Enhancing acceleration of droplets moving on inclined surfaces by decreasing density of projection for microfluidic devices to transport droplets

Abstract

In the fields of medicine, biotechnology, and chemical analysis, the need often arises to transport a very small amount of liquid. The characteristics of sliding acceleration by inclined surfaces with arrayed micro-pillars were examined. This work related to the surface technology required for the actuator of a microfluidic device such as a μ-TAS to improve the movement of droplets. The ease of movement of the droplets, 5 to 20 μl as the target volume, was evaluated by the acceleration in a relatively short distance from the beginning of movement. The acceleration of the droplets was also measured when the viscosity and surface tension of the droplet were changed. The size of the projection, made of silicon, was 15 μm square (height: about 20 μm). The area ratio of the projection was mainly used to evaluate the surface performance. The area ratio was varied from 0.9% to 25%. The relationship between the area ratio and the sliding acceleration when the inclined angle was 20° was measured. The water droplets slide almost had a spherical shape for a low area ratio. The sliding acceleration increased with a decrease in the area ratio; when the area ratio was approximately less than 2%, it was close to the theoretical sliding acceleration with no friction. When the area ratio was less than 1 %, the water droplets fell down between the projection gaps. When the area ratio was more than approximately 21%, the acceleration was greatly reduced. Lowering the area ratio was also useful for increasing the sliding acceleration, even for a solution with high viscosity or low surface tension. It was confirmed that the acceleration was significantly enhanced by decreasing the projection density.