Motion of liquid on chemically patterned surfaces is of vital interest in designing microfluidic components and devices for biological applications such as cell analyzers. Of particular importance is how droplets and liquid ridges move along regularly structured surfaces: Do they stay intact or break up owing to instabilities? One scenario is that the drops or ridges remain stuck to areas that are “hydrophilic” until some external force causes depinning and the liquid object moves to another location. In a paper in Physical Review E, Philippe Beltrame at the University of Avignon, France, and colleagues in the US, Germany, and the UK report their numerical studies of how ridges and droplets behave under various conditions of driving force and patterning.
Beltrame et al. model these scenarios by calculating how liquids move past parallel water-repelling stripes when a force is applied parallel to the substrate and perpendicular to the stripes. The driven liquid ridges move in the direction of the force (for instance, down an inclined substrate). The authors find that in some cases the ridges become depinned and pass the stripe intact, or in other cases may break up into droplets before depinning, and only then move towards the next stripe. All in all, Beltrame et al. find seven distinct transition regimes for this behavior. This diversity shows that in addition to requiring a clear understanding of the different kinds of heterogeneities that may cause the pinning, the complex coupling between pinning force and surface of the fluid also has to be taken into account. – David Voss