Fuquan Tu

Shape-Changing and Amphiphilicity-Reversing Janus Particles with pH-Responsive Surfactant Properties

Janus particles are biphasic colloids that have two sides with distinct chemistry and wettability. Because of their amphiphilicity, Janus particles present a unique opportunity for stabilizing multiphasic fluid mixtures such as emulsions. Our work is motivated by one class of molecular amphiphiles that change their surfactant properties in response to environmental stimuli. Depending on the environmental conditions, these stimuli-responsive molecular amphiphiles are able to assemble into different structures, generate emulsions with different morphologies, and also induce phase inversion emulsification. We present a new synthesis method utilizing a combination of polymerization-induced phase separation and seeded emulsion polymerization, which allows for the bulk synthesis of highly uniform pH-responsive Janus particles that are able to completely reverse their surfactant properties in response to solution pH. One side of these Janus particles is rich in a hydrophobic monomer, styrene, whereas the other side is rich in a pH-sensitive hydrophilic repeating unit, acrylic acid. These Janus particles change their aggregation/dispersion behavior and also transform into different shapes in response to pH changes. Furthermore, we demonstrate that these Janus particles can stabilize different types of emulsions (oil-in-water and water-in-oil) and, more importantly, induce phase inversion of emulsions in response to changes in solution pH. The pH-responsive aggregation/dispersion behavior of these Janus particles also allows us to tune the interactions between oil-in-water emulsion droplets without inducing destabilization; that is, emulsion drops with attractive or repulsive interactions can be generated by changing the pH of the aqueous phase. Our study presents a new class of colloidal materials that will further widen the functionality and properties of Janus particles as dynamically tunable solid surfactants.

Amphiphilic Janus particles at fluid interfaces

Janus particles are colloids that have both hydrophilic and hydrophobic faces. Recent advances in particle synthesis enable the generation of geometrically and chemically anisotropic Janus particles with high uniformity and precision. These amphiphilic particles are similar to molecular surfactants in many aspects; they self-assemble in bulk media and also readily attach to fluid interfaces. These particles, just like molecular surfactants, could potentially function as effective stabilizers for various multiphasic systems such as emulsions and foams. In particular, just as the shape and chemical composition have a significant impact on the surfactancy of molecular amphiphiles, the ability to control the shape and wetting properties of Janus particles could provide a unique opportunity to control their surface activity. In this review, we first examine the recent developments in using amphiphilic Janus particles as colloid surfactants to stabilize multiphasic mixtures such as emulsions. These results have motivated a number of detailed investigations aimed at understanding the behaviour of Janus particles at fluid–fluid interfaces at the microscopic level, which we highlight. This review also discusses the importance of controlling the shape of Janus particles, which has a drastic impact on their behaviour at fluid interfaces. We conclude this review by presenting outlook on the future directions and outstanding problems that warrant further study to fully enable the utilization of Janus particles as colloid surfactants in practical applications.

Thermodynamically stable emulsions using Janus dumbbells as colloid surfactants.

One of the most important properties of emulsions is their stability. Most emulsions stabilized with molecular surfactants tend to lose their stability over time via different mechanisms. Although the stability of emulsions stabilized with homogeneous particles have been shown to be superior to that of surfactant-stabilized emulsions, these Pickering emulsions nevertheless are only kinetically stable and thus can undergo destabilization. Janus particles that have two opposite wetting surfaces have shown promise in imparting emulsions with long-term stability because of their strong attachment to the oil-water interface. In this theoretical study, we consider thermodynamics of emulsion stabilization using amphiphilic Janus dumbbells, which are nonspherical particles made of two partially fused spherical particles of opposite wettability. These amphiphilic dumbbells are attractive candidates as colloid surfactants for emulsion stabilization because highly uniform Janus dumbbells can be synthesized in large quantities; thus, their application in emulsion stabilization can become practical. Our theoretical calculation demonstrates that Janus dumbbells can indeed generate thermodynamically stable Pickering emulsions. In addition, we also find that there exists a total oil-water interfacial area that results in the lowest energy state in the system, which occurs when Janus dumbbells available in the system are completely consumed to fully cover the droplet interfaces. We show that the geometry of dumbbells as well as the composition of the emulsion mixtures has significant influences on the average size of dumbbell-stabilized emulsions. We also investigate the effect of asymmetry of Janus dumbbells on the average droplet radius. Our results clearly show that amphiphilic Janus dumbbells provide unique opportunities in stabilizing emulsions for various applications.

