Stimuli-Responsive Hybrid Polymer for Enhanced Solid-Liquid Separation of Industrial Effluents.

In the current study, a novel stimuli-responsive hybrid polymer with aluminum hydroxide colloids incorporated into a cationic copolymer of N-isopropylacrylamide and N-[3-(dimethylamino)propyl]methacrylamide was synthesized to enhance the settling and filtration performance of fine clay suspensions. The conformation of the synthesized hybrid copolymer was shown to respond to changes in both temperature and pH. Compared with a cationic copolymer of similar structure without aluminum hydroxide colloids, settling and filtration rates were significantly enhanced using the hybrid copolymer, which is attributed to the synergy between the inorganic aluminum hydroxide cores and organic copolymer. While the ideal treatment protocol for the hybrid polymer involved the addition of the polymer at room temperature, followed by heating to 45 °C for enhanced settling and dewatering, the synergistic effect between colloidal cores and polymer also allowed the hybrid polymer to perform well when added at temperatures above the LCST, demonstrating the robustness of the hybrid polymer to the process environment. The ideal treatment protocol resulted in an optimal adsorption of polymer on clays before inducing a coil-globule transition to form large and dense flocs, resulting in a more porous filter cake.

Measurement and modeling on hydrodynamic forces and deformation of an air bubble approaching a solid sphere in liquids.

The interaction between bubbles and solid surfaces is central to a broad range of industrial and biological processes. Various experimental techniques have been developed to measure the interactions of bubbles approaching solids in a liquid. A main challenge is to accurately and reliably control the relative motion over a wide range of hydrodynamic conditions and at the same time to determine the interaction forces, bubble-solid separation and bubble deformation. Existing experimental methods are able to focus only on one of the aspects of this problem, mostly for bubbles and particles with characteristic dimensions either below 100 µm or above 1 cm. As a result, either the interfacial deformations are measured directly with the forces being inferred from a model, or the forces are measured directly with the deformations to be deduced from the theory. The recently developed integrated thin film drainage apparatus (ITFDA) filled the gap of intermediate bubble/particle size ranges that are commonly encountered in mineral and oil recovery applications. Equipped with side-view digital cameras along with a bimorph cantilever as force sensor and speaker diaphragm as the driver for bubble to approach a solid sphere, the ITFDA has the capacity to measure simultaneously and independently the forces and interfacial deformations as a bubble approaches a solid sphere in a liquid. Coupled with the thin liquid film drainage modeling, the ITFDA measurement allows the critical role of surface tension, fluid viscosity and bubble approach speed in determining bubble deformation (profile) and hydrodynamic forces to be elucidated. Here we compare the available methods of studying bubble-solid interactions and demonstrate unique features and advantages of the ITFDA for measuring both forces and bubble deformations in systems of Reynolds numbers as high as 10. The consistency and accuracy of such measurement are tested against the well established Stokes-Reynolds-Young-Laplace model. The potential to use the design principles of the ITFDA for fundamental and developmental research is demonstrated.

Nonmonotonous variation of DNA angular separation during asymmetric pulsed field electrophoresis.

Asymmetric pulsed field electrophoresis within crystalline arrays is used to generate angular separation of DNA molecules. Four regimes of the frequency response are observed, a low frequency rise in angular separation, a plateau, a subsequent decline, and a second plateau at higher frequencies. It is shown that the frequency response for different sized DNA is governed by the relation between pulse time and the reorientation time of DNA molecules. The decline in angular separation at higher frequencies has not previously been analyzed. Real-time videos of single DNA molecules migrating under high frequency-pulsed electric field show the molecules no longer follow the head to tail switching, ratchet mechanism seen at lower frequencies. Once the pulse period is shorter than the reorientation time, the migration mechanism changes significantly. The molecule reptates along the average direction of the two electric fields, which reduces the angular separation. A freely jointed chain model of DNA is developed where the porous structure is represented with a hexagonal array of obstacles. The model qualitatively predicts the variation of DNA angular separation with respect to frequency.

Interaction of divalent cations with basal planes and edge surfaces of phyllosilicate minerals: muscovite and talc.

