Reconfigurable interactions and three-dimensional patterning of colloidal particles and defects in lamellar soft media

Abstract

Colloidal systems find important applications ranging from fabrication of photonic crystals to direct probing of phenomena typically encountered in atomic crystals and glasses. New applicationssuch as nanoantennas, plasmonic sensors, and nanocircuits-pose a challenge of achieving sparse colloidal assemblies with tunable interparticle separations that can be controlled at will. We demonstrate reconfigurable multiscale interactions and assembly of colloids mediated by defects in cholesteric liquid crystals that are probed by means of laser manipulation and three-dimensional imaging. We find that colloids attract via distance-independent elastic interactions when pinned to the ends of cholesteric oily streaks, line defects at which one or more layers are interrupted. However, dislocations and oily streaks can also be optically manipulated to induce kinks, allowing one to lock them into the desired configurations that are stabilized by elastic energy barriers for structural transformation of the particle-connecting defects. Under the influence of elastic energy landscape due to these defects, sublamellarsized colloids self-assemble into structures mimicking the cores of dislocations and oily streaks. Interactions between these defectembedded colloids can be varied from attractive to repulsive by optically introducing dislocation kinks. The reconfigurable nature of defect-particle interactions allows for patterning of defects by manipulation of colloids and, in turn, patterning of particles by these defects, thus achieving desired colloidal configurations on scales ranging from the size of defect core to the sample size. This defect-colloidal sculpturing may be extended to other lamellar media, providing the means for optically guided self-assembly of mesoscopic composites with predesigned properties.

Topological colloids

“…This allows us to establish and experimentally test the procedure for assignment and summation of topological charges in threedimensional director fields. Our findings lay the groundwork for new applications of colloids and liquid crystals that range from topological memory devices 14 , through new types of self-assembly [15][16][17][18][19][20][21][22][23] , to the experimental study of low-dimensional topology 6,7,[11][12][13] .Although a coffee mug and a doughnut look different to most of us, they are topologically equivalent solid tori or handlebodies of genus g = 1, both being different from, say, balls and solid cylinders of genus g = 0, to which they cannot be smoothly morphed without cutting 11,12 . In a similar way, molecules can form topologically distinct structures including rings, knots and other molecular configurations satisfying the constraints imposed by chemical bonds 24 .…”

“…Optical manipulation and three-dimensional imaging of samples were performed with an integrated setup of holographic optical tweezers and 3PEF-PM ( Supplementary Fig. 2) 23,30 built around an inverted microscope IX 81 (Olympus) and using a 100 oil-immersion objective Acknowledgements. …”

“…This allows us to establish and experimentally test the procedure for assignment and summation of topological charges in threedimensional director fields. Our findings lay the groundwork for new applications of colloids and liquid crystals that range from topological memory devices 14 , through new types of self-assembly [15][16][17][18][19][20][21][22][23] , to the experimental study of low-dimensional topology 6,7,[11][12][13] .…”

“…Although the topology of shapes, fields and defects is important in many phenomena and in theories ranging from the nature of elementary particles to early-Universe cosmology 25,26 , topological aspects of colloidal systems (composed of particles larger than molecules and atoms but much smaller than the objects that we encounter in our everyday life) are rarely explored. Typically dealing with particles with surfaces homeomorphic to spheres, recent studies 8,9,15,18,[20][21][22][23] demonstrate that the topology of curved surfaces dictates the formation of defects during two-dimensional colloidal crystallization at fluid interfaces as well as inside liquid crystal droplets and around spherical inclusions in liquid crystals. However, despite the fact that several techniques for scalable fabrication of particles with complex geometric shapes and g > 0 have recently been introduced 18,19,[27][28][29] , the potential impact of particle topology on colloidal alignment, self-assembly and response to fields remains unexplored.…”

“…Beyond the exploration of the topology of colloids, fields and defects, the experimental arena we have developed may enable the design of topology-dictated elastic colloidal interactions and reconfigurable self-assembly in liquid crystals 18 , the entrapment and scaffolding of nanoparticles by particle-induced defects 23 , the selfassembly of reconfigurable topological memory devices 14 , and electrooptic and photonic devices based on bistable switching between different states with distinct director configurations and orientations of particles.…”