Electrokinetics and Electrohydrodynamics in Microsystems
June 22, 2009 — June 26, 2009
- Antonio Ramos (University of Seville, Spain)
The manipulation of colloidal particles and fluids in microsystems has many existing and potential applications. Among the most promising techniques to handle small objects at the micrometre scale are those that employ electrical forces, which have the advantages of voltage-based control and dominance over other forces. The latter is a clear example of the scaling laws of physical systems: in the range above a few millimetres the electrical forces are rather ineffective, but in the micrometre (and submicrometre) scale the electrical forces dominate.
The aim of the course is to provide state-of-the-art knowledge on both theoretical and applied aspects of the electrical manipulation of colloidal particles and fluids in microsystems. To achieve this goal, the course will cover the following topics: Dielectrophoresis (DEP), Electrowetting, Electrohydrodynamics (EHD) in microsystems, and Electrokinetics of fluids and particles.
The lectures dedicated to the DEP manipulation of particles will have a short introduction on electrical forces on particles in suspension, impedance detection of particles and dielectric spectroscopy. The relation between dielectric spectroscopy of suspensions and DEP measurements will be shown. The theory of Dielectrophoresis will be examined in depth: the dielectric forces and torques on particles and particle-particle interaction will be presented from first principles. Different analytical and numerical solution methods will be covered. Other effects, such as Brownian motion of nanoparticles, will be studied in the context of the DEP manipulation. Practical applications of DEP in Microsystems and the Lab-on-Chip, such as DEP manipulation and separation of cells and nanoparticles, will also be analysed. Details of the fabrication process of microelectrodes and microfluidic chips will be given.
The electrical manipulation of fluids in microsystems can be divided loosely into two categories: digital microfluidics, where the liquid is subdivided into droplets that are manipulated (e.g. Electrowetting); and continuous microfluidics, where the liquid flows through conduits (e.g. Electrohydrodynamics and Electrokinetics).
The part of the course dedicated to Electrowetting will start with the basic concepts of wetting in the framework of the sharp interface model. The statics and dynamics of electrowetting will then be examined. Applications of electrowetting in Lab-on-Chip, Optics, displays and MEMS will be presented.
Another part of the course will give an overview of EHD micropumps. The lectures will start with basic concepts of electrical conduction in liquids, electrical forces and electro-mechanical equations. The different kinds of EHD actuation will be described: from forces in the liquid bulk to forces in the electrical double layer. We will analyse the range of liquid conductivity that each micropump can actuate.
Most of microfluidic applications are designed for electrolytic solutions (such as aqueous solutions). They are in the so-called ohmic regime, where quasi-electroneutrality holds. The ohmic regime of EHD flows will be established by presenting the Taylor-Melcher leaky-dielectric model. The important issue of Electrokinetic flow instabilities will then be analysed. In addition, the physics of the EHD cone-jet transitions will be discussed.
An important part of electrically induced flows are generated by forces in the electrical double layer. The course will provide an introduction of Induced Charge Electrokinetics of particles and fluids. After presenting the classical picture of electroosmosis and electrophoresis, we will examine the basic equations of induced charge electroosmosis, induced charge electrophoresis and ac electroosmosis. The theoretical problems of electrokinetics at large induced voltages will be exposed.
The course is addressed to doctoral students, young or senior researchers, chemical engineers and/or biotechnologists with an interest in Microfluidics, Lab-on-Chip or MEMS.