Smooth basal plane and edge surfaces of two platy phyllosilicate minerals (muscovite and talc) were prepared successfully to allow accurate colloidal force measurement using an atomic force microscope (AFM), which allowed us to probe independently interactions of divalent cations with phyllosilicate basal planes and edge surfaces. The Stern potential of basal planes and edge surfaces was obtained by fitting the measured force profiles with the classical DLVO theory. The fitted Stern potential of the muscovite basal plane became less negative with increasing Ca(2+) or Mg(2+) concentration but did not reverse its sign even at Ca(2+) or Mg(2+) concentrations up to 5 mM. In contrast, the Stern potential of the muscovite edge surface reversed at Ca(2+) or Mg(2+) concentrations as low as 0.1 mM. The Stern potential of the talc basal plane became less negative with 0.1 mM Ca(2+) addition and nearly zero with 1 mM Ca(2+) addition. The Stern potential of talc edge surface became reversed with 0.1 mM Ca(2+) or 1 mM Mg(2+) addition, showing not only a different binding mechanism of talc basal planes and edge surfaces with Ca(2+) and Mg(2+), but also different binding mechanism between Ca(2+) and Mg(2+) ions with basal planes and edge surfaces.

A systematic evaluation of the role of crystalline order in nanoporous materials on DNA separation.

The role of order within a porous separation matrix on the separation efficiency of DNA was studied systematically. DNA separation was based on a ratchet mechanism. Monodisperse colloidal suspensions of nanoparticles were used to fabricate highly ordered separation media with a hexagonal close-packed structure. Doping with a second particle size yielded structures with different degrees of disorder, depending upon the volume fraction of each particle size. Radial distribution functions and orientational order parameters were calculated from electron micrographs to characterize the scale of disorder. The peak separation distance, band broadening, and separation resolution of DNA molecules was quantified for each structure. DNA separation parameters using pulsed fields and the ratchet effect showed a strong dependence on order within the porous nanoparticle array. Ordered structures gave large separation distances, smaller band broadening and better resolution than highly disordered, nearly random, porous structures. The effect dominated these three parameters when compared to the effect of pore size. However, the effect of order on separation performance was not monotonic. A small, but statistically significant improvement was seen in structures with short range order compared to those with long range order.

Determination of anisotropic surface characteristics of different phyllosilicates by direct force measurements.

To fundamentally understand the electrokinetic behavior of clay minerals, it is necessary to study the anisotropic surface charge properties of clay surfaces. In this study, two 2:1 layer natural minerals, talc and muscovite, were chosen as representatives of magnesium and aluminum phyllosilicate minerals, respectively. The molecularly smooth basal planes of both platy minerals were obtained by cleavage along the basal planes, while suitable edge surfaces were prepared by an ultramicrotome cutting technique. Silicon nitride atomic force microscopy tip was used as a probe to study the interaction forces between the tip and clay basal/edge surfaces in aqueous solutions of various pH values. The measured interaction force profiles between the tip and clay basal/edge surfaces were fitted with the classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, which allows direct determination of electrical surface potential of talc and muscovite surfaces. The surface potential of muscovite basal planes was found to be significantly more negative than the basal plane of talc, both being pH insensitive. In contrast, the surface potential of edge surfaces was highly pH-dependent, exhibiting a point of zero charge (PZC) at pH 7.5 and 8.1 for edges of muscovite and talc, respectively. The observed differences in surface potential of basal planes and edge surfaces for both talc and muscovite are closely related to their crystal structure and ionization characteristics. The protonation reactivity and the contribution of each surface group to the surface charging behavior are modeled using their protonation constants.

Flocculation kinetics and aggregate structure of kaolinite mixtures in laminar tube flow.

