Lifting a sessile oil drop from a superamphiphobic surface with an impacting one

Bouncing Dynamics of Drops' Successive Off-Center Impact

Bouncing dynamics of a trailing drop off-center impacting a leading drop with varying time intervals and Weber numbers are investigated experimentally. Whether the trailing drop impacts during the spreading or receding process of the leading drop is determined by the time interval. For a short time interval of 0.15 ≤ Δt* ≤ 0.66, the trailing drop impacts during the spreading of the leading drop, and the drops completely coalesce and rebound; for a large time interval of 0.66 < Δt* ≤ 2.21, the trailing drop impacts during the receding process, and the drops partially coalesce and rebound. Whether the trailing drop directly impacts the surface or the liquid film of the leading drop is determined by the Weber number. The trailing drop impacts the surface directly at moderate Weber numbers of 16.22 ≤ We ≤ 45.42, while it impacts the liquid film at large Weber numbers of 45.42 < We ≤ 64.88. Intriguingly, when the trailing drop impacts the surface directly or the receding liquid film, the contact time increases linearly with the time interval but independent of the Weber number; when the trailing drop impacts the spreading liquid film, the contact time suddenly increases, showing that the force of the liquid film of the leading drop inhibits the receding of the trailing drop. Finally, a theoretical model of the contact time for the drops is established, which is suitable for different impact scenarios of the successive off-center impact. This study provides a quantitative relationship to calculate the contact time of drops successively impacting a superhydrophobic surface, facilitating the design of anti-icing surfaces.

Effect of Wettability on the Collision Behavior of Acoustically Excited Droplets

Acoustic droplet ejection (ADE) is a noncontact technique for micro-liquid handling (usually nanoliters or picoliters) that is not restricted by nozzles and enables high-throughput liquid dispensing without sacrificing precision. It is widely regarded as the most advanced solution for liquid handling in large-scale drug screening. Stable coalescence of the acoustically excited droplets on the target substrate is a fundamental requirement during the application of the ADE system. However, it is challenging to investigate the collision behavior of nanoliter droplets flying upward during the ADE. In particular, the dependence of the droplet's collision behavior on substrate wettability and droplet velocity has yet to be thoroughly analyzed. In this paper, the kinetic processes of binary droplet collisions were investigated experimentally for different wettability substrate surfaces. Four states occur as the droplet collision velocity increases: coalescence after minor deformation, complete rebound, coalescence during rebound, and direct coalescence. For the hydrophilic substrate, there are wider ranges of Weber number (We) and Reynolds number (Re) in the complete rebound state. And with the decrease of the substrate wettability, the critical Weber and Reynolds numbers for the coalescence during rebound and the direct coalescence decrease. It is further revealed that the hydrophilic substrate is susceptible to droplet rebound because the sessile droplet has a larger radius of curvature and the viscous energy dissipation is greater. Besides, the prediction model of the maximum spreading diameter was established by modifying the droplet morphology in the complete rebound state. It is found that, under the same Weber and Reynolds numbers, droplet collisions on the hydrophilic substrate achieve a smaller maximum spreading coefficient and greater viscous energy dissipation, so the hydrophilic substrate is prone to droplet bounce.

Drop splashing after impact onto immiscible pools of different viscosities

Droplet impact onto liquid pools is a canonical scenario relevant to numerous natural phenomena and industrial processes. However, despite their ubiquity, multi-fluid systems with the drop and pool consisting of different liquids are far less well understood. Our hypothesis is that the post-impact dynamics greatly depends on the pool-to-droplet viscosity ratioμ_p/μ_d, which we explore over a range of six orders of magnitude using a combination of experiments and theoretical approaches (mathematical modelling and direct numerical simulation). Our findings indicate that in this scenario the splashing threshold and the composition of the ejecta sheet are controlled by the viscosity ratio. We uncover that increasing the pool viscosity decreases the splashing threshold for high viscosity pools (μ_p/μ_d≳35) when the splash comes from the droplet. By contrast, for low viscosity pools, the splash sheet comes from the pool and increasing the pool viscosity increases the splashing threshold. Surprisingly, there are conditions for which no splashing is observed under the conditions attainable in our laboratory. Furthermore, considering the interface velocity together with asymptotic arguments underlying the generation of the ejecta has allowed us to understand meaningful variations in the pressure during impact and rationalise the observed changes in the splashing threshold.

Wetting Effect on Patterned Substrates

A droplet deposited on a solid substrate leads to the wetting phenomenon. A natural observation is the lotus effect, known for its superhydrophobicity. This special feature is engendered by the structured microstructure of the lotus leaf, namely, surface heterogeneity, as explained by the quintessential Cassie-Wenzel theory (CWT). In this work, recent designs of functional substrates are overviewed based on the CWT via manipulating the contact area between the liquid and the solid substrate as well as the intrinsic Young's contact angle. Moreover, the limitation of the CWT is discussed. When the droplet size is comparable to the surface heterogeneity, anisotropic wetting morphology often appears, which is beyond the scope of the Cassie-Wenzel work. In this case, several recent studies addressing the anisotropic wetting effect on chemically and mechanically patterned substrates are elucidated. Surface designs for anisotropic wetting morphologies are summarized with respect to the shape and the arrangement of the surface heterogeneity, the droplet volume, the deposition position of the droplet, as well as the mean curvature of the surface heterogeneity. A thermodynamic interpretation for the wetting effect and the corresponding open questions are presented at the end.

