Deposition of non-porous calcium phosphate shells onto liquid filled microcapsules.

The efficient encapsulation of small molecule active ingredients has been a challenge for many decades across many commercial applications. Recently, successful attempts to address this issue have included deposition of thin metal shells onto liquid filled polymer microcapsules or emulsion droplets to provide an impermeable barrier to diffusion. In this work we have developed a novel method to protect small molecule active ingredients by deposition of thin mineral shells. Platinum nanoparticles are used to catalyse and direct growth of a calcium phosphate shell onto liquid filled polymer microcapsules under various reaction conditions. Findings indicate that a non-porous protective shell is formed on the majority of the microcapsule population, with small concentrations of the core material being released only from those microcapsules with defects, over a 7 days period, when conducting forced release studies into a solvent for the core oil. The resulting microcapsules show no significant cell toxicity when exposed to HEK 293 cells for 72 h.

Analytical Model for Diffusive Evaporation of Sessile Droplets Coupled with Interfacial Cooling Effect.

Current analytical models for sessile droplet evaporation do not consider the nonuniform temperature field within the droplet and can overpredict the evaporation by 20%. This deviation can be attributed to a significant temperature drop due to the release of the latent heat of evaporation along the air-liquid interface. We report, for the first time, an analytical solution of the sessile droplet evaporation coupled with this interfacial cooling effect. The two-way coupling model of the quasi-steady thermal diffusion within the droplet and the quasi-steady diffusion-controlled droplet evaporation is conveniently solved in the toroidal coordinate system by applying the method of separation of variables. Our new analytical model for the coupled vapor concentration and temperature fields is in the closed form and is applicable for a full range of spherical-cap shape droplets of different contact angles and types of fluids. Our analytical results are uniquely quantified by a dimensionless evaporative cooling number Eo whose magnitude is determined only by the thermophysical properties of the liquid and the atmosphere. Accordingly, the larger the magnitude of Eo, the more significant the effect of the evaporative cooling, which results in stronger suppression on the evaporation rate. The classical isothermal model is recovered if the temperature gradient along the air-liquid interface is negligible ( Eo = 0). For substrates with very high thermal conductivities (isothermal substrates), our analytical model predicts a reversal of temperature gradient along the droplet-free surface at a contact angle of 119°. Our findings pose interesting challenges but also guidance for experimental investigations.

Measurements of Submicron Particle Adsorption and Particle Film Elasticity at Oil-Water Interfaces.

The influence of particle adsorption on liquid/liquid interfacial tension is not well understood, and much previous research has suggested conflicting behaviors. In this paper we investigate the surface activity and adsorption kinetics of charge stabilized and pH-responsive polymer stabilized colloids at oil/water interfaces using two tensiometry techniques: (i) pendant drop and (ii) microtensiometer. We found, using both techniques, that charge stabilized particles had little or no influence on the (dynamic) interfacial tension, although dense silica particles affected the "apparent" measured tension in the pendent drop, due to gravity driven elongation of the droplet profile. Nevertheless, this apparent change additionally allowed the study of adsorption kinetics, which was related qualitatively between particle systems by estimated diffusion coefficients. Significant and real interfacial tension responses were measured using ∼53 nm core-shell latex particles with a pH-responsive polymer stabilizer of poly(methyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (pMMA-b-pDMAEMA) diblock copolymer. At pH 2, where the polymer is strongly charged, behavior was similar to that of the bare charge-stabilized particles, showing little change in the interfacial tension. At pH 10, where the polymer is discharged and poorly soluble in water, a significant decrease in the measured interfacial tension commensurate with strong adsorption at the oil-water interface was seen, which was similar in magnitude to the surface activity of the free polymer. These results were both confirmed through droplet profile and microtensiometry experiments. Dilational elasticity measurements were also performed by oscillation of the droplet; again, changes in interfacial tension with droplet oscillation were only seen with the responsive particles at pH 10. Frequency sweeps were performed to ascertain the dilational elasticity modulus, with measured values being significantly higher than previously reported for nanoparticle and surfactant systems, and similar in magnitude to protein stabilized droplets.

Printing small dots from large drops.

Printing of droplets of pure solvents containing suspended solids typically leads to a ring stain due to convective transport of the particles toward the contact line during evaporation of the solvent. In mixtures of volatile solvents, recirculating cells driven by surface tension gradients are established that lead to migration of colloidal particles toward the center of the droplet. In favorable cases, a dense disk of particles forms with a diameter much smaller than that of the droplet. In the latter stages of drying, convective transport of the particles radially toward the contact line still occurs. Two strategies are described to fix the distribution of particles in a compact disk much smaller than the initial diameter of the drying droplet. First, a nanoparticulate clay is added to induce an evaporation-driven sol-gel transition that inhibits convective flow during the latter stages of drying. Second, a nonadsorbing polymer is added to induce depletion flocculation that restricts particle motion after the particles have been concentrated near the center of the droplet. The area of the resulting deposit can be as little as 10% of the footprint of the printed droplet.

Facile synthesis of gold core-polymer shell responsive particles.

The free adsorption of an end-functionalised weak polybase, poly dimethylaminoethyl methacrylate (pDMAEMA), on the surface of colloidal gold nanoparticles (AuNPs) as a route to produce a responsive core-shell nanoparticle is explored here. Optimal conditions for the physisorption of the polymeric chains onto the colloidal nanoparticles are explored. A dense coverage is facilitated by rapidly mixing the well solvated pH responsive homopolymer, at low pH, into a relatively poor solvent environment, at higher pH, containing a stable dispersion of charge-stabilised gold nanoparticles. The rapid pH change causes the polymer chains to concurrently collapse and adsorb onto the gold nanoparticles. In order to achieve sterically stable, monodisperse and responsive core shell nanoparticles, a crucial factor is the pH difference of the systems prior to their mixing. Once adsorbed, end-functional thiol groups on the adsorbed polymer chains can form more permanent covalent attachments with the core particles. Dynamic light scattering coupled with mobility data of pH titration experiments show that the core-shell particles exhibit a responsive character consistent with the observed potentiometric titration data of the polymer. The same particles demonstrate reversible aggregation when cycled between pH extremes. This is confirmed by shifts in the SPR peak of the corresponding UV-Vis absorption profile. The ease and flexibility of this strategy for core-shell particle production, coupled with the stability and responsiveness of the product, make this a promising colloidal coating mechanism.

Drop penetration into porous powder beds.

The kinetics of drop penetration were studied by filming single drops of several different fluids (water, PEG200, PEG600, and HPC solutions) as they penetrated into loosely packed beds of glass ballotini, lactose, zinc oxide, and titanium dioxide powders. Measured times ranged from 0.45 to 126 s and depended on the powder particle size, viscosity, surface tensions, and contact angle. The experimental drop penetration times were compared to existing theoretical predictions by M. Denesuk et al. (J. Colloid Interface Sci.158, 114, 1993) and S. Middleman ("Modeling Axisymmetric Flows: Dynamics of Films, Jets, and Drops," Academic Press, San Diego, 1995) but did not agree. Loosely packed powder beds tend to have a heterogeneous bed structure containing large macrovoids which do not participate in liquid flow but are included implicitly in the existing approach to estimating powder pore size. A new two-phase model was proposed where the total volume of the macrovoids was assumed to be the difference between the bed porosity and the tap porosity. A new parameter, the effective porosity epsilon(eff), was defined as the tap porosity multiplied by the fraction of pores that terminate at a macrovoid and are effectively blocked pores. The improved drop penetration model was much more successful at estimating the drop penetration time on all powders and the predicted times were generally within an order of magnitude of the experimental results.