Structural Forces near Phase Transitions of Liquid Crystals

Dynamics of order reconstruction in a nanoconfined nematic liquid crystal with a topological defect

At the wall in a hybrid nematic cell with strong anchoring, the nematic director is parallel to one wall and perpendicular to the other. Within the Landau-de Gennes theory, we have investigated the dynamics of s = ±1/2 wedge disclinations in such a cell, using the two-dimensional finite-difference iterative method. Our results show that with the cell gap decreasing, the core of the defect explodes, and the biaxiality propagates inside the cell. At a critical value of dc* ≈ 9ξ (where ξ is the characteristic length for order-parameter changes), the exchange solution is stable, while the defect core solution becomes metastable. Comparing to the case with no initial disclination, the value at which the exchange solution becomes stable increases relatively. At a critical separation of dc ≈ 6ξ, the system undergoes a structural transition, and the defect core merges into a biaxial layer with large biaxiality. For weak anchoring boundary conditions, a similar structural transition takes place at a relative lower critical value. Because of the weakened frustration, the asymmetric boundary conditions repel the defect to the weak anchoring boundary and have a relatively lower critical value of da, where the shape of the defect deforms. Further, the response time between two very close cell gaps is about tens of microseconds, and the response becomes slower as the defect explodes.

Detection of DNA targets hybridized to solid surfaces using optical images of liquid crystals

In this paper, we report a method of detecting DNA targets hybridized to a solid surface by using liquid crystals (LC). The detection principle is based on different interference colors of LC supported on surfaces decorated with single-stranded DNA (ssDNA) or double-stranded DNA (dsDNA). However, the contrast between the ssDNA and dsDNA is not obvious, unless DNA-streptavidin complexes are introduced to the dsDNA to increase the surface mass density. Two different approaches of introducing streptavidin to the system are studied and compared. We find that by premixing the biotin-labeled DNA targets with streptavidin prior to the DNA hybridization, branched-streptavidin complexes are formed and clear LC signal can be observed. This LC-based DNA detection principle represents an important step toward the development of a simple, instrument- and fluorophore-free DNA detection method.

Detecting DNA targets through the formation of DNA/CTAB complex and its interactions with liquid crystals

In this paper, we report the formation of a DNA/cetyl trimethylammonium bromide (CTAB) complex on a solid surface and its interaction with a thin layer of liquid crystals (LC) supported on the surface. Our results show that when the surface is decorated with DNA only, the LC gives a bright image, but when the surface is decorated with the DNA/CTAB complex, the LC becomes dark when the surface density of CTAB is above 5.25 ± 0.13 × 10(13)/cm(2). To exploit this phenomenon for detecting DNA targets, we used a surface decorated with electroneutral PNA probes for capturing DNA targets, and then treated the surface with 0.1 mM of CTAB. In the presence of DNA targets, a PNA/DNA/CTAB complex is formed and that leads to a dark image on the thin layer of the LC supported on the surface. Moreover, DNA targets with a complementary, 1-base mismatch and non-complementary sequence can be differentiated by using this method. This study provides a new principle for the label-free detection of DNA targets without any fluorescent labels.

Direct nanomechanical measurement of layer thickness and compressibility of smectic liquid crystals

Using an atomic force microscope (AFM) we confined a smectic-A liquid crystal (LC) between a flat glass plate and a 10-μm glass sphere attached to the free end of the AFM cantilever. Both surfaces were treated with a surfactant that induces normal alignment of the LC molecules. We measured the force F acting on the cantilever while varying the plate-sphere distance D with subnanometer precision. For D < 50 nm, the force was periodically oscillating and decayed as D was increased. Analyzing the force in the framework of a simple model of elastic deformation of the smectic layers, we have evaluated the undeformed layer thickness a(0) and compressibility modulus B. Compared to other techniques used to determine a(0) and B, AFM measurements are faster and require a much smaller amount (microliters) of LC. Moreover, they are based on purely mechanical deformations of the LC structure and do not require any static or radiative electromagnetic field.

Anisotropic reversible aggregation of latex nanoparticles suspended in a lyotropic nematic liquid crystal: effect of gradients of biaxial order

We studied the anisotropic aggregation of spherical latex particles dispersed in a lyotropic liquid crystal presenting three nematic phases: calamitic, biaxial, and discotic. We observed that in the nematic calamitic phase aggregates of latex particles are formed, which become larger and anisotropic in the vicinity of the transition to the discotic phase, due to a coalescence process. Such aggregates are weakly anisotropic and up to 50 microm long and tend to align parallel to the director field. At the transition to the discotic phase, the aggregates dissociated and re-formed when the system was brought back to the calamitic phase. This shows that the aggregation is due to attractive and repulsive forces generated by the particular structure of the nematic phase. The surface-induced positional order was investigated by surface force apparatus experiments with the lyotropic system confined between mica surfaces, revealing the existence of a presmectic wetting layer around the surfaces and oscillating forces of increasing amplitude as the confinement thickness was decreased. We discuss the possible mechanisms responsible for the reversible aggregation of latex particles, and we propose that capillary condensation of the NC phase, induced by the confinement between the particles, could reduce or remove the gradient of order parameter, driving the transition of aggregates from solidlike to liquidlike and gaslike.

