Energy-based model for capillary spreading of power-law liquids on a horizontal plane

Separation of emulsified crude oil from produced water by gas flotation: A review

The development and production of oil and gas fields would eventually result in a considerable amount of oily generated water, posing serious risks to humans and the environment. Nowadays, the oil concentration in the drainage stream of the produced water is strictly regulated, and many countries have established strict emission standards. As an indispensable oily wastewater treatment technology, flotation technology has attracted much attention because of its maturity, economy, practicality, and relative efficiency. Firstly, this paper summarizes and compares flotation techniques, such as dissolved gas flotation, induced gas flotation, electroflotation, and compact flotation units widely used in produced water treatment offshore in recent years. Considering the complexity of the mechanism of oil removal by air flotation, the mechanism of the oil droplet-bubble interaction is further discussed. The effects of flocculant, PH, and salinity on the oil droplet-bubble interaction in the flotation process were summarized from the perspective of the microscopic colloidal interface, which has a specific guiding role in improving the oil removal efficiency in the gas flotation process. Finally, the research status of produced water treatment by air flotation is summarized, and the feasible research direction is put forward.

Comparative Study on the Spreading Behavior of Oil Droplets over Teflon Substrates in Different Media Environments

This paper comparatively investigated the spreading process of an oil droplet on the surface of highly hydrophobic solid (Teflon) in air and water media using a high-speed imaging technology, and analyzed their differences in spreading behavior from the perspective of empirical relations and energy conservation. Furthermore, the classical HD and MKT wetting models were applied to describe the oil droplet spreading dynamics to reveal the spreading mechanism of oil droplets on the Teflon in different media environments. Results showed that the entire spreading process of oil droplets on Teflon in air could be separated into three stages: the early linear fast spreading stage following θ(t)=θ0+kt , the intermediate exponential slow spreading stage obeying θ(t)=bt−3α, and the late spreading stage described by θ(t)=θeq+a×exp(−t/T). However, the dynamics behavior of dynamic contact angle during the oil droplet spreading on Teflon in water could be well described by these expressions, θ(t)=θ0+kt and θ(t)=θeq+a×exp(−t/T). Clearly, a significant difference in the oil droplet spreading behavior in air and water media was found, and the absence of the intermediate exponential spreading stage in the oil–water–Teflon system could be attributed to the difference in the dissipated energy of the system because the dissipation energy in the oil–water–solid system included not only the viscous dissipation energy of the boundary layer of oil droplet, but also that of the surrounding water which was not included in the dissipation energy of the oil–air–solid system. Moreover, the quantitative analysis of wetting models suggested that the MKT model could reasonably describe the late spreading dynamics of oil droplets (low TPCL velocities), while the HD model may be more suitable for describing the oil droplet spreading dynamics at the early and intermediate spreading stages (high TPCL velocities).

Effect of Moving Contact Line's Curvature on Dynamic Wetting of non-Newtonian Fluids

The curvature of the contact line is always changing with the dynamic wetting condition. Using a modified Wilhelmy plate method and the sessile drop method, this study experimentally investigated the dynamic wetting process of several kinds of Newtonian and non-Newtonian fluids. The results show that the curvature of the moving contact line strongly affects the relationship θ_D = f( U) for non-Newtonian fluids but has no effect on Newtonian fluids. The effect is more obvious with the stronger non-Newtonian fluids. The theoretical relationship derived from the Navier-Stokes equations established for spontaneous spreading indicates that the moving contact line curvature affects the relationship θ_D = f( U) for shear-thinning fluids and shear-thickening fluids in a different way, which agrees with the forced wetting experimental results for shear-thinning fluids in both this work and the previous one on the fluid showing shear-thickening rheology. A force balance relation of the braking force and driving force for the moving contact line is used to explain the internal mechanism about how the curvature of the contact line affects θ_D during wetting process.

Dynamic Spreading of Droplets on Lyophilic Micropillar-Arrayed Surfaces

The wetting kinetics of droplets on lyophilic pillar-arrayed substrates is the driving mechanism of several natural phenomena (e.g., insect capturing by Nepenthes) and many industrial technologies (e.g., gas-liquid separation). For a lyophilic pillar-arrayed surface, a fringe film is formed ahead of the contact line, resulting in distinct wetting kinetics, which needs further investigation. In this study, Si(100) substrates with square micropillars were used to investigate the early spreading of droplets on lyophilic pillar-arrayed surfaces through the droplet-spreading method. A fringe film was observed ahead of the contact line for micropillar-arrayed surfaces. The spreading radius was enhanced by micropillars and mainly caused by liquid penetration into the pillar forest, resulting in alteration of the dissipation mechanism. The early spreading of droplets on lyophilic micropillar-arrayed surface was affected only by the solid fraction and independent of the pillar height. A semitheoretical model without adjustable parameters was established on the basis of the global energetic equation, considering the local dissipation, viscous dissipation, and the dissipation in the precursor film. The prediction of the model agrees with the experimental results. Our semitheoretical model may aid in predicting the wetting kinetics on lyophilic pillar-arrayed substrates and assist the design of pillar-arrayed surfaces in practical applications.

