The theme of this special issue of JFE is the flow behavior and Rheology of complex fluids and electric phenomena at the micro and nano scale. The papers are drawn from those presented at the IMECE2006 and FEDSM 2007 in related Symposia sponsored by the Fluids Engineering Division and the Materials Division organized by Dennis Siginer.
It is well known that many fluids in engineering applications, such as polymer melts and solutions, muds and drilling fluids in petroleum industry, food products, cosmetics, paints and others, present non-Newtonian behavior. They exhibit features such as shear-thinning or shear-thickening, normal stress differences in shearing flows, viscoplasticity, extension-hardening and memory effects due to elasticity. The first seven papers in this collection address issues at the forefront of this area. The next set of five papers explores aspects of the increasingly important applications of electrical effects both at the macro and micro level such as electro-chemical machining, electrowetting and dielectrophoresis, and the last set of three papers look at cold spray, a very promising new coating technique, and the Ludwig-Soret effect which plays an important role in reservoir engineering.
The rate of material processing in film casting and fiber spinning, widely used multi-purpose industrial manufacturing processes, is limited by the draw resonance instability. Beyond a critical draw ratio, the ratio of the take-up velocity to the velocity at the die exit, stable operation is impossible as the instability results in a spatio-temporal periodic variation in film thickness. Since take-up velocity is much greater than the velocity at the die exit, the conservation of mass requires change in the cross-sectional area. Thus, the film thickness is reduced to a desired value at the take-up point by choosing the appropriate draw ratio. Inertial effects on film casting have been widely ignored in the literature. Inertia can, however, have a significant effect on the stability. Radoslav German and Roger E. Khayat examine the effects of inertia, elasticity and boundary conditions on the film casting of a PTT fluid and show that inertia plays an important role in the process and has a stabilizing effect on the film-casting flow. They also demonstrate that the choice of stress boundary conditions becomes important with increasing fluid elasticity.
The study of confined and/or free surface swirling flows due to rotating top and bottom covers of cylindrical containers yields a wealth of information about the fundamental behavior of viscoelastic fluids and instabilities in strong flows. This geometry is also used in testing the validity of the predictions of constitutive equations. Shinji Tamano et al. report an experimental study of strong flows of surfactant solutions much less prone to degradation than polymeric fluids in cylindrical casings of aspect ratio one and two using a sectional flow visualization technique and a two-component laser Doppler velocimetry (LDV) system and compare their results to the behavior of polymer solutions. Perhaps more importantly they find that the popular Giesekus model cannot predict the flow structure.
Products made from liquid crystalline polymers (LCP) have many advantages over conventional polymers, higher modulus and higher heat resistance among others. It is generally thought that the alignment structure of the LCP molecules is the cause of these useful properties. However, it is difficult to control the LCP structure in a molding process, because the flow behavior of LCP strongly depends on its deformation history and there is a variety of complicated geometries used in the molding flows. Takatsune Narumi et al. report an experimental study of the unstable behavior which is manifested as wavy textures in elongational flows in a curved slit geometry with a right-angle corner in an L-shaped channel of a solution of LCP, hydroxyl-propylcellulose (HPC).
Applications of liquid thread breakup are widespread in industry such as encapsulation processes for controlled drug delivery, inkjet printing, spray drying of starches, spray painting, and emulsification. The physics of Newtonian liquid thread breakup is understood well enough. In contrast, the analysis of viscoelastic liquid threads and the breakup of purely viscous, shear-thinning non-Newtonian liquid threads are still in their infancy. While similarity solutions explain the singularity of the jet at breakup, they are unable to predict drop sizes or drop shapes in shear-thinning jets. To address this limitation V. Dravid et al. report the effect of power-law shear-thinning behavior when the filament is two-dimensional and axisymmetric by solving the entire Navier–Stokes equations at Re=5 and compare model predictions against experimental data obtained using jets of power-law non-Newtonian fluids exiting from capillary tubes. Drop formation from these capillary jets is captured using a high speed digital camera. They report good agreement.
Surfactant solutions are extensively used in many industrial processes, examples of which are foaming, jet printing, emulsification and coating. The practical importance of surfactants is based on the ability of these molecules to quickly reach an equilibrium state at the freshly created solution/air interface, thus decreasing the surface tension from the value of bulk solution to the equilibrium value at the surface (DST). Hasegawa et al. propose a new model based on the concept that surfactant molecules rotate during the process of reaching the equilibrium surface state. This is different from the conventional adsorption theory. They obtain a simple expression of DST as a function of the surface age. In addition, an experiment is carried out to determine DST by measuring the period and weight of droplets falling from a capillary. The predictions of the proposed model are compared to their own experimental data and to those reported previously by several other authors, and good agreement is shown. Furthermore, the characteristic time in the model is shown to be correlated with the concentrations of solutions regardless of the type of solution examined.
Welan gum, a commercially available highly stable biopolymer, is extensively pumped through straight and coiled tubing in various petroleum installations. It is suitable for drag reduction and viscosity enhancement in many oil and gas production operations including hydraulic fracturing, acidizing, wellbore cleanup, cementing and drilling. Fluid flow behavior in coiled tubing differs significantly from that in straight tubing. Asubiaro and Shah report an extensive experimental investigation of the behavior of Welan gum suspensions of different concentrations in straight and coiled tubing and develop correlations for the friction factor.
