Magnetic droplet microfluidic system incorporated with acoustic excitation for mixing enhancement

Abstract

This paper describes a new type of magnetic droplet microfluidic system incorporated with acoustic excitation for the enhancement of mixing performance. The oscillation amplitudes of acoustically oscillating droplets with different volumes (6μl and 8μl) in different frequencies are measured by high-speed images. When a droplet is acoustically excited, it only strongly responds to the droplet’s natural frequency. Internal flow inside an acoustically oscillating droplet is visualized using fluorescent particles (15μm diameter). Mixing performance is investigated using a methylene blue and glycerol (5wt%) mixture droplet. The mixture droplet (6μl) is completely mixed within 60s with acoustic excitation; whereas, the mixture droplet remains unmixed without the excitation. The mixing efficiency is quantitatively evaluated through mixing indexes obtained by image analysis. The transportation of a magnetic droplet (6μl) containing magnetic particles (500nm dia.) is separately demonstrated using a neodymium magnet beneath a chip. The maximum manipulation speeds of magnetic droplets (approximately 25mm/s for 6μl droplet and 20mm/s for 8μl droplet) are measured by the magnetic droplet rotation test. As proof of concept, the manipulation of four magnetic droplets with two different volumes (3μl and 4μl) and colors (red and blue) is experimentally achieved by incorporating magnetic actuation for the transportation of droplets and acoustic excitation for the oscillation of droplets. This result shows the highly reliable manipulation of magnetic droplets with high mixing performance.