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A Capillary Force Microgripper for Microassembly Using Electrowetting-on-Dielectric (EWOD)

Vasudev, Abhay
http://rave.ohiolink.edu/etdc/view?acc_num=akron1239374149

Abstract Details

2009, Master of Science, University of Akron, Mechanical Engineering.
Current MEMS devices are fabricated by monolithic micromachining in which all components are fabricated in one sequential process. Hybrid microsystems having complex 3-D geometries and multiple micro components cannot be manufactured using monolithic micromachining. In these situations, assembly of micron-sized parts is necessary. Gripping techniques using micro-grippers and manipulation tools are needed to accomplish micro-assembly tasks. Conventional mechanical grippers tend to scratch and indent micro components during assembly, which may destroy critical features on the components. Capillary and surface forces, which are dominant forces at the micro scale, can be utilized as the gripping mechanism to overcome drawbacks of mechanical grippers. Capillary grippers take advantage of capillary lifting forces evolving from a liquid bridge between two solid surfaces. In order to realize the pick-up, hold and release functions, the capillary lifting force needs to be varied and controlled dynamically. The capillary force needs to be greater than the weight of the micro component during the pick-up phase and hold phase so that the micro component can be positioned at the target location; subsequently, the lifting force needs to be reduced to a level where the weight of the micro component is greater than the lifting force to release the object. In this thesis, electrowetting is used to dynamically change the capillary forces to facilitate object pick-up and release. Electrowetting is a phenomenon that is used to dynamically change the contact angle of a liquid droplet at a liquid-solid interface by applying an electric potential. The liquid bridge capillary force, which is dependent on the contact angle the liquid bridge makes with the gripper surface, can thus be dynamically varied by electrowetting. The microgripper consists of interdigitated radial coplanar gold electrodes across which the driving voltage is applied and a thin hydrophobic insulator that separates the droplet from electrode. The higher the electric potential applied across the electrodes, the lower the contact angle of the liquid. The lifting force is at a maximum for the lowest contact angle and reduces to a minimum at the highest contact angle. In this thesis, first the change in contact angle of a de-ionized (DI) water droplet by electrowetting is demonstrated. The capillary lifting force of the microgripper is characterized using a digital electronic balance. Results indicate that electrowetting dynamically changes the capillary lifting force evolved from a liquid bridge. The influence of liquid bridge height on lifting forces is also experimentally studied. Using a 0.8µm Teflon AF coating serving as insulation and also providing a hydrophobic surface, the microgripper is capable of picking up and releasing micro glass beads having a mass ranging from 77µN to 136µN. For the 136µN glass bead, the pick-up and release voltages are 120V and 58V respectively. Experiments were conducted to determine the response time of electrowetting using a micro-liter droplet and the response time of the lifting force of liquid bridge. To reduce the driving voltage, a 0.5µm Barium Strontium Titanate (BST) layer as the dielectric insulating layer is used. A thin coating of Teflon AF provides the hydrophobic surface. Experiments indicated that the use of BST as the dielectric insulation enables a low voltage microgripper, which can pick-up and release a 136µN glass bead at 28V and 8V respectively. A study is carried out to determine the feasibility of use of room temperature ionic liquids (RTILs) as the liquid for microgripping using electrowetting. Although the total contact angle change for RTILs were found to be inferior to that of aqueous solutions, the properties of RTILs, such as high ionic conductivity, negligible volatility, non-flammability, thermal stability and usage in a wide temperature range offer distinct advantages over aqueous liquids for electrowetting applications. Further, the response time and lifting force of the RTIL based microgripping is also characterized. The maximum lifting force generated was 140µN.
Jiang Zhe, PhD (Advisor)
98 p.
Mechanical Engineering
electrowetting; microgripper; capillary

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Citations

  • Vasudev, A. (2009). A Capillary Force Microgripper for Microassembly Using Electrowetting-on-Dielectric (EWOD) [Master's thesis, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1239374149

    APA Style (7th edition)

  • Vasudev, Abhay. A Capillary Force Microgripper for Microassembly Using Electrowetting-on-Dielectric (EWOD). 2009. University of Akron, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1239374149.

    MLA Style (8th edition)

  • Vasudev, Abhay. "A Capillary Force Microgripper for Microassembly Using Electrowetting-on-Dielectric (EWOD)." Master's thesis, University of Akron, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=akron1239374149

    Chicago Manual of Style (17th edition)

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© 2009, all rights reserved.
This open access ETD is published by University of Akron and OhioLINK.
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