Droplet formation and breakup dynamics in microfluidic flow-focusing devices: From dripping to jetting
Highlights
▸ Micro-PIV technique was applied to quantify the flow field around a droplet. ▸ Droplet could be generated in either dripping or jetting regimes. ▸ The dynamics of droplet formation in both regimes was studied. ▸ The most unstable instability of the oil jet was analyzed.
Introduction
Droplets are often encountered in multiphase microfluidics and have many potential applications in emulsions, drug encapsulation, crystallization, materials synthesis, chemical mixing and reaction (Baroud et al., 2010, Mark et al., 2010, Squires and Quake, 2005, Whitesides, 2006). Droplets are usually generated in a microfluidic T-junction or a flow-focusing device, with different mechanisms (Cubaud and Mason, 2008, Garstecki et al., 2006, Garstecki et al., 2005). The confinement is one of the most important factors influencing droplet formation in microfluidic devices as the propagating thread is restricted within the microchannels and is affected by both the geometry and size of the channel (Abate et al., 2009, Dollet et al., 2008, van Steijn et al., 2009). The typical microfluidic flow-focusing devices can be categorized into three types according to various confinements (Fig. 1): (I) Axisymmetric capillary co-flowing device, the dispersed phase is flowed through a capillary tube positioned upstream near an exit orifice through which the continuous fluid is forced (Gañán-Calvo and Gordillo, 2001, Hua et al., 2007, Umbanhowar et al., 2000); (II) the flow-focusing junction composes a central channel and two outside channels, which is aligned to a small and short orifice, and then to a wide outlet collection. In this kind of device, the dispersed liquid is flowed into the central channel and the continuous liquid is flowed into the two outside channels, and the dispersed thread usually breaks up at the small and short orifice (Anna et al., 2003, Garstecki et al., 2006, Garstecki et al., 2005, Nie et al., 2008); (III) cross-junction geometry, which consists of four microchannels with the same geometry and size that intersects at right angles. The dispersed phase is fed from the inlet channel and the continuous phase is fed from the two lateral channels (Cubaud and Mason, 2008, Funfschilling et al., 2009). The confinement degree in these devices differs and results in different scaling laws for the size of formed droplets.
Generally, droplets can be formed in either dripping or jetting regimes. One should firstly classify the regimes before controlling the size of formed droplets perfectly. The dripping–jetting transition can be obtained either by comparing the capillary collapse time in Rayleigh–Plateau instability and the growth time of the dispersed thread (Utada et al., 2005), or by comparing the capillary time for the growth of interfacial disturbance and the convective flow time of the fluid (Zhou et al., 2006). Therefore, the map of flow patterns is usually constructed based on the capillary number or the Weber number of both phases (Cubaud and Mason, 2008, Utada et al., 2007). Furthermore, the dripping–jetting transition can be regarded also as the transition from convective to absolute instability for flows at low Reynolds numbers (Guillot et al., 2007) and is affected by the capillary number, the degree of confinement and the viscosity ratio of both phases (Cramer et al., 2004, Guillot et al., 2007). Besides dripping and jetting mechanism, Anna and Mayer (2006) reported a geometry-controlled regime for droplet formation at low capillary numbers. And they also classified a thread formation mechanism for surfactant-covered water droplets formed in oils, which was controlled by flow rates and concentration of the surfactant.
