The violent collapse of vapor bubbles in cryogenic liquids.

Vapor bubbles in cryogenic fluids may collapse violently under subcooled and pressurized conditions. Despite important implications for engineering applications such as cavitation erosion in liquid propellant rocket engines, these intense phenomena are still largely unexplored. In this paper, we systematically investigate the ambient conditions leading to the occurrence of violent collapses in liquid nitrogen and analyze their thermodynamic characteristics. Using Brenner's time ratio χ, the regime of violent collapse is identified in the ambient pressure-temperature parameter space. Complete numerical simulations further refine the prediction and illustrate two classes of collapses. At 1 < χ < 10, the collapse is impacted by significant thermal effects and attains only moderate wall velocity. Only when χ > 10 does the collapse show more inertial features. A mechanism analysis pinpoints a critical time when the surrounding liquid enters supercritical state. The ultimate collapse intensity is shown to be closely associated with the dynamics at this moment. Our study provides a fresh perspective to the treatment of cavitation in cryogenic fluids. The findings can be instrumental in engineering design to mitigate adverse effects arising from intense cavitational activities.

Penetration of hydroxyl radicals in the aqueous phase surrounding a cavitation bubble.

In the sonochemical degradation of nonvolatile compounds, the free radicals must be delivered into the aqueous solution from the cavitation bubble to initiate reduction-oxidation reactions. The penetration depth in the liquid becomes an important parameter that influences the radical delivery efficiency and eventual treatment performance. However, the transport of radicals in the liquid phase is not well understood yet. In this paper, we focus on the most reactive OH radical and numerically simulate its penetration behavior. This is realized by solving the coupled equations of bubble dynamics, intracavity chemistry, and radical dispersion in the aqueous phase. The results present both the local and global penetration patterns for the OH radicals. By performing simulations over a wide range of acoustic parameters, we find an undesirable phenomenon that the penetration can be adversely suppressed when strengthening the radical production. A mechanistic analysis attributes this to the excessively vigorous recombination reactions associated with high radical concentrations near the bubble interface. In this circumstance, the radicals are massively consumed and converted into molecular species before they can appreciably diffuse away. Our study sheds light on the interplay between radical production inside the bubble and dispersion in the outside liquid. The derived conclusions provide guides for sonochemical applications from a new perspective.

An inverse method to fast-track the calculation of phase diagrams for sonoluminescing bubbles.

A sound driven air bubble can be transformed into an argon bubble emitting light pulses stably. The very foundation to investigate the sonoluminescing bubble is to accurately determine the ambient radius and gas composition in the interior. The conventional approach is to model the air-to-argon transformation process through a large number of bubble dynamics simulations to obtain the physical parameters of the ultimate argon bubble. In this paper, we propose a highly efficient method to pinpoint this information in a phase diagram. The method is based on the diffusive equilibrium for each species inside the bubble and derives the ambient radius and composition inversely. To calculate the former parameter, the bisection algorithm is employed to consecutively narrow down the searching range until the equilibria is approached. Afterward, several cycles of full dynamics simulations are conducted to refine the composition. The method is validated using published experimental data. The calculated ambient radii deviate from the test results by less than 1 µm, which falls within the margin of measurement error. The advantages of this method over the semi-analytical approach reported by Hilgenfeldt et al. [J. Fluid Mech. 365 (1998)] are also discussed. Our study provides a standard procedure to calculate the ambient radius and composition and is beneficial for the numerical simulation of sonoluminescing bubbles.

An innovative experimental apparatus for the analysis of natural gas hydrate erosion process using cavitating jet.

