Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing OPEN ACCESS

This collection of selected papers is drawn from those presented at the IMECE 2005 in Orlando, Florida at the Symposia on “Rheology & Fluid Mechanics of Non-Linear Materials” and “Electric and Magnetic Phenomena in Micro- and Nano-Scale Systems” sponsored by the Fluids Engineering Division and the Materials Division as well as the Symposium on “Flows in Manufacturing Processes” held at the 2nd Joint US-European Fluids Engineering Summer Meeting in Miami, Florida in July 2006.

The first group of eight papers in this collection investigates electric and magnetic effects in flows at the micro and nano scales. The next group of seven papers on flows in manufacturing processes and complex industrial flows at the macro scale deals with issues relevant to paper making, coating, mixing, high-speed water jets, drag reduction, and heat transfer, respectively.

At very small scales, electric and magnetic field effects, such as dielectrophoresis and electro-osmosis are gaining greater focus as important tools in flow control, and magneto-electro-micro-fluidics is increasingly viewed as one of the most promising techniques for separating, controlling, and manipulating fine particles, cells, and micro-organisms in flowing suspensions. The issue is gaining in magnitude as sophisticated micro-fabrication technologies make miniaturization of a range of analytical devices possible, all requiring effective actuation mechanisms. Effective and rapid mixing of liquids in small scale devices is essential in many applications, such as hand-held pollution monitoring devices, drug delivery, DNA analysis and sequencing, pheromone synthesis in micro bio-reactors, and biological and chemical agent detection. However, mixing in micro-devices is a challenge as low Reynolds number flows encountered in these systems are in general not turbulent, and diffusion dominates mixing processes. Improving the efficiency of mixing through diffusion alone in the absence of turbulence is impractical. For instance, the diffusion coefficients of large molecules such as proteins and DNA are of the order of $O(10−10)m2∕s$ and even smaller. Hence, the mixing time can be prohibitively long, not to speak of the mixing length. A highly efficient fast micro-level mixer will greatly benefit a number of critical applications, such as immunological studies, DNA hybridation, and cytometric analysis. In this context, flow manipulation through judicious use of magnetic and electric fields or with actuators and internal micro-pumps becomes a central issue in applications. It should be noted that the bulk motion of electrolyte solutions driven by electric fields, electro-osmotic flows, requires large electrostatic potentials. In contrast, magnetic field driven effects do not, and may be preferable in some applications.

Two novel ways to achieve effective flow manipulation are through the organized motion of particles in a controllable manner and through the phenomenon of electrowetting. In the former, the motion of suspended magnetic or paramagnetic particles are driven by the application of magnetic fields, leading to the formation of controllable self assembling aggregates. The latter takes advantage of the dominance of surface tension at the micro scale and utilizes the modulation of surface tension as an effective actuation mechanism in micro-devices. Electrowetting was first advanced by Berge in France in the early 1990s. It is based on the modification of the wetting property of a liquid droplet resting on a dielectric surface by an external electric field set up when a voltage is applied between the liquid droplet and a counter-electrode beneath the solid dielectric layer. Low voltage, large actuation amplitude, and high reversibility make electrowetting a promising actuation technique for MEMS devices. Micro-pumps driven by electro-hydrodynamic and electro-osmotic effects are also the focus of intensive research to achieve effective mixing and flow manipulation. They tend to have a simpler design and are easily controllable, as opposed to diaphragm micro-pumps, which are more complicated actuation mechanisms and are affected by fatigue. These issues are investigated in the first four papers. The dynamic self-assembly of neutrally buoyant particles rotating in a plane in a viscous fluid driven by a magnetic field is studied by Climent et al.  The electrowetting actuation of droplets of mercury on dielectric insulation films is the focus of the investigation of Wan et al.  The characteristics of a Y-form hybrid electro-kinetic-passive micro-mixer are presented in the work of Wang et al.  The concept and operation of a novel electro-hydrodynamic menisci pump is introduced by Herescu and Allen.

