Numerical analysis of anisotropic wetting of chemically striped surfaces

Anisotropic behaviours of droplets impacting on dielectrowetting substrates

The spreading of a sessile droplet on a solid substrate is enhanced if a non-uniform electric field is applied at the contact-line region. This so-called dielectrowetting effect holds great potential in controlling the spreading of droplets by varying the strength of the electric field. In this paper, we experimentally and theoretically study the effect of the dielectrowetting on the dynamics of droplets impacting on a solid surface having electrodes to impose the non-uniform electric field to the liquid. We experimentally study the anisotropic behaviours in both the spreading and retracting stages: the droplets spread more but retract with significantly smaller rates in the direction parallel to the electrodes. We provide a theoretical explanation for the spreading enhancement caused by dielectrowetting by decoupling it from inertia-induced spreading. We also theoretically account for the reduction in retraction rate using force balance at the contact line. The theoretical analysis in both the spreading and retracting stages is verified experimentally.

1D Roughness Driven Depinning of Self-Assembly of Liquid Droplets

The conventional hexagonal, uniform breath figure pattern formed over smooth substrates and substrates with constraints of the order of 50 μm is distorted when the underlying constraints are down to 1 μm. This paper explores this phenomenon further and concludes that, in addition to topology-based arguments presented by other authors previously, it is necessary to invoke the depinning effects of the three-phase contact line in order to explain the same. The influence of surface constraints on the self-assembly of liquid droplets is investigated. A semiquantitative explanation for large-scale pattern formation consisting of small-scale closely arranged droplets inside the large-scale distorted ring of droplets is presented in this paper. The scale at which the influence of constraints becomes dominant is also determined in this study. It is seen that the underlying roughness has a larger impact than the nature of polymer on pore size. Comparative studies of pore patterns formed on smooth and constrained substrates are reported. The simulated energy-minimized shapes of the droplets on smooth and constrained substrates are obtained using Surface Evolver.

Modeling and Measurement on the Sliding Behavior of Microgrooved Surfaces

The sliding behavior of anisotropic surfaces is a crucial property to their applications from fundamental research to practical fields. Herein, we propose a theoretical model for analyzing the sliding behavior based on the concept of adhesion energy. Surface Evolver simulation is conducted to determine the adhesion energy per unit area. The microgrooved surfaces are fabricated and characterized to validate the proposed theory. It is found that the apparent contact angle measured along the direction parallel to the strips increases with the increase of microgroove width, while the corresponding sliding angles exhibit an opposite trend. The adhesion energy per unit area has a constant value regardless of the droplet volume. The different sliding behaviors of anisotropic surfaces along the perpendicular and parallel directions are attributed to the difference in the corresponding adhesion energies per unit area. The proposed model is expected to be used for predicting the sliding behavior of anisotropic surfaces.

Stability of Evaporating Droplets on Chemically Patterned Surfaces

The stability of water droplets on striped surfaces exposing regions of different wettability is studied experimentally, numerically, and based on a scaling model. Different values of the stripe widths and different contact angle contrasts between the hydrophilic and hydrophobic stripes are considered. The boundary between the contact angle contrasts leaving the droplets intact and those leading to droplet breakup is computed numerically. The minimum contrast for which breakup occurs increases with increasing hydrophobic contact angle. The existence of an unstable and a stable regime is confirmed experimentally. In the unstable regime, when approching droplet breakup, a configuration with two liquid fingers on the hydrophilic stripes connected by a capillary bridge on the hydrophobic stripe is found. For decreasing volumes, the width of this capillary bridge decreases until a critical value is reached at which the droplet breaks up. The critical width depends on the ratio of the hydrophilic and the hydrophobic stripe width. A simple scaling model is presented with which the critical width can be predicted. According to the model, the droplet becomes unstable when the increasing Laplace pressure inside the bridge can no longer be balanced by the pressure inside the liquid fingers on the hydrophilic stripes.