13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Droplet Ejection and Liquid-solid Separation from Covered Microfluidic Systems Weiqiang Wang1,2,*, Thomas B. Jones2 1 School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 2 Department of Electrical and Computer Engineering, University of Rochester, Rochester, New York 14627, USA * Corresponding author: wangweiqiang@njust.edu.cn Abstract In droplet-based microfluidics, two different types of microsystems are used for droplet manipulation - covered systems based on parallel-plate devices, and open systems based on single-plate devices. Ejecting droplets from covered systems into an open system is an essential operation to build a complete analysis system combining the advantages of both covered and open systems. This paper presents theoretical study and experimental results for ejecting a water and/or oil droplet from a covered into an open system. We use a force analysis method to determine the condition required for a droplet to cross the open/covered edge and demonstrate droplet movements using electric actuation. A bevelled edge structure is developed to facilitate the final liquid-solid separation process, greatly reducing the required actuation voltage. Keywords electrowetting, electrowetting-on-dielectric (EWOD), microfluidics, dielectrophoresis 1. Introduction Droplet-based microfluidics (or digital microfluidics) is a fast-developing liquid-handling technology that manipulates liquids in the form of discrete droplets. In integrated devices for droplet-based microfluidics, liquid droplets in contact with dielectric surfaces are moved by applying AC or DC potentials across electrodes patterned beneath the dielectric. There are two types of microsystems for electric actuation of liquid droplets: the covered system, in which voltages are applied across electrodes in both the top and bottom plates, [1-5] and the open system, in which voltages are applied across electrodes on a single plate. [6-10] The mechanism for droplet actuation has been attributed to electrowetting (EW) forces for conductive liquids [1-12], and to dielectrophoretic (DEP) forces for dielectric liquids. [13-16] When voltage is applied across electrodes in a device, a liquid droplet can move towards the activated electrode. Study on droplet manipulation is usually done in either covered microsystems or open microsystems. The covered system is capable of achieving all the basic fluidic operations- droplet dispensing [3-5,18], moving [1,2], splitting [3,5] and merging [3], and controls droplet volume reliably through dispensing. [4,18] The open system, while lacking the capacity of dispensing or splitting droplets, [5] allows direct access to liquid handling tools on the surface and analytical equipments. Thus, the concept of dual open/covered microsystem combining the advantages of both covered and open systems is developed. [20,21] A covered section dispenses liquid droplets with desired volume and further analytical operation or liquid droplet handling is done on an open section. To achieve this scheme, it is essential to eject a droplet from a covered into an open section of a chip. In this paper, we analyze the possibility of such a motion for both water and oil droplets, and demonstrate experimental results to compare with the results of modeling. A bevelled edge on the top plate makes it possible to minimize the contact area between droplet and the top plate, thus easily separating the liquid-solid interface. 2. Water droplet ejection An integrated device consisting of a covered section and an open section is illustrated in top and
RkJQdWJsaXNoZXIy MjM0NDE=