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    Drop formation in non-planar microfluidic devices

    Year: 2012

    Journal: Lab on a chip 2012, 12 (21) pp 4263-4268, 20121211

    Authors: Assaf Rotem , Adam R. Abate , Andrew S. Utada , Volkert Van Steijn, David A. Weitz

    Organizations: Department of Physics and School of Engineering and Applied Sciences, Harvard University, 11 Oxford St., Cambridge, MA 02138, United States; Department of Bioengineering and Therapeutic Sciences, California; Institute for Quantitative Biosciences, University of California, SanFrancisco, 505 Parnassus Avenue, San Francisco, CA 94122, United States; Bioengineering Department, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, United States; Delft University of Technology, Department of Chemical Engineering, Julianlaan 136, 2628 BL Delft, the Netherlands; Department of Physics and School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138, United States

    Microfluidic devices can be used to produce single or multiple emulsions with remarkably precise control of both the contents and size of the drops. Since each level of a multiple emulsion is formed by a distinct fluid stream, very efficient encapsulation of materials can be achieved. To obtain high throughput, these devices can be fabricated lithographically, allowing many devices to operate in parallel. However, to form multiple emulsions using a planar microfluidic device, the wettability of its surface must switch from hydrophobic to hydrophilic on the scale of micrometers where the drops are formed; this makes the fabrication of the devices very difficult. To overcome this constraint, we introduce non-planar microfluidic devices with graduated thicknesses; these can make drops even when their wetting properties do not favor drop formation. Nevertheless, the dependence of drop formation on the device geometry, the flow rates and the properties of the fluids, particularly in the case of unfavorable wetting, is very complex, making the successful design of these devices more difficult. Here we show that there exists a critical value of flow of the continuous phase above which drop formation occurs; this value decreases by two orders of magnitude as the wetting to the device wall of the continuous phase improves. We demonstrate how this new understanding can be used to optimize device design for efficient production of double or multiple emulsions.