Flocculation is commonly used in various solid-liquid separation processes in chemical and mineral industries to separate desired products or to treat waste streams. This paper presents an experimental technique to study flocculation processes in laminar tube flow. This approach allows for more realistic estimation of the shear rate to which an aggregate is exposed, as compared to more complicated shear fields (e.g. stirred tanks). A direct sampling method is used to minimize the effect of sampling on the aggregate structure. A combination of aggregate settling velocity and image analysis was used to quantify the structure of the aggregate. Aggregate size, density, and fractal dimension were found to be the most important aggregate structural parameters. The two methods used to determine aggregate fractal dimension were in good agreement. The effects of advective flow through an aggregate's porous structure and transition-regime drag coefficient on the evaluation of aggregate density were considered. The technique was applied to investigate the flocculation kinetics and the evolution of the aggregate structure of kaolin particles with an anionic flocculant under conditions similar to those of oil sands fine tailings. Aggregates were formed using a well controlled two-stage aggregation process. Detailed statistical analysis was performed to investigate the establishment of dynamic equilibrium condition in terms of aggregate size and density evolution. An equilibrium steady state condition was obtained within 90 s of the start of flocculation; after which no further change in aggregate structure was observed. Although longer flocculation times inside the shear field could conceivably cause aggregate structure conformation, statistical analysis indicated that this did not occur for the studied conditions. The results show that the technique and experimental conditions employed here produce aggregates having a well-defined, reproducible structure.

Mechanistic study on demulsification of water-in-diluted bitumen emulsions by ethylcellulose.

In our previous study, ethylcellulose (EC), an effective, nontoxic, and biodegradable natural polymer, was found effective in dewatering water-in-diluted bitumen emulsions. In this study, the demulsification mechanism of water-in-diluted bitumen emulsions by EC is investigated. In situ experiments using a micropipet apparatus provided direct evidence on both flocculation and coalescence of water droplets in diluted bitumen by EC. The addition of EC was found to decrease naphtha-diluted bitumen-water interfacial tension significantly. At the molecular level, AFM imaging revealed disruption of the continuous interfacial films formed from surface-active components of bitumen by EC. Our study clearly indicates that the demulsification by EC is through both flocculation and coalescence of water droplets, attained by competitive adsorption of EC at the oil-water interface and disruption of the original protective interfacial films formed from the surface-active components of bitumen.

Probing surface charge potentials of clay basal planes and edges by direct force measurements.

The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.

Analysis of electroosmotic flow of power-law fluids in a slit microchannel.

Electroosmotic flow of power-law fluids in a slit channel is analyzed. The governing equations including the linearized Poisson-Boltzmann equation, the Cauchy momentum equation, and the continuity equation are solved to seek analytical expressions for the shear stress, dynamic viscosity, and velocity distribution. Specifically, exact solutions of the velocity distributions are explicitly found for several special values of the flow behavior index. Furthermore, with the implementation of an approximate scheme for the hyperbolic cosine function, approximate solutions of the velocity distributions are obtained. In addition, a generalized Smoluchowski velocity is introduced by taking into account contributions due to the finite thickness of the electric double layer and the flow behavior index of power-law fluids. Calculations are performed to examine the effects of kappaH, flow behavior index, double layer thickness, and applied electric field on the shear stress, dynamic viscosity, velocity distribution, and average velocity/flow rate of the electroosmotic flow of power-law fluids.

Study of Al(OH)3-polyacrylamide-induced pelleting flocculation by single molecule force spectroscopy.

A nanocomposite polymer, Al(OH)3-polyacrylamide (Al-PAM) hybrid, was tested as a flocculant. This novel hybrid polymer was found to induce pellet-like floccules, leading to a more effective solid-liquid separation than common polyacrylamide (PAM)-based flocculants. To understand the mechanism of Al-PAM-induced pelleting flocculation, the molecular structure of this hybrid polymer was studied using an atomic force microscope (AFM). The interactions between Al-PAM molecules and a silica surface were measured using single molecule force spectroscopy (SMFS). The Al-PAM molecules were found to have a star-like structure with Al(OH)3 colloidal particles as cores connecting PAM chains. The SMFS results showed a strong attachment of the Al(OH)3 cores to the silica surface with an adhesion force of approximately 1250 pN, in contrast to a weaker adhesion force of only approximately 250 pN for PAM chains on the silica surface. The Al-PAM-induced pelleting flocculation is attributed to its star-like structure.

Electrokinetic Phenomena in concentrated disperse systems: general problem formulation and Spherical Cell Approach.

