Self-Transport and Manipulation of Aqueous Droplets on Oil-Submerged Diverging Groove

We report experimental study of self-transport of aqueous droplets along an oil-submerged diverging groove structure. The migration phenomenon is illustrated, and the effect of various parameters such as droplet size d, oil layer thickness h, groove angle 2θ, and groove thickness δ on the droplet transport behavior (i.e., migration velocity and length) is investigated. Our study reveals that complete engulfment of aqueous droplets in the oil layer, that is attributed to a positive spreading parameter (S > 0), is a prerequisite for the droplet transport. The results show that only droplets of diameter larger than the oil layer thickness (i.e., d ≥ h) get transported owing to a differential Laplace pressure between the leading and trailing faces of a droplet because of the diverging groove. Using experimental data, the variation of droplet migration velocity with distance along the diverging groove is correlated as U(x) = ψx-0.9, where ψ = d0.32θ-2.2h-1.5δ0.7. The submerged groove structure was used to demonstrate simultaneous and sequential coalescence and transport of multiple droplets. Finally, the submerged groove structure was employed for extraction of aqueous droplets from oil. The proposed technique opens up a new avenue for evaporation and contamination free transport and coalescence of droplets for chemical and biological applications.