How Droplets Move on Surfaces with Directional Chemical Heterogeneities

The nature of droplet adhesion to surfaces has long been a subject of scientific discussion. Quantification and prediction of these adhesion forces become even more challenging on surfaces with complex chemistry and wettability. However, comprehending these forces is crucial for deriving design principles for artificial interfaces and for describing liquid motion in nature. Here, it is demonstrated that the adhesion of droplets, both in the static and dynamic regimes, is very sensitive to the direction of chemical heterogeneities. Experiments using bending beams and droplet roll-off are used to quantify the droplet-surface adhesion forces and fluorescent microscopy is used to directly observe the shape of the droplet contact line with the surface. We elucidate the origin of direction-dependent adhesion on surfaces with hydrophilic areas embedded in a hydrophobic background and show it is the result of the fluid-surface contact line geometry on the microscale. In other words, one should look at the shape of the receding part of a droplet contact line and the way it pins to surface heterogeneities.