Controlling the Stability and Size of Double-Emulsion-Templated Poly(lactic-co-glycolic) Acid Microcapsules

The stability and size of poly(lactic-co-glycolic)acid (PLGA)-containing double emulsions and the resulting PLGA microcapsules are controlled by varying the composition of highly monodisperse water-in-oil-in-water (W/O/W) double emulsions. We propose that the basic inner phase of W/O/W double emulsions catalyzes the hydrolysis of PLGA and the ionization of carboxylic acid end groups, which enhances the surface activity of PLGA and facilitates the stabilization of the double emulsions. The size of PLGA-containing double emulsions and that of resulting microcapsules can be readily tuned by osmotic annealing, which depends on the concentration ratio of a solute in the inner and outer phases of double emulsions. The internal volume of PLGA microcapsules can be changed by more than 3 orders of magnitude using this method. This approach also overcomes the difficulty in generating monodisperse double emulsions and microcapsules over a wide range of dimensions using a single microfluidic device. The osmotic annealing method can also be used to concentrate encapsulated species such as colloidal suspensions and biomacromolecules.

Directed assembly of particles using microfluidic droplets and bubbles

Assembly of particles into three dimensional structures is critical for a variety of advanced applications including photonics, optics, catalysis, MEMS, drug delivery and biosensing. Needless to say, a precise control over the structure and properties of three-dimensional particle assemblies is essential in maximizing the functionality that is afforded by the particles in these structures. One method that enables rapid, inexpensive and potentially scalable assembly of particles involves using fluid droplets and bubbles as structure directing agents. Recent advances in microfluidics allow for the formation of highly uniform and structured droplets and bubbles that can be used to direct the assembly of particles into three dimensional structures. In this review, we introduce the recent developments in using microfluidic techniques to generate highly uniform and complex droplets and bubbles, which are subsequently used to direct the assembly of various nano and microparticles. We also highlight a number of functional supraparticles that have been shown to exhibit unique photonic and sensing properties. We conclude this review by providing an outlook on the current challenges and opportunities that need to be addressed for these microfluidic-based approaches to have a broader impact as widely applicable methods for directed assembly of particles.

Curvature-Driven Migration of Colloids on Tense Lipid Bilayers

Inspired by proteins that generate membrane curvature, sense the underlying membrane geometry, and migrate driven by curvature gradients, we explore the question: Can colloids, adhered to lipid bilayers, also sense and respond to membrane geometry? We report the migration of Janus microparticles adhered to giant unilamellar vesicles elongated to present spatially varying curvatures. In our experiments, colloids migrate only when the membranes are tense, suggesting that they migrate to minimize membrane area. By determining the energy dissipated along a trajectory, the energy field is inferred to depend on the local deviatoric curvature, like curvature driven capillary migration on interfaces between immiscible fluids. In this latter system, energy gradients are larger, so colloids move deterministically, whereas the paths traced by colloids on vesicles have significant fluctuations. By addressing the role of Brownian motion, we show that the observed migration is analogous to curvature driven capillary migration, with membrane tension playing the role of interfacial tension. Since this motion is mediated by membrane shape, it can be turned on and off by dynamically deforming the vesicle. While particle-particle interactions on lipid membranes have been considered in many contributions, we report here an exciting and previously unexplored modality to actively direct the migration of colloids to desired locations on lipid bilayers.