Robin Singh

Microencapsulation and Stimuli-Responsive Controlled Release of Particles Using Water-in-Air Powders

In this work, we report a facile, one-step method to encapsulate hydrophilic particles (HP) (micro- or nanosize) using water-in-air powders. Hydrophobic silica nanoparticles were mixed with an aqueous phase containing HP in the presence of air under high shear, resulting in the self-assembly of silica nanoparticles on water droplets to make water-in-air powders with HP encapsulated in the aqueous phase within the silica shell. The encapsulated HP can be released on the basis of an external stimulus such as adding an external aqueous phase of a certain pH or a surfactant solution that alters the wettability of the encapsulating silica nanoparticles. A contact angle study was performed using surface-hydrophobized glass slides, which acted as a proxy for hydrophobic silica nanoparticles, to investigate the effect of these stimuli on surface hydrophobicity. Such encapsulation and a stimuli-responsive controlled release system has promising potential in subsurface petroleum engineering such as the delayed swelling of particles for conformance control and delayed acid stimulation.

Foams Stabilized by In-Situ Surface Activated Nanoparticles in Bulk and Porous Media

Foams for subsurface applications are traditionally stabilized by surfactants. The goal of this work is to study foam stabilization by nanoparticles—in particular, by in-situ surface-hydrophobization of hydrophilic nanoparticles. The interfacial properties of the nanoparticles were modulated by the attachment of short-chain surface modifiers (alkyl gallates) that render them partially hydrophobic, but still fully dispersible in water. First, static foams were generated with nanoparticles with varying concentrations of surface modifiers. The decay of foam height with time was measured, and half-lives were determined. Optical micrographs of foam stabilized by surface-modified nanoparticles (SMNPs) and surfactant were recorded. Second, aqueous foams were created insitu by coinjecting the SMNP solutions with nitrogen gas through a Berea sandstone core at a fixed quality. Pressure drop across the core was measured to estimate the achieved resistance factor. These pressure-drop results were then compared with those of a typical surfactant (alpha olefin sulfonate, alkyl polyglucoside) under similar conditions. Finally, oil-displacement experiments were conducted in Berea cores with surfactant and SMNP solutions as foaming agents (coinjection with nitrogen gas). A Bartsch shake test revealed the strong foaming tendency of SMNPs even with a very low initial surface-modifier concentration (0.05 wt%), whereas hydrophilic nanoparticles alone could not stabilize foam. The bubble texture of foam stabilized by SMNPs was finer than that with surfactants, indicating a stronger foam. As the degree of surface coating increased, the resistance factor of SMNP foam in a Berea core increased significantly. The corefloods in the sandstone cores with a reservoir crude oil showed that immiscible foams with SMNP solution can recover a significant amount of oil (20.6% of original oil in place) over waterfloods.

Multistimuli-Responsive Foams Using an Anionic Surfactant

In this work, we report a novel class of a commercially available surfactant which shows a multistimuli-responsive behavior toward foam stability. It comprises three components-a hydrophobe (tristyrylphenol), a temperature-sensitive block (polypropylene oxide, PO), and a pH-sensitive moiety (carboxyl group). The hydrophobicity-hydrophilicity balance of the surfactant can be tuned by changing either the pH or temperature of the system. At or below pH 4, the carboxyl functional group is dominantly protonated, resulting in zero foamability. At higher pH, the surfactant exhibits good foamability and foam stability marked with a fine bubble texture (∼200 μm). Foam destabilization could be achieved rapidly by either lowering the pH or bubbling CO2 gas. At a fixed pH in the presence of salt, increasing the temperature to 65 °C resulted in rapid defoaming because of the increased hydrophobicity of the PO chain. This stimuli-induced stabilization and destabilization of foam were found to be reversible. We envisage the use of such a multi-responsive foaming system in diverse applications such as foam-enhanced oil recovery and environmental remediation where spatial and temporal control over foam stability is desirable. The low-cost commercial availability of the surfactant further makes it lucrative.