Osmotic pressure and chemical potential of silica nanoparticles in aqueous poly(ethyleneoxide) solution

Wetting Dynamics of Nanoparticle Dispersions: From Fully Spreading to Non-sticking and the Deposition of Nanoparticle-Laden Surface Droplets

Controlled transport of liquid droplets on solid surfaces is critical in many practical applications, such as self-cleaning surfaces, coating, drug delivery, and agriculture. Non-adhesive liquid drops levitate on solid surfaces; therefore, they are highly mobile and directed toward desired locations by external stimuli. Although research on liquid-repellent surfaces has proliferated, the existing methods are still limited to creating surface roughness or coating the liquid droplets. Here, we create non-contact aqueous drops on hydrophilic surfaces in an oleic environment and use them to deposit submicrometer droplets encapsulating nanoparticles on solid surfaces. A glass surface is buried under an oil phase that contains a high concentration of Span 80 surfactants, and a drop of silica nanoparticle dispersion is released on the solid surface. We study the effect of surfactant concentration in oil and nanoparticle concentration in water on wetting dynamics and report a plethora of droplet spreading regimes from fully wetting to non-wetting. We find a threshold Span 80 concentration above which surfactant assemblies are formed on the solid and prevent the direct contact of the drop with the surface. At the same time, water-in-oil emulsions are generated at the drop-oil interface. The drop moves and leaves a trace of emulsions with encapsulated nanoparticles on the solid. We demonstrate the possibility of local surface coating with hydrophilic nanoparticles in a hydrophobic medium. The developed methodology in this study is a generic approach facilitating the droplet patterning in numerous applications, from pharmaceutical polymetric carriers to the formulation of cosmetics, insecticides, and biomedical diagnoses.

Thermodynamic stability condition can judge whether a nanoparticle dispersion can be considered a solution in a single phase

Establishing that a nanoparticle dispersion can, in fact, be treated as a solution has an important practical ramification, namely the application of solubility theories for solvent selection. However, what distinguishes a solution and dispersion has remained ambiguously understood. Based on the recent progress in statistical thermodynamics on multiple-component solutions, here we establish the condition upon which a nanoparticle dispersion can be considered a single-phased solution. We shall provide experimental evidence already found in the literature showing the solution nature of nanoparticle dispersions.

AWE-somes: All Water Emulsion Bodies with Permeable Shells and Selective Compartments

Living cells exploit compartmentalization within organelles to spatially and temporally control reactions and pathways. Here, we use the all aqueous two phase system (ATPS) of poly(ethylene glycol) (PEG) and dextran to develop all water emulsion bodies, AWE-somes, a new class of encapsulated double emulsions as potential cell mimics. AWE-somes feature rigid polyelectrolyte (PE)/nanoparticle (NP) shells and double emulsion interiors. The shells form via complexation of PE and NP at interfaces of ATPS. The NPs, excluded from the drop phase, create an osmotic stress imbalance that removes water from the encapsulated phase and draws droplets of external PEG phase into the shells to form the double emulsion interior. We demonstrate that molecules can permeate the AWE-some shells, selectively partition into the internal droplets, and undergo reaction. AWE-somes have significant potential for creating functional, biocompatible protocell systems.

Adsorption of poly(ethylene oxide)-containing amphiphilic polymers on solid-liquid interfaces: Fundamentals and applications

The adsorption of amphiphilic molecules of varying size on solid-liquid interfaces modulates the properties of colloidal systems. Nonionic, poly(ethylene oxide) (PEO)-based amphiphilic molecules are particularly useful because of their graded hydrophobic-hydrophilic nature, which allows for adsorption on a wide array of solid surfaces. Their adsorption also results in other useful properties, such as responsiveness to external stimuli and solubilization of hydrophobic compounds. This review focuses on the adsorption properties of PEO-based amphiphiles, beginning with a discussion of fundamental concepts pertaining to the adsorption of macromolecules on solid-liquid interfaces, and more specifically the adsorption of PEO homopolymers. The main portion of the review highlights studies on factors affecting the adsorption and surface self-assembly of PEO-PPO-PEO block copolymers, where PPO is poly(propylene oxide). Block copolymers of this type are commercially available and of interest in several fields, due to their low toxicity and compatibility in aqueous systems. Examples of applications relevant to the interfacial behavior of PEO-PPO-PEO block copolymers are paints and coatings, detergents, filtration, and drug delivery. The methods discussed herein for manipulating the adsorption properties of PEO-PPO-PEO are emphasized for their ability to shed light on molecular interactions at interfaces. Knowledge of these interactions guides the formulation of novel materials with useful mesoscale organization and micro- and macrophase properties.

Osmotic Bulk Modulus of Charged Colloids Measured by Ensemble Optical Trapping

The optical-bottle technique is used to measure osmotic bulk moduli of colloid suspensions. The bulk modulus is determined by optically trapping an ensemble of nanoparticles and invoking a steady-state force balance between confining optical-gradient forces and repulsive osmotic-pressure forces. Osmotic bulk moduli are reported for aqueous suspensions of charged polystyrene particles in NaCl solutions as a function of particle concentration and ionic strength, and are compared to those determined by turbidity measurements under the same conditions. Effective particle charges are calculated from the bulk moduli and are found to increase as a function of ionic strength, consistent with previously reported results.

Angle- and strain-independent coloured free-standing films incorporating non-spherical colloidal photonic crystals

Colloidal photonic crystals (CPCs) provide a convenient way to generate structural colour with high stability against degradation under environmental factors. For a number of applications including flexible electronic and energy devices, it is important to generate flexible structural colour that maintains its colour regardless of the angle of observation and the extent of mechanical deformation. However, it is challenging to simultaneously achieve these goals because anisotropy in typical CPC structures (e.g., CPC films) tends to lead to angle-dependent photonic properties and also changes in the lattice constant due to mechanical deformation lead to changes in the photonic properties of CPCs. To overcome these challenges, we present a means of fabricating large-area free-standing films of CPC structures that exhibit angle- and strain-independent photonic characteristics. First, monodisperse double emulsions encapsulating colloidal crystal arrays are prepared using a microfluidic device. By inducing crystallization of highly charged polystyrene particles in the core of double emulsions using osmotic annealing, we generate angle independent colloidal photonic crystal (CPC) supraparticles. Moreover, the shape and crystallinity of the CPC supraparticles can be tuned by changing the concentration of salt in the solution used for osmotic annealing. Subsequently, an array of CPC supraparticles is embedded inside an elastomeric matrix to form a flexible free-standing film, which exhibits structural colours that are independent of viewing angles and externally applied strain.