Earlier studies have shown that for cavities present on any heater surface to become active nucleation sites during boiling, they should entrap gas. The liquid penetrates the cavity due to the capillary and surface forces, but the exact physical mechanisms have not been fully quantified. The physical mechanisms of the gas entrapment process in closed-end microchannels, representing nucleation sites, are investigated in this study. Aside from the fluid properties, the width, length and depth of the cavities, as well as the static contact angle of the test liquid with the solid are considered as main parameters that influence the gas entrapment process. Test pieces consisted of micromachined silicon dices with glass bonded on top. Widths of 50, 30, 15 and 5μm were chosen based on size distribution probability. The mouth angle was 90° in all cases. Test pieces were held horizontally under a microscope equipped with a CCD camera. A drop of liquid was placed at the entrance of the microchannel and capillary and surface forces drive the liquid into the microchannel. Experiments show two main filling behaviors: (1) A uniform meniscus forms at the entrance and moves inwards, (2) Two menisci: one at the entrance and the other at the closed end of the microchannel. In some cases droplet formation at the walls was observed. A single meniscus typically forms for higher contact angles, while two menisci form for lower contact angles. In all cases, after a sufficient time interval (hours to days) the microchannel was completely flooded. In general, for a given depth, wider microchannels take longer to fill. Surface cleanliness and fabrication process also play a role in modifying the contact angle and hence the time taken to fill the microchannel. A comparison of the interface advancement in the microchannel with a simple mass diffusion model shows reasonable agreement.
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