Capillary force-driven fluid flow of a wetting liquid on a surface with multiple parallel open microchannels

Fluid flow driven by the capillary force is omnipresent in nature and important in many engineering technologies. The focus of this work is capillary force-driven fluid flow of a wetting liquid in open microchannels when a liquid droplet is gently introduced to a metal surface on which multiple parallel microchannels with an open rectangular cross section are formed. It is found that, aided with a high-speed camera, the capillary-force driven fluid behavior consists of uni-directional spreading of the bulk droplet on the microchannel fins and liquid penetration into the microchannels. The kinetics of fluid flow due to the liquid penetration into the microchannels can be divided into three distinct stages: initial stage, transition stage, and Washburn stage; only in the Washburn stage, the flow has a penetration lengthtime dependence in proportion to square root of time as described by the Washburn's equation. Comparison with liquid spreading on a plain surface having only one microchannel (the same geometry and size) revealed that the bulk droplet spreading on the microchannel fins, after elapse of the initial stage, has little effect on the fluid flow kinetics in the multiple microchannels. Some analytical results shed more insights into the capillary forcedriven fluid flow in open microchannels.