Electrokinetic Phenomena in concentrated disperse and colloid systems have been studied employing Spherical Cell Approach for over three decades. The critical review of the advances in this area, which is conducted in the present paper, demonstrates a number of contradictions between the results reported by different authors. These contradictions are largely associated with imposition of boundary conditions at the outer boundary of the representative Spherical Cell. In order to establish a correct version of the Spherical Cell Approach, in the present paper, the theory of Electrokinetic Phenomena in concentrated suspensions is revisited by primarily focusing on the boundary conditions employed at the Spherical Cell outer boundary. To this end, a general mathematical problem is formulated for addressing the behavior of a planar layer of a macroscopically homogeneous disperse system under simultaneous influence of the pressure difference, gravitation and applied electric fields. On the basis of the general problem formulation, we present strict definitions of the kinetic coefficients which describe the system behavior. Making use of such definitions, some general relationships are rederived for the kinetic coefficients, namely, the Smoluchowski asymptotic expressions and the Onsager irreversible thermodynamic relationships. The general problem is reformulated for describing the electric, hydrodynamic and ion concentration fields inside the representative Spherical Cell. Using an original approach, a complete set of the boundary conditions is derived by employing the only assumption: the average over the disperse system volume is equal to the average over a representative Spherical Cell volume. A general method for predicting the kinetic coefficients is developed by employing the solution of the formulated problem. The developed method is combined with the method of small perturbation parameter using the normalized zeta potential. Final expressions for the kinetic coefficients are obtained while accounting for the terms proportional to zeta potential. The predictions are compared with results of other publications. On this basis, conclusions are made about the validity of different models proposed in the literature.

Single molecule force spectroscopy of asphaltene aggregates.

Asphaltene aggregation and deposition cause severe problems in nearly all phases of petroleum processing. To resolve those problems, understanding the aggregation mechanisms is a prerequisite and has attracted the interest of a great number of investigators. However, to date, the nature and extent of asphaltene aggregation remain widely debated. In the present study, we attempt to investigate asphaltene aggregation from a completely new perspective. The technique of single molecule force spectroscopy (SMFS) was used to investigate the response of single asphaltene aggregates under an external pulling force. Force curves representing the stretching of single asphaltene aggregates were obtained in simple electrolyte solutions (KCl and calcium) and organic solvents (toluene and heptane). These force curves were well-fitted by the modified worm-like chain model, indicating that those asphaltene aggregates acted like long-chain polymers under pulling by an external force. It was found that lower solution pH values and the presence of divalent cations resulted in a lower bending rigidity of the formed aggregates. The information retrieved from the force curves suggests that asphaltene molecules with a structure featuring small aromatic clusters connected by aliphatic chains do exist and that asphaltene aggregation could occur through a linear polymerization mechanism. The current study extends the application scope of SMFS.

Stability and disintegration of ultrathin heptane films in water: molecular dynamics simulations.

Molecular dynamics simulations of ultrathin heptane films (less than 5 nm in thickness) in water were conducted to study their stability and disintegration behavior. The density distributions of heptane and water molecules across the film were determined for different equilibrium film thicknesses ranging from 1.5 to 4 nm. The potential energy of the system was computed as a function of the heptane number fraction, and the results were employed to determine the excess energy of mixing of heptane in water. The diffusion coefficients of heptane and water obtained from the MD simulations were also compared with experimental data. A good agreement was found between the heptane self-diffusivity obtained from the MD simulations and its literature reported value. Following an analysis of the equilibrium properties of the heptane films and associated structures, we performed simulations where the shapes of the heptane films were initially perturbed. Different perturbations of these ultrathin films led to formation of various associated structures, including cylindrical rodlike heptane droplets, films with holes, and intact films. The different shapes are formed in systems with the same heptane/water composition. An analysis of this behavior is presented showing the possibility of multiple associated structures with similar total energy in these highly confined systems.

Particle deposition onto charge heterogeneous surfaces: convection-diffusion-migration model.

The role of surface charge heterogeneity of planar collectors on particle deposition and distribution was investigated in the vicinity of a heterogeneous surface for a radial impinging jet flow geometry. The charge heterogeneity was modeled as concentric circular stripes bearing different surface charges. Particle deposition was studied employing the Eulerian approach (convection-diffusion-migration equation). It was observed that when the collector was completely unfavorable, the presence of small amounts of charge heterogeneity in the form of a small fraction of favorably charged stripes enhanced the deposition rate substantially. In contrast, when the collector was completely favorable, the presence of small amounts of charge heterogeneity in the form of a small fraction of unfavorably charged stripes did not affect the particle deposition rate significantly.

Colloidal interactions between Langmuir-Blodgett bitumen films and fine solid particles.

