Amr I. Abdel-Fattah

Storage and separation applications of nanoporous metal–organic frameworks

This Highlight explores the progress and perspective in studies of metal–organic frameworks (MOFs), a new class of nanoporous materials, particularly suited for storage and separation applications related to energy utilization and environmental remediation. Since the discovery of the first MOF compound, hundreds of different MOFs have been developed and reported. MOFs are generally synthesized by self-assembly of metal ions/clusters as coordination centers and organic ligands as linkers. They possess intriguing chemical and physical properties and are structurally tunable, thermally stable and mechanically sound. MOFs are increasingly proving to be a superior class of materials for state-of-the-art applications in crystal engineering, chemistry, and materials science. In this Highlight, we present general routes for MOFs synthesis, discuss reticular design of their pore structures, and show some of their remarkable applications, especially in the areas of storage and separation.

On colloidal particle sorption onto a stagnant air–water interface

Abstract This paper reviews reported experimental investigations of colloidal particle sorption onto a stagnant air–water interface and presents the results of some new work performed by the authors. A general consensus has emerged, based on the reported data, that particle sorption onto the interface is irreversible. Such irreversibility has been attributed to a capillary trapping caused by a net negative change in the interfacial energies, associated with transferring a fully immersed particle to the interface. The colloidal particle sorption onto an air–water interface depends on the energy barriers between sorbing particles and the interface and between particles in the bulk solution. The heights of these energy barriers, and the separation distances at which they occur, have been determined, based on the DLVO theory, after incorporating the effects of the solvation zone around particles and of the air–water interfacial region in calculating the van der Waals interaction energies.

Dispersion Stability and Electrokinetic Properties of Intrinsic Plutonium Colloids: Implications for Subsurface Transport

Subsurface transport of plutonium (Pu) may be facilitated by the formation of intrinsic Pu colloids. While this colloid-facilitated transport is largely governed by the electrokinetic properties and dispersion stability (resistance to aggregation) of the colloids, reported experimental data is scarce. Here, we quantify the dependence of ζ-potential of intrinsic Pu(IV) colloids on pH and their aggregation rate on ionic strength. Results indicate an isoelectric point of pH 8.6 and a critical coagulation concentration of 0.1 M of 1:1 electrolyte at pH 11.4. The ζ-potential/pH dependence of the Pu(IV) colloids is similar to that of goethite and hematite colloids. Colloid interaction energy calculations using these values reveal an effective Hamaker constant of the intrinsic Pu(IV) colloids in water of 1.85 × 10(-19) J, corresponding to a relative permittivity of 6.21 and refractive index of 2.33, in agreement with first principles calculations. This relatively high Hamaker constant combined with the positive charge of Pu(IV) colloids under typical groundwater aquifer conditions led to two contradicting hypotheses: (a) the Pu(IV) colloids will exhibit significant aggregation and deposition, leading to a negligible subsurface transport or (b) the Pu(IV) colloids will associate with the relatively stable native groundwater colloids, leading to a considerable subsurface transport. Packed column transport experiments supported the second hypothesis.

Peristaltic particle transport using the Lattice Boltzmann method

Peristaltic transport refers to a class of internal fluid flows where the periodic deformation of flexible containing walls elicits a non-negligible fluid motion. It is a mechanism used to transport fluid and immersed solid particles in a tube or channel when it is ineffective or impossible to impose a favorable pressure gradient or desirous to avoid contact between the transported mixture and mechanical moving parts. Peristaltic transport occurs in many physiological situations and has myriad industrial applications. We focus our study on the peristaltic transport of a macroscopic particle in a two-dimensional channel using the lattice Boltzmann method. We systematically investigate the effect of variation of the relevant dimensionless parameters of the system on the particle transport. We find, among other results, a case where an increase in Reynolds number can actually lead to a slight increase in particle transport, and a case where, as the wall deformation increases, the motion of the particle becomes non-negative only. We examine the particle behavior when the system exhibits the peculiar phenomenon of fluid trapping. Under these circumstances, the particle may itself become trapped where it is subsequently transported at the wave speed, which is the maximum possible transport in the absence of a favorable pressure gradient. Finally, we analyze how the particle presence affects stress, pressure, and dissipation in the fluid in hopes of determining preferred working conditions for peristaltic transport of shear-sensitive particles. We find that the levels of shear stress are most hazardous near the throat of the channel. We advise that shear-sensitive particles should be transported under conditions where trapping occurs as the particle is typically situated in a region of innocuous shear stress levels.

