Directional Spreading of Viscous Droplets on Wettability-Patterned Diverging Tracks

Spontaneous pumpless transport of liquid droplets on wettability-patterned tracks is important for a number of diverse applications such as rapid transport and mixing of fluid droplets, enhancing dropwise condensation on surfaces, and in the biomedical sector as well. Recent studies point to the fact that, on an open surface, a superhydrophilic diverging track laid on a superhydrophobic background results in pumpless transport of water from the narrow end to the wide end at unprecedented rates. However, the interplay between the driving capillary force and the resisting viscous force, which governs the spreading behavior of liquid droplets on such surfaces, have so for not been characterized. Potential applications for transporting organic liquids and in point-of-care devices hence calls for understanding the spreading behavior of viscous droplets on such surfaces. An effort to do the same has been made in the present work by experimentally observing the spreading of liquid droplets of different viscosities and surface tensions on the aforementioned wettability-patterned diverging or wedge-shaped tracks. An universal relationship of the spreading behavior in terms of two dimensionless variables is obtained. The liquid spreading front was found to follow three distinct temporal scales, transitioning from a Washburn-type spreading to a much faster Laplace-pressure driven spreading, and finally to an extremely slow, density augmented-Tanner-type spreading.