Helmholtz Resonance Based Micro Electrostatic Actuators for Compressible Gas Control: A Microjet Generator and a Gas Micro Pump

This paper reports Helmholtz-resonance based micro electrostatic actuators to control compressible gaseous fluids in the micro scale. Particularly, it discusses design, fabrication, and testing results of two electrostatic actuators: a micro jet generator and an integrated peristaltic multistage micro pump. These electrostatic actuators vibrate a micro membrane in a micro chamber at a high frequency (>10 kHz), and easily induce the resonant behavior of compressible gases in the chamber. Such resonant behavior, often called the Helmholtz resonance, can repeatedly create instantaneous pressure in equilibrium between the inside and outside of the chamber and cause gas to rapidly exit the chamber, forming a collimated jet. The developed micro jet generator consists of multiple acoustic chambers in parallel; produces directional gas momentum from each chamber by utilizing the Helmholtz resonance; and collectively entrains nearby gas molecules to form a gas stream. The fabricated micro jet generator has a footprint of 1.6 × 1.6 cm2 and contains 25 acoustic micro thrusters. It operates using a 140 V and 70 kHz sinusoidal signal and produces a thrust of 55.6 μN, a maximum air velocity of 1.2 m/s, and consumes power of 3.11 mW. The developed micro pump consists of multiple acoustic chambers in series and produces a high total pressure by accumulating pressures across the multiple chambers, while maintaining high flow rates utilizing the fluidic resonance of each pumping chamber. The fabricated 18-stage pump produces the maximum air flow rate of 4.0 sccm and maximum pressure differentials of 17.5 kPa with total power consumptions of only 57 mW. Its total package volume is 25.1 × 19.1 × 1 mm3. It is notable that these electrostatic actuators, with their actuation membranes, acoustic chambers, fluidic channels, and micro valves, are fabricated into a single silicon chip by developing low temperature wafer bonding techniques to protect the polymer structures inside.