The goal of this project is to develop a new class of smart bubbles that selectively enhance the acoustic contrast of specific fluids, or of specific fluid-fluid interfaces. The essential concept of these materials is the use of small gas bubbles; these have enormous acoustic contrast, and can enhance both the scattering and adsorption of acoustic waves. The utility of such bubbles is well known for ultrasonic imaging within the body; however, it is extremely difficult to stabilize a gas bubble in a fluid for sufficient time to be of use for reservoir purposes. Therefore, a strong stabilizing principle is required to prevent bubble coalescence, and Ostwald ripening.
To accomplish this, we use colloidal particles; these particles have a propensity to adsorb at an interface as they can lower the surface energy between two fluids or between a gas and a fluid [1, 2], as illustrated and calculated in Figure 1. The strong interface adsorption is the key to the advanced stability provided by colloidal particles; however, it there is a large energy barrier that prevents particles from getting onto the gas-liquid interface.
Figure 1: Left: Nanoparticle adsorbed to gas-liquid interface. Right: Sample calculation of adsorption energy.
To overcome this energy barrier, we use a double-emulsion-template strategy, as illustrated in Figure 2. We build a colloid-stabilized structure of an inner gas core that is surrounded by a shell of a volatile organic solvent containing a hydrophobic colloidal suspension; this combination is immersed in a third fluid. When the solvent evaporates, the formerly suspended nanoparticles jam at the interface producing a bubble stabilized by a shell of densely-packed solid colloidal particles .
Figure 2: Schematic of double emulsion template strategy for producing particle-stabilized bubbles
This strategy requires the use of monodisperse particles that have specific surface functionalities; these can be obtained through commercial sources, but only with limited diameters and fixed volume fractions. For example, hydrophobically treated 15-nm-diameter silica nanoparticles (Nissan Chemical America Corporation) are useful for suspending in solvents for the double-emulsion-template approach; however, they are available only with diameters of 15 nm. To make this particle-based stabilizing strategy more robust, we synthesize monodisperse nanoparticles in sizes that are not available through commercial sources; furthermore, we modify their surface chemistry to make them suitable for the desired strategy.
We synthesize monodisperse fluorescently labeled silica nanoparticles, as shown in Figure 3, by a sol-gel method, in which an alkyl silicate is hydrolyzed and condensed in the presence of an alcohol. Ethanol (aq) is thoroughly mixed with fluoresceinisothicyanate FITC dye with a pH adjusting catalyst. The addition of tetraethyl orthosilicate (TEOS) to this mixture produces monodisperse FITC labeled nanoparticles. Simple adjustments to reaction conditions allow us to easily tune the diameters of these nanoparticles to increase or decrease potential interfacial adsorption energies.
Figure 3: SEM image shows monodisperse 250nm silica nanoparticles obtained by the sol-gel reaction.
To test our double-emulsion-template strategy, we use 700-nm-diameter nanoparticles synthesized and functionalized by the methods described earlier. We vigorously mix a suspension of nanoparticles suspended in toluene in a 50 mL centrifuge tube for 1 min. After the addition of a stabilizing surfactant solution, the mixture is sheared again for an additional minute to obtain a mixture of stabilized bubbles and nanoparticle emulsions as seen in Figure 4. Here, we not only see the individual nanoparticles stabilizing the gas interface, but we also see their incomplete surface coverage. The results are quite promising, yet there is still a need for greater control over fabrication to obtain discrete size distributions and complete surface coverage, both of which will increase the stability of these nanobubbles.
Figure 4: Brightfield image of 700nm FITC silica nanobubble showing incomplete nanoparticle surface coverage.
Double emulsion glass capillary devices are the most appropriate means of producing monodisperse gas-in-oil-in-water nanobubbles. We use a glass double emulsion glass capillary device that is shown in Figure 5 to produce particle stabilized bubbles; these are shown in the right half of Figure 5. We can precisely control the shell thickness and bubble size to control bubble stability by independently varying the flow rates, and device geometries; however, the stability of these bubbles is limited because the synthesized nanoparticles do not effectively adsorb to the interface.
Figure 5: Brightfield images of monodisperse, nanoparticle-stabilized bubbles. Left: production of bubbles within a microfluidic device. Right: Optical micrograph of bubbles on a glass slide.
The stability of the nanobubbles depends on the adsorption of the nanoparticles on the interface, and their jamming there to form an interlocking solid. If this does not occur, the nanobubbles will not remain stable; this can occur if the coverage of the nanoparicles on the interface is incomplete, or if there remains a layer of solvent from the shell that does not completely disappear. Our custom synthesized and functionalized nanoparticles can play a strong role as we can increase the solvent volatility to more easily induce nanoparticle jamming. Also, we can increase the volume fraction of nanoparticles to increase the packing density. To further enhance nanobubble stability, we must use a stronger driving force to direct the nanoparticles to the gas-water interface, and we must ensure that the intermediate solvent is completely removed. This will be an important goal for the next phase of the work. We continue to use glass capillary microfluidics to test prospective nanobubble designs with greater potential for stability.
