Experiments were performed to study the flow behaviour of de-ionized water and nitrogen gas through round capillary rubes having an inner diameter of 100µm. At steady state, the single-phase pressure drop along the glass microchannel was measured and analysed. To compare with conventional flow theory, an evaluation was made of the friction factor constant for laminar flow and critical Reynolds number for the transition from laminar to turbulent flow. The liquid flow data were well predicted by the conventional friction factor equations for larger channels, and the critical Reynolds number was close to the traditional macro-scale value. For single-phase gas flow, the measured friction factors were found to agree with theory if compressibility effects are taken into account. The addition of compressibility yields a non-linear pressure distribution that arises from the density change of the gas in the channel. Unlike liquid flow in microchannels, the gas friction factor constant depends on the Reynolds number, which changes along the channel length. Moreover, compressibility caused the velocity to vary all along the length of the channel and prevented the flow from being fully-developed. The neglect of the slip-flow boundary condition and compressibility may account for the discrepancy between the experimental results of various researchers.

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