Some investigations have been devoted to explore the formation mechanism for droplets or bubbles in either dripping or jetting regimes and to examine the parameters influencing the size of formed droplets and bubbles. For bubbles formed in microfluidic flow-focusing devices with a small and short orifice, at low capillary numbers, the neck of the gaseous thread proceeds a linear thinning process in the collapse stage (Garstecki et al., 2005), followed by a non-linear unstable thinning process during the final pinch-off (Dollet et al., 2008). And the final collapse of the thinning process is controlled by inertia of both phases (Dollet et al., 2008). This mechanism is quite similar to the blocking–pinching mechanism proposed for droplets formed in a microfluidic T-junction (Guillot and Colin, 2005). Funfschilling et al. (2009) obtained the information about the velocity distribution in the continuous phase around the dispersed thread measured by a micro-particle image velocimetry (micro-PIV) system to support the blocking–pinching mechanism. In all of the three types of microfluidic flow-focusing devices, the size of droplets formed in the dripping regime usually depends on the channel size, the viscosities and flow rates of both phases, as well as the interfacial tension (Hua et al., 2007, Umbanhowar et al., 2000). However, the effect of the viscosity ratio of both phases on the droplet size is contradictive in the literature (Cramer et al., 2004, Nie et al., 2008). Some results showed that the droplet size is not influenced by the viscosity ratio of both phases in axisymmetric capillary co-flowing device (Cramer et al., 2004), whereas, others found that the viscosity of the dispersed phase affects dramatically the droplet size in microfluidic flow-focusing devices with a small and short orifice (Nie et al., 2008). Moreover, the scaling law varies with droplet size in the dripping regime in microfluidic flow-focusing devices with cross junction geometries (Cubaud and Mason, 2008). The mechanism for the pinch-off of the dispersed thread in the jetting regime is similar to that in the dripping regime (Zhou et al., 2006). And the droplet size formed in the jetting regime can be estimated through the product of the volumetric flow rates of the dispersed phase and the characteristic time for the pinch-off, which is related to the diameter of the jet, the viscosity of the continuous phase and the surface tension in axisymmetric capillary co-flowing devices (Utada et al., 2005). Or the droplet size is only related to the flow rate ratio of both phases in this regime in microfluidic flow-focusing devices with cross junction geometries (Cubaud and Mason, 2008). Although several scaling laws were proposed to predict the droplets size in both dripping and jetting regimes in microfluidic flow-focusing devices with cross junction geometries (Cubaud and Mason, 2008, Liu and Zhang, 2011), however, to our best knowledge, there is little information about the breakup dynamics during the droplet formation in microfluidic flow-focusing devices with cross junction geometries.
This work aims at studying experimentally the droplet formation in microfluidic flow-focusing devices with cross junction geometries, in particular, the droplet breakup dynamics and various parameters affecting the droplet formation, such as flow rates and viscosity ratio of both phases, and the channel size. Compared to previous studies (Cubaud and Mason, 2008, Utada et al., 2007), the present study focused on the viscosity ratio ranging from 1 to 112 between dispersed and continuous phases.
Section snippets
Experimental
The microfluidic device was fabricated in a plate (45×27.5×2 mm) of polymethyl methacrylate (PMMA) by precision milling and sealed with another thin PMMA plate. The square sections were 600 μm wide×600 μm deep and 400 μm wide×400 μm deep, designated as +600 and +400, respectively. Stainless steel tubes (di=1 mm) were used to connect the inlets and outlet of the microchannel to tygon tubes (ID=1.02 mm), which were employed to connect the microchannel device with liquid supplies. Liquids were
Flow patterns at the cross junction
Various flow patterns were observed at the cross-junction for a wide range of flow rates Qo and Qw (Fig. 2). At low flow rate of the continuous phaseviscous displacement appeared (Fig. 2a). The low momentum of the continuous phase was insufficient to drive the dispersed phase, and the dispersed phase invades the lateral channels,
Conclusions
This work investigated the droplet formation and breakup dynamics in microfluidic flow-focusing devices with a high speed camera and the micro-PIV technique. Six various flow patterns were observed at the cross-junction of the device and a flow patterns map was constructed by using the Weber number of the dispersed oil phase and the capillary number of the continuous phase. The transition between the dripping and jetting regimes was analyzed, as well as between the plug and monodispersed
Acknowledgments
The financial supports for this project from the National Natural Science Foundation of China (Nos. 21106093, 20911130358), the Research Fund for the Doctoral Program of Higher Education (No. 20110032120010) and the Program of Introducing Talents of Discipline to Universities (Grant No. B06006) are gratefully acknowledged. T. Fu appreciates the aid from D. Funfschilling for the experiments, and the financial aid from both the China Scholarship Council and the French Embassy in China.
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