Topography- and topology-driven spreading of non-Newtonian power-law liquids on a flat and a spherical substrate

The spreading of a cap-shaped spherical droplet of non-Newtonian power-law liquids on a flat and a spherical rough and textured substrate is theoretically studied in the capillary-controlled spreading regime. A droplet whose scale is much larger than that of the roughness of substrate is considered. The equilibrium contact angle on a rough substrate is modeled by the Wenzel and the Cassie-Baxter model. Only the viscous energy dissipation within the droplet volume is considered, and that within the texture of substrate by imbibition is neglected. Then, the energy balance approach is adopted to derive the evolution equation of the contact angle. When the equilibrium contact angle vanishes, the relaxation of dynamic contact angle θ of a droplet obeys a power-law decay θ∼t^{-α} except for the Newtonian and the non-Newtonian shear-thinning liquid of the Wenzel model on a spherical substrate. The spreading exponent α of the non-Newtonian shear-thickening liquid of the Wenzel model on a spherical substrate is larger than others. The relaxation of the Newtonian liquid of the Wenzel model on a spherical substrate is even faster showing the exponential relaxation. The relaxation of the non-Newtonian shear-thinning liquid of Wenzel model on a spherical substrate is fastest and finishes within a finite time. Thus, the topography (roughness) and the topology (flat to spherical) of substrate accelerate the spreading of droplet.

Spreading law of non-Newtonian power-law liquids on a spherical substrate by an energy-balance approach

The spreading of a cap-shaped spherical droplet of non-Newtonian power-law liquids, both shear-thickening and shear-thinning liquids, that completely wet a spherical substrate is theoretically investigated in the capillary-controlled spreading regime. The crater-shaped droplet model with the wedge-shaped meniscus near the three-phase contact line is used to calculate the viscous dissipation near the contact line. Then the energy balance approach is adopted to derive the equation that governs the evolution of the contact line. The time evolution of the dynamic contact angle θ of a droplet obeys a power law θ∼t^{-α} with the spreading exponent α, which is different from Tanner's law for Newtonian liquids and those for non-Newtonian liquids on a flat substrate. Furthermore, the line-tension dominated spreading, which could be realized on a spherical substrate for late-stage of spreading when the contact angle becomes low and the curvature of the contact line becomes large, is also investigated.

Kinetics of Wetting and Spreading of Droplets over Various Substrates

There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and spreading of non-Newtonian liquid (blood) over porous substrates. We showed that in spite of the enormous complexity of blood, the spreading over porous substrate can be described using a relatively simple model: a power low-shear-thinning non-Newtonian liquid. (iv) The kinetics of spreading of surfactant solutions. In this part, new results related to various surfactant solution mixtures (synergy and crystallization) are discussed, which shows some possible direction for the future revealing of superspreading phenomena. (v) The kinetics of spreading of surfactant solutions over hair. Fundamental problems to be solved are identified.

The effects of manganese ions on rheology and thermal properties of polyether ether ketone

Polyether ether ketone (PEEK) mixed with manganese (Mn) of different valences was prepared to simulate conditions of impurity. The melt flow rate (MFR) and rheological behaviour were systematically studied to characterize the mobility from different angles. MFR test was used for the preliminary exploration of mixtures of Mn ions. The results showed that Mn, Mn(II), and Mn(IV) had little impact on the mobility of PEEK contrary to Mn(VII) and that long chain branching or cross-linking was suspected. Mn(VII) was determined to be the main object of this research. Capillary rheometer test demonstrated that the apparent shear viscosity constantly reduced with the increase of apparent shear rate, and that the addition of Mn(VII) could worsen the mobility of PEEK. Similarly, parallel plate rheometer studies proved that the complex viscosity increased along with the content of Mn(VII) and LCB was confirmed. Thermal analysis revealed that LCB occurred internally and that thermal stability was worse.

A Critical Review of Dynamic Wetting by Complex Fluids: From Newtonian Fluids to Non-Newtonian Fluids and Nanofluids

Dynamic wetting is an important interfacial phenomenon in many industrial applications. There have been many excellent reviews of dynamic wetting, especially on super-hydrophobic surfaces with physical or chemical coatings, porous layers, hybrid micro/nano structures and biomimetic structures. This review summarizes recent research on dynamic wetting from the viewpoint of the fluids rather than the solid surfaces. The reviewed fluids range from simple Newtonian fluids to non-Newtonian fluids and complex nanofluids. The fundamental physical concepts and principles involved in dynamic wetting phenomena are also reviewed. This review focus on recent investigations of dynamic wetting by non-Newtonian fluids, including the latest experimental studies with a thorough review of the best dynamic wetting models for non-Newtonian fluids, to illustrate their successes and limitations. This paper also reports on new results on the still fledgling field of nanofluid wetting kinetics. The challenges of research on nanofluid dynamic wetting is not only due to the lack of nanoscale experimental techniques to probe the complex nanoparticle random motion, but also the lack of multiscale experimental techniques or theories to describe the effects of nanoparticle motion at the nanometer scale (10(-9) m) on the dynamic wetting taking place at the macroscopic scale (10(-3) m). This paper describes the various types of nanofluid dynamic wetting behaviors. Two nanoparticle dissipation modes, the bulk dissipation mode and the local dissipation mode, are proposed to resolve the uncertainties related to the various types of dynamic wetting mechanisms reported in the literature.

Induction and coverage times for crude oil droplets spreading on air bubbles

The interactions between crude oil droplets and air bubbles were studied by the droplet-bubble micromanipulator technique. Eight crude oils were investigated, and some aspects of the involved mechanisms were discussed. The induction time was measured for air bubbles approaching crude oil droplets in different aqueous phases. Distinct differences were observed in the presence and absence of salts, which showed the importance of long-ranged electrostatic repulsive forces on thin-film stability. The results also suggested that adsorption of dissolved hydrocarbons at air bubble surfaces may increase the potential energy barrier in the thin liquid film. Furthermore, the time needed for crude oil droplets to spread over the air bubble surfaces (referred to as coverage time) was determined for the crude oils. The results showed that the spreading velocity decreased with increasing viscosity of the crude oil. The detailed understanding of this type of interaction is considered to be a precursor for improving the oil removal efficiency during the flotation process.