Magnetic fluids are suspensions of fine stable single domain magnetic particles in non-conducting fluids. They are not found in nature and must be artificially synthesized. The particles are coated with a surfactant and Brownian motion keeps the fine particles from settling under gravity. They lead to innovative applications and have been used in seals, as dampers in steeper motors and shock absorbers, to cool loud-speaker coils, as a lubricant in various machines, and in non-invasive circulatory measurements as a tracer of blood flow. There are two distinct families of magnetic fluids: ferrofluids and magnetorheological (MR) fluids. Conventional MR fluids date back to their discovery in 1948 by Rabinow right after electrorheological fluids in 1947. Ferrofluids are colloidal suspensions in a non-conducting liquid carrier whose surfactant stabilized and permanently magnetized magnetite particle constituents are $10–20nm$ in size. In some ways they behave like a paramagnetic gas of high permeability. The Cauchy body stress field is asymmetric and the medium is anisotropic through magnetization. A general theory for ferrofluids with internal rotation and vortex viscosity, particle-particle interaction, magnetization relaxation, and couple stresses has been only recently formulated by Rosensweig in 2004. In contrast, magnetorheological fluids are suspensions of micrometer-sized magnetic particles dispersed in a carrier fluid such as a mineral or silicon oil. Rheological and thermal transport properties of MR fluids can be changed reversibly through the application of an external magnetic field, which explains their appeal as fluid clutches for automotive and sports equipment applications. The formation of chainlike structures under a strong magnetic field increases the thermal conductivity of the MR fluid several-fold, thus dissipating the heat energy resulting from viscous work. Due to their constitutive structure, the response of a MR fluid to a magnetic field is quite different from that of a ferrofluid. A new generation of MR fluids that offer potentially unparalleled performance is based on carbon nano-tubes (CNTs) magnetized through hybridization with other magnetic materials either encapsulated or incorporated within the walls or deposited on the outer surface of the nano-tube. Successful magnetization makes CNT manipulation possible at very small scales by magnetic actuation, thus creating a new class of MR fluids that incorporates the highly desirable electrical, thermal, and mechanical properties of CNTs. The papers by Chaves et al.  and Lloyd et al.  investigate aspects of ferrofluids and MR fluids, respectively, and the work of Samouhos and McKinley explores the properties and feasibility of CNT based MR fluids. A potentially promising application of strong magnetic fields concerns oxygen separation from air. Evidence indicates that the efficiency of fuel cells is increased by pure oxygen or oxygen enriched air. In the last paper concerning magnetic effects, Asako and Suzuki study by Monte Carlo simulation the characteristics of oxygen separation and enrichment from atmospheric air.

The turbulent flow of the flexible fiber suspension in the paper-making production line determines the distribution, the orientation, and the aggregation of the fibers in the suspension, thus affecting the properties and the quality of the final product. The mechanical properties of the manufactured paper are strongly dependent on the fiber orientation and on the degree of the uniformity of the distribution of cellulose fibers suspended in water with mass concentration typically below 1% used in paper making. The word “floc” refers in industry terminology to groups of fibers clumped together, which may break and self-assemble again during the course of processing depending on whether they are hard or soft flocs. Flocculation of fibers is widely observed in paper-making industries, and floc strength and size as a function of added chemicals and of applied shear are pivotal factors in designing the process. Turbulence is the most important mechanism behind the formation and destruction of fiber flocs. Knowledge of the flexible fiber suspension behavior, including fluid-fiber and fiber-fiber interaction, and the physics of floc formation and destruction are very helpful in designing any unit operation in the paper-making process. In addition, the lamellas used in the head box to damp out turbulence create shear layers affecting the orientation and the concentration of the fibers, which, close to the solid boundary are likely to pole vault and perform Jeffrey orbits. The papers dedicated to paper-making issues in this collection present dynamic simulations of viscoelastic fibers, and introduce a multiscale numerical simulation of the turbulent suspension flow using a hybrid method between direct numerical simulations and large eddy simulations that takes into account fibrous structure interactions (Arezou et al. ), and an experimental investigation of the orientation and concentration of fibers suspended in a shear flow over a solid wall (Carlsson et al. ).

