Effects of Nanodroplet Sizes on Wettability, Electrowetting Transition, and Spontaneous Dewetting Transition on Nanopillar-Arrayed Surfaces

Impingement of binary nanodroplets on rough surfaces: a molecular dynamics study

Roughness or texture endow the solid surface with the ability of some particular property of water repellency that has been employed in a variety of practical applications, including self-cleaning, icing-resistant, and so forth. However, the understanding of the dynamic evolution of impacting binary droplets on rough surfaces is not satisfactory, especially at the nanoscale. In this work, we investigate the impact process of the binary droplet system, a suspending droplet impacts a sessile one deposited on hydrophobic textured surfaces, via molecular dynamics (MD) simulations. Dynamic evolutions from MD simulations under various impact conditions are discussed, including coalescence, spreading, retraction and vibration, and bouncing. The free energy variation during the impacting process is calculated to reveal the mechanisms behind the impact dynamics. The effect of the surface texture on the spreading and retraction is investigated, and the corresponding maximum spreading diameter is also discussed. Finally, we investigate the effect of the surface texture on bouncing behavior, which is found to promote the droplet bouncing at low We range but suppress the bouncing behavior at high We range.

Freezing-Melting Mediated Dewetting Transition for Droplets on Superhydrophobic Surfaces with Condensation

The water-repellence properties of superhydrophobic surfaces make them promising for many applications. However, in some extreme environments, such as high humidities and low temperatures, condensation on the surface is inevitable, which induces the loss of surface superhydrophobicity. In this study, we propose a freezing-melting strategy to achieve the dewetting transition from the Wenzel state to the Cassie-Baxter state. It requires freezing the droplet by reducing the substrate temperature and then melting the droplet by heating the substrate. The condensation-induced wetting transition from the Cassie-Baxter state to the Wenzel state is analyzed first. Two kinds of superhydrophobic surfaces, i.e., single-scale nanostructured superhydrophobic surface and hierarchical-scale micronanostructured superhydrophobic surface, are compared and their effects on the static contact states and impact processes of droplets are analyzed. The mechanism for the dewetting transition is analyzed by exploring the differences in the micro/nanostructures of the surfaces, and it is attributed to the unique structure and strength of the superhydrophobic surface. These findings will enrich our understanding of the droplet-surface interaction involving phase changes and have great application prospects for the design of superhydrophobic surfaces.

Coalescence of multiple droplets induced by a constant DC electric field

In this work, the electro-coalescence process of three nanodroplets under a constant DC electric field is investigated via molecular dynamics simulations (MD), aiming to explore the electric manipulation of multiple droplets coalescence on the molecular level. The symmetrical and asymmetrical dynamic evolutions of electrocoalescence process can be observed. Our MD simulations show that there are two types of critical electric fields to induce the special dynamics. The chain configuration can be formed, when one of the critical electric field is exceeded, referred to as Ecc. On the other hand, there is another critical electric field to change the coalescence pattern from complete coalescence to partial coalescence, the so-called Ecn. Finally, we find that the use of the pulsed DC electric field can overcome the drawbacks of the constant DC electric field in the crude oil industry, and the mechanisms behind the suppressed effect of the water chain or non-coalescence are further revealed.

Total Liquid Transfer with Enhanced Contact Line Slippage

A new surface treatment method is developed to achieve total liquid transfer. The transfer process of a liquid droplet is recorded through high-speed photography and analyzed via image analysis to investigate the hydrodynamic interactions. For a pristine PMMA surface, a viscous and viscoelastic liquid facilitates transfer by increased viscous and inertial forces and delayed liquid bridge breakage but is limited by slow contact line slippage. Hydrophobic surface treatments can increase contact line slippage and the receding angle to achieve transfer ratios up to 98%. However, pinning and contact angle hysteresis from surface roughness features limit liquid transfer, especially for smaller droplets and higher separation velocities. A lubricant-infused surface treatment with PDMS and a thin layer of less viscous silicone oil provides a smooth, homogeneous surface with fast slippage, low contact angle hysteresis, and only a slight oil wetting ridge. Liquid could then transfer at high ratios (∼99.9%), regardless of droplet size and separation velocity. Finally, complete transfer liquid from indented cells is demonstrated to show the potential of this surface modification method for gravure printing.

Oblique Impacts of Nanodroplets upon Surfaces

In this work, oblique impacts of nanodroplets impacting surfaces in a wide range of impact angles (α) are investigated in detail via molecular dynamics simulations. Five outcomes are observed, including deposition, prompt splashing, break-up, separation, and shattering. With increasing impact angle, the outcomes of prompt splashing, break-up, separation, and shattering are enlarged but the one of deposition is compressed. By drawing a We_n ∼ α phase diagram, the outcome regimes and corresponding boundaries of them can be successfully identified, and the boundary between the deposition and other outcome regimes is theoretically modeled and shows good agreement with the phase diagram, where We_n is the normal impact Weber number. For further understanding of the oblique impacts, the maximum spreading factor, as the feature parameter of spreading, is investigated. Asymmetry spreading behaviors are observed, noting that β_max,∥ is always larger than β_max,⊥. β_max,⊥ is tested that it only depends on We_n with wide impact angles and could be predicted by the scaling law of β_max,⊥ = 0.7We_n^1/4. However, β_max,∥ depends on not only We_n but also impact angles. A modified model is proposed for predicting β_max,∥ as 0.7We_n^1/4 + 0.001(We_n tan² α)^3/2, which shows good agreement with data on surfaces with θ from 73 to 105° in wide We_n and α ranges.