Melting of lamellar phases in temperature sensitive colloid-polymer suspensions

Equation of state of colloidal membranes

In the presence of a non-adsorbing polymer, monodisperse rod-like colloids assemble into one-rod-length thick liquid-like monolayers, called colloidal membranes. The density of the rods within a colloidal membrane is determined by a balance between the osmotic pressure exerted by the enveloping polymer suspension and the repulsion between the colloidal rods. We developed a microfluidic device for continuously observing an isolated membrane while dynamically controlling the osmotic pressure of the polymer suspension. Using this technology we measured the membrane rod density over a range of osmotic pressures than is wider that what is accessible in equilibrium samples. With increasing density we observed a first-order phase transition, in which the in-plane membrane order transforms from a 2D fluid into a 2D solid. In the limit of low osmotic pressures, we measured the rate at which individual rods evaporate from the membrane. The developed microfluidic technique could have wide applicability for in situ investigation of various soft materials and how their properties depend on the solvent composition.

Condensation and dissolution of nematic droplets in dispersions of colloidal rods with thermo-sensitive depletants

Nematic droplets are droplets composed of elongated molecules that tend to point in the same direction but do not have any positional order. Such droplets are well known to adopt a spindle shape called tactoid. How such droplets condensate or melt and how the orientational symmetry is broken remains however unclear. Here we use a colloidal system composed of filamentous viruses as model rod-like colloids and pnipam microgel particles to induce thermo-sensitive depletion attraction between the rods. Microscopy experiments coupled to particle tracking reveal that the condensation of a nematic droplet is preceded by the formation of a new phase, an isotropic droplet. As the viruses constitute an excellent experimental realization of hard rods, it follows that the phenomenology we describe should be relevant to diverse micro- and nano-sized rods that interact through excluded volume interactions. This transition between isotropic and nematic droplets provides a new and reversible pathway to break the symmetry and order colloidal rods within a droplet with an external stimulus, and could constitute a benchmark experiment for a variety of technologies relying on reconfigurable control of rods.

Self-assembly of colloidal polymers via depletion-mediated lock and key binding

We study the depletion-induced self-assembly of indented colloids. Using state-of-the-art Monte Carlo simulation techniques that treat the depletant particles explicitly, we demonstrate that colloids assemble by a lock-and-key mechanism, leading to colloidal polymerization. The morphology of the chains that are formed depends sensitively on the size of the colloidal indentation, with smaller values additionally permitting chain branching. In contrast to the case of spheres with attractive patches, Wertheim's thermodynamic perturbation theory fails to provide a fully quantitative description of the polymerization transition. We trace this failure to a neglect of packing effects and we introduce a modified theory that accounts better for the shape of the colloids, yielding improved agreement with simulation.

Temperature as an external field for colloid-polymer mixtures: 'quenching' by heating and 'melting' by cooling

We investigate the response to temperature of a well-known colloid-polymer mixture. At room temperature the gas-liquid critical value of the second virial coefficient of the effective pairwise colloid-colloid interaction for the Asakura-Oosawa model predicts the onset of gelation observed experimentally with remarkable accuracy. Upon cooling the system the effective attraction between colloids induced by polymer depletion is reduced, because the polymer radius of gyration decreases as the θ-temperature is approached. Paradoxically this raises the effective temperature, leading to 'melting' of colloidal gels. We find that the Asakura-Oosawa model of effective colloid interactions, together with a simple description of the polymer temperature response, provides a quantitative description of the observed location of the fluid-gel transition in the colloid volume fraction polymer reservoir number density plane. Further, we present evidence for enhancement of crystallization rates in the vicinity of the metastable critical point.

