From colloids in liquid crystals to colloidal liquid crystals

ABSTRACT

Liquid crystals in combination with nanoparticles are a fascinating topic of research, because of the wealth of aspects and questions to study. These range from simple effects of nanoparticles on phase transitions and phase diagrams, to the tuning of physical properties, adding of novel functionalities, all the way to the formation of spontaneous order by nanoparticles themselves and the possibilities that templating has for future materials design and applications. This article intends to provide a flavour of the multiplicity, variety and diversity that these thermotropic and lyotropic systems have to offer in the area of materials development, which we believe will become increasingly important, especially for switchable non-display applications and nanotechnology. It is not intended to provide a conclusive overview, which would be a presumptuous attempt considering the limited space available, but rather to place our own work into a wider context and to point out some more recent developments and trends in liquid crystal – nanoparticle dispersions.     

Active broadband polymer stabilized liquid crystals

We consider the realization and the electro-optical behaviour of a polymer network stabilized liquid crystal whose reflection band has been broadened. We particularly emphasize the reflective properties (mean value of the selective reflection and bandwidth, and the parameters of broadening) and their evolution with voltage. Broadband active films can be obtained under special conditions of polymerization. A mechanism concerning the broadening of the reflection band is put forward based on inhomogeneous consumption of a chiral monomer within the samples.     

Active broadband polymer stabilized liquid crystals

We consider the realization and the electro-optical behaviour of a polymer network stabilized liquid crystal whose reflection band has been broadened. We particularly emphasize the reflective properties (mean value of the selective reflection and bandwidth, and the parameters of broadening) and their evolution with voltage. Broadband active films can be obtained under special conditions of polymerization. A mechanism concerning the broadening of the reflection band is put forward based on inhomogeneous consumption of a chiral monomer within the samples.     

Photonic control of surface anchoring on solid colloids dispersed in liquid crystals

The anchoring of liquid-crystal (LC) mesogens to the surfaces of colloids is an important factor in determining intercolloidal interactions and the symmetry of the ensuing colloidal assembly in nematic colloids. The dynamic control of surface anchoring could therefore provide a handle to tune the colloidal organization and resulting properties in these systems. In this article, we report our results on the study of thermotropic nematic LC (E7) dispersions of silica and glass microcolloids bearing photosensitive surface azobenzene groups. By the photoinduced modulation of the colloidal-LC interfacial properties, due to the trans-cis isomerization of azobenzene units, we tune the anchoring on silica colloids from homeotropic (trans-azobenzene) to homogeneous planar (cis-azobenzene) reversibly. In tune with the change in surface anchoring, the interparticle interactions were also dictated by dipolar and quadrupolar symmetries for homeotropic and homogeneous planar anchoring, respectively. In our experiments, we find that, in addition to the isomerization state of the surface-bound azobenzene units, the nature of the colloid plays a crucial role in determining the anchoring state obtained on applying photostimuli. We also study the LC anchoring on colloids as a function of the azobenzene surface density and find that beyond a threshold value the anchoring properties remain invariant.     

Formation of self-supporting reversible cellular networks in suspensions of colloids and liquid crystals

In mixtures of thermotropic liquid crystals with spherical poly (methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transformation. To characterize the process of network formation in terms of morphology, phase transformation kinetics, and mechanical properties, we have combined data from polarization and laser scanning confocal microscopy with calorimetric, NMR, and rheological results. Our data suggest that the mechanism of network formation is dominated by a broadened temperature and time interval of phase transformation rather than by particle size or concentration. The observation that the width of the transformation interval strongly depends on sample preparation supports the hypothesis that a third component, most likely alkane remnants slowly liberated from the particles, plays a crucial role. In addition, calorimetric findings for liquid crystal/colloid mixtures, heated and cooled up to 13 times, point to separation of the liquid crystal into two compartments with different phase transformation kinetics. This could be explained by redistribution and enrichment of alkane in the particle-composed network walls. A further increase of the storage modulus, G', and incomplete dissolution of the networks in the isotropic state indicate that the formation of two compartments during repeated temperature cycles stabilizes the network and confers strong memory effects.     