Natural Gas Hydrate (NGH) develops and exists in pores of soil sediments under deep seabed and permafrost regions. A cavitation jet is an efficient method of rock breaking, especially for soft hydrate sediment erosion. This paper presents an experimental apparatus that was developed to synthesize NGH and hydrate-bearing sediments and analyze the drilling efficiency of the cavitation jet. The visualization study of fluid flow and breaking mechanism can be conducted over a temperature range varying from -20 °C to 100 °C and up to a maximum confining pressure of 20 MPa. This apparatus is mainly composed of the pressure control and injection system, the cooling system, the cavitation system, and the reaction vessels into which the lab-fabricated temperature/pressure/resistivity sensor probe is inserted. The basic principles of this apparatus are discussed, and a series of experiments were performed to verify that the cavitating jet can be practically applied in the exploitation of NGH reservoirs.

Core-Shell Nanohydrogels with Programmable Swelling for Conformance Control in Porous Media.

Conformance control during waterflooding in an oil reservoir is utilized to redistribute water and increase the sweep efficiency and hence oil production. Using preformed gel particles can effectively redirect the flow by blocking the high-permeability zones and forcing water into low-permeability zones where the oil is trapped. However, the size of such gel particles can limit their applications deeper within the reservoir and can result in shear-induced degradation near the well bore. Here, we fabricate core-shell nanohydrogels with delayed swelling behavior; their volume increases by a factor of 200 after about 30 days in brine under reservoir conditions. We study their effect on the flow behavior in a three-dimensional porous medium micromodel consisting of randomly packed glass beads. Using confocal microscopy, we directly visualize the spatial variations of flow in the micromodel before and after nanohydrogel injection and swelling. The swollen nanohydrogels block some pores reducing the permeability of the micromodel and diverting the water into low-permeability regions. A core flood experiment further confirms that the nanohydrogels can significantly reduce the permeability of a reservoir sample and divert the fluid flow. Our results demonstrate that these core-shell nanohydrogels might be useful for flow control in porous media and can be used as a conformance control agent.

Metal Coordination-Mediated Functional Grading and Self-Healing in Mussel Byssus Cuticle.

Metal-containing polymer networks are ubiquitous in biological systems, and their unique structures enable a variety of fascinating biological behaviors. Cuticle of mussel byssal threads, containing Fe-catecholate complexes, shows remarkably high hardness, high extensibility, and self-healing capability. Understanding strengthening and self-healing mechanisms is essential for elucidating animal behaviors and rationally designing mussel-inspired materials. Here, direct evidence of Fe3+ and Fe2+ gradient distribution across the cuticle thickness is demonstrated, which shows more Fe2+ inside the inner cuticle, to support the hypothesis that the cuticle is a functionally graded material with high stiffness, extensibility, and self-healing capacity. The mechanical tests of the mussel threads show that both strength and extensibility of the threads decrease with increasing oxygen contents, but this property degradation can be restored upon removing the oxygen. The first-principles calculations explain the change in iron coordination, which plays a key role in strengthening, degradation, and self-healing of the polymer networks. The oxygen absorbs on metal ions, weakening the iron-catecholate bonds in the cuticle and collagen core, but this process can be reversed by sea water. These findings can have important implications in the design of next-generation bioinspired robust, highly extensible materials, and catalysis.

Enhanced Adhesion of Carbon Nanotubes by Dopamine Modification.

According to the fact that gecko-inspired vertically aligned carbon nanotubes (VA-CNTs) exhibit ultrastrong adhesion, dopamine is utilized to make a modification to this traditional biomimetic material. The composite material is tested for adhesion performance under different environmental conditions by an atomic force microscope. The adhesion force of the modified VA-CNTs does not show obvious fluctuation during the gradual heating process; however, the material gains improved adhesion when increasing the ambient humidity. In addition, the modified CNTs show a stronger adhesion force than the original CNTs in their performance tests. The dopamine polymer has a good combination with CNTs, which is responsible for the aforementioned excellent performance. Overall, this modification method is simple, convenient, efficient, and environmentally friendly, which all indicates a promising future in its application. The modified CNTs are expected to be used for super-adhesion in harsh environments, as well as in the field of microelectronics.

Nanoscale Surface Properties of Organic Matter and Clay Minerals in Shale.

