Local in-situ hydrophilic treatment of micro-channels using surface discharge

In this paper, we describe a new method of local hydrophilic treatment of polymer micro-channel using surface discharge generated by a micro-electrode array. The method uses a pair of interdigitating electrodes embedded beneath a hydrophobic polymer (e.g. PDMS) micro-channel. When RF voltage is applied to the electrodes, surface discharge occurs inside the channel, and its inside surface is modified to be hydrophilic. Because the diffusion length of the generated plasma is only a few tens of micrometers, the treatment is local and selective. By using such a hydrophobic/hydrophilic patterning method, a droplet dispenser is developed. A straight PDMS channel having a side branch at the middle is fabricated, and the hydrophilic treatment is applied to the straight part. Due to capillary force, water solution fed from one end of the channel fills the entire straight channel, and when air is fed from the side, the liquid is divided into two parts, one ejected from the outlet. The ejection releases the pressure, and the channel is refilled, ready for the next ejection. The process produces a series of droplets, whose volume is precisely determined by the volume of the hydrophilic channel.