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Freeze drying and re-dispersibility of oil-in-water (o/w) emulsions is important from the perspective of storage, transportation and usability. A set of stable and re-dispersible o/w emulsions using polyacryloyl hydrazide (PAHz) capped Ag nanoparticles (NP) as the stabilizer is reported in which the NP size (D) and PAHz concentration collectively controlled the stability and re-dispersibility of the emulsion system. O/w emulsions prepared using different concentrations of PAHz (0.05-0.25 g mL) and sizes of Ag NPs (10-25 nm) were analyzed by DLS, IFT, contact angle, SEM, and rheological studies. The emulsion stabilized by 0.05 g mL of PAHz and Ag NPs (D ≈ 20 nm, 5 wt% PAHz-Ag NPs) was unstable against coalescence, exhibited maximum oil leakage during freeze-drying and lacked re-dispersibility. The stability of the Pickering emulsions was inversely proportional to the D of the Ag NPs. The presence of Ag NPs possessing D ≈ 10 nm and 0.25 g mL of PAHz in the aqueous phase (25 wt% PAHz-Ag NPs) stabilized the Pickering emulsions for up to 30 days without any sign of creaming and the oil powders (oil content ≈ 98%) obtained after freeze drying exhibited adequate re-dispersibility in aqueous media. In addition, the emulsion stabilized by 25 wt% PAHz-Ag NPs showed maximum recovery of the viscosity value after re-dispersion and exhibited similar shear-thinning behavior to that of the original sample. Furthermore, the trends of moduli vs. frequency for the re-dispersed samples were similar to that of the original samples suggesting that the structural arrangements between Ag NPs and oil droplets were least affected by the drying process. Thus, we conclude that o/w emulsions stabilized by PAHz-Ag NPs can be a potential alternative to produce stable oil powders or gels for industrial applications.

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Thermal stability is becoming a barrier for silica nanofluid use at high temperatures in several industrial applications including oil fields. Homoagglomeration of SiO 2 nanoparticles (NPs), which leads to premature sedimentation of large NP clusters, is one of the reasons for the loss in thermal stability. However, this can be improved by incorporating a thermally conductive co-stabilizer, which not only reduces the homoagglomeration of SiO 2 −SiO 2 NPs but also improves their rheological properties. Thus, this study reports the use of titania (TiO 2 ) NPs in improving the thermal stability and rheological properties of silica nanofluids for high-temperature applications. TiO 2 concentration was kept low and constant (0.1 wt %), whereas the SiO 2 concentration was varied from 0.1 to 1.0 wt %. In nanofluid synthesis, watersoluble partially hydrolyzed polyacrylamide of 1000 ppm is used as a viscosity enhancer. Different techniques such as visual inspection, dynamic light scattering, ultraviolet−visible (UV−vis) spectroscopy, thermogravimetric analysis, and field emission scanning electron microscopy were used to characterize nanofluids. The thermal stability and rheological properties of HS nanofluids were not only affected by agglomeration and high temperature but also showed least dispersion (45 days). TiO 2 inclusion controlled the rate of homoagglomeration in silica nanofluids, resulting in SiO 2 −TiO 2 nanocomposites of least size, and better dispersion (71 days) and thermal stability (only 78% mass loss) were observed in HTS nanofluids. Thus, thermally stable silica nanofluids of improved flow behavior are proposed for oil field applications where conventional nanofluids may find limitations. Finally, the enhanced oil recovery potential of silica nanofluids is studied and compared with the ones of silica and titania at real oil field conditions of high temperature (90 °C) and saline environment of 5 wt % NaCl. The oil recovery potential of conventional silica nanofluids increased by 12% original oil in place, with the inclusion of 0.1 wt % TiO 2 in the system.

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