Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability

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Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability

Authors: Adam R. Abate, Amber T. Krummel, Daeyeon Lee, Manuel Marquez, Christian Holtzed and David A. Weitz

Lab Chip, 2008, 8, 2157–2160

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By Alex Epstein

Abstract from the original paper

For many applications in microfluidics, the wettability of the devices must be spatially controlled. We introduce a photoreactive sol–gel coating that enables high-contrast spatial patterning of microfluidic device wettability.

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Fig. 1 SEM images of channel cross sections; scale bars denote 5 mm. (a) Uncoated PDMS channel cross-section and (b) magnified view of upper right corner. (c) Coated, PAA functionalized PDMS channel and (d) magnified view of upper right corner; the corner is rounded because the sol–gel wets the surface and collects in regions of high curvature.
Fig. 2 Contact angle measurement of water droplets in air on (a) sol–gel coated substrate with contact angle 105 deg and (b) PAA grafted substrate with contact angle 22 deg. AFM images of (c) sol–gel coated and (d) PAA grafted microchannels. The images show a 10 deg 20 mm area at high resolution; the dark to light color scale maps to feature heights of -150 to 150 nm. (e) Surface concentrations of atoms on sol–gel coated and PAA functionalized substrates, measured with XPS; fluorine (F 1s), oxygen (O 1s), carbon (C 1s), and silicon (Si 2s).
Fig. 3 (a) Photomicrograph of a sol–gel coated channel. PAA has been grafted to the right half of the channel using UV-initiated graft polymerization. The grafted polymer is dyed with toluidine blue, a dye that preferentially stains PAA. (b) Average grayscale intensity across the channel as a function of location along the channel.

Fig. 4 (a) Diagram of sol–gel coated device. The upper half of the device is hydrophilic due to graft-polymerization of PAA. The bottom half of the device is hydrophobic due to the default properties of the sol–gel coating. (b) Photomicrograph of double emulsions being formed (R1) and flowing out of the microfluidic device (R2 and R3). (c) Magnified view of the double emulsion flow-focusing junction in R1. (d) Photomicrograph of O/W/O double emulsions; the drops crystallize due to their high monodispersity. The scale bars for all figures denote 100 mm.