How Droplets Move on Surfaces with Directional Chemical Heterogeneities

Soft wetting: Substrate softness- and time-dependent droplet/bubble adhesion

HYPOTHESIS

The droplet/bubble adhesion characteristics depend on the length of the droplet/bubble three-phase contact line. Since the deformation caused by the liquid-gas interfacial tension on the soft substrate, referred as to the wetting ridge, retards contact line spreading and retraction, we conjecture that the droplet/bubble adhesion characteristics depend also on the substrate softness.

EXPERIMENTS

Soft substrates with various shear moduli are prepared and characterized by the spreading and receding dynamics of water droplets and underwater bubbles. Snap-in and normal adhesion forces of droplets/bubbles on such soft substrates are directly measured along with the visualized droplet/bubble shape profiles.

FINDINGS

The droplet/bubble snap-in force, which corresponds to the short-time spreading dynamics, decreases with a decrease in the substrate shear modulus because of the retarded contact line spreading. The droplet maximal adhesion force on a soft substrate can be counterintuitively either smaller or larger than its counterpart on the rigid substrate depending on different dwelling times, i.e., the droplet/bubble-substrate contact time before droplet/bubble-substrate separation. The former is attributed to the retarded contact line spreading, whereas the latter is attributed to the retarded contact line retraction. The substrate softness- and dwelling time-dependent droplet/bubble adhesion reported in this study will benefit various applications related to soft substrates.

Soft wetting: an analytical model for pillar topography- and softness-dependent droplet depinning force

The extent to which a droplet pins on a textured substrate is determined by the dynamics of the contact line and the liquid-vapor interface. However, the synergistic contribution of contact line sliding and interface distortion to the droplet depinning force remains unknown. More strikingly, current models fail to predict the depinning force per unit length of droplets on soft pillar arrays. Therefore, we fabricate soft pillar arrays with varying geometrical dimensions and mechanical properties and measure the depinning forces per unit length by allowing droplets to evaporate on such substrates. We then analyze the decrease in excess Gibbs free energy of the apparent droplet caused by the detachment of the droplet boundary from the previously pinned pillars. In contrast to prior notions, based on the measured decreases in excess Gibbs free energy, we find that the coefficient, that governs the ratio of interface distortion's contribution to the depinning force to that of the sliding contact line, increases with a decrease in pillar packing density. By considering the combined contribution from contact line sliding, liquid-vapor interface distortion, and pillar deflection, we introduce an analytical model to predict the droplet depinning force per unit length and corroborate the model using experimental data reported in this and prior studies.

Soft Wetting: Droplet Receding Contact Angles on Soft Superhydrophobic Surfaces

Despite intensive investigations on the droplet receding contact angle on superhydrophobic surfaces, i.e., a key parameter characterizing surface wettability and adhesion, the quantitative correlation between the surface structure mechanical properties (softness) and the droplet receding contact angles remains vague. By systematically varying the geometric dimensions and mechanical properties of soft pillar arrays, we find that the droplet receding contact angles decrease with the decrease in the pillar spring constant. Most surprisingly, the densely packed pillar arrays may result in larger receding contact angles than those on sparsely packed pillars, opposing the understanding of rigid pillar arrays, where the receding contact angles increase with a decrease in the packing density of pillars. This is attributed to the collective effects of capillarity and elasticity, where the energy consumed by the sliding contact line, the energy stored in the distorted liquid-vapor interface, and the energy stored in the deflected pillar contribute to the droplet depinning characteristics. We develop an analytical model to predict the droplet receding contact angles on soft superhydrophobic pillar arrays with knowledge of the material intrinsic receding contact angle, the pillar geometry, and the pillar mechanical properties. The predictions are corroborated by the experimental data measured in this and prior studies.

Percolation in Networks of Liquid Diodes

Liquid diodes are surface structures that facilitate the spontaneous flow of liquids in a specific direction. In nature, they are used to increase water collection and uptake, reproduction, and feeding. However, large networks with directional properties are exceptional and are typically limited up to a few centimeters. Here, we simulate, design, and 3D print liquid diode networks consisting of hundreds of unit cells. We provide structural and wettability guidelines for directional transport of liquids through these networks and introduce percolation theory in order to identify the threshold between a connected network, which allows fluid to reach specific points, and a disconnected network. By constructing well-defined networks with uni- and bidirectional pathways, we experimentally demonstrate the applicability of models describing isotropically directed percolation. We accurately predict the network permeability and the liquid final state. These guidelines are highly promising for the development of structures for spontaneous, yet predictable, directional liquid transport.

The temperature dependent dynamics and periodicity of dropwise condensation on surfaces with wetting heterogeneities

HYPOTHESIS

Biphilic surfaces, namely surfaces comprising hydrophilic areas with a (super)hydrophobic background, are used in nature and engineering for controlled dropwise condensation and liquid transport. These, however, are highly dependent on the surface temperature and subcooling.

EXPERIMENTS

Here, biphilic surfaces were cooled inside a rotatable environmental chamber under controlled humidity. The condensation dynamics on the surface was quantified, depending on the subcooling, and compared to uniform superhydrophobic (USH) surfaces. Rates of condensation and transport were analyzed in terms of droplet number and size, covered area and fluid volume over several length scales. Specifically, from microscale condensation to macroscale droplet roll-off.

FINDINGS

Four phases of condensation were identified: a) initial nucleation, b) droplets on single patches, c) droplets covering adjacent patches and d) multi-patch droplets. Only the latter become mobile and roll off the surface. Cooling the surface to temperatures between T = 2-16 °C shows that lowering the temperature shortens some of the condensation parameters linearly, while others follow a power law, as expected from the theory of condensation. The temperature dependent condensation dynamics on (super)biphilic surfaces is faster in comparison to uniform superhydrophobic surfaces. Nevertheless, within time intervals of a few hours, droplets are mostly immobile. This sets guiding lines for using biphilic surfaces in applications such as water collection, heat transfer and separation processes. Generally, biphilic surfaces are suitable for applications in which fluids should be collected, concentrated and immobilized in specific areas.