Microelectromechanical Systems (MEMS)
Modeling Gas and Liquid Flows Through Microdevices
Interest in microelectromechanical systems (MEMS) has experienced explosive growth during the past few years. Such small devices typically have characteristic size ranging from 1 mm down to 1 micron, and may include sensors, actuators, motors, pumps, turbines, gears, ducts and valves. Microdevices often involve mass, momentum and energy transport. Modeling gas and liquid flows through MEMS may necessitate including slip, rarefaction, compressibility, intermolecular forces and other unconventional effects.
In this research, we provide a methodical approach to flow modeling for a broad variety of microdevices. The continuum-based Navier-Stokes equations---with either the traditional no-slip or slip-flow boundary conditions---work only for a limited range of Knudsen numbers above which alternative models must be sought. These include molecular dynamics (MD), Boltzmann equation, Direct Simulation Monte Carlo (DSMC), and other deterministic/probabilistic molecular models. The research broadly survey available methodologies to model and compute transport phenomena within microdevices.
Micropumps Smaller Than a Grain of Sand
For some medical and processing applications it might be desired to move gases and liquids using exceedingly small pumping devices. We introduce a novel approach for pumping fluids at extremely low Reynolds numbers. Presently the only mechanical pumps that can be used for microelectromechanical applications are of the positive-displacement type. The present pump is a much simpler alternative. Its operation is based on the rotation of a cylinder placed asymmetrically in a narrow duct, so that the differential viscous resistance between the small and large gaps causes a net flow along the channel.
Our version of a micropump is particularly suited for microelectromechanical system (MEMS) applications, and the concept has already been disclosed as a United States patent. To test the idea, two-dimensional, time-dependent Navier-Stokes calculations are carried out with Reynolds numbers in the range of 0-100. Questions related to the physics of the pumping mechanism, its efficiency, and parameter optimization are answered during this phase of the research. If the device shows promise, a subsequent phase will include experimental verification. Low-Reynolds number experiments will be carried out first using a viscous oil and a cm-scale polygon driven by a conventional electric motor, and then using air or water as the working fluid and a microfabricated cylinder and motor. Issues addressed during the latter experiment will include slip-flow and Knudsen number effects as well as the effect of interfacial forces.