Active colloids in complex fluids

We review recent work on active colloids or swimmers, such as self-propelled microorganisms, phoretic colloidal particles, and artificial micro-robotic systems, moving in fluid-like environments. These environments can be water-like and Newtonian but can frequently contain macromolecules, flexible polymers, soft cells, or hard particles, which impart complex, nonlinear rheological features to the fluid. While significant progress has been made on understanding how active colloids move and interact in Newtonian fluids, little is known on how active colloids behave in complex and non-Newtonian fluids. An emerging literature is starting to show how fluid rheology can dramatically change the gaits and speeds of individual swimmers. Simultaneously, a moving swimmer induces time dependent, three dimensional fluid flows that can modify the medium (fluid) rheological properties. This two-way, non-linear coupling at microscopic scales has profound implications at meso- and macro-scales: steady state suspension properties, emergent collective behavior, and transport of passive tracer particles. Recent exciting theoretical results and current debate on quantifying these complex active fluids highlight the need for conceptually simple experiments to guide our understanding.     

Active colloids on fluid interfaces

We review recent work on active colloids at interfaces, including self-propelled colloids that move by generating a propulsive force, and driven colloids that move under external fields. Features unique to fluid interfaces alter the flows generated at interfaces by active colloid motion, and hydrodynamic interactions with these layers. We emphasize recent observations of natural swimmers, like bacteria, and bio-mimetic colloids including self-propelled phoretic and Marangoni swimmers, and magnetically driven colloids. We discuss active colloid interaction with boundaries and with each other. We conclude with a discussion of open issues and opportunities to design active colloids as active surface agents that manipulate interfacial properties and the transport in the vicinity of interfaces.     

Forces in nematic liquid crystals: from nanoscale interfacial forces to long-range forces in nematic colloids

In this paper we give an overview of experiments that provided an insight into the nature of forces between surfaces and objects in a nematic liquid crystal. These forces, also called ‘structural forces’, are the consequence of the long-range orientational order and orientational elasticity of nematic liquid crystals. Owing to their fundamental as well as technological importance, forces between objects in liquid crystals have been a subject of growing interest during the last decade. Experimental observations and studies of structural forces are described from nanoscale interfacial forces, measured by an atomic force microscope, to the micro-scale forces between colloidal particles in nematics, studied by laser tweezers and optical video microscopy.     

Self-assembly of photonic crystals from polymer colloids

This article reviews recent developments in self-assembly of polymer colloids into colloidal crystals, a good candidate material for photonic crystals. Self-assembly strategy has developed as a facile and efficient method to fabricate colloidal crystals. Much research work has been focused on controlling the morphology and improving the quality, as well as finding applications of the colloidal crystals.     

Active structuring of colloids through field-driven self-assembly

In recent years self-assembly has become progressively more “active”, i.e. the focus of research gradually has shifted towards field-manipulation of matter in order to form temporary states rather than creating static architectures. The desire for time-programmed control of materials certainly originates from the unmatched complexity of natural systems that orchestrate multiple components across length scales. Although artificial self-assembly still lacks control comparable to natural systems, there has been impressive progress in a concerted approach from physicists, chemists, biologists, and engineers. This review summarizes the current trend in colloidal assembly advancing from static assembly of isotropic particles towards active structuring of anisotropic particles with heterogeneous (patchy) surfaces, and ultimately, to complex behavior in dissipative dynamic systems. We focus both on the formation of static structures and on temporary states due to response to magnetic, electric, or optic stimulation. We give examples of nano- and microparticle assembly where the temporary state may adopt equilibrium order or a continuously changing dynamic pattern.     

Mesoscopic monodisperse ferromagnetic colloids enable magnetically controlled photonic crystals

We report here the first synthesis of mesoscopic, monodisperse particles which contain nanoscopic inclusions of ferromagnetic cobalt ferrites. These monodisperse ferromagnetic composite particles readily self-assemble into magnetically responsive photonic crystals that efficiently Bragg diffract incident light. Magnetic fields can be used to control the photonic crystal orientation and, thus, the diffracted wavelength. We demonstrate the use of these ferromagnetic particles to fabricate magneto-optical diffracting fluids and magnetically switchable diffracting mirrors.     

