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Key Words low-Reynolds-number hydrodynamics, electro-osmosis, nanofluidics, microdevices, mixing
Abstract Microfluidic devices for manipulating fluids are widespread and finding uses in many scientific and industrial contexts. Their design often requires unusual geometries and the interplay of multiple physical effects such as pressure gradients, electrokinetics, and capillarity. These circumstances lead to interesting variants of well-studied fluid dynamical problems and some new fluid responses. We provide an overview of flows in microdevices with focus on electrokinetics, mixing and dispersion, and multiphase flows. We highlight topics important for the description of the fluid dynamics: driving forces, geometry, and the chemical characteristics of surfaces.
1. INTRODUCTION
Microfluidics refers to devices and methods for controlling and manipulating fluid flows with length scales less than a millimeter. Studies of such fluid-related phenomena have long been part of the fluid mechanical component of colloid science (e.g., Russel et al. 1989) and plant biology (Canny 1977) and draw on many classical features of the dynamics of viscous flows (e.g., Happel & Brenner 1965, Batchelor 1977). However, the subject has received enormous recent attention because of (a) the availability of methods for fabricating individual and integrated flow configurations with length scales on the order of tens and hundreds of microns and smaller (e.g., Ho & Tai 1998, Stone & Kirn 2001, Whitesides & Stroock 2001), (b) rapid developments in biology and biotechnology for which manipulations on the cellular length scale (and below) and the ability to detect small quantities and manipulate very small volumes (typically less than 1 microliter) offer advantages (Voldman et al. 1999, Jain 2000, Beebe et al. 2002), (c) the quest for cheap portable devices able to perform simple analytical tasks, and (d) the potential use of microsystems to perform fundamental studies of physical, chemical, and biological processes. This trend is continuing; moreover, the term nanofluidics emphasizes the desire to manipulate flows on the scale of DNA strands, other biopolymers, and large proteins.
There are many journals now reporting applications at the micron scale, though they may not be familiar to fluid dynamicists (e.g., Lab-on-a-Chip, Sensors and Actuators, and Analytical Chemistry). Indeed, the microfluidics literature contains descriptions of many kinds of functional elements including valves, pumps, actuators, switches, sensors, dispensers, mixers, filters, separators, heaters, etc., some of which may motivate...