Magnetic droplet manipulation incorporated with acoustic excitation

This paper describes a new type of magnetic droplet microfluidic system incorporated with acoustic excitation for the enhancement of mixing performance. Internal flows inside an acoustically oscillating droplet are visualized using fluorescent particles (15 μm diameter). When a droplet is acoustically excited, it only strongly responds to the droplet´s natural frequency. The oscillation amplitudes of acoustically oscillating droplets with different volumes (6 μl and 8 μl) in different frequencies are measured by high-speed images. Mixing performance is investigated using a methylene blue and glycerol (5 wt%) mixture droplet. The mixture droplet (6 μl) is completely mixed in 60 seconds with acoustic excitation; whereas, the mixture droplet remains unmixed in 60 seconds without the excitation. And the mixing efficiency is quantitatively evaluated through mixing indexes obtained by image analysis. The transportation of a magnetic droplet containing magnetic particles is separately demonstrated using a neodymium magnet. As proof of concept, the manipulation of magnetic droplets is experimentally achieved by incorporating magnetic forces for the transportation of droplets and acoustic excitation for the oscillation of droplets. This result shows the highly reliable manipulation of magnetic droplets with the high mixing performance.