects are both important to keep in mind upon the construction of 3D patterns. Ivan I. Smalyukh and his team in the University of Colorado, Boulder have developed a system that enables the generation and control of 3D patterns found in knotted nematic colloids. This work can be used to predict configurations of looped line defects and the interplay of topologies of knotted surfaces, fields and defects.

Rigid particles were obtained by using a two-photon photopolymerization with spatially patterned pulsed femtosecond laser light. This method allowed the team to construct particles with the topology of colloidal knots. Consisting of polymeric tubes, the particles are looped p times with q revolutions, T(p,q), about the colloidal rotational symmetry axis. Examples of colloidal knots can be seen in Fig. 2. Various configurations of knots even became mutually tangled when constructed in large quantities. The particles’ molecular orientations and points of incompatibility with the nematic liquid crystal resulted in point defects called “boojums”.

The team found that boojums around different knots could be specifically characterized both by the number of times the director rotates as one circumnavigates the defect core and by the bulk topological charge. The overall interplay of these particle topologies with the liquid crystal was controlled by the varying surface boundary conditions. This approach to a predictable way of experimenting with knotted colloids can lead to uses involving self-assembly of metal and semiconductor nanoparticles possibly applied to information displays, metamaterials and data storage.

The full article can be found at the Nature Materials website.

-Marcus Rice

 


 

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