(358j) The Motion of Long Accelerating Bubble Trains In Microchannels

The motion of long bubble trains through microfluidic channels is relevant to several applications in microchemical syntheses of molecules involving reactants in two or more fluid phases. One of the main challenges in such applications is to design the microchannel geometry such that uniform and controllable multiphase flow is obtained. There is a large amount of literature that examines fluid dynamics on the scale of single bubbles or droplets. However, relatively few studies examine the rich nonlinear ensemble dynamics of large bubble trains that are relevant to microchemical applications.

In this paper, we study the motion of uniform segmented bubble trains that comprise large numbers of bubbles flowing through long microchannels of rectangular cross section. We specifically work with liquids that wet the microchannel walls either partially (small non-zero contact angle) or completely (zero contact angle). A T-junction gas injection geometry is used to produce nearly monodisperse bubble populations. We report measurements of bubble size, speed, liquid slug size, and bubbling frequency at low capillary numbers (<10-2) along several axial locations in a microchannel, and simultaneously measure the system pressure drop. For fixed liquid flow rate, the system pressure drop is a nonlinear function of gas flow rate. Gas compressibility leads to axial bubble expansion and consequent speeding up of the bubble train along the length of the microchannel. We analyze our results using a simple model for system pressure drop. This model makes use of well established analyses on single-bubble scale phenomena and a scaling description of bubble generation to build a system-level picture of bubble train dynamics. Our work is especially relevant in emerging applications involving large multiphase microfluidic networks where the pressure drop is large enough to cause significant changes in dynamical properties along the microchannel network.