Impact Forces of Water Drops Falling on Superhydrophobic Surfaces

A falling liquid drop, after impact on a rigid substrate, deforms and spreads, owing to the normal reaction force. Subsequently, if the substrate is nonwetting, the drop retracts and then jumps off. As we show here, not only is the impact itself associated with a distinct peak in the temporal evolution of the normal force, but also the jump-off, which was hitherto unknown. We characterize both peaks and elucidate how they relate to the different stages of the drop impact process. The time at which the second peak appears coincides with the formation of a Worthington jet, emerging through flow focusing. Even low-velocity impacts can lead to a surprisingly high second peak in the normal force, even larger than the first one, namely when the Worthington jet becomes singular due to the collapse of an air cavity in the drop.

Particulate-Droplet Coalescence and Self-Transport on Superhydrophobic Surfaces

Particulate transport from surfaces governs a variety of phenomena including fungal spore dispersal, bioaerosol transmission, and self-cleaning. Here, we report a previously unidentified mechanism governing passive particulate removal from superhydrophobic surfaces, where a particle coalescing with a water droplet (∼10 to ∼100 μm) spontaneously launches. Compared to previously discovered coalescence-induced binary droplet jumping, the reported mechanism represents a more general capillary-inertial dominated transport mode coupled with particle/droplet properties and is typically mediated by rotation in addition to translation. Through wetting and momentum analyses, we show that transport physics depends on particle/droplet density, size, and wettability. The observed mechanism presents a simple and passive pathway to achieve self-cleaning on both artificial as well as biological materials as confirmed here with experiments conducted on butterfly wings, cicada wings, and clover leaves. Our findings provide insights into particle-droplet interaction and spontaneous particulate transport, which may facilitate the development of functional surfaces for medical, optical, thermal, and energy applications.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.

Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate

We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this scenario the triple Leidenfrost effect.

Drop-on-Drop Impact Dynamics on a Superhydrophobic Surface

Investigation of multiple droplet interactions is vital due to its importance in various applications, including sprays. In this context, the interaction of a sessile droplet (droplet #1) interaction with an oncoming droplet (droplet #2), in a vertically aligned, drop-on-drop configuration, on a superhydrophobic surface is investigated, both experimentally and computationally. Three droplet impact regimes, with low We number of the impacting droplet (i.e., We_droplet #2 ∼ 1.0) is observed, viz., gentle merging, late merging, and droplet bounce-off without merging. Complementary simulation of the gentle merging regime shows that a very high-pressure gradient is generated between the neck and the bulk region immediately after the contact, which acts as a driving force for the subsequent rapid evolution of the droplet shape. With a larger We number (We_droplet #2 ∼ 8.2 and 14.8), there is always a unique outcome of the impact experiments. The two droplets merge, spread, recede, and bounce-off; there are multiple instances of rebound of the combined droplet mass from the surface. The estimated shear stress emanating during the impact and subsequent dynamics is large enough to cause deterioration of chemically coated superhydrophobic surfaces. The difference in the shape of moving droplet #2 at the instance of impact for We_droplet #2 ∼ 1, as compared to We_droplet #2 ∼ 8.2 and 14.8 is also highlighted. Important time scales during coalescence and beyond are the inertial-capillary time scale (∼16 ms) and viscous-capillary time scale (∼8 s). We also highlight the role of the droplet #1 deposition boundary condition, influence of the We number of droplet #2, and substrate wettability on drop-on-drop interactions, which has direct relevance to the transport mechanisms happening in various engineering applications like spraying pesticides, spray cooling, and rain interactions, etc.

Droplet Dynamics on a Wettability Patterned Surface during Spray Impact

Wettability patterning of a surface is a passive method to manipulate the flow and heat transport mechanism in many physical processes and industrial applications. This paper proposes a rational wettability pattern comprised of multiple superhydrophilic wedges on a superhydrophobic background, which can continuously remove the impacted spray droplets from the horizontal surface. We observed that the spray droplets falling on the superhydrophilic wedge region spread and form a thin liquid film, which is passively transported away from the surface. However, most of the droplets falling on the superhydrophobic region move towards the wedge without any flooding. The physics of the passive transport of the liquid film on a wedge is also delved into using numerical modelling. In particular, we elucidate the different modes of droplet transport in the superhydrophobic region and the interaction of multiple droplets. The observed droplet dynamics could have profound implications in spray cooling systems and passive removal of liquid from a horizontal surface. This study’s findings will be beneficial for the optimization of efficient wettability patterned surfaces for spray cooling application.

How ambient conditions affect the Leidenfrost temperature

By sufficiently heating a solid, a sessile drop can be prevented from contacting the surface by floating on its own vapour. While certain aspects of the dynamics of this so-called Leidenfrost effect are understood, it is still unclear why a minimum temperature (the Leidenfrost temperature T_L) is required before the effect manifests itself, what properties affect this temperature, and what physical principles govern it. Here we investigate the dependence of the Leidenfrost temperature on the ambient conditions: first, by increasing (decreasing) the ambient pressure, we find an increase (decrease) in T_L. We propose a rescaling of the temperature which allows us to collapse the curves for various organic liquids and water onto a single master curve, which yields a powerful tool to predict T_L. Secondly, increasing the ambient temperature stabilizes meta-stable, levitating drops at increasingly lower temperatures below T_L. This observation reveals the importance of thermal Marangoni flow in describing the Leidenfrost effect accurately. Our results shed new light on the mechanisms playing a role in the Leidenfrost effect and may help to eventually predict the Leidenfrost temperature and achieve complete understanding of the phenomenon, however, many questions still remain open.