Large-area optoelastic manipulation of colloidal particles in liquid crystals using photoresponsive molecular surface monolayers

Abstract

Noncontact optical trapping and manipulation of micrometer-and nanometer-sized particles are typically achieved by use of forces and torques exerted by tightly focused high-intensity laser beams. Although they were instrumental for many scientific breakthroughs, these approaches find few technological applications mainly because of the small-area manipulation capabilities, the need for using high laser powers, limited application to anisotropic fluids and low-refractive-index particles, as well as complexity of implementation. To overcome these limitations, recent research efforts have been directed toward extending the scope of noncontact optical control through the use of optically-guided electrokinetic forces, vortex laser beams, plasmonics, and optofluidics. Here we demonstrate manipulation of colloidal particles and self-assembled structures in nematic liquid crystals by means of single-molecule-thick, light-controlled surface monolayers. Using polarized light of intensity from 1,000 to 100,000 times smaller than that in conventional optical tweezers, we rotate, translate, localize, and assemble spherical and complex-shaped particles of various sizes and compositions. By controlling boundary conditions through the monolayer, we manipulate the liquid crystal director field and the landscape of ensuing elastic forces exerted on colloids by the host medium. This permits the centimeter-scale, massively parallel manipulation of particles and complex colloidal structures that can be dynamically controlled by changing illumination or assembled into stationary stable configurations dictated by the "memorized" optoelastic potential landscape due to the last illumination pattern. We characterize the strength of optically guided elastic forces and discuss the potential uses of this noncontact manipulation in fabrication of novel optically-and electrically-tunable composites from liquid crystals and colloids.optical manipulation | photoresponsive surface monolayers | self-assembly R econfigurable self-assembly of micrometer-and nanometersized particles of various shapes and chemical compositions is of great interest from the standpoints of both fundamental science and practical applications (1-14). The use of anisotropic liquid crystal (LC) fluids as host media for such colloidal selfassembly is currently perhaps one of the most promising approaches (2-14). It not only allows one to engender anisotropic long-range interaction forces and achieve oriented self-assembly guided by the long-range orientational order of the LC host (2), but also enables control of the medium-mediated interparticle forces by means of varying temperature, applying external fields, and utilizing the response of LC alignment to the presence of various chemical substances (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Properties of such tunable self-assembled LC-based composite micro-and nanostructured materials can be further engineered by controlling positions and orientations of constituent particles of desired material compositions, shapes, and s...

Nematic liquid crystal boojums with handles on colloidal handlebodies

“…Future studies will also explore dynamics of nucleation and annihilation of defects as particles are continuously rotated by external magnetic and optical torques, the influence of elastic constant anisotropy on the stability of different stable and metastable configurations, and hysteresis in switching between different states, as well as how spatial localization of boojums is related to intrinsic curvature of colloidal surfaces. The experimental arena we have developed, combined with the ability to achieve optical control of surface anchoring on colloidal surfaces (40) and the use of chiral LCs (7), may become a test ground for applying and probing a number of newly developed theories describing the topology of nematic defects, such as the CoparZumer theory (41), and may be extended to topologically nontrivial LC confinement geometries (12).…”

Magnetically responsive gourd-shaped colloidal particles in cholesteric liquid crystals

“…Elastic properties of LCs trigger anisotropic interactions between colloids, [1][2][3][4][5][6] directing their self-assembly into macrostructures [7][8][9][10][11][12][13][14] useful, for example, in photonics applications. [11][12][13] During the last decades, experimental and theoretical studies covered different aspects of elastic interactions between spherical colloids mostly in nematic LCs.…”

“…[11][12][13] During the last decades, experimental and theoretical studies covered different aspects of elastic interactions between spherical colloids mostly in nematic LCs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Two- [7][8][9][10][11] and three-dimensional 14 crystals were designed in nematics using different elastic dipoles and quadrupoles, and singular defects were tailored using spherical 16 and topologically non-trivial 17 colloidal particles. Recent studies also showed the importance of size [18][19][20][21][22][23][24] and shape [24][25][26][27][28][29][30][31] in elastic interactions and ensuing assemblies of colloidal particles in nematics.…”

Nematic films at chemically structured surfaces

“…Additionally, equation (10) implies that the interfacial deformation in the limit qw p → 0 has the scaling form…”

“…Apart from an academic interest, a fundamental understanding of such systems will pave the way for the design and manufacturing of novel materials, devices, and applications. Examples include smart surfaces with self-cleaning [7], enhanced heat transfer [8], reduced fluid drag [9] or photo-responsive properties [10]; templatedirected assembly of colloidal particles [11][12][13]; microfluidics [14,15] and optofluidics [16][17][18].…”