Elsevier

Chemical Engineering Journal

Volume 227, 1 July 2013, Pages 90-96
Chemical Engineering Journal

Coalescences of microdroplets at a cross-shaped microchannel junction without strictly synchronism control

https://doi.org/10.1016/j.cej.2012.09.060Get rights and content

Abstract

In addition to the generation of micrometer scaled droplets, microchannel device can also serve as unique platform to control microdroplet coalescence. In this study, a cross-shaped microchannel junction is designed to realize head-on microdroplet coalescence without strict control of droplet synchronism. The flow characters of contacting microdroplets in confined microchannels are carefully examined and a critical capillary number (Ca  0.005) for the occurrence of coalescence is identified for different working systems. Experimental analysis on the film drainage time and the droplet contact time are carefully discussed and a comparison between microdroplet coalescences in confined and free flowing spaces are taken for the in-depth understanding of microdroplet coalescence process.

Highlights

► Head-on microdroplet coalescence is obtained without synchronism control. ► The critical Ca for microdroplet coalescence is identified as 0.005. ► The critical Ca is larger in confined microchannel than in free flowing space.

Introduction

Microchannel platforms are effective tools for versatile, fundamental research on the basic studies of many application fields including chemistry [1], medicine [2], energy [3] material science [4] and etc. The use of micrometer-scaled chemical device offers a number of advantages over the conventional reactor or separator, especially for the enhancement of heat and mass transfer processes [5], [6], based on the high surface area and the small amounts of fluid in them [7], [8]. In microchannel devices, droplet-based microflows have shown good potentials for the control, sorting and manipulation of nanolitre reagents, acting as isolated micromixers and microreactors [9], [10]. Microflow chemical technology has become an important method for the basic research of new chemical engineering processes and has worked as a new approach for the enhancement of chemical reactions [11].

For the controllable application of microfluidic chemical devise, the fluid dynamics of multiphase flow at micrometer scale is an importantly fundermatal principle. Many investigations have been reported in the last decade concerning the generation of microdroplets in microchannels [12], [13], but relative few articles talked about the coalescence of microdroplets. Also important as the microdroplet generation rules, the microdroplet coalescence laws are key issues for the microflow chemistry study. For example, locally enlarged microchannels are a commonly used to realize microdroplet coalescence [14]. In those microchannels, droplet coalescence often occurs at the end of the channel before the droplets starting to leave each other [15], [16]. Using locally wetting wall of dispersed phase has been proven to be a good method for positioning downstream microdroplet and inducing fusion as the approach of upstream droplet [17]. In those devices, coalescence is mainly affected by the capillary number of incoming fluids [18]. For the head-on collision of microdroplets in the T-junction microchannels, a critical capillary number describing the relative magnitude of viscous stresses and interfacial tension for the occurring of coalescence is found [19]. Coalescence only takes place as the operating capillary number below the critical capillary number. Aside from these passive coalescence methods, several active approaches have also been explored for selective droplet fusion [20], [21]. As an important demulsification method, microchannels can facilitate strong electric fields due to their small widths. Several studies have already shown that applying electrical field in a microchannel results in droplets quickly merging over the electrode region [22], [23].

Base on these basic studies of microdroplet coalescence process, microdroplet coalescence devices have become effective tools in many areas of chemical researches such as, reagent mixing [24], particle synthesis [25], DNA and blood analysis [26], [27] during recent years. Precisely controlled droplet contact is important for the application of microdroplet coalescence technology. However, it is not easy to realize in the actual operating process, especially for the head-on collision of microdroplets [28], since the time synchronism of flowing droplets is hard to maintain. Usually, the contact time of microdroplets is very short in the microchannels, and the time difference of arriving droplets should be controlled to a very low level in operation – less than the residence time in the contact region – such as our previous experiment for the study of microbubble coalescence in T-shaped microchannel junctions [29].

The synchronism problem of flowing droplets increases the controlling difficulty of microdroplet coalescence process and finding a microchannel structure permitting some timing difference of arrival droplets is important for the development of new microfluidic coalescence technology. In this work, the cross-shaped microfluidic junction is tested for the coalescence of head-on moving microdroplets. This kind of microfluidic structure can provide stable and symmetrical flow field for arriving droplet, better than the T-shaped microchannel structure [19], [29]. The critical capillary number for microdroplet coalescence at the cross-shaped microchannel junction are discussed and the film drainage theory [30], which describes the droplet contacting process with the droplet contact time and the film drainage time, is introduced to give an explanation on the occurrence of the critical capillary number.

Section snippets

Cross-shaped microchannel device

The cross-junction microchannel device was fabricated on a polymethyl methacrylate (PMMA) plate with end mills as shown in Fig. 1. The width and height of its main channels are both 600 μm (W = H = 600 μm). This square section ensures the spherical shape of droplet in the microchannels. Two flow-focusing generators are used to produce monodispersed microdroplets with equal size from opposite directions. The orifice length in both generators is 500 μm and the orifice height and width are both 300 μm (w = h

Coalescence of microdroplets with different arrival time

In the experiment, water droplets were produced from opposite directions using the two flow-focusing generators. We did not give any control on the arrival of droplets to the junction center, thus the arriving time of opposite flowing droplets was random in different tests. Experimental results indicate at low average velocity of two-phase fluids (u=(QC + QD)/WH), coalescence occurs after the droplet contact. Fig. 3 gives some examples of microdroplet coalescence processes with different arriving

Conclusion

In summary, the microdroplet coalescence at a cross-shaped microchannel junction is studied with four different working systems. The advantage of this cross-shaped droplet coalescence structure is that it provides a more stable and symmetrical flow field for the waiting droplet at the junction center and allows some time difference for the subsequent droplet arrival. Microdroplet coalescences are successfully obtained at arriving time difference less than 1.4W/u. The main forces dominating the

Acknowledgments

We would like to acknowledge the support of the National Natural Science Foundation of China (21036002, 21106076) and the Postdoctoral Science Foundation of China (20100480283, 201104095) for this work.

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