The effect of interfacial tension on droplet formation in flow-focusing microfluidic device
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- Peng, L., Yang, M., Guo, S. et al. Biomed Microdevices (2011) 13: 559. doi:10.1007/s10544-011-9526-6
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Interfacial tension plays an important role in microfluidic emulsification, which is the process of preparing emulsions. A promising method which controls droplet behavior according to the function of the interfacial tension in the process of microfluidic emulsification is reported. The droplet size and generation frequency changed regularly to obtain appropriate concentrations of surfactant. This method could be of great help for setting up the size-controllable droplet generation systems, and ameliorating the emulsification technology. The interfacial tension effect was first analyzed by computational simulation before the real experiment, which significantly improved the efficiency of the whole research process.
Depth of the microchannel
Width of the microchannel
Hydraulic diameter for the microchannel
Level set function
Viscosity of the oil
Flow volume of oil
Volume of the droplet
Flow volume of water
The generating zone of a single droplet (φ > 0.5)
Average velocity of oil
The interfacial tension σ is a critical parameter that affects the evolution of the interface of the two phases when the droplets are shaping. Normally, a tensiometer (DSA100) is used to measure σ in an invariable environment. But things can be quite different in a microscale system with dynamic interfacial tension as in microfludic systems. In order to find the best way to build up the size-controllable droplet generation system, researchers tried various methods to figure out the relationship among interfacial tension, surfactant concentration and the droplet properties in μTAS systems (Serra et al. 2007; Wang et al. 2009a). However, the perfect solution never comes out, because the interfacial tension dynamicity was never approached directly.
With developed analysis software, computer simulation is able to play a more and more important role in the physics research and industrial design, especially for prediction and possibility analysis. They can tightly analyze physical domains, act as a validation tool for hypothesis-driven experiments to understand physical mechanisms, validate analytical models, and optimize device design at low cost (Boy et al. 2008). And the explicit visual results of a well setup flow-mechanics model are valuable to the lab-on-a-chip researches.
In this article, we focus on the effect of interfacial tension σ in the microfluidic flow-focusing devices. The same geometry of the microfluidic chip and fluid flow rates are applied, so that the droplet size and generation frequency will only be dependent on the interfacial tension. The droplet size was controlled only by adjusting the concentration of surfactant. During the experiment, the coefficient of variation is approximately 4% and droplet diameter is from 10 to 50 μm. According to the simulation and related experimental results, it can be concluded that the interfacial tension between two immiscible fluids has an important influence on the droplet formation, especially the shape property, at the same flow rate.
2 Materials and methods
2.1 Device design
2.2 Materials and measurement
Soybean oil was used as the continuous phase and 0%, 0.005%, 0.01%, 0.2%, and 1% tween20 water solution were chosen as the disperse phase, respectively. The viscosity of the sample was measured by Rheometrics ARES (TA Instruments Inc.), and the temperature was controlled by Julabo FS18 (A heating and refrigerator circulation).
It is generally accepted that a tiny portion of surfactant can significantly lower the interfacial tension (thereby facilitating breakup) and prevent coalescence (Hu et al. 2000). In this study, a common surfactant tween20 (Garstecki et al. 2004) was used to decrease the interfacial tension between the two phases, because it was an efficient nonionic surfactant which could be dissolved in both water and organic solvent, remarkably change the interfacial tension with negligible influence on the viscosity and material density, and be of little harm to the biological environment. The surfactant was of greatest efficiency in a low range of concentration and less efficient closer to Critical Micelle Concentration (CMC). The CMC of tween20 was 8.04 × 10−5 M at 20°C, approximately 0.008% concentration. However, this data was relatively high at the microscale due to the much higher ratio of the surface to the volume than the normal standard. Consequently, the concentrations of the surfactants were determined from the preliminary experiment.
In order to test the efficiency of tween20, two separate measurements were made: First, the tension for the interface between oil and pure water was measured, and the result was 29.8 mN/m; then 1% concentration (much higher than the CMC to make sure the interfacial tension was invariable in the saturated state) tween20 was added to the water and another measurement value as 8.2 mN/m was derived. All these values were obtained from a KRUSS tensiometer (Drop Shape Analyzer DSA100, KRUSS GmbH, Hamburg, Germany).
2.3 Numerical method
Parameters used for simulations
Density of water
Viscosity of water
Density of oil
Viscosity of oil
Flow volume of water
Flow volume of oil
2.4 Chip experiment
3 Result and discussion
It is generally accepted that interfacial tension is a significant element in the process of emulsification where one or more immiscible liquid intimately disperse in another insolvable liquid in the form of droplets. When two immiscible liquids contact with each other, they tend to maintain as small an interface as possible, this is due to the nature that a substance tends to maintain the smallest energy on the surface.
The interesting discovery from the simulation actually proposed a method to obtain different droplet behavior by changing the interfacial tension. And in practice, various droplet shape and frequency could be achieved by adjusting the surfactant concentration in the fluid.
In the experiment, the coefficient of variation(CV) for the droplet size was lower than 4%, which was good for the experiment analysis, and the simulation showed steady results.
Both the experimental and simulation results agreed at the same point that the interfacial tension apparently affected on the characters of microfluidic droplet formations. Although results were still qualitative, the simulation results matched the experimental results well and exhibited the same trends. It could be confirmed that the simulation used as a prediction and validation tool for experimental could be adequately refined to describe the interfacial tension dependence in droplet formation. The experimental results demonstrated the initial assumption in this paper.
In this research, the effect of interfacial tension on the droplet formation within the microfluidic flow-focusing devices was illustrated. Monodisperse droplets of different size were obtained by adjusting the concentration of surfactant. Interfacial tension can be expected to decrease with decrease in droplet size and generation rate in a wide range of circumstances from both simulation and experimental results. The results were of interest in view of the important role of interfacial tension in determining the behavior of small droplets in microfluidic emulsification. The effect of wide range of parameters referred to droplet formation in lab-on-a-chip systems could be also explored, e.g. viscosity of fluids and channel hydrophoblicity, in order to study dynamics in microfluidics based on the behavior of droplets prepared.
We thank the financial support of National Natural Science Foundation of China under Grant No. 10804087.