Explosive micro-bubble actuator

Microactuators are key components in numerous microsystems, and in many applications strong and fast microactuators are required. The principles used to generate forces in the current actuators are not capable of fulfilling both requirements at the same time, so new principles have to be investigated. One promising technique is based on explosive evaporation. Explosive evaporation occurs when a liquid is exposed to extremely high heat-fluxes. Within a few microseconds the liquid reaches a temperature close to the critical point. At these temperatures spontaneous nucleation can take place; the nucleation barrier is lowered significantly and small fluctuations in the liquid density will act as bubble nuclei. A large number of small bubbles will be formed at numerous sites. Nucleated bubbles instantly coalesce forming a vapour film followed by rapid expansion due to the pressure impulse. This effect of explosive evaporation, which already has proven its use in current thermal inkjet technology, can be used to produce mechanical work. Here the force generated by bubble formation and growth will be utilized to deflect a flexible membrane. Within this thesis the design and fabrication of the explosive micro-bubble actuator is described. The influence of different operating conditions and design parameters on the performance of the actuator is determined. The behaviour of the actuator is highly dependent on the generation and dynamics of the created bubbles. The study of the bubbles created due to explosive evaporation increases the insight in the complex dynamics and provides experimental data to verify the validity of newly developed models. This information is used to find the best operating conditions and create design rules for future devices. The actuator is fabricated using MEMS technology. A pyrex substrate with thin film heaters of platinum form the bottom of a cavity etched in silicon. The top of the cavity is sealed by a flexible silicon-nitride membrane. This cavity is filled with degassed ethanol. Inside the actuator, bubbles are created by applying potential difference across a micrometer sized heater for a few microseconds. The power generated by these current pulses is dissipated inside and around the heater resulting in extremely high heat-fluxes up to several hundreds of MW/m2 and temperature rise rates of 108K/s. These conditions result in spontaneous nucleation. A high degree of superheat is concentrated in a small area. The fact that only a small volume is heated leads to short cooling times and fast bubble collapse. This microsecond heating time, together with a high reproducibility makes the device extremely fast for a thermo-pneumatic actuator. Information on the bubble generation and the bubble dynamics inside the bubble actuator was gained from images taken by stroboscopic imaging and by monitoring the heater temperature during the heating pulse with resistance thermometry. The influence of different operating and design parameters, such as heating power, pulse length and heater geometry on the moment of nucleation, the nucleation temperature, the bubble growth speed and the bubble size was investigated. In contrast to the research on the thermal bubble jet printers and most other studies on bubble generation under high heatflux conditions, the bubbles in the explosive micro-bubble actuator are generated in a closed system. Therefore the influence of the pressure inside the cavity on the bubble generation and dynamics were also examined. The experimental results are used to verify the validity of models on the heat-transfer in system and the effect of a non-uniform current density though the heater. A bubble formed in this closed system by a short heating pulse will result in a dynamic deflection of the membrane. This dynamic membrane displacement is measured in realtime with a scanning laservibrometer. The membrane deflection is not only affected by the dynamics of the bubble, but also depends on the design of the actuator, the material properties of the membrane and the pressure inside the cavity. A constant pressure difference across the membrane will result in a permanent membrane deflection, which can be measured with a white light interferrometer. Measurements on this constant deflection were used to calibrate the membrane. After a bubble collapses the membrane is left in free-vibration and the properties of the membrane and the liquid inside the cavity will determine the movement of the membrane. The maximum repetition frequency is reached when the previous bubble has just vanished at the moment a new bubble is created. At higher frequencies a permanent bubble will form and the performance of the actuator will decrease drastically. Although the ethanol is degassed before use, a small amount of residual gas is still present. A bubble created by explosive evaporation will therefore not only contain vapour, but also some gas. This gas has to dissolve into the liquid and this process is much slower than condensation. The amount of gas inside the bubble determines the bubble collapse time. An effective way to increase the maximum repetition frequency is to increase the pressure inside the cavity. This will increase solubility of the gas in the liquid and will change the dynamics of the bubble; the time it will take a bubble to collapse decreases. Although a small decrease in bubble size and membrane deflection will also occur, it will lead to a considerable increase in maximum repetition frequency. The best performance achieved in this research is a 2μm dynamic deflection for a repetition frequency of 5kHz at a cavity pressure of nearly 3bar. At low repetition frequencies and ambient pressure even a higher membrane deflection of 6μm and a membrane displacement speed of 1.3m/s were measured. The total energy required to generate a bubble is several micro Joules. Only a fraction of this thermal energy is converted to mechanical energy, but due to the high energy density, the actuator is still powerful. The pressure inside the initial bubble is estimated at 25 bar and the growth speed of a bubble is several tens of m/s. The local heating also enables a fast bubble collapse. A reduction of the amount of residual gas, for example by an improved degassing procedure, could increase the maximum frequency to more than 50kHz. The explosive micro-bubble actuator proves to be fast and can create a considerable deflection. It is not possible for this actuator to provide a continuous deflection; it can only give pulses. Therefore on can think about applications such as fast micro-pumps, loudspeakers, micro-compressors and inchworm actuators