Fast confocal technique shows internal flow of a droplet
The smaller the fluid samples scientists can work with, the more control they have over the analysis time of chemical mixing and reactions. Thus smaller samples can lead to better medicine and microfluidics research. A combined technique has allowed researchers to investigate the complex circulating flow inside a single liquid droplet. Understanding this flow could further development of fluid operation methods based on discrete flows, rather than on the available continuous-flow-based methods.
Continuous-flow devices require an excess amount of sample liquid for a reaction to take place. In a microchannel —such as a capillary — a chemical reaction (such as one created by a drug) will currently occur only by molecular diffusion of a liquid (such as blood). This means that a large amount of liquid must be used to transport the sample mixture to the target area, even though the actual amount needed for the chemical reaction is quite small. But scientists have continued to use continuous-flow-based methods because they understand how to control this type of fluid flow.
Researchers discovered that they could measure the internal flow of a droplet by combining a high-speed confocal scanner with microparticle image velocimetry. The image depicts the 3-D views the scientists obtained of the velocity distributions (colors show velocity in each direction) inside a moving droplet. Reprinted with permission from Lab on a Chip.
Microparticle image velocimetry has been a powerful tool for measuring the velocity distributions of continuous fluid flows, allowing scientists to design the appropriate microchannels for diffusion. However, the method previously has been unable to measure the internal flow of a single liquid droplet — information that could help to create even smaller channels and devices for more efficient mixing methods and faster chemical reactions.
Haruyuki Kinoshita and his colleagues from the Institute of Industrial Science at the University of Tokyo discovered that they could measure the internal flow of a droplet by combining a high-speed confocal scanner with microparticle image velocimetry. Kinoshita explained that the scanning rate of standard confocal microscopy is too slow to image moving particle-size objects — it would take several seconds to a few minutes just to get a single static image. However, with a high-speed scanner, the researchers found that they could take cross-sectional images of microscale flows.
Using a Nipkow disk-type confocal scanner from Yokogawa Electric Corp. in Japan, the researchers measured the 3-D distributions of three-component velocities of a glycerol droplet traveling in a 100 × 58-μm square-shaped channel. The scanner enabled them to obtain a sequence of sharp high-contrast cross-sectional particle images at 2000 fps — the key to the research team’s ability to measure flow at such a small size. It could do this because it has more than 2000 microlenses arrayed on the upper disk and the same number of pinholes following the same pattern on the lower disk — providing several confocal spots in the focal plane. Each confocal spot scans the droplet with disk rotation, providing simultaneous multispot confocal imaging. The scanner rotates at 5000 rpm, producing the images at 2000 fps in about 0.5 ms.
The droplet contained tracer fluorescent polymer microspheres that emitted red light when illuminated by a green 532-nm laser. A dichroic mirror within the scanner allowed the green light to pass through, but it reflected the red fluorescent light from the tracer particles. The scanner extracted the light from the tracer particles in the area around the focal plane but eliminated other light from unfocused particles. Therefore, when it took a cross-sectional image of the droplet, it revealed the particles’ distribution and flow.
With the 3D-image data, the researchers measured the three velocity components of the droplet and investigated the flow pattern around the surface of the droplet. They found that the fluid inside a droplet has a complex 3-D circulation resulting from the drag force on the contact surfaces with the surrounding walls of the channel (when a droplet moves in a square-shaped microchannel).
Kinoshita explained that understanding this information on droplet flow will help sientists perform more effective and faster mixing or reactions by controlling a droplet’s internal flow.
Lab on a Chip, March 2007, pp. 338-346.
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