Universal scales of droplet spreading on wettability-patterned wedge tracks

Spontaneous transport of liquid droplets on open-surface microfluidic platforms is important for a wide range of applications, and can be facilitated by having a difference of wettability on different spatial domains of the substrate. Recent studies have shown that a trapezoidal or wedge-shaped superhydrophilic track on a superhydrophobic substrate can transport microvolumes of fluid from the narrower to the wider end of the track at velocities of the order of several hundreds of mm/s and without the use of a pump or any external actuation system. Application areas of such tracks include, among others, the transport of droplets of complex biofluids in point-of-care devices, which calls for the knowledge of the spreading behavior of viscous droplets on wettability-patterned surfaces. The wetting behavior of droplets of different viscosities was observed, and a universal relationship was obtained between two dimensionless variables, which accurately described the spreading characteristics of a droplet regardless of its viscosity. Three distinct droplet spreading regimes were observed - the spreading exhibiting transition initially from a Washburn-type to a Laplace-pressure driven flow, and finally to a Tanner-type regime.