Theoretical and experimental study on capillary flow in a grooved microchannel with hydrophilic or hydrophobic walls

In this work, we investigate the capillary flow in hydrophilic and hydrophobic microchannels with grooved structure by theoretical and experimental methods. Based on Young-Laplace equation, we build a theoretical model to compute the capillary pressure on the meniscus as it traverses through several important locations of the groove, including four corners and three walls. The results indicate that the meniscus may only be stopped at the first corner of the hydrophilic groove whereas at the forward facing wall of the hydrophobic groove as the capillary pressure decreases to a negative value. More importantly, we sheds light on the roles of the geometric size and the contact angles on the stopping effect of the groove. Based on it, we build the geometric conditions required for unstopped capillary flow in the hydrophilic and hydrophobic channels.To validate our theoretical model, we perform experiments on the grooved microchannels with adjustable geometric size fabricated on a laser cutting machine. We use the plasma cleaner and hydrophobic agent to modify the contact angles of the hydrophilic and hydrophobic surfaces, respectively. The experimental observation reaches good agreements with the theoretical prediction on the geometric conditions, demonstrating our model to be a useful tool for designing both active and passive valves.