Towards strong and fast micro-actuators
Microactuators are key components of microsystems playing the role of motors. Mechanical actuators use a variety of principles to generate forces, but nonetheless all of the established principles have specific drawbacks. For example, electrostatic actuators are fast but weak, actuators working in the thermal domain are strong but slow, electromagnetic actuators cannot be scaled down in size, piezoelectric actuators are notorious for monolithic integration issues. However, many applications require strong and fast microactuators. Examples are positioning of microoptical components such as mirrors or lenses, instruments in micromanipulation, actuators for pumps and valves in microfluidics, loudspeakers and ultrasound generators. Therefore relentless efforts are made to search for alternative actuation principles.
Fast reactions in nanobubbles
Electrochemical decomposition of water was used to produce large pressure in a closed chamber in a rather short time, but it was not possible to get rid of the produced gas, eventually leading to slow actuators. Fast termination of gas was the main question of the STW project “Exploding gas electrochemical actuators” completed in 2013. In this project it was found that the reaction between hydrogen and oxygen gases is ignited spontaneously in nanobubbles (<200 nm) if they are filled with a stoichiometric mixture of gases. Using a special regime of the short time electrolysis it turns out to be possible to get rid of gas faster than it is produced. This finding opens the way for fabrication of fast and strong actuators based on this new physical phenomenon. Already our first devices demonstrated overpressures of 1-3 bar operating frequencies of 5 kHz in a chamber with dimensions of only 100×100×5 µm³, as can be seen in the figure below.
In image (a), you can see side and top views of the actuator chamber, with the electrodes clearly visible. In (b), the deflection of the membrane (in the center) when alternating voltage pulses of f = 100 kHz are applied to the electrodes. The inset shows enlarged view nearby t = 400 µs where the pulses are switched off. (c) Show the original signal of the vibrometer, which demonstrates the correlation between driving electrical pulses and movement of the membrane. The signal increases due to heat produced by the reaction. Deflection of the membrane is this signal integrated over time.
Current Goals
In this project we propose to fabricate and investigate actuators based on this new physical principle. We expect to produce micro-devices (lateral size 100 – 500 µm) working at low AC voltage < 10 V (DC component is zero) with actuation frequencies of the order of 10 kHz, pressure in the chamber up to 10 bar, and modest power consumption 10 – 100 mW. These are unprecedented characteristics for an actuator, which is well suited for applications in microfluidic systems and for precision control systems. Moreover, for medical applications we are planning to fabricate a prototype of a micro-pump working at a flow rate 10 – 100 µL/min with the record small dosing of 20 – 200 pL/pulse.