Surface properties of shale play an essential role in adsorption, transport, and production of hydrocarbons from shale reservoirs. Nanoscale surface properties of kerogen and minerals of shale were examined by a series of techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIRS), and atomic force microscopy (AFM). The results show that aluminosilicate is the main component of inorganic matter, while kerogen chiefly consists of carbon. FTIRS and XPS analysis indicate that the chemical bonds of the kerogen surface are O-H, C-C, C-O, pyrrolic, and so on. In contrast to kerogen, illite's bonds are mainly Si-O and Al-O. AFM results indicate that the adhesion force of kerogen is higher than that of illite in shale. In addition, at a preloading force of 2500 nN, the adhesion force of kerogen increases from 40.8 to 118.2 nN when retraction velocity increases from 500 to 2500 nm/s. The adhesion forces of montmorillonite, calcite, and muscovite are 33.7 ± 6.28, 23.8 ± 11.8, and 105.1 ± 9.1 nN, respectively. The chemical composition and bonds have a profound effect on the adhesion force of shale, which further reveals the transport and adsorption mechanism of methane in kerogen.

Surface Properties of Organic Kerogen in Continental and Marine Shale.

The adhesion energy of kerogen in continental and marine shale was innovatively discovered using the colloid probe technique with atomic-force microscopy (AFM). AFM results indicated that the adhesion force of kerogen was higher than the inorganic material in both the continental and marine shale samples. The chemical elements in the two kinds of samples were measured by energy-dispersive X-ray analysis with scanning electron microscopy (SEM). The chemical compositions of kerogen involved C═C bonding, C═O bonding, pyridine nitrogen, and pyrrole nitrogen, whereas the primary constituent involving inorganic matter was Si-O bonding. These results were confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The high percentages of C═C and C═O bonding in kerogen are attributed to the large dipole on the kerogen surface which allowed kerogen to contain liquid and gaseous hydrocarbons.

Single-component multiphase lattice Boltzmann simulation of free bubble and crevice heterogeneous cavitation nucleation.

This work serves as an important extension of previous work on cavitation simulation [Sukop and Or, Phys. Rev. E 71, 046703 (2005)10.1103/PhysRevE.71.046703]. A modified Shan-Chen single-component multiphase lattice Boltzmann method is used to simulate two different heterogeneous cavitation nucleation mechanisms, the free gas bubble model and the crevice nucleation model. Improvements include the use of a real-gas equation of state, a redefined effective mass function, and the exact difference method forcing scheme. As a result, much larger density ratios, better thermodynamic consistency, and improved numerical accuracy are achieved. In addition, the crevice nucleation model is numerically investigated using the lattice Boltzmann method. The simulations show excellent qualitative and quantitative agreement with the heterogeneous nucleation theories.

A Comprehensive Model for Real Gas Transport in Shale Formations with Complex Non-planar Fracture Networks.

A complex fracture network is generally generated during the hydraulic fracturing treatment in shale gas reservoirs. Numerous efforts have been made to model the flow behavior of such fracture networks. However, it is still challenging to predict the impacts of various gas transport mechanisms on well performance with arbitrary fracture geometry in a computationally efficient manner. We develop a robust and comprehensive model for real gas transport in shales with complex non-planar fracture network. Contributions of gas transport mechanisms and fracture complexity to well productivity and rate transient behavior are systematically analyzed. The major findings are: simple planar fracture can overestimate gas production than non-planar fracture due to less fracture interference. A "hump" that occurs in the transition period and formation linear flow with a slope less than 1/2 can infer the appearance of natural fractures. The sharpness of the "hump" can indicate the complexity and irregularity of the fracture networks. Gas flow mechanisms can extend the transition flow period. The gas desorption could make the "hump" more profound. The Knudsen diffusion and slippage effect play a dominant role in the later production time. Maximizing the fracture complexity through generating large connected networks is an effective way to increase shale gas production.