These bubbles must be able to withstand reservoir pressures; therefore, we investigate their stability at elevated pressures to assess their response to pressures typically encountered in reservoirs. To accomplish this, we use a custom built pressure setup at Schlumberger Doll Research (SDR) is used to test the stability of our double emulsion bubbles under hydrostatic pressures. The SDR pressure setup is equipped with an Isco pump to drive the pressure and flow in the vessel. A lipstick camera is positioned at the top window of the vessel to allow for ease of recorded monitoring with the TV/DVD setup, as shown in Figure. To pressurize nanobubbles in the SDR pressure setup, the nanobubble dispersion is added to demonized water in the pressure vessel. The vessel is filled from a water reservoir by an Isco dyne syringe pump at 25 psi. Each sample is pressurized to one final pressure in a matter of seconds. The bubbles remain at the user-defined pressure for 2 minutes then depressurized at 25psi, and finally atmosphere.
Figure 6: SDR pressure vessel. The Isco pump (500D) draws fluid from the reservoir. Bubbles are dispersed in the pressure vessel with a backfilling of water from the Isco pump. The pressurization is visualized through a battery powered lipstick camera and monitored and recorded on a TV monitor and DVD player.
After the bubbles have been pressurize, the remaining bubbles or nanobubble shells are collected and imaged with confocal microscopy and scanning electron microscopy. In response to the hydrostatic pressures, the bubbles buckle. The majority of the thin shelled bubbles tested in this system buckle under pressure, as seen in Figure 5.
Figure 7: Three sets of double emulsion bubbles with thin nanoparticle coatings buckle under hydrostratic pressures of 500 and 1000 psi.
As an alternate to the SDR test bed, we use evaluate the stability of individual bubbles in a diamond anvil cell. The diamond anvil cell is constructed from two flat polished diamond cutlets that have been mounted, aligned and expoxied into a stainless steel gasket with a hole of diameter of 300 µm and a thickness of 200 µm. The ruby placed inside the hole for pressure measurements along with one bubble. The force exerted is controlled by turning the screws of the diamond anvil cells. The ruby responds to pressure by shifting in fluorescence and thus we are able to construct a pressure calibration curve. To seal the bubble, ruby and liquid in the gasket of the diamond anvil, we exert an initial pressure of 5 psi and image the bubble optically through the diamond. We increase the pressure to 20 kpsi and observe only slight changes in bubble diameter and shell thickness of our bubble as shown in Figure 6 . The integrity of the bubbles tested in the diamond anvil is maintained at high pressures.
Figure 8: Double emulsion template bubble withstands hydrostatic pressure from (A)5 kpsi to (B)20 kpsi in a diamond anvil cell.
We have had one success and several failures in our tests for stability under pressure. Similar to our reasoning for bubble instability in the synthesis section, nanobubble instability can be attributed to insufficient nanoparticle interface coverage. Most importantly, we believe, is that the nanoparticles have not fully coated the air-water interface; moreover, we believe there is still some solvent left that prevents a fully jammed particle interface. Future work will focus on refining the fabrication of the nanobubbles to ensure that they all remain stable at higher pressure. As we learn and develop more effective strategies to drive nanoparticles to interfaces, we will couple these learning’s to produce nanobubbles stable to solvent evaporation but also nanobubbles more stable under hydrostatic pressures.
We do not possess the necessary acoustic equipment in our lab; instead, these measurements must be done in collaboration with academic and industrial collaborators. We perform ultrasonic testing of the double emulsion based nanobubbles with Terason, a clinical ultrasonic system. A baseline measurement of a water-filled bag is first used to set the time gain compensation to show zero enhancement prior to acquiring the signal from the bubbles. We suspend our bubbles in a sealed thin layered plastic water bag. We submerge a bag with a suspension of bubbles with a diameter of 200µm and a shell thickness of 35 µm, in a water tank. The clinical 128 element phased array ultrasonic transducer with a 5-10 MHz bandwidth is used to image the bubbles and to obtain radiofrequency data.
The stored radiofrequency data is converted to a brightness mode image representative of the nanobubble effect on increasing contrast in the studied frequency range. The raw radio frequency data is saved in a proprietary manufacturer format and post-converted in Matlab with a Hilbert transform envelope detection. The average image brightness of a selected region shown of interest in Figure 7 is calculated as 32 dB in the brightness mode image obtained. Based on the size frequency of the bubbles to the sound wavelength, the brightness increase is due to specular reflection (λ≈a) and scattering (λ>>a). Regardless of the mode of increased contrast, our bubbles effectively enhance acoustic contrast at ultrasonic frequencies.
Figure 9: Ultrasonic B-mode image shows 200 micron double emulsion enhance echo by 32 dB.
he goals of the experiments at Petrobras were to investigate the behavior of the nanobubbles in rock cores and perform initial acoustic measurements of the nanobubbles. The sound speed of two cleaned Berea sandstone and carbonate plugs was measured in dry and water (DI) saturated states. Nanobubble (d~300µm) suspensions were injected into Berea sandstone and carbonate plugs using low flow rates. We estimated the seismic attenuation of the both rocks, in three states of saturation: dry, water-saturated, and water and bubble saturated. The bubbles were expected to significantly attenuate the signal relative to the water saturated rock. In both the Berea and Dolomite rocks, the amplitude of the waveform from highest to lowest was the dry rock, the water-saturated rock followed by the water-bubble saturated rock as shown in Figure 8. The decreased amplitude indicated attenuation due to the presence of bubbles.
Figure 10: Waveforms for Dolomite rock. The water saturation (blue) is less the dry rock (orange). The amplitude for the rock with bubbles (purple) is lower than the water saturated and dry rock thus indicating attenuation.
The strong attenuation is likely to be due to the compressibility of the gas compared to the fluid and solid rock core. The attenuation and scattering response of these bubbles clearly demonstrates the potential of these nanobubbles to serve as excellent contrast enhancers for acoustic waves.
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