Rolling performance of liquid marbles enhanced by optimizing particles as digital microfluidics actuators

This study investigated the use of liquid marbles to transport small volumes of water as digital microfluidics actuators by rolling the marbles down an inclined substrate. Hydrophobic magnetic particles were chosen as the encapsulating material because they are readily separated from the inner liquid using a magnet. The rolling angles at which the marbles on an inclined surface began to move, as well as the marble rolling characteristics, were assessed for various particle sizes and materials. Magnetic liquid marbles from 0.6 (0.1 μl) to 4.5 mm in diameter started rolling at small incline angles from 1.5 to 15° on Si and glass surfaces. It was revealed that the rolling angles and the rolling accelerations were highly influenced by the size of particles and the fine magnetic particles, approximately 10 μm in diameter, exhibited excellent rolling performances. Friction on rolling angle, rolling resistance, and rolling condition with slip of liquid marbles were also investigated in terms of particles and a design guideline on optimizing a particle size and an inclination angle (using fine particles and a small inclination angle) was presented. The excellent performance of the fine magnetic particles was related to high mobility of the particles which lowered the rolling resistance. The high mobility was probably derived from high cohesiveness of the particles based on observations of the structure of particles.