Capillary flow in polymer microfluidic chips

Disposable polymer microfluidic chips have been used more and more in miniaturized analytical devices. The surface of the polymers often needs to be treated to acquire specific properties. This study investigates the characteristics of capillary flow in three microfluidic chips under different surface conditions and the aim is to understand how the surface property could affect the capillary flow over the shelf life of the chips. The channel surfaces of polymer chips were treated using air plasma. The interface pattern and velocity were measured by a photographic technique and a micron Particle Imaging Velocimetry (MicroPIV) method. The glass chip could maintain a capillary flow velocity of around 3.0 mm/s and showed little reduction with time. The velocity agreed well with theory by Washburn. The PDMS chip surfaces could be easily modified and the capillary flow rate could reach 4 mm/s. However, the hydrophilicity decreased rapidly over time and was lost completely within a few hours. The polycarbonate chips need more powerful surface treatment. Once modified, the surface could sustain for much longer time. It took one month for the capillary flow velocity to decrease by 50%.