Kinetics of Crystallization in Polydisperse Emulsions

Nucleation of melt: From fundamentals to dispersed systems

The most evident aspects of a first order transition of a system from an old to a new phase, are the presence of a discontinuity at the interface between both phases and the thermal effects related to the latent heat exchanged with the surrounding environment. These effects are the result of a sequence of events promoted by thermodynamic conditions persisting over the equilibrium in a metastable state. The breakdown of metastability is promoted by infinitesimal energy fluctuations resulting in the germination of clusters of the new phase that can grow to a critical size (nucleus) and then develop or vanish. Examples of these sequences are common in various technological fields such as combustion, food processing, pharmaceutical manufacturing, condensation, and phase change heat transfer, etc. This work aims to highlight a logical path that leads the readers from the fundamental phenomenology to the most intricated aspects of the nucleation within dispersed systems such as oil-in-water emulsions. Differences between the homogeneous and heterogeneous mechanisms are, under the light of the Classical Nucleation Theory (CNT), presented in bulk and confined systems until defining a minimum confinement size. By collecting insights coming from a rich scientific literature mostly focused on the stability of emulsified systems, the discussion is then on the aspects related to the surface related mechanisms. Two main aspects are then considered: a) the wettability of the nucleating cluster by the surrounding melt; b) the affinity between the adsorbed layer, where a surfactant is located, and the oil melt phase (mainly n-alkanes and triacylglycerols with different moieties). In cases where nucleation is dominating over the dewetting of the nucleus, the contact angle can be considered as a constant value. The affinity in terms of molecular features between the surfactant and the oil phase can promote the template effect. Several factors seem to play a role in this interaction such as the thermal characteristics of the surfactant and comparable dimensions between the molecule (or fractions) of the dispersed compound and the tail of the surfactant.

Crystallization kinetics of colloidal model suspensions: recent achievements and new perspectives

Colloidal model systems allow studying crystallization kinetics under fairly ideal conditions, with rather well-characterized pair interactions and minimized external influences. In complementary approaches experiment, analytic theory and simulation have been employed to study colloidal solidification in great detail. These studies were based on advanced optical methods, careful system characterization and sophisticated numerical methods. Over the last decade, both the effects of the type, strength and range of the pair-interaction between the colloidal particles and those of the colloid-specific polydispersity have been addressed in a quantitative way. Key parameters of crystallization have been derived and compared to those of metal systems. These systematic investigations significantly contributed to an enhanced understanding of the crystallization processes in general. Further, new fundamental questions have arisen and (partially) been solved over the last decade: including, for example, a two-step nucleation mechanism in homogeneous nucleation, choice of the crystallization pathway, or the subtle interplay of boundary conditions in heterogeneous nucleation. On the other hand, via the application of both gradients and external fields the competition between different nucleation and growth modes can be controlled and the resulting microstructure be influenced. The present review attempts to cover the interesting developments that have occurred since the turn of the millennium and to identify important novel trends, with particular focus on experimental aspects.

Fabrication of advanced particles and particle-based materials assisted by droplet-based microfluidics

Recent advances in the fabrication of complex particles and particle-based materials assisted by droplet-based microfluidics are reviewed. Monodisperse particles with expected internal structures, morphologies, and sizes in the range of nanometers to hundreds of micrometers have received a good deal of attention in recent years. Due to the capability of generating monodisperse emulsions and of executing precise control and operations on the suspended droplets inside the microchannels, droplet-based microfluidic devices have become powerful tools for fabricating complex particles with desired properties. Emulsions and multiple-emulsions generated in the microfluidic devices can be composed of a variety of materials including aqueous solutions, gels, polymers and solutions containing functional nanoparticles. They are ideal microreactors or fine templates for synthesizing advanced particles, such as polymer particles, microcapsules, nanocrystals, and photonic crystal clusters or beads by further chemical or physical operations. These particles are promising materials that may be applicable for many fields, such as photonic materials, drug delivery systems, and bio-analysis. From simple to complex, from spherical to nonspherical, from polymerization and reaction crystallization to self-assembly, this review aims to help readers be aware of the many aspects of this field.

