We demonstrate the complete filling of both deionized water (DI water) and liquid metal (eutectic gallium-indium, EGaIn) into closed-end microchannels driven by a constant pressure at the inlet. A math-ematical model based on gas diffusion through a porous polydimethylsiloxane (PDMS) wall is developed to unveil the physical mechanism in the filling process. The proposed theoretical analysis based on our model agrees well with the experimental observations. We also successfully generate traveling surface acoustic waves by actuating interdigitated microchannels filled with EGaIn. Our work provides significant insights into the fabrication of liquid electrodes that can be used for various acustofluidics applications.