Small particles find a wide range of applications, from drug delivery and sensing to serving as fillers for altering the properties of materials. The particle size and the size distribution are often critical and so it is common to seek to separate particles by size from a mixture of particles with various sizes. As one example is blood, where it is often desired to separate the large white blood cells from the somewhat smaller flexible red blood cells from the even smaller platelets. It remains a challenge to perform such separations in many circumstances. Here the investigators demonstrate a new idea for accomplishing such particle-size separations by flowing a long gas bubble in a cylindrical tube filled with a suspension. The motion of the bubble is a well-understood fluid dynamics problem that we exploit in a new way. In particular, we show that the speed of the bubble selects a critical size of smaller particles that can flow past the bubble while larger particles remain in front of the bubble. Thus, the bubble acts as a filter for particles. Several variants of this problem are studied for different channel geometries and the efficiency of the new separation process will be characterized.
Technologies are needed to separate particles by size from a suspension, and it remains a challenge to separate particles in the micron and sub-micron range. A long bubble translating in a tube is separated from the boundaries by a thin liquid film, whose thickness varies with flow speed, as is well known in the fluid mechanics literature. Here this thin film is shown to act as a speed-dependent filter for separating particles by size from a suspension. Thus, this configuration is exploited to (1) study the efficiency of separation by size for a bubble translating through a horizontal channel filled with a bidisperse suspension, (2) study the thin films formed when one or more bubbles translates in curved channels, and (3) study the separation efficiency of bubbles translating through suspension-filled curved channels and vertical channels, the latter potentially allowing the most effective continuous flow process for such separations.
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