Nucleation and solidification in static arrays of monodisperse drops

The precise measurement of nucleation and non-equilibrium solidification are vital to fields as diverse as atmospheric science, food processing, cryopreservation and metallurgy. The emulsion technique, where the phase under study is partitioned into many droplets suspended within an immiscible continuous phase, is a powerful method for uncovering rates of nucleation and dynamics of phase changes as it isolates nucleation events to single droplets. However, averaging the behavior of many drops in a bulk emulsion leads to the loss of any drop-specific information, and drop polydispersity clouds the analysis. Here we adapt a microfluidic technique for trapping monodisperse drops in planar arrays to characterize solidification of highly supercooled aqueous solutions of glycerol. This system measured rates of nucleation between 10(-5) and 10(-2) pL(-1) s(-1), yielded an ice-water interfacial energy of 33.4 mJ m(-2) between -38 and -35 degrees C, and enabled the specific dynamics of solidification to be observed for over a hundred drops in parallel without any loss of specificity. In addition to the physical insights gained, the ability to observe the time and temperature of nucleation and subsequent growth of the solid phase in static arrays of uniform drops provides a powerful tool to discover thermodynamic protocols that generate desirable crystal structures.

Microfluidic droplet method for nucleation kinetics measurements

In this work, we developed a microfluidic equivalent of the classical droplet method for investigating nucleation kinetics. Our microfluidic device allows us to store hundreds of droplets of small volume ( approximately 100 nL) and to accurately control their temperature. We also monitor directly all the stored droplets, and thus perform statistical measurements on a large number of nucleation events. In the case of aqueous solutions of potassium nitrate, we manage to investigate nucleation kinetics and polymorphs and quantify the influence of impurities. The use of small droplets is crucial in such experiments, since it allows the sample to reach high supersaturations and to separate all the nucleation events. Moreover, we compare our results to the classical nucleation theory, and we demonstrate unambiguously using direct observations of the droplets that nucleation in aqueous solutions of potassium nitrate always occurs using heterogeneous mechanisms.

Microfluidic crystallization

Microfluidics offers a wide range of new tools that permit one to revisit the formation of crystals in solution and yield insights into crystallization processes. We review such recent microfluidic devices and particularly emphasize lab-on-chips dedicated to the high-throughput screening of crystallization conditions of proteins with nanolitre consumption. We also thoroughly discuss the possibilities offered by the microfluidic tools to acquire thermodynamic and kinetic data that may improve industrial processes and shed a new light on nucleation and growth mechanisms.

Nucleation rate measurement of colloidal crystallization using microfluidic emulsion droplets

An emulsion crystallization method has been demonstrated to measure the nucleation rate of a thermoresponsive colloidal poly-N-isopropylacrylamide (PNIPAM) system. The colloidal PNIPAM suspension was injected into a microfluidic flow-focusing device to generate monodispersed droplets in oil. The temperature was controlled to fine tune the volume fraction of the PNIPAM particles, and the microfluidic flow rate was varied to change the droplet sizes, thus altering the nucleation volume. Using independent droplets, we can isolate the nucleation events to eliminate the interactions among crystallites that existed in bulk or large droplet systems. Therefore, we were able to carry out accurate nucleation rate measurements of colloidal crystals. This emulsion crystallization method is promising for bridging the gap among theories, simulations, and experiments for nucleation kinetics studies.

