Role of geometry and fluid properties in droplet and thread formation processes in planar flow focusing

Abstract

Droplet formation processes in microfluidic flow focusing devices have been examined previously and some of the key physical mechanisms for droplet formation revealed. However, the underlying physical behavior is still too poorly understood to utilize it for generating droplets of precise size. In this work, we formulate scaling arguments to define dimensionless variables which capture all the parameters that control the droplet breakup process, including the flow rates and the viscosities of the two immiscible fluids, the interfacial tension between the fluids and the numerous dimensions in the flow focusing device. To test these arguments, we perform flow focusing experiments and systematically vary the dimensional parameters. Through these experiments, we confirm the validity of the scaling arguments and find a power law relationship between the normalized droplet size and the capillary number. We demonstrate that droplet formation can be separated into an upstream process for primary droplet formation and a downstream process for thread formation. These results are key to the ability to tune the flow focusing process for specific applications that require monodisperse micron and submicron droplets and particles.

Dynamics of microfluidic droplets

“…Other reports however state that squeezing/dripping droplet breakup depends solely on the upstream geometry and associated flow field, and not on the geometry of the channel downstream of the flow focusing orifice. 37 By contrast, the elongation and breakup of the fine thread during the thread formation mode of breakup depends solely on the geometry and flow field in the downstream channel. In light of these recent papers and despite the widespread use of flow-focusing devices, it is clear that the understanding of their detailed dynamics still warrants further research.…”

Single Droplet Break-up in Controlled Mixed Flows

“…This is however, not a general break-up mechanism and can mostly be attributed to the presence of impurities or a nonuniform surfactant distribution along the drop surface (14,15). Other break-up mechanisms have been described in literature, for instance when dealing with viscoelastic components (16,17) or when using microfluidic devices in which confinement effects become important (18,19). Here, however, we focus on bulk behavior of systems with Newtonian components.…”

A Tunable Microfluidic Device for Drug Delivery

“…The key advantage of FF technique is precise control in producing droplets into the range of hundreds of nanometers (Anna et al, 2003;Thiele et al, 2010), but ambiguity remains regarding the lower limit of droplet sizes that can be achieved, the size distribution and continuity of the droplet threads still remain unreported due in part to the difficulty of in-line droplet size analysis and the length of the droplet thread (Anna et al, 2003;W. Lee et al, 2009).…”

“…Emulsion droplets of very narrow size distributions can be strategically generated by harnessing hydrodynamic behaviors within microfluidic systems (Anna et al, 2003;Martin- Lee et al, 2009). FF geometries are used in mixing immiscible phases or encapsulating one phase within a second sheathing phase.…”

“…The inner phase is traditionally termed the dispersed phase while the outer phase is termed the continuous phase when they meet at an orifice. Most microfluidic emulsions involve a single droplet dispersed in a continuous phase such as water-in-oil (W/O) and oil-in-water (O/W) (Ha and Yang, 1999;Anna et al, 2003;W. Lee et al, 2009;Kiss et al, 2011).…”