As a promising flow pattern in microchannels for process intensification,gas-liquid slug flow has attracted a lot of researchers attention.It is characterized by regular sized gas bubbles and liquid slugs flowing alternately through the channel.The gas bubbles are separated from the channel walls by a thin liquid film of which the thickness is a function of Ca.In circular microchannels,as the liquid film thickness is usually very small,film velocity can be neglected as dissipation of mechanical energy is much higher than in the bulk.In rectangular microchannels,there is a large space between a gas bubble and channel corners due to surface tension.Therefore,gas bubbles behave like leaky pistons and liquid may bypass through the corner gutters,which is defined as leakage flow.Leakage flow is of great importance for the heat or mass transfer while it is still not directly observed and the konwledge is rather limited.