Industrial processes such as paper and photographic film manufacturing, wire coating, and the iron and steel industries use a coating technique based on the deposition of a very thin liquid film on a solid substrate. Hot-dip galvanization is widely used in steel industry to coat steel strips with a thin layer of zinc against oxidation. Coating thickness and uniformity is controlled by the jet wiping technique also referred to as air-knife coating. The technique based on two-dimensional high-speed gas jets impinging on the liquid layer on the moving substrate yields a thin and even coating of constant thickness. However, an instability called splashing, which occurs downstream of the gas jet nozzles, limits productivity increases by limiting the strip speed. Gosset and Buchlin present a new analysis of air-knife coating and derive the optimum conditions to improve the efficiency.

Positive displacement gear pumps consume relatively more energy and are not used on an industrial scale for mixing and blending of additives. However, they are widely used for viscous flow metering in both plastics and food manufacturing. Thus, although the installation of a new gear pump for mixing and blending may not be cost effective, if there is one already in use for metering purposes, to use it for mixing would be advantageous. Numerical investigation of the mixing process in gear pumps requires a boundary fitted mesh with automatically created and agglomerated cells to maintain grid quality and to accommodate changes in the shape of the base geometry. Strasser investigates numerically the performance of an existing in-service industrial-scale gear pump in blending additives.

High-speed water jets are proven to be feasible replacements for cutting tools, be it surgical scalpels or rock cutting. Among the myriad of applications in various industries, the capability of high-speed water jets and of water∕ice slurries to perfectly clean contaminated surfaces in a relatively inexpensive and environmentally sound way is particularly noteworthy. For instance, paint can be neatly blasted off any surface, say a metallic can, without any damage to the surface. Surface finish describes the exterior features of the surface such as roughness, texture, and pits, while surface integrity defines the condition of a surface layer, properties such as micro-structural transformations, hardness alteration, residual stress distribution, and the depth of induced plastic deformation. Manufacturing processes on material surfaces are controlled by surface finish and integrity. Chillman et al.  evaluate and discuss the surface characteristics induced by different jet conditions.

Drag reduction by polymeric additives helps to reduce the cost of pumping in pipelines, to stabilize and increase the range of jets in fire fighting, to prevent cavitation in turbo machines, to reduce noise, and to save energy in ship and submarine propulsion, to name but a few of the applications. The physics of drag reduction is not completely understood and the root cause remains controversial. Small concentrations of anisotropic additives drastically reduce the efficiency of the transversal transport of momentum by turbulent fluctuations. The stress anisotropy created translates into a substantial decrease of the friction factor. The merits of the competing theories behind the physics, the role of stress anisotropy due to polymer extension versus elasticity in governing drag reduction, is still debated. Lumley was the first to suggest that the phenomenon has less to do with the viscous sublayer, and that an effective viscosity increase of the turbulent flow caused by the extensional viscosity of additives is responsible for the phenomenon. On the other hand, De Gennes and Joseph, among others, argue that polymer elasticity governs drag reduction. Elastic energy is transported in the boundary layer thanks to the relaxation time. The article by Cunha and Andreotti contributes to the characterization of the mechanism of drag reduction with low volume fractions of anisotropic additives in turbulent channel flow.

Studies of the flow past a square heated cylinder with its longitudinal axis aligned normal to the direction of the flow are motivated, aside by the fundamental significance, by their importance in applications such as combustion chambers in chemical processes, flow dividers in polymer processing, cooling of electronic components, and compact heat exchangers. Many non-Newtonian fluids of industrial interest can be adequately characterized by their shear rate-dependent viscosity behavior. Mixed convection from hot square rods to inelastic generalized Newtonian fluids has not received the attention it deserves. Dhiman et al.  investigate numerically mixed convection from a two-dimensional rod.

In closing, I would like to thank the numerous reviewers and the authors who made this issue possible.