The colloidal stability of nanoparticles (NPs) stabilized by grafted polyelectrolyte (PE) brushes in concentrated divalent ion solutions, at either ambient or high temperature, is of interest in a wide variety of applications including medicine, personal care products, oil and gas recovery, reservoir imaging, and environmental remediation. Previous attempts to determine the length of PE brushes at these conditions have been limited by lack of colloidal stability particularly when divalent ions form complexes with the charges on the brushes. We find that brushes of highly acidic strong PE poly­(2-acrylamido-2-methyl­propane­sulfonate, AMPS) end-grafted to silica NPs provide colloidal stability at salinities up to 4.5 M CaCl2 or NaCl. Thus, the brush behavior could be studied with dynamic light scattering (DLS) and the electrophoretic mobility by phase analysis light scattering (PALS) from the salt-free condition to the extreme salinities of 4.5 M. In monovalent NaCl solutions, the highly extended poly­(AMPS) brushes at low salt concentration (C s) collapse monotonically with increasing C s. On the other hand, in divalent CaCl2 solutions the brushes underwent four distinct regimes of (i) a low C s collapse regime, (ii) a relatively broad plateau regime (0.1 M ≤ C s < 1 M), (iii) a weak reswelling regime, and (iv) a high C s collapse regime. The novel behavior in regimes ii–iv may be attributed to weak interactions of the poly­(AMPS) brushes with Ca2+. We also find that the brushes are more extended at 90 °C as thermal energy weakens interchain bridging, which is consistent with the behavior of free polymer chains dissolved in CaCl2 solutions at extreme salinities.

Numerical simulations supplemented by experiments together uncovered that strategic integration of discrete electric fields in a non-invasive manner could substantially miniaturize the droplets into smaller parts in a pressure driven oil-water flow inside microchannels. The Maxwell's stress generated from the electric field at the oil-water interface could deform, stretch, neck, pin, and disintegrate a droplet into many miniaturized daughter droplets, which eventually ushered a one-step method to form water-in-oil microemulsion employing microchannels. The interplay between electrostatic, inertial, capillary, and viscous forces led to various pathways of droplet breaking, namely, fission, cascade, or Rayleigh modes. While a localized electric field in the fission mode could split a droplet into a number of daughter droplets of smaller size, the cascade or the Rayleigh mode led to the formation of an array of miniaturized droplets when multiple electrodes generating different field intensities were ingeniously assembled around the microchannel. The droplets size and frequency could be tuned by varying the field intensity, channel diameter, electrode locations, interfacial tension, and flow ratio. The proposed methodology shows a simple methodology to transform a microdroplet into an array of miniaturized ones inside a straight microchannel for enhanced mass, energy, and momentum transfer, and higher throughput.

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Superparamagnetic nanoparticles with a high initial magnetic susceptibility χ are of great interest in a wide variety of chemical, biomedical, electronic, and subsurface energy applications. In order to achieve the theoretically predicted increase in χ with the cube of the magnetic diameter, new synthetic techniques are needed to control the crystal structure, particularly for magnetite nanoparticles larger than 10 nm. Aqueous magnetite dispersions (FeO) with a χ of 3.3 (dimensionless SI units) at 1.9 vol %, over 3- to 5-fold greater than those reported previously, were produced in a one-pot synthesis at 210 °C and ambient pressure via thermal decomposition of Fe(II) acetate in triethylene glycol (TEG). The rapid nucleation and focused growth with an unusually high precursor-to-solvent molar ratio of 1:12 led to primary particles with a volume average diameter of 16 nm and low polydispersity according to TEM. The morphology was a mixture of stoichiometric and substoichiometric magnetite according to X-ray diffraction (XRD) and Mössbauer spectroscopy. The increase in χ with the cube of magnetic diameter as well as a saturation magnetization approaching the theoretical limit may be attributed to the highly crystalline structure and very small nonmagnetic layer (∼1 nm) with disordered spin orientation on the surface.

Superparamagnetic iron oxide nanoparticles (IONPs), which have been investigated extensively as contrast-enhancing agents in biology, are being explored for subsurface applications such as electromagnetic tomography, fracture mapping, and enhanced oil recovery. However, two key challenges must be addressed: (a) high magnetic susceptibility and (b) colloidal stability and mobility under harsh reservoir conditions of high salinity and temperature. Herein, we synthesize IONPs grafted with poly(2-acrylamido-3-propanesulfonate-co-acrylic acid) poly(AMPS-co-AA) to achieve a high surface grafting density of polymer (49%) with minimal aggregation to yield sub-50 nm IONPs. The IONPs were found to be colloidally stable at 120 °C for a period of one month at pH 8. In crushed Berea sandstone, polymer-grafted IONPs exhibited significantly high mass breakthrough (84%) and low retention (149 μg/g) when used with a sacrificial polymer preflood (0.1% v/v). Intact Berea core experiments showed an 8-fold improvement in mass breakthrough (65%) and a two-thirds reduction in retention (from 433 μg/g to 160 μg/g) when compared to previous studies with IONPs synthesized via coprecipitation. The high grafting density of polymeric stabilizer and small nanoparticle size contribute to the improved mobility in consolidated porous media at high salinity.