Intense interest has been shown in the development of “smart nanofluids” because they can dramatically change their flow properties in complex fluid systems. We introduce a robust and straightforward approach to develop a smart nanofluid system, in which associative silica nanoparticles (ASNPs) were incorporated to regulate their flow properties in rock pores. ASNPs were synthesized by covalently coating a hydrophobically associative hygroscopic zwitterionic poly­[2-methacryloyloxyethyl phosphorylcholine (MPC)-co-divinylbenzene (DVB)] shell layer on 20 nm sized silica nanoparticles (NPs) by surface-mediated living radical polymerization. The essence of our approach is to copolymerize DVB, which endows the polymer shell of ASNPs with a weak long-range hydrophobic attraction. This leads to the favorable formation of a wedge film as a result of structural disjoining pressure. This wedge film made the oil adsorbed on the rock surface more dewettable, thus enhancing the efficiency of oil recovery. As a result of this unique behavior, the ASNP nanofluid developed in this study remarkably improved the fluidity of complex oil fluids; ∼5 vol % oil recovery enhancement and ∼0.3 psi pressure reduction were also achieved in comparison to neat nanofluid controls.

This study reports a Pickering emulsion flooding system, in which the oil–water interface is structurally stabilized by a complex colloidal layer consisting of silica nanoparticles, dodecyltrimethylammonium bromide (DTAB), and poly­(4-styrenesulfonic acid-co-maleic acid) sodium salt (PSS-co-MA). The colloidal layer was generated by adsorption of PSS-co-MA on the silica nanoparticles as a result of the van der Waals attraction and by adsorption of DTAB onto the PSS-co-MA layer as a result of the electrostatic attraction, thus providing the mechanically robust, stable interface. To demonstrate a practical applicability to the enhanced oil recovery, the complex colloidal dispersion fluid was injected into the Berea sandstone for a core flooding experiment. The result revealed that the colloidal dispersion significantly increased the oil recovery by ∼4% compared to the case of flooding water. This means that the emulsion drops in situ produced in the core could readily flow through the rock pores. We attribute this to the fact that the oil–water interface made with the complex colloidal phase not only increased the structural stability of the emulsion drops but also provided them deformability without any drop breakup or coalescence.

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Cellulose fibrils, unique plant-derived semicrystalline nanomaterials with exceptional mechanical properties, have significant potential for rheology modification of complex fluids due to their ability to form a physically associated semiflexible fibrillary network. Here, we report new associative cellulose nanocrystals (ACNCs) with stress-responsive rheological behaviors in an aqueous solution. The surface-mediated living radical polymerization was employed to graft poly(stearyl methacrylate-co-2-methacryloxyethyl phosphorylcholine) brushes onto the nanofibrils, and then 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation was conducted to produce nanoscale ACNCs in the aqueous solution. The ACNCs displayed interfibril association driven by the hydrophobic interaction that resulted in the formation of a nanofibrillar crystalline gel phase. We observed that the viscosity of the ACNC fluid showed reversible shear thinning and temperature-induced thickening in response to applied shear stress and thermal shock. Moreover, thanks to generation of a mechanically robust nanofibrillar crystalline gel network, the ACNC suspension showed extraordinary stability to changes in salinity and pH. These results highlighted that the interfibril hydrophobic association of ACNCs was vital and played an essential role in regulation of stimuli-responsive sol-gel transitions.

Microbially enhanced oil recovery involves the use of microorganisms to extract oil remaining in reservoirs. Here, we report fabrication of microgel particles with immobilized Bacillus subtilis for application to microbially enhanced oil recovery. Using B. subtilis isolated from oil-contaminated soils in Myanmar, we evaluated the ability of this microbe to reduce the interfacial tension at the oil-water interface via production of biosurfactant molecules, eventually yielding excellent emulsification across a broad range of the medium pH and ionic strength. To safely deliver B. subtilis into a permeable porous medium, in this study, these bacteria were physically immobilized in a hydrogel mesh of microgel particles. In a core flooding experiment, in which the microgel particles were injected into a column packed with silica beads, we found that these particles significantly increased oil recovery in a concentration-dependent manner. This result shows that a mesh of microgel particles encapsulating biosurfactant-producing microorganisms holds promise for recovery of oil from porous media.

Novel associative nanoparticles (ANPs) are synthesized by grafting poly(2-methacryloyloxyethyl phosphorylcholine-co-stearyl methacrylate) polymer brushes on silica nanoparticles by using surface-mediated living radical polymerization.

This study introduces a new type of associative nanoparticle (ANP) that provides controlled chain-to-chain attraction with an associative polymer rheology modifier (APRM) to produce highly stable Pickering emulsions. The ANPs were synthesized by grafting hydrophobically modified hygroscopic zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine-co-stearyl methacrylate) brushes onto 20 nm sized silica NPs via surface-mediated living radical polymerization. The ANP-stabilized Pickering emulsions show significant viscosity enhancement in the presence of the APRM. This indicates that the ANPs act as particulate concentration agents at the interface owing to their hydrophobic association with the APRM in the aqueous phase, which leads to the generation of an ANP-mediated complex colloidal film. Consequently, the described ANP-reinforced Pickering emulsion system exhibits improved resistance to pH and salinity changes. This coacervation approach is advantageous because the complex colloidal layer at the interface provides the emulsion drops with a mechanically robust barrier, thus guaranteeing the improved Pickering emulsion stability against harsh environmental factors.