Mobilization of Colloidal Particles by Low-Frequency Dynamic Stress Stimulation

Naturally occurring seismic events and artificially generated low-frequency (1 to 500 Hz) elastic waves have been observed to alter the production rates of oil and water wells, sometimes increasing and sometimes decreasing production, and to influence the turbidity of surface and well water. The decreases in production are of particular concern, especially when artificially generated elastic waves are applied as a method for enhanced oil recovery. The exact conditions that result in a decrease in production remain unknown. Although the underlying environment is certainly complex, the observed increase in water well turbidity after natural seismic events suggests the existence of a mechanism that can affect both the subsurface flow paths and the mobilization of in situ colloidal particles. This article explores the macroscopic and microscopic effects of low-frequency dynamic stress stimulations on the release of colloidal particles from an analog core representing an infinitesimal section along the propagation paths of an elastic wave. Experiments on a column packed with 1 mm borosilicate beads and loaded with polystyrene microparticles demonstrate that axial mechanical stress oscillations enhance the mobilization of captured microparticles. Increasing the amplitude of the oscillations increases the number of microparticles released and can also result in cyclical spikes in effluent microparticle concentration during stimulation. Under a prolonged period of stimulation, the cyclical effluent spikes coincided with fluctuations in the column pressure data and continued at a diminished level after stimulation. This behavior can be attributed to rearrangements of the beads in the column, resulting in possible changes in the void space and/or tortuosity of the packing. Optical microscopy observations of the beads during low-frequency oscillations reveal that individual beads rotate, thereby rubbing against each other and scraping away portions of the adsorbed microparticles. These results support the theory that mechanical interactions between porous matrix grains are important mechanisms in flow path alteration and the mobilization of naturally occurring colloidal particles during elastic wave stimulation. These results also point to both continuous and discrete en masse releases of colloidal particles, perhaps because of circulation cells within the packing material.

Diffusiophoretic Focusing of Suspended Colloids

Using a microfluidic system to impose and maintain controlled, steady-state multicomponent pH and electrolyte gradients, we present systems where the diffusiophoretic migration of suspended colloids leads them to focus at a particular position, even in steady-state gradients. We show that naively superpositing effects of each gradient may seem conceptually and qualitatively reasonable, yet is invalid due to the coupled transport of these multicomponent electrolytes. In fact, reformulating the classic theories in terms of the flux of each species (rather than local gradients) reveals rather stringent conditions that are necessary for diffusiophoretic focusing in steady gradients. Either particle surface properties must change as a function of local composition in solution (akin to isoelectric focusing in electrophoresis), or chemical reactions must occur between electrolyte species, for such focusing to be possible. The generality of these findings provides a conceptual picture for understanding, predicting, or designing diffusiophoretic systems.

Final report of the Peña Blanca natural analogue project

The Pena Blanca region, 50 km north of Chihuahua City, Chihuahua, Mexico, was a target of uranium exploration and mining by the Mexican government. After mining ceased in 1981, researchers became interested in this region as a study area for subsurface uranium migration with relevance to geologic disposal of nuclear waste. Many studies related to this concept were conducted at the Nopal I mine site located on a cuesta (hill) of the Sierra Pena Blanca. This site has geologic, tectonic, hydrologic, and geochemical similarities to Yucca Mountain, Nevada, a formerly proposed site for a high-level nuclear-waste repository in the unsaturated zone. The U.S. Department of Energy (U.S. DOE), Office of Civilian Radioactive Waste Management (OCRWM), sponsored studies at Nopal I in the 1990s and supported the drilling of three research wells – PB1, PB2, and PB3 – at the site in 2003. Beginning in 2004, the Pena Blanca Natural Analogue Project was undertaken by U.S. DOE, OCRWM to develop a three-dimensional conceptual model of the transport of uranium and its radiogenic daughter products at the Nopal I site.

Microscopic Behavior Of Colloidal Particles Under The Effect Of Acoustic Stimulations In The Ultrasonic To Megasonic Range

It is well known that colloid attachment and detachment at solid surfaces are influenced strongly by physico‐chemical conditions controlling electric double layer (EDL) and solvation‐layer effects. We present experimental observations demonstrating that, in addition, acoustic waves can produce strong effects on colloid/surface interactions that can alter the behavior of colloid and fluid transport in porous media. Microscopic colloid visualization experiments were performed with polystyrene micro‐spheres suspended in water in a parallel‐plate glass flow cell. When acoustic energy was applied to the cell at frequencies from 500 kHz to 5 MHz, changes in colloid attachment to and detachment from the glass cell surfaces were observed. Quantitative measurements of acoustically‐induced detachment of 300‐nm microspheres in 0.1M NaCl solution demonstrated that roughly 30% of the colloids that were attached to the glass cell wall during flow alone could be detached rapidly by applying acoustics at frequencies in t...

Low‐Frequency Dynamic‐Stress Effects On Core‐Scale Porous Fluid Flow Due To Coupling With Sub‐Pore‐Scale Particle Interactions

It has been observed repeatedly that low‐frequency (1–500 Hz) seismic stress waves can enhance oil production from depleted reservoirs and contaminant extraction from groundwater aquifers. The physics coupling stress waves to fluid flow behavior in porous media is still poorly understood. Numerous underlying physical mechanisms have been proposed to explain the observations. Core‐scale experiments were performed to investigate one of these proposed mechanisms, the coupling of dynamic stress to sub‐pore size particle (colloid) interactions with solid surfaces. This is an important mechanism because it can produce profound changes in porous matrix permeability due to either accumulation or release of natural colloids. Core‐scale porous flow experiments demonstrated that both natural (in‐situ) and artificial (injected) colloids can be released from the pores by applying dynamic stress to sandstone cores at frequencies below 100 Hz. Results are shown for release of in‐situ particles from Fontainebleau sandsto...