MM18-2 SICE02-0475
Formation of Droplets Using Branch Channels in a Microfluidic Circuit Takasi Nisisako, Toru Torii, Toshiro Higuchi The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
[email protected] Abstract: This paper presents a new method for preparing micro droplets inside the liquid layer at a T-junction in a microchannel network. The relations between droplet size, flow speed, and channel size are studied. The droplet size is easily varied by changing the flow conditions in the microchannels. The size distribution of the resulting droplets is very narrow. Keywords: droplet formation, microchannel, emulsion, micro reactor, µTAS droplets is very narrow.
1. Introduction Emulsions have a number of applications in various industrial fields, such as the food, cosmetic, pharmaceutical industries. Additional new aspects of emulsions are now emerging. For instance, a PCR method using micro droplets as DNA containers has recently been proposed. Also, an innovative method for organic reactions that utilize oil-in-water emulsions as small reaction chambers has been paid much attention 1). On the other hand, chemical and biochemical experiments have been performed using pico/nanoliter-sized discrete droplets inside the liquid layer in microfabricated devices 2)-6). Thus, the droplet-based chemical reactor is under intensive study, and the preparation of small droplets having accurately specified volume will become increasingly important. Several techniques for emulsification processes have recently been reported using a microchannel device. At IMM (Institut für Mikrotechnik, Mainz), various types of micromixers have been developed which can be used to mix otherwise immiscible liquids 7). The resulting emulsions have a narrow size distribution. Nakajima et al. produced regular-sized emulsion cells by permeating a dispersed phase into the continuous phase through an array of parallel microchannels or microfabricated through-holes 8) . In this method, droplets are formed spontaneously by their own surface tension, and no shear force by the flow is required. The droplet size depends on the microchannel structure and cannot be altered. This method is therefore unsuitable for metering varying volumes of chemical samples. Also, the diameter of the generated droplets is larger than the channel size (width or depth). The small channel size required for droplets less than 1 µm in diameter is not easy to fabricate. We present a novel method for preparing micro droplets using a branch connection in a microchannel network. With oil as the continuous phase and water as the dispersed phase, pico/nanoliter-sized water droplets can be generated in continuous phase flow at a junction. The droplet size is easily varied by changing the flow conditions in the microchannels. The size distribution of the resulting
SICE 2002 Aug. 5–7, 2002, Osaka
2. Experimental 2.1 Microchannel fabrication Microchannels that include a T-junction were fabricated on a plate of polymetyl metacrylate (PMMA) using a 100 µm diameter end mill. Figure 1 shows the microchannel network schematically. The dispersed phase flow channel is vertical to the continuous phase flow channel. To study the effect of channel size, two devices were prepared with differing channel sizes. One device has a wider (500 µm) continuous phase flow channel, and the other has a narrower channel (200 µm). The other dimensions were the same in both devices: depth 100 µm, and dispersed phase channel width 100 µm. The channels to the pressure sensors were 100 µm in width. The bottom surface of the channels was smooth, and the surface roughness (Ra) was less than 1.0 µm. The channels were rectangular in cross section (Fig. 2). The channel length for the continuous phase was 15 mm, and for the dispersed phase was 5 mm. A T-junction component was positioned 10 mm from the supply port of the continuous phase flow. A deeper and wider region (depth 0.5 mm, width 1.0 mm) was made for observing the size of the droplets as spheres. The channels were enclosed by thin PMMA plates (thickness 0.5 mm).
Fig. 1 A Schematic of the fabricated channels.
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2.2 Equipment and Procedures
3.1 Effect of Flow Speed on Droplet Size
Syringe pumps (KDS100; Kd SCIENTIFIC) were employed for injection of the dispersed and continuous phase flow. Ultra-pure water (15.0MΩ･cm) was used as the water phase, and high oleic sunflower oil (75.0 mPa･s at 293 K) was used as the oil phase. Both were injected using syringe pumps. Semi-conductor pressure sensors (PMS-5M; TOYODA MACHINE WORKS, Japan) were set in the plate to study the conditions for droplet formation and to confirm that the flow rate was constant. Droplet formation at the T-junction was observed using a microscope (BX50; OLYMPUS, Japan) and a high-speed video camera (FASTCAM-ultima; PHOTORON, Japan). 4500 gray-scale images per second could be recorded at full frame size (height: 256 pixels, width: 256 pixels). The droplet size was measured by counting pixels.
Figure 3 shows the relation between the continuous phase flow speed (average velocity) and the droplet size (diameter). For a given flow rate of the dispersed phase (0.01 ml/h), the droplet size decreases as the average velocity increases of the continuous phase flow. These graphs show that the rate of size reduction decreases as the continuous phase flow speed increases. This result is explicable by the velocity profile in the continuous phase channel, which for smooth viscous flow is parabolic. The flow is fastest at the center and falls essentially to zero at the channel wall. As the flow rate is varied, the change in velocity near the channel wall is therefore small. Consequently, as the continuous phase flow speed increases and the size of generated droplets decreases, the shear force involved in droplet formation decreases, and reduction of the droplet size becomes less pronounced. Error bars added to markers show the standard deviation, showing a narrow distribution of droplet size (less than 1.0 µm). The standard deviation was kept to be small, even if the mean size of droplets was varied widely. At higher continuous phase flow velocities, far smaller droplets (a few µm or smaller in diameter) should be generated. However, at the higher pressures involved, the energy efficiency becomes to be a serious problem. Modification of the channel design to reduce the pressure drop would be helpful.
3. Results and discussion With sunflower oil as the continuous phase flow and water as the dispersed phase flow, regular-sized water-in-oil (W/O) droplets were generated at the T-junction, and the resulting droplets implies good reproducibility (Fig. 2). Flow in the channels is laminar (Re
