Inititiation, Flow Rate, and Routing of Capillary-Driven Flow of Liquid Moving through Microchannels on a Slipchip

Abstract

This study is devoted to controlling the initiation and flow rate of spontaneous liquid-liquid flow passing through microfluidic channels in response to capillary action on a non-wetting droplet. Aqueous droplets were generated on a hydrophobic SlipChip in a shallow channel, and then a stepwise change in capillary force was induced by connecting the shallow channel to a deeper channel filled with immiscible oil. A model to predict the rate of spontaneous flow was developed based on the balance of net capillary pressure with viscous flow resistance; the inputs to the model were the liquid-liquid surface tension, advancing and receding contact angles at the three-phase aqueous-oil-surface interface, and the geometry of the device. The effects of contact angle hysteresis, presence or absence of a lubricating oil layer, and adsorption of surfactants at liquid-liquid or liquid-solid interfaces were quantified. Two different regimes of flow were studied. Faster (mm/s) flow rates were obtained when oil being displaced by the aqueous could escape through connected channels, and slower (μm/s) flow rates were obtained when displaced oil could escaped only through a μm-scale gap between the plates of the SlipChip (“dead-end flow”). Both diluted salt solutions and complex biological media such as human blood plasma were found to flow using this approach, and we anticipate it being useful in the future for control of flow in microfluidic designs that do not require external power, valves, or pumps. Approaches based on initiation of spontaneous flow would be useful for design and operation of the SlipChip platform as well as for other droplet-based and plug-based microfluidic devices.