The current study examined the foaming behavior of poly(vinylpyrrolidone) (PVP)-silica composite nanoparticles. Individually, the two components, PVP and silica nanoparticles, exhibited very little potential to partition at the air-water interface, and as such, stable foams could not be generated. In contrast, combining the two components to form silica-PVP core-shell nanocomposites led to good "foamability" and long-term foam stability. Addition of an electrolyte (NaSO) was shown to have a marked effect on the foam stability. By varying the concentration of electrolyte between 0 and 0.55 M, three regions of foam stability were observed: rapid foam collapse at low electrolyte concentrations, delayed foam collapse at intermediate concentrations, and long-term stability (∼10 days) at the highest electrolyte concentration. The observed transitions in foam stability were better understood by studying the microstructure and physical and mechanical properties of the particle-laden interface. For rapidly collapsing foams the nanocomposite particles were weakly retained at the air-water interface. The interfaces in this case were characterized as being "liquid-like" and the foams collapsed within 100 min. At an intermediate electrolyte concentration (0.1 M), delayed foam collapse over ∼16 h was observed. The particle-laden interface was shown to be pseudo-solid-like as measured under shear and compression. The increased interfacial rigidity was attributed to adhesion between interpenetrating polymer layers. For the most stable foam (prepared in 0.55 M NaSO), the ratio of the viscoelastic moduli, G'/G″, was found to be equal to ∼3, confirming a strongly elastic interfacial layer. Using optical microscopy, enhanced foam stability was assessed and attributed to a change in the mechanism of foam collapse. Bubble-bubble coalescence was found to be significantly retarded by the aggregation of nanocomposite particles, with the long-term destabilization being recognized to result from bubble coarsening. For rapidly destabilizing foams, the contribution from bubble-bubble coalescence was shown to be more significant.
The polymerization of a styrene in water (o/w) emulsion was performed at 30 °C (±5 °C) under ultrasonic irradiation in the absence of any added chemical initiator. The radicals formed as a consequence of the cavitation process were sufficient to cause polymerization. The latices formed in this way were stable bluish-white dispersions. The polymerization rate increased through a maximum at about 30% conversion before decreasing; no region with a steady rate was seen. The final latex diameters were around 50 nm and the polystyrene molecular weights were high (>106), suggesting at most a few polymer chains per particle. Calculations of the particle numbers as a function of monomer conversion suggested that there was a continuous nucleation of the particles. The small particle sizes, high polymerization rates, and continuous nucleation are postulated to be due to the continuous formation of very small oil droplets in the ultrasonic field which can efficiently scavenge the radicals formed during the cavitation process.
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 E whose magnitude is determined only by the thermophysical properties of the liquid and the atmosphere. Accordingly, the larger the magnitude of E, 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 ( E = 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.
Using ultra-thin surface coatings of water-soluble polymers to modify interfacial friction is relatively new, but may offer routes to form beneficial coatings whilst using significantly lower polymer concentrations. In the current study, silica surfaces were modified by the physisorption of poly(vinylpyrrolidone) (PVP) from water solution. Four polymer samples with different molecular weights ranging from 8 kDa to 1300 kDa, were examined here. Optical reflectivity measurements showed that the saturated surface excess for each PVP sample was ~1 mg/m 2. The amount of trapped water within the 8 kDa PVP film (~ 10 wt%) was found to be much less than the trapped water (40-55 wt%) in films formed from higher molecular weight PVPs (40 kDa, 360 kDa, and 1300 kDa). In addition, QCM dissipation values for the 8 kDa PVP film was more than four times smaller than those measured for the higher molecular weight PVPs, suggesting that the 8 kDa PVP conforms to a flat film (predominantly train orientation), while the high molecular weight PVPs slowly reorganize resulting in more lossy films (increased Sauerbrey film thickness). 2 Colloid-probe AFM lateral force measurements showed that 8 kDa PVP films exhibited similar lateral resistance to that seen for uncoated silica surfaces in water, whereas higher molecular weight PVP films showed significantly reduced lateral forces. This lubrication effect, induced by the adsorbed higher molecular weight PVP samples was explored further by measuring the rheology of concentrated particle suspensions. Suspension yield stress data for PVP-coated particles showed a reduction by a factor of 2 in the yield stress when compared to the uncoated particles for suspension concentrations above 60 vol%, i.e. approaching the close-packed limit of spheres.
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