Gravity-driven hydrodynamic particle separation in digital microfluidic systems

In the present study, a hydrodynamic particle manipulation method for digital microfluidic platforms is introduced to separate and concentrate non-buoyant particles in micro-droplets. The proposed method utilizes the combined effects of the gravitational forces and the fluid flow inside the droplet, and it depends only on the hydrodynamic properties of the system without the need of external electrical or magnetic modules. The desired hydrodynamic effect is created by manipulating the droplet in a controlled fashion on a circular pattern of electrodes using the electrowetting-on-dielectric technique. Two successful electrode designs (Star Design and Square Design) are presented. To investigate the feasibility of the proposed particle separation technique, suspensions of silica and polystyrene beads are tested. The fabricated devices successfully focus the non-buoyant silica beads of 1 mm and 5 mm and polystyrene beads of 15 mm in a region on the central electrode; whereas the focusing behavior is not observed for the polystyrene beads of 5 mm. A capturing efficiency of 86% and 94% is achieved for the 5 mm silica particles and 15 mm polystyrene particles, respectively. A parametric study is conducted to investigate the dependency of the focusing regimes on the particle size and density, the droplet size and the electrode geometry, and the actuation scheme. To understand the focusing mechanism, the fluid flow inside the droplet is simulated numerically, and the observed time scales are verified through analytical calculations. Finally, the application of the current particle focusing technique is illustrated for detection of the presence of a low concentration of DNA (18 mg mL(-1)).