The physical mechanisms of the gas entrapment process in closed-end microchannels were investigated. Deionized water was the test fluid. The test pieces consisted of micromachined silicon squares with glass bonded on top. The microchannels had widths varying from 50 to 5 mu m and had a mouth angle of 90 degrees. Experiments show two main filling behaviors: (1) A single meniscus at the entrance, (2) Two or more menisci: one at the entrance and the other near the closed end. A single meniscus typically forms for higher contact angles (phi > 50 degrees), while two or more menisci form for lower contact angles (0 < 30 degrees). For 30 degrees <= phi <= 50 degrees, one or two interfaces were observed. In all cases, after sufficient time (hours to, days), the microchannel was completely flooded. In general, increasing the depth and/or width increases the time taken to fill. On the other hand, decreasing the contact angle decreases the time taken to fill. Comparison of experimental data with predictions based on a simple mass diffusion model shows reasonable agreement. (c) 2005 Elsevier Ltd. All rights reserved.