Entropy driven self-assembly of nonamphiphilic colloidal membranes

We demonstrate that homogeneous monodisperse rods in the presence of attractive interactions assemble into equilibrium 2D fluid-like membranes composed of a one-rod length thick monolayer of aligned rods. Unique features of our system allow us to simultaneously investigate properties of these membranes at both continuum and molecular lengthscales. Analysis of thermal fluctuations at continuum lengthscales yields the membranes' lateral compressibility and bending rigidity and demonstrates that the properties of colloidal membranes are comparable to those of traditional lipid bilayers. Fluctuations at molecular lengthscales, in which single rods protrude from the membrane surface, are directly measured by comparing the positions of individual fluorescently labeled rods within a membrane to that of the membrane's continuum conformation. As two membranes approach each other in suspension, protrusion fluctuations are suppressed leading to effective repulsive interactions. Motivated by these observations, we propose an entropic mechanism that explains the stability of colloidal membranes and offers a general design principle for the self-assembly of 2D nanostructured materials from rod-like molecules.

Two-step mechanism of rotational relaxation in lamellar phases of rods: accelerating effect of the addition of spheres

By using computer simulation on a model colloidal rod-sphere mixture in its lamellar phase, the mechanism responsible for the rod rotational relaxation has been definitely identified and characterized. It consists of two steps: first, a rod, parallel to the director, has to escape from the layer in which it is located and go into the interlayer region mostly populated by spheres, perpendicular to the director; then, it has to insinuate again into one of the adjacent layers, with 50% of probability of resulting antiparallel to the director. While this mechanism is also operating in a pure smectic phase of rods, the presence of spheres increases notably its efficacy, thus promoting the rotational relaxation and facilitating its observation.

Direct measurement of the twist penetration length in a single smectic A layer of colloidal virus particles

In the 1970s, deGennes discussed the fundamental geometry of smectic liquid crystals and established an analogy between the smectic A phase and superconductors. It follows that smectic layers expel twist deformations in the same way that superconductors expel magnetic field. We make a direct observation of the penetration of twist at the edge of a single isolated smectic A layer composed of chiral fd virus particles subjected to a depletion interaction. Using the LC-PolScope, we make quantitative measurements of the spatial dependence of the birefringence due to molecular tilt near the layer edges. We match data to theory for the molecular tilt penetration profile and determine the twist penetration length for this system.

Isotropic-to-nematic nucleation in suspensions of colloidal rods

Using computer simulations, we study the isotropic-to-nematic nucleation in a fluid of colloidal hard rods as well as in a mixture of colloidal rods and non-adsorbing polymer. In order to follow the transformation of the system from the isotropic to the nematic phase, we use a new cluster criterion that enables us to distinguish the nematic clusters from the isotropic fluid phase. Applying this criterion in Monte Carlo simulations, we find two different regimes depending on the supersaturation. At low supersaturation we find nucleation and growth, while at higher supersaturation spinodal decomposition is observed. We determine the height of the nucleation barrier, and we study the structure as well as the shape of the nematic clusters. We discuss our simulation results in the light of classical nucleation theory.

Kinetic pathways for the isotropic-nematic phase transition in a system of colloidal hard rods: a simulation study

We study the kinetic pathways for the isotropic-to-nematic transition in a fluid of colloidal hard rods. In order to follow the formation of the nematic phase, we develop a new cluster criterion that distinguishes nematic clusters from the isotropic phase. Applying this criterion in Monte Carlo simulations, we find spinodal decomposition as well as nucleation and growth depending on the supersaturation. We determine the height of the nucleation barrier and we study the shape and structure of the cluster. More specifically, we find ellipsoidal nematic clusters with an aspect ratio of about 1.7 and a homogeneous nematic director field. Our results are consistent with theoretical predictions on the shape and director field of nematic tactoids. Classical nucleation theory gives reasonable predictions for the height of the nucleation barrier and the critical nucleus size.

Imaging the Sublimation Dynamics of Colloidal Crystallites

We studied the kinetics of sublimating crystals with single-particle resolution by experiments with colloidal spheres and by computer simulations. A short-range attraction between spheres led to crystallites one to three layers thick. The spheres were tracked with optical microscopy while the attraction was reduced and the crystals sublimated. Large crystallites sublimated by escape of particles from the perimeter. The rate of shrinkage was greatly enhanced, however, when the size decreased to less than 20 to 50 particles, depending on the location in the phase diagram. At this size, the crystallites transformed into a dense amorphous structure, which rapidly vaporized. The enhancement of kinetics by metastable or unstable phases may play a major role in the melting, freezing, and annealing of crystals.