Normal capillary forces

A liquid meniscus between two lyophilic solid surfaces causes an attractive force, the capillary force. The meniscus can form by capillary condensation or by accumulation of adsorbed liquid. Under ambient conditions and between hydrophilic surfaces, capillary forces usually dominate over other surface forces. They are relevant in many processes occurring in nature and technical applications, for example the flow of granular materials and friction between surfaces. Here we review normal capillary forces, focusing on a quantitative description with continuum theory. After introducing the capillary force between spherical surfaces, we extend the discussion to other regular and irregular surfaces. The influence of surface roughness is considered. In addition to capillary forces at equilibrium, we also describe the process of meniscus formation. Assumptions, limits, and perspectives for future work are discussed.

Optical imaging of surface-immobilized oligonucleotide probes on DNA microarrays using liquid crystals

In this paper, we report a new label-free method for the imaging of immobilized oligonucleotide probes on DNA microarrays. The imaging principle is based on the disruption of orientations of nematic liquid crystals (LCs), 4-cyano-4'-pentylbiphenyl (5CB), by the immobilized oligonucleotides on a surface. Because LCs are birefringent materials, disruption of their orientations by the immobilized oligonucleotides can manifest as optical signals visible to the naked eye. LC cells with two homeotropic boundary conditions, which align 5CB perpendicularly to both surfaces, were developed to deliver a distinct contrast between a dark background and a bright image caused by the immobilized oligonucleotides. This design also allows the quantification of immobilized oligonucleotide concentrations through the interference colors of LCs. The LC-based imaging method has a good signal-to-noise ratio and a clear distinction between positive and negative results and is nondestructive to the immobilized oligonucleotides. These advantages make it a promising means of assessing the quality of DNA microarrays.

The role of interparticle and external forces in nanoparticle assembly

The past 20 years have witnessed simultaneous multidisciplinary explosions in experimental techniques for synthesizing new materials, measuring and manipulating nanoscale structures, understanding biological processes at the nanoscale, and carrying out large-scale computations of many-atom and complex macromolecular systems. These advances have led to the new disciplines of nanoscience and nanoengineering. For reasons that are discussed here, most nanoparticles do not 'self-assemble' into their thermodynamically lowest energy state, and require an input of energy or external forces to 'direct' them into particular structures or assemblies. We discuss why and how a combination of self- and directed-assembly processes, involving interparticle and externally applied forces, can be applied to produce desired nanostructured materials.

Liquid-crystal-mediated force between a cylindrical nanoparticle and substrate

Using classical density functional theory, the structure of a molecular fluid around a cylindrical nanoparticle near a solid substrate is studied. The solvent-mediated force between the nanoparticle and the substrate is calculated in both the nematic and isotropic phases of the solvent. In the nematic phase, the force is short ranged and arises due to interaction between high-density regions near the substrate and nanoparticle. In the isotropic phase, the formation of a nematic bridge between the substrate and nanoparticle gives rise to an attractive force between them. The potential between the nanoparticle and substrate as a function of separation calculated numerically is compared to that calculated from the Derjaguin approximation. In the isotropic phase these are found to be in reasonable agreement at low separations, while the agreement is poorer in the nematic phase.

Interfacial nematodynamics of heterogeneous curved isotropic-nematic moving fronts

The early stages of liquid crystal phase ordering upon thermal quenches of isotropic phases into unstable and metastable temperature ranges is studied using two-dimensional (2D) computational solutions of the governing Landau-de Gennes (L-dG) equations for low molar mass nematic liquid crystals and analysis based on the corresponding interfacial nematodynamic model. The early phase ordering stage, for both unstable and metastable quenches of the isotropic phase, is shown to lead to highly textured nematic spherulites through a mechanism of interfacial defect nucleation. The underlying mechanisms of interface-driven texturing are elucidated using complementary 2D computational parametric studies of the bulk L-dG equation and analysis of the IN model. It is shown that for highly curved nanodomains and realistic elastic anisotropy, sharp interfacial transitions between uniaxial and biaxial states arise and are resolved by interfacial defect nucleation, which upon subsequent migration into the spherulite's interior leads to strong texturing. This paper shows that texture formation in the early stages of phase ordering is interface driven, and due to low interface tension, elastic anisotropy, and large curvature. Interfacial defect shedding in highly curved, low tension, anisotropic interfaces is a significant defect nucleation mechanism that needs to be taken into account when considering texturing processes.

Capillary bridging and long-range attractive forces in a mean-field approach

When a mixture is confined, one of the phases can condense out. This condensate, which is otherwise metastable in the bulk, is stabilized by the presence of surfaces. In a sphere-plane geometry, routinely used in atomic force microscope and surface force apparatus, it can form a bridge connecting the surfaces. The pressure drop in the bridge gives rise to additional long-range attractive forces between them. By minimizing the free energy of a binary mixture we obtain the force-distance curves as well as the structural phase diagram of the configuration with the bridge. Numerical results predict a discontinuous transition between the states with and without the bridge and linear force-distance curves with hysteresis. We also show that similar phenomenon can be observed in a number of different systems, e.g., liquid crystals and polymer mixtures.

Capillary condensation in liquid-crystal colloids

We study capillary condensation between two spherical particles dispersed in the isotropic phase of a nematic liquid crystal. Within the Landau-de Gennes theory, we calculate interaction energies due to the formation of capillary bridges that reproduce experimental observations. Close to the critical point of the transition line separating the no-bridge from the bridge configuration, fluctuations in the particle cluster might be described by an effective two-state system. We show that the transition line vanishes for small particles and that the shape of the interaction potential depends on particle size.