Computational methods to solve non-Newtonian flow problems are afflicted by numerical instabilities and spurious oscillations. In the case of purely viscous inelastic fluids highly steep and non-smooth viscosity models are the cause of locally advection dominated regions and severe gradients. Zinani and Frey present a Galerkin least-squares (GLS) multi-field finite element formulation for extra-stress, velocity and pressure as primal variables for the approximation of inelastic non-Newtonian fluid flows. GLS enhances the stability of the classical Galerkin approximation for incompressible flows and also circumvents compatibility between the approximation functions of stress, velocity and pressure.
Electro-Chemical Machining (ECM) is an advanced machining technology widely used in aerospace, defense and medical industries among others. The applications of ECM in the automobile and turbo-machinery manufacturing also have been on the rise because ECM has no tool wear and difficult-to-cut metal parts and complex geometries can be machined with relatively high accuracy and extremely smooth surfaces. Fujisawa et al. develop a multi-physics model and the associated numerical procedure to predict the ECM process. The model and the numerical procedure satisfactorily simulated a typical ECM process for a two-dimensional flat plate and for a three-dimensional compressor blade.
Light-emitting diodes (LED) are an attractive alternative to incandescent bulbs and fluorescent lamps. The typical life span of LEDs is about ten years, twice as long as fluorescent lamps and approximately twenty times longer than the incandescent bulbs. They generate much less thermal energy than incandescent bulbs with the same light output and are free of environmental pollutants such as neon, helium, and argon discharged from fluorescent lamps. Lighting modules with combination of red, green, and blue LEDs can emit light of an intended color without additional color filters that traditional lighting methods adopt. Kim et al. investigate a combination of discrete red, green, and blue LEDs to realize a high efficacy white LED. Compared to red and blue LEDs, green LED leaves much more room for improvement in luminescent efficacy. The production of green LEDs by metal organic vapor phase epitaxy (MOVPE) is closely related to the ability to grow InGaN/GaN multi-quantum-wells (MQWs) with high indium compositions. Kim et al. report their study of the characterization of three different commercial MOVPE reactors for performance enhancement.
The manipulation of discrete droplets has seen rapid development from engineering to life sciences including variable focus lenses, display technology, fiber-optics, and lab-on-a-chip devices. Accurate description of actuation forces and resultant droplet velocities must be available when designing an integrated device making use of discrete flows. Currently the leading methods for electrostatically actuating microdroplets in microfluidic devices are electrowetting on dielectric (EWOD) for conductive droplets and dielectrophoresis (DEP) for electrically insulating droplets. In each case, a transverse electric field is used to create an electrostatic pressure giving rise to the transport of individual liquid slugs. Young and Mohseni examine the nature of the force distribution for both EWOD and DEP actuated droplets and the effect of system parameters such as contact angle and electrode length on the shape of the force density.
Micro-pumps have many applications in medical devices, portable fuel cells and electronic cooling among others. Mechanical micro-pumps with moving parts are subject to high friction wear, generate noise and are not appropriate for applications with high pressure requirements such as chromatography and micro-cooling of power electronics. In contrast electro-kinetic (EK) pumps, which convert electrical energy into kinetic energy in the fluid do not suffer from these disadvantages. One of the most popular EK pumps is the electro-osmotic (EO) pump. It is very cheap to fabricate in comparison to other EK pumps and can generate high pressure with a small volume substrate. Nevertheless, because the shear stress is confined to a thin Debye layer, EO pumps suffer from high viscous dissipation and exhibit a very low thermodynamic efficiency like all other EK pumps. In addition, Faradaic reactions at the electrodes generate bubbles by electrolysis and change the pH of the working fluid limiting the continuous operation of EO pumps to a few hours. Berrouche et al. present a new theory for optimizing the thermodynamic efficiency of an EO pump with a large surface area and highly charged nano-porous silica disk substrate.
Dielectrophoresis (DEP) and traveling wave dielectrophoresis (twDEP) are very effective AC electrokinetic techniques for the manipulation and separation of particles in micro- and nano-fluidics for instance biological particles such as DNA, cells, and bacteria. AC field suppresses undesirable electrolytic effects such as Faradaic reactions and electro-convection in the liquid, and employs polarization forces that are insensitive to the particle charge making dielectrophoresis a vastly superior method. Song and Bennett solve the electric potential equation with mixed type of boundary conditions for dielectrophoresis and traveling wave dielectrophoresis generated by an interdigitated parallel electrode array.
The cold spray is a novel and promising coating technology and was originally developed in the mid-1980s at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk. In the conventional cold spray process, powder particles are accelerated through the momentum transfer from the supersonic gas jet. The temperature of supersonic gas jet is always lower than the melting point of the powder material. Thus the coating is formed from the particles in solid state and the problems associated with traditional thermal spray methods are eliminated. The adhesion of the particles in a cold spray process occurs only when the kinetic energy is large enough to cause their extensive plastic deformation at the contact surface. Takana et al. explore the use of electrostatic force to assist the acceleration of the particles in addition to the acceleration imparted by supersonic flow, and Samareh and Dolatabadi examine the effect of the presence of a dense particulate flow on the supersonic gas.
Ludwig-Soret effect refers to the phenomenon of component separation in a convection free liquid or gaseous mixture under a temperature gradient. This separation mechanism is of importance in petroleum engineering applications as it can in tandem with natural convection greatly influence the composition distribution in hydrocarbon reservoirs. Jaber et al. report a numerical study based on a non-equilibrium irreversible thermodynamic model of the Soret effect for a ternary mixture in a porous cavity.
In closing I would like to convey my thanks and appreciation to the authors and the anonymous reviewers for their contributions to this special issue.