Gravity driven flow for the precise control of hydrodynamic focusing

Abstract

Hydrodynamic focusing is a key operation in microfluidics, withapplications that range from cell sorting and counting to mixing and reactions.For most of these operations, both stability and precise handling of thefocused stream are essential. However, these requirements are hardly attainedwhen fluids are supplied by syringe pumps, due to the unavoidable fluctuationsassociated to the driving mechanical system, namely the stepper motor and thelead screw. As an alternative, this work presents a platform for controlledhydrodynamic focusing driven by gravity. In advance to previous works were theuse of hydrostatic pressure has been proposed, here we describe a mathematicalmodel to quantitatively handle the flow stream widths as a function of thefluid reservoir heights. The model enables a fine tuning of flow rates,provided the hydrodynamic resistance of the whole circuit is known.Focusing experiments were carried out at different flow rate ratios inPMMA/OCA film hybrid chips with slit microchannels forming cross-shapedintersections. Results were compared to those obtained when fluids are infusedby syringe pumps to the same chips. It is demonstrated that the gravity driven systemis successful for attaining highly stable and well-defined flow streams. Then,the crystallization of calcium carbonate was implemented in the gravity-drivensystemas example of application. The reaction take place at the interface ofco-flowing streams that transport the respective reactants, hence differentfinal products are attained for different reservoir heights. Apart fromenabling accurate fluid handling and stability, the model-controlled platformis highly versatile to design new experiments, as well as to assist theoperator in practice.