In oil sand processing, accumulation of surface-active compounds at various interfaces imposes a significant impact on bitumen recovery and bitumen froth cleaning (i.e., froth treatment) by altering the interfacial properties and colloidal interactions among various oil sand components. In the present study, bitumen films were prepared at toluene/water interfaces using a Langmuir-Blodgett (LB) upstroke deposition technique. The surface of the prepared LB bitumen films was found to be hydrophobic, comprised of wormlike aggregates containing a relatively high content of oxygen, sulfur, and nitrogen, indicating an accumulation of surface-active compounds in the films. Using an atomic force microscope, colloidal interactions between the LB bitumen films and fine solids (model silica particles and clay particles chosen directly from an oil sand tailing stream) were measured in industrial plant process water and compared with those measured in simple electrolyte solutions of controlled pH and divalent cation concentrations. The results show a stronger long-range repulsive force and weaker adhesion force in solutions of higher pH and lower divalent cation concentration. In plant process water, a moderate long-range repulsive force and weak adhesion were measured despite its high electrolyte content. These findings provide more insight into the mechanisms of bitumen extraction and froth treatment.

Spherical cell approach for the effective viscosity of suspensions.

In the present paper, the spherical cell approach is employed for addressing the effective viscosity of suspensions of spherical particles. The proposed derivation is based on the only assumption which constitutes the essence of the spherical cell approach: a representative part of the suspension is a spherical cell which contains a particle surrounded by the continuous phase. In contrast with the previous studies on this topic, no additional assumptions are used in the present analysis. The general method of derivation and the final result, which represents the effective viscosity as a function of the solid-phase volume fraction, are compared with earlier studies where the spherical cell approach was applied for describing the effective viscosity.

Adhesion of single polyelectrolyte molecules on silica, mica, and bitumen surfaces.

In a recent study (Energy Fuels 2005, 19, 936), a partially hydrolyzed polyacrylamide (HPAM) was used as a process aid to recover bitumen from oil sand ores. It was found that HPAM addition at the bitumen extraction step not only improved bitumen recovery but also enhanced fine solids settling in the tailings stream. To understand the role of HPAM, single-molecule force spectroscopy was employed for the first time to measure the desorption/adhesion forces of single HPAM molecules on silica, mica, and bitumen surfaces using an atomic force microscope (AFM). Silicon wafers with an oxidized surface layer and newly cleaved mica were used, respectively, to represent sand grains and clays in oil sands. The force measurements were carried out in deionized water and in commercial plant process water under equilibrium conditions. The desorption/adhesion forces of HPAM obtained on mica, silica, and bitumen surfaces were approximately 200, 40, and 80 pN in deionized water and approximately 100, 50, and 40 pN in the plant process water, respectively. The measured adhesion forces together with the zeta potential values of these surfaces indicate that the polymer would preferentially adsorb onto clay surfaces rather than onto bitumen surfaces. It is the selective adsorption of HPAM that benefits both bitumen recovery and tailings settling when the polymer was added directly to the bitumen extraction process at an appropriate dosage.

Particle deposition onto micropatterned charge heterogeneous substrates: trajectory analysis.

A trajectory analysis of particles near a micropatterned charged substrate under radial impinging jet flow conditions is presented to investigate the effect of surface charge heterogeneity on particle trajectory and deposition efficiency. The surface charge heterogeneity is modeled as concentric bands of specified width and pitch having positive and negative surface potentials. The flow distribution is obtained using finite element analysis of the governing Navier-Stokes equations. The particle trajectory analysis takes into consideration the hydrodynamic interactions, gravity, van der Waals and electrostatic double layer interactions. The presence of surface charge heterogeneity on the substrate gives rise to an oscillating particle trajectory near the collector surface due to repulsive and attractive forces. As a result of the coupled effects of hydrodynamic and colloidal forces, the particle trajectories and deposition efficiencies are increasingly affected by surface charge heterogeneity as one moves radially away from the stagnation point. The results indicate that it is possible to render collectors with up to 50% favorable surface fraction completely unfavorable by modifying the ratio of the radial to normal fluid velocity. Utilizing the real favorable area fraction of the collector, the patch model expression for calculating the deposition efficiency is modified for impinging jet flow geometry.