The objective of this research is to investigate a fundamentally new and innovative technology for generating actuation forces in microelectromechanical systems. The approach is to exploit the change in capillary pressure of a liquid bridge when electrowetting occurs. Capillary force actuators are capable of delivering ten to one hundred times greater force than currently-favored technologies and have the potential to revolutionize the development of microsystems. In this research, instrumented capillary force actuators will be microfabricated, experimentally characterized, and compared to electrohydrodynamic models.
Intellectual merit: This research will advance capillary force actuation technology along several fronts essential to its adoption in microelectromechanical applications. The understanding gained will result in accurate models of capillary force actuators that relate dynamic behavior to design choices. The experimental results obtained will confirm the dynamic models and provide demonstration of the potential of this technology for microelectromechanical systems. Finally, the fabrication and testing of several prototype devices will provide knowledge critical to the development of practical, manufacturable actuators.
Broader impacts: The effort will provide both undergraduate and graduate students a highly interdisciplinary research opportunity, ideal for the development of broad fundamental knowledge and practical know-how. The undergraduate students will produce podcasts for outreach to underrepresented high school students that bring the research experience to life. Through the involvement of undergraduates from underrepresented groups, this effort will directly impact the training of a diverse engineering workforce. The effort will enable the development of new microdevices that were previously unachievable and result in products that benefit society.
Carl R. Knospe, Associate Professor, Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904, knospe@virginia.edu 1. Introduction Actuators play a critical role in micro electromechanical systems (MEMS), converting signals from electronic components into forces and motion. While a variety of actuation technologies have been proposed for MEMS, they all suffer from low force production and small ranges of motion. As part of this grant we have demonstrated for the first time a new type of microactuator, based upon the electrical manipulation of capillary forces, which can achieve much greater (50x) forces and motions than previously possible. As illustrated in Figure 1, capillary force actuators (CFA) employ a conducting liquid bridge between two surfaces. The surfaces contain electrodes covered with a very thin insulating (dielectric) layer. Upon application of a voltage across the electrodes, the liquid’s contact angle upon the surface changes. The change in contact angle results in a change in the bridge’s capillary pressure and therefore force acting on the surfaces. This research project developed methods to design, fabricate, and test capillary force actuators. Furthermore, it greatly increased our understanding of how to mathematically model these actuators and predict their behavior. 2. Research Outcomes Several significant advances were made as part of the research effort. Here, we review one aspect, actuator manufacture and testing. Prototype actuators were assembled from two microfabricated silicon wafers. The moving part (platen) was a 2 milimeter square supported by eight 250 micron long micro-machined beams (3 micron thick x 30 micron wide), - see Figure 2. The beams allowed motion perpendicular to the wafer surface. The opposing wafer in the actuator assembly has no moving parts and does not contain any micro-machined components. A 30 nanometer silicon dioxide layer was thermally grown on one surface of both wafers, and these were then coated with a 30 nanometer hydrophobic fluoropolymer film. A 0.1 microliter drop of a water / propylene glycol mixture was placed on the stationary wafer surface and the two wafers were brought together so that the drop made contact with the platen surface and spread as the gap between the surfaces was narrowed. Spacers placed between the wafers set the assembled gap to 100 microns. The resulting liquid bridge was approximately 1 milimeter in diameter. Electrical potential was applied between the two wafers and stepped from 0 to 40 Volts in 2 Volt increments. The platen moved in response to each step, reaching a maximum deflection of 5 microns, see Figure 3. From the measurement, it is clear that the platen motion was quite fast; the response time was shorter than could be measured with available instrumentation, less than 0.02 seconds. With 40 Volts applied, the microfabricated actuator has achieved 5 micron displacement, which requires 200 microNewtons of actuation force. For comparison, with the same area and electrode gap, a conventional electrostatic actuator would achieve only 3 microNewtons at this voltage level. Thus, these experimental results demonstrate that CFAs can achieve actuation forces more than 60 times greater than those achievable by conventional microscale actuators. 3. Training The grant supported two doctoral students in part, Ms. Huihui Wang and Mr. Ali Nezamodinni. Ms. Wang, who led the experimental efforts, received training in a vast number of techniques, including microfabrication, mechanical testing, electrical measurement, interferometry, ellipsometry, microscopy, photolithography, and electroplating. Four undergraduate students were also supported over the course of the grant and received extensive experimental training, including annealing, microscopy, photolithography, ellipsometry, and contact angle measurement.
