Volume Control of Droplets in Vertical Contact-Separation Process Using Radius Difference of Solid–Liquid Interface on Substrate

We investigated the contact-separation behavior of micro fluids, which is an efficient candidate for dispensing mechanisms that can replace pipetting in biochemical assays. During the Vertical contact-separation process (VCSP) of droplets, gravity causes a volume difference, ΔV. To solve this problem, we designed the radius difference of the solid–liquid interface, δR, and proposed to manipulate droplets against gravity. Systematic observations showed that the ΔV monotonically decreased with an increase in the δR, and the droplet volume was maintained (ΔV = 0) at a critical δR*. This behavior quantitatively correlates with a theoretical model based on the force balance between gravity and surface tension under asymmetric boundary conditions. Thus, after the VCSP, the ΔV was maintained at zero and the arbitrary volume of droplets was controlled using the δR. The results showed that the proposed mechanism can suppress the ΔV and quantitatively control the droplet volume. This study contributes to the three-dimensional and accurate manipulation of droplets.