Wettability Distribution on the Surface Treated by Plasma Jet at Different Flow Rates for Microfluidic Applications

This article investigates the wettability distribution of a surface treated by an atmospheric-pressure plasma jet at different flow rates and aims to apply on microfluidic channel plotting. It is well known that plasma is capable of modifying the wettability of a surface, and a plasma jet, due to its finite nozzle size, has a great advantage that can modify only a local area on a surface. The feasibility of plotting wall-less microfluidic channel has been confirmed by our recent works. In this work, we present a quantitative study on the wettability distribution with different flow rates. Argon is used as the working gas, and six different flow rates, including 0, 0.1, 0.5, 1, 3, and 5 L/min, have been tested. According to the results, the wettability distribution is significantly affected by the flow rate. A higher flow rate would lead to a wider range of modified area, as well as more hydrophilic. The flow rates are also employed for plotting wall-less microfluidic channels, and the width of the channel can be directly controlled by the flow rates. Optical emission spectroscopy (OES) is used to verify the plasma dosages at both the nozzle outlet and on the treated surface. Based on the OES results, the intensity of OH radical is positively correlated with the flow rate. The features and properties of the modification with different flow rates are discussed.