Directional Liquid Wicking in Regular Arrays of Triangular Posts

Abstract

Wicking of wetting liquids into micro-patterns of posts with homogeneous triangular cross-section is studied in experiments and by numerical energy minimizations. To test for directional wicking we fabricated regular arrays of posts with various combinations of line fractions and aspect ratios using standard photolithography processes. In agreement with numerical energy minimizations of the liquid film morphology, we find spontaneous wicking in the experiments only for line fractions and aspect ratios where the homogeneous liquid film represents the state of lowest interfacial free energy and where no local energy minimum could be detected in our numerical energy minimizations. The numerical results further demonstrate that the stability of a certain morphology of the terminal meniscus controls the direction of wicking relative to the orientation of the triangular posts. The observed selectivity of spontaneous wicking in respect to the meniscus orientation can be exploited to build a micro-fluidic rectifier for partially wetting liquids.

De-wetting of evaporating drops on regular patterns of triangular posts

“…15,16 Chemical modification of certain parts of the surface can also be employed to generate uni-, bi-, and tri-directional wicking in micro-patterns of topographic features that conform to the reflection symmetry of the underlying lattice. 17,18 Directional wicking in periodic arrays of posts with triangular cross section has been studied in wetting experiments, 19,20 in simulations, 21,22 and in numerical energy minimizations. 20 An example of a topographic structure that promotes directional wicking in nature is the skin of the Texas horned lizard.…”

“…17,18 Directional wicking in periodic arrays of posts with triangular cross section has been studied in wetting experiments, 19,20 in simulations, 21,22 and in numerical energy minimizations. 20 An example of a topographic structure that promotes directional wicking in nature is the skin of the Texas horned lizard. 23 Several studies derive necessary conditions for liquid wicking on micro-patterned surfaces from simple thermodynamic considerations.…”

“…Because of their simplicity, thermodynamic arguments do not account for the possibility that a liquid interface at the edge of the film is trapped in one of the numerous local energy minima. 20,24 Those metastable states of the interface can cause an arrest of the liquid film edge, even if the interfacial energies averaged over length scales larger than the typical dimensions of the topographies would allow wicking or de-wicking of the liquid. 20,24 In the present work, we consider the de-wetting of evaporating drops on surfaces with micro-patterns of triangular posts employed in a previous study on directional wicking.…”

“…20,24 Those metastable states of the interface can cause an arrest of the liquid film edge, even if the interfacial energies averaged over length scales larger than the typical dimensions of the topographies would allow wicking or de-wicking of the liquid. 20,24 In the present work, we consider the de-wetting of evaporating drops on surfaces with micro-patterns of triangular posts employed in a previous study on directional wicking. 20 Directional wicking and de-wicking of liquid films in a periodic lattice of triangular posts can be understood from the morphology of the interface close to the three-phase contact line and the mechanisms that lead to interfacial advance on the length-scale of the posts.…”

“…20,24 In the present work, we consider the de-wetting of evaporating drops on surfaces with micro-patterns of triangular posts employed in a previous study on directional wicking. 20 Directional wicking and de-wicking of liquid films in a periodic lattice of triangular posts can be understood from the morphology of the interface close to the three-phase contact line and the mechanisms that lead to interfacial advance on the length-scale of the posts. 20 In a certain range of density and aspect ratio of the posts, the orientation of a film edge relative to the micro-pattern determines whether the film advances, recedes, or is arrested.…”