Amphotropic liquid crystals

The liquid crystalline state is a fundamental organization of matter, which combines order and mobility on a molecular, supramolecular and macroscopic level. In many cases the molecules can show both thermotropic and lyotropic liquid-crystalline (LC) phases, which is described as amphotropic behavior. Block-copolymers, polyhydroxy amphiphiles, disc-like, rod-like, polycatenar and banana-shaped LC molecules are discussed with respect to their amphotropic behavior with specific and non-specific solvents. The interactions of salts with polyether chains, leading to halotropic mesophases, and the interaction of aromatic electron acceptor molecules with electron-rich aromatic molecular parts are discussed in relation to lyotropic mesomorphism induced by classical solvent molecules. Polyphilic amphotropic materials showing more complex mesophase morphology and amphiphiles showing a hierarchical order of different levels of order are pointed out as future directions.     

Mineral liquid crystals

The contemporary state of studying mineral liquid crystals has been analyzed. Such crystals are lyotropic aqueous or water–organic colloidal solutions, the dispersed phases of which are represented by nano- and microsized crystalline particles. The methods of production, structure, and physicochemical properties of these systems, as well as the influence of electric and magnetic fields on them, have been discussed in detail.     

Germanium liquid crystals

Liquid-crystalline compounds containing germanium atoms were synthesised and assessed for liquid-crystalline properties. These new compounds generally possess smectic C phases, and many also possess nematic, smectic A and higher order smectic phases. The germanium-containing liquid crystals were incorporated into smectic C mixtures. These mixtures tend to exhibit little change in smectic C*?layer thickness over temperature. This characteristic is associated with de Vries smectic A materials, but measurements show that, although they have high smectic C stability, the materials' smectic cone angles are small. Measurement of smectic cone angle versus temperature of an exemplar material and its analogues containing carbon and silicon in place of the germanium, all show small cone angles which fall smoothly and extrapolate to zero as the smectic C*?to smectic A transition is approached. These measurements largely explain the observed small layer changes and establish that the materials are not first-order de Vries materials. They must be located elsewhere along the de Vries-orthogonal continuum of smectic A phases.     

Mineral liquid crystals

This review describes recent developments in the field of liquid–crystalline suspensions of mineral nanoparticles. New families of chemical compounds have been investigated in the last few years. The most common mesophases (nematic, lamellar and columnar) have now been discovered in dispersions of disc-like and rod-like nanoparticles. New research thrusts presently focus on more subtle thermodynamic effects such as those of polydispersity and gravity. The specific physical properties brought by the mineral building blocks to the liquid–crystalline phases are now being examined. Mesomorphic ordering of the nanoparticles is increasingly used in materials science for templating and for preparing composites.     

Nucleus growth in liquid crystals

The phase ordering kinetics of various liquid crystalline materials from well known series (PCH, CCH, nO. m, BCH and CB) when turning from a disordered to an ordered phase induced by a rapid temperature quench have been investigated. The process of phase ordering can generally be described by the growth of a characteristic length L. The exponents n of the theoretically predicted universal growth law L (t) tn have been determined. In this paper we present in particular the results for the kinetics of phase formation which takes place under the mechanism of nucleus growth. As characteristic length we chose the diameter of the nuclei. We obtained values for n between 0.5 and 1 dependent on the depth of the temperature quench. These results for mainly spherical shaped nematic and smectic germs are discussed in terms of the Allen-Cahn theory for the movement of an antiphase boundary (APB). As a lower limiting value, 0.5 confirms the classical theory, while 1 as an upper limiting value can be explained by the introduction of a volume driving force.     

Polymers in lyotropic liquid crystals

Polymers can be incorporated into lamellar liquid crystalline systems without the phenomena of macroscopic phase separation occurring. By playing with the inter-membrane interactions and the polymer–bilayer interactions the properties of the lamellar system can be modified significantly. This is not only of interest as a fundamental prerequisite to life, but it also opens new fields of application.     

Arch-texture in cholesteric liquid crystals

We have observed an arch-texture in cholesteric liquid crystals sandwiched between two glass plates making a small wedge angle. The anchoring is homeotropic on one plate and planar on the opposite one. This texture is locally periodic and composed of parallel stripes whose average direction rotates by 180° each time the sample thickness increases by p/2, where p is the equilibrium pitch of the cholesteric phase. This texture is due to a periodic modulation of the elastic boundary layer which forms near the plate treated for homeotropic anchoring.