Phase Transitions In Suspensions Research Articles

Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots

A review is given of the field of mineral colloidal liquid crystals: liquid crystal phases formed by individual mineral particles within colloidal suspensions. Starting from their discovery in the 1920s, we discuss developments on the levels of both fundamentals and applications. We conclude by highlighting some promising results from recent years, which may point the way towards future developments.

Self-assembly and magnetically induced phase transition of three-dimensional colloidal photonic crystals

Charged superparamagnetic colloidal Fe(3)O(4)@SiO(2) core-shell particles were chosen as model dipolar soft spheres to study their crystallization and magnetically induced phase transition in suspensions. The 3D colloidal crystals feature excellent magnetically responsive photonic properties with strong diffraction, fast response and wide tunability.

Non-equilibrium phase transitions in suspensions of oppositely driven inertial particles

The structural evolution of a two-specie suspension of inertial particles under opposite driving forces is investigated by molecular dynamics simulations. The effects of the driving force ( F) and the total number density of the particles ( ρ) on the final structure are explored. When F is increased under a high ρ, the system starts with a frozen phase, passes through an ordered phase characterized by two demixed lanes moving in opposite directions, and finally returns to a disorder phase. When ρ is increased under a low F, a novel re-entrant phase transition is found: more than two lanes parallel to the driving forces are observed first, followed by a disordered phase with different kinds of particles blocking each other, and then an ordered state with all particles separating into two demixed lanes. We comment on the possible mechanisms underlying these phase transitions in terms of the compromise between the directional driven motion and random thermal fluctuation.

Gelation versus liquid crystal phase transitions in suspensions of plate-like particles

Gelation is a common effect in aqueous suspensions of charged colloidal clay platelets at concentrations as low as 1 wt%. However, in systems of charged gibbsite [Al(OH)3] platelets, gelation can be delayed to concentrations as high as 50 wt% depending on the ionic strength. We investigated the phase behaviour of this system approaching the state of gelation in the delicate region between attractive and repulsive states that originate from competition between Coulomb repulsion and van der Waals attraction. As a function of the ionic strength, isotropic-nematic, nematic-columnar and isotropic-columnar phase separations were observed. Moreover, compression by gravitational forces allowed us to observe phase separation that is arrested by gelation in the homogeneous suspensions.

Isotropic-nematic phase transition in suspensions of filamentous virus and the neutral polymer Dextran.

We present an experimental study of the isotropic-nematic phase transition in an aqueous mixture of charged semiflexible rods ( fd virus) and neutral polymer (Dextran). A complete phase diagram is measured as a function of ionic strength and polymer molecular weight. At high ionic strength we find that adding polymer widens the isotropic-nematic coexistence region with polymers preferentially partitioning into the isotropic phase, while at low ionic strength the added polymer has no effect on the phase transition. The nematic order parameter is determined from birefringence measurements and is found to be independent of polymer concentration (or equivalently the strength of attraction). The experimental results are compared with the existing theoretical predictions for the isotropic-nematic transition in rods with attractive interactions.

Combined negative dielectrophoresis and phase separation in nondilute suspensions subject to a high-gradient ac electric field.

Experiments were conducted on concentrated suspensions of neutrally buoyant particles which exhibit negative dielectrophoresis. We found that, due to interparticle electrical interactions, such suspensions undergo a phase separation when subjected to a high-gradient ac field (approximately kV/mm) and form a propagating distinct front between the regions enriched with and depleted of particles. A generalization of our theory for the thermodynamics of the field-induced phase transitions in suspensions of polarized particles [Phys. Rev. E 52, 1669 (1995); 54, 5428 (1996); 60, 3015 (1999)] is proposed for the front propagation, and its predictions are shown to be consistent with the experiments even though the model contains no fitting parameters. The combination of field-induced dielectrophoresis and phase transition provides a method for strongly concentrating particles in prespecified regions of dielectrophoretic devices.

Liquid–crystal phase transitions in suspensions of plate–like particles

We study the liquid–crystalline phase behaviour of suspensions of hard colloidal platelets, in particular, the relationship between their size polydispersity and the stability of a nematic, columnar and smectic phase. A first–order nematic–to–columnar transition is observed for suspensions up to 25% polydispersity in platelet diameter (σD). However, at densities slightly above nematic–columnar coexistence, the columnar order appears to vanish in the case of σD = 25%, while columnar stability persists for σD = 17%. The observed tolerance of polydispersity in the columnar phase, being a two–dimensional crystal, seems remarkable in light of current predictions for the terminal polydispersity for hard–sphere crystallization and hard–rod smectic ordering.

Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration. We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought.

Phase transitions in suspensions of needle-like particles in liquid crystals

The thermodynamical properties of suspensions of rigid stretched particles in liquid crystal solvent are studied with the help of modification of Mc Millan and Maier-Saupe theories. We have shown that the presence of particles increases the temperatures of phase transitions isotropic medium-nematic and nematic-smectic A in a solvent as well. The indirect interaction between particles through nematic solvent effectively acts as an attraction. This attraction may induce their condensation and phase transition of type “gas-liquid” in the system of particles.

Angular correlations and the isotropic-nematic phase transition in suspensions of tobacco mosaic virus.

The specific magnetic-field-induced birefringence is measured in aqueous suspensions composed of the charged rodlike particle tobacco mosaic virus (TMV) as a function of temperature, TMV concentration, ionic strength, and TMV polydispersity over the entire isotropic range. This quantity is proportional to the magnitude of the interparticle angular correlations at zero field. Theoretical expressions for the field-induced birefringence for both the mono- and polydisperse samples are derived based on extensions of the Onsager model [L. Onsager, Ann. N.Y. Acad. Sci. 51, 627 (1949)] and compare well with experiment. In addition, the isotropic-nematic phase coexistence concentrations are measured as a function of ionic strength and temperature. The agreement between experiment and theory indicates that the TMV particles interact primarily through electrostatic repulsion and that attractive forces are negligible.