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This study addresses air-water, two-phase flows in micro-channels of aspect ratios 1, 2 and 3 with fixed hydraulic diameter, micro-milled on poly-methyl- methacrylate (PMMA) with walls that are partially non-wetting (typical static contact angle 65o in stock form) and not molecularly smooth. Flow-maps obtained using the same gas-liquid injection geometry and method for the three aspect ratio micro-channels are constructed, and regime boundaries are compared with those found by other investigations. The results indicate that the bubbly flow regime boundary is shifted to higher liquid and/or lower gas superficial velocities for the higher aspect ratio channels, while transition to the Annular and Annular-Dry regimes remains the same to within experimental uncertainty. The emphasis of what is presented is on the Segmented flow regime. Regular and irregular Segmented flow regimes are assessed on the basis of the statistical variation in the associated phase length scales from flow observations over a substantial channel length. Two mechanisms responsible for development of irregular Segmented flow under steady gas and liquid flow rates are identified. One is related to progressive mutual approach of bubble pairs along the channel resulting in coalescence subject to small perturbations. This mechanism occurs mostly at relatively high liquid volumetric flow ratios. The other mechanism is attributed to unsteady injection because of the transition from the smaller mixing channel to the test channel and possibly feedback within the gas injection and mixing system. The variability in the size of the liquid plug separating gas bubbles in Segmented flow is found to be substantially higher than that of the bubbles even when the flow is regular (low variability of bubble size). The average bubble length of most Segmented flows, regular and irregular alike is shown to scale with the capillary number to the 2/3 power (liquid volumetric flow ratio to the -2/3). Irregular Segmented flow is favoured by higher liquid superficial velocities, lower liquid volumetric flow ratios and lower channel aspect ratios. Of the three aspect ratios examined, the micro-channel with aspect-ratio 3 displayed the broadest window of regular Segmented flow.