Volume difference in microdroplets in vertical contact control under electrowetting-induced asymmetric boundary condition

Three-dimensional manipulation of microdroplets is attractive for biochemical science applications. In vertical contact control, we contact a microdroplet and an opposite droplet to diffuse chemicals between microdroplets. Then, we separate coalescent droplets into two microdroplets. However, vertical contact control of opposite microdroplets results in a volume difference due to gravity. We integrate electrowetting into the wetting pattern substrates to control the volume difference. When voltage is applied to electrowetting substrates, dielectric interaction generates an attractive force in the opposite direction to gravity. We quantitatively investigated the volume difference after vertical contact control under applied voltage. Thus, we discovered that the volume difference monotonously decreases as the voltage increases. Next, our force balance theory, in which the effect of electrowetting is an asymmetric boundary condition, quantitatively correlates with experimental results.