Influence of initial conditions on homogeneous nucleation kinetics in a closed system

The formation of nuclei of a new phase from the supersaturated mother phase in a closed system is studied. The depletion of the mother phase due to phase transition is taken into account. Basic kinetic equations describing such process are solved numerically to determine the number density of nuclei of newly forming phase and nucleation rate. It is shown that in contrary to the standard nucleation model, when the depletion of the mother phase is not taken into account, the initial size distribution of the clusters affects considerably the nucleation process at higher supersaturations. Our model starts with the equilibrium size distribution of clusters up to various cluster sizes in the undercritical region. At lower supersaturation the formation of nuclei is similar to the standard model because of the low depletion of the mother phase. At higher supersaturation, the depletion of the mother phase plays an important role and some extremal value appears at the size distribution of nuclei, which is not observed in the standard model. The extremum in the size distribution is not a consequence of the coalescence process itself, but it is caused rather by the depletion of the mother phase during the phase transformation.

Effects of addition of hydrophobic sucrose fatty acid oligoesters on crystallization rates of n-hexadecane in oil-in-water emulsions

Ultrasonic velocity measurements were made on crystallization rates of n-hexadecane dispersed in an oil-in-water (O/W) emulsion (20 wt.% oil and 80 wt.% water) in which Tween 20 was employed for emulsification. Highly hydrophobic emulsifiers, sucrose fatty acid oligoesters involving stearic acid (S-170), lauric acid (L-195) and oleic acid (O-170) moieties, were added to n-hexadecane in an attempt to modify the crystallization rate of n-hexadecane. The crystallization process of n-hexadecane was monitored by variations in the ultrasonic velocity values, which increase with increasing amount of crystal fractions in the oil phase of the emulsion. In comparison with the results of the O/W emulsion systems with the additive P-170 (a sucrose palmitate) (N.Kaneko et al., J. Crystal Growth 197 (1999) 263), the following results were obtained: (a) the addition of S-170 accelerated the nucleation in the emulsion system in the same manner as P-170, no acceleration was revealed with the additive O-170, and L-195 showed moderate effects; (b) the rate of crystal growth was retarded by S-170 and L-195, but not by O-170; (c) the effects of acceleration of nucleation occurred singly in the emulsion system, but not in the bulk system; and (d) the acceleration of nucleation was exhibited through two stages with increasing concentrations of the additives. These results showed the remarkable influence of the fatty acid chain structures of sucrose oligoesters on the acceleration of heterogeneous nucleation of n-hexadecane in the O/W emulsions. The heterogeneous nucleation effected by the addition of S-170 and P-170 was discussed taking into account the adsorption at the oil-water interface and the formation of reversed micelles of the sucrose oligoesters added in the oil phase.

Kinetics of Crystallization in n-Hexadecane and Cocoa Butter Oil-in-Water Emulsions Accounting for Droplet Collision-Mediated Nucleation

Droplet collision is shown to be important in the propagation of nucleation through supercooled oil-in-water emulsions by the use of a novel membrane technique. On the other hand, nucleation mediated by Tween 20 micelles is shown to be of relatively much less importance in both n-hexadecane and cocoa butter oil-in-water emulsions. The droplet collision phenomenon probably occurs via a surfactant bridge between the colliding droplets. When this process is taken into account we show that the Turnbull model for crystal nucleation kinetics explains very well nucleation in cocoa butter oil-in-water emulsions by seed crystals. On the basis of this model we characterized the seed crystals in cocoa butter via isothermal crystallization studies at 14.2, 15.0, 15.5, and 15.8 degrees C. We suggest that there are few seed crystals whose size exceeds 0.28 µm at 80 degrees C. In our cocoa butter samples there were between 10(16) and 10(17) seed crystals m(-3) whose average size we inferred to be less than 0.09 µm. A value of 0.133 mJ m(-2) is obtained for the Gibbs free energy of the nucleating surface in our West African cocoa butter samples. There is evidence that the alpha-polymorph of POS comprises the nucleating layer in the seed crystal. There is no evidence that surfactant influences the primary nucleation of oil crystals. However, the surfactant has a big effect on the kinetics of the secondary nucleation process, mediated by droplet collision. Copyright 2000 Academic Press.