David Weitz and colleagues at Harvard University, Arizona State University, and Phillip Morris USA recently developed a precise picoliter-volume microreactor for surfactant-stabilized aqueous droplets in carrier oil. The mechanism allowed the researchers to coalesce droplets in a controlled, high-speed manner. The researchers fabricated a microfluidic device with two T junctions where droplets of water in hexadecane were formed independently. Surfactant was added to stabilize the droplets. Small droplets (approx25 (mu)m in diam) were formed at one junction and large droplets (approx50 (mu)m in diam) at the other. When the two streams of droplets were brought together in a single channel, the small droplets caught up to the large ones after traveling approx1 mm, depending on the initial spacing and the velocity difference. However, the droplet pairs did not coalesce into single droplets until they reached the electrodes downstream, where an electric field was applied.
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