Successive Rebounds of Impinging Water Droplets on Superhydrophobic Surfaces

Directional Continuous Bouncing Behavior of Water Droplets on a Surface with a Chemical Gradient

Manipulation of directional bouncing behavior of liquid droplets after impacting solid surfaces is highly significant for biological, agricultural, engineering, and industrial applications. Here, we prepared a surface with a chemical gradient on a Ti-6Al-4V substrate, on which directional multiple bouncing of droplets and long-range movement has been achieved. The wetting gradient of the vapor-deposited surface reached 2.5° mm-1 by finely controlling the distribution of low surface energy functional groups. Droplet adhesion force analysis was carried out to visualize the variation of surface wettability. On this surface with inhomogeneous wettability, the droplet repeated the impacting and rebounding 8 times along the direction of the chemical gradient, displaying an interesting phenomenon of "droplet trampoline". The maximal rebound height and the horizontal jumping distance reached 8.36 and 10.19 mm, respectively. Additionally, the underlying mechanism behind this consecutive bouncing behavior of droplets was thoroughly elucidated from energy and force perspectives. This research is anticipated to advance the understanding of directional continuous bounce behavior of a droplet and provide a promising strategy to delicately manipulate the movement of droplets.

Droplet impact dynamics on the surface of super-hydrophobic BNNTs stainless steel mesh

The 'gas‒liquid‒solid' mechanism annealing method was used to create a superhydrophobic boron nitride nanotube (BNNT) stainless steel mesh in a tube furnace at 1250 °C in an NH3 environment. Fe powder was used as a catalyst, and B:B2O3 = 4:1 was used as the raw material. The water droplets on the surface of the superhydrophobic material had a contact angle of approximately 150° and a slide angle of approximately 3°. By using molecular dynamics (MD) simulation technology, a three-dimensional braided physical model of nanodroplets and superhydrophobic BNNT mesh surfaces with the same contact angle and rolling angle was prepared via the function weaving method. The Weber number (We) was used as the entrance point to establish the relationship between macroscale experimental studies and nanoscale MD simulation analysis on the basis of these efforts. A study was conducted on the dynamic behaviour of droplets impacting a superhydrophobic BNNT filter surface. We suggest that the wettability, substrate structure, and impact velocity are connected to the impact dynamic behaviour of droplets on the basis of the data obtained at various scales. The findings demonstrate that when the droplet impact velocity increases, several droplet phenomena-such as impact-rebound, impact-spread-rebound, and impact-spread-breaking-polymerisation-spatter-appear on the substrate surface sequentially. The mechanism of impact behaviour at various scales is explained in light of these events. Furthermore, a better theoretical model is proposed to assess the droplet wetting transition at the nanoscale. This model accurately predicts the boundary Weber number that starts the wetting transition. Moreover, the connections among the impact velocity, spreading diameter, and contact time (or We) are examined. The tendencies found via MD simulations match the outcomes of the experiments. Our discoveries and outcomes broaden our understanding of how droplet impact affects the dynamic behaviour of superhydrophobic surfaces. A scientific foundation for examining the dynamic behaviour of droplets is provided by combining simulations and experiments.

Probing the contact time of droplet impacts: From the Hertz collision to oscillation regimes

Droplet rebound on nonwetting surfaces is a common phenomenon. However, the underlying physics regulating the contact time remains unclear. In this work, we investigate droplet impacts on superamphiphobic surfaces through experiments and theoretical analyses. By analyzing the spreading and retraction of droplet impinging processes over a wide range of Weber numbers (We), it is revealed that droplet impacts experience three regimes as We is varied, which are denoted as the Hertz collision (We<1), transition (110) regimes. In the Hertz collision regime, the droplet impinging process is temporally symmetric, i.e., the spreading time, t_{S}, and the retraction time, t_{R}, are almost the same. Furthermore, t_{S} and t_{R} decrease with increasing We and follow a power-law dependence, which is different from previous theories. In the transition regime, t_{S} remains dominated by the Hertz collision, while t_{R} is governed by droplet oscillation. In the oscillation regime, t_{S}, t_{R}, and the total contact time, t_{C}, become independent of We. These three regimes are valid for both monophase and compound droplets. The findings in this work advance the understanding and offer a clear picture of droplet impact dynamics.

Experimental investigation on the bouncing dynamics of a liquid marble during the impact on a hydrophilic surface

Liquid marbles are droplets coated by hydrophobic particles. At low Weber numbers (We), when impacting a hydrophilic surface, the marble may bounce on the substrate repeatedly without any rupturing until the quiescence condition is achieved. The marble bouncing has gained far less attention, although its rich underlying physics is due to the interaction between liquid core, hydrophobic grain, and surrounding air. Accordingly, this research experimentally scrutinizes the marble impact and subsequent bouncing on a hydrophilic surface for the first time. Additionally, the conversion of kinetic, gravitational potential, inertial, and surface energies occurring regularly during the impact is exhaustively surveyed. Moreover, the effect of Weber and gravitational Bond numbers (Bo) on the bouncing time, maximum spreading time, maximum spreading ratio, maximum elongation ratio, and maximum restitution are investigated, which characterize the marble impact and bouncing dynamics. This study is one of the limited investigations exploring the effects of the gravitational Bond number on the results. Dimensionless correlations are proposed for the mentioned parameters based on the experimental data. Furthermore, utilizing the simplifying theoretical presumptions, correlations are suggested based on the scale analysis for the spreading time and maximum spreading ratio. The results imply that the mentioned parameters behave differently at low and moderate Weber numbers, though the distinction is more pronounced in the case of the bouncing time, maximum spreading time and maximum spreading ratio. Although increasing with the Weber number when WeWecr. In addition, the maximum elongation ratio linearly grows with the Weber number.

Droplet jump from a particle bed

Drop tower experiments have been performed to study droplet jump from a particle bed across a wide range of fluid viscosities. Here the droplet jumps from the particle bed in response to the apparent step reduction from terrestrial gravity to microgravity when the experiment is dropped and enters free fall. The presence of a particle layer has been found to affect contact line dissipation and the overall jumping behavior of droplets. Additionally, the study has identified the impact of the Ohnesorge number (Oh) on droplet morphology. The investigation has yielded results that not only validate a modified version of the spring-mass-damper model for droplet rebound [Jha et al., Soft Matter, 2020, 16, 7270] but also extend its applicability to previously unexplored initial conditions. In particular, the model predicts droplet jump time and velocity. Moreover, the presence of particle layers has been found to effectively eliminate contact line dissipation without introducing substantial additional forms of dissipation. Experiments have been conducted at the Dryden Drop Tower facility at Portland State University. Particle beds have been constructed using polyethylene and polystyrene poly-dispersed spheres with diameters ranging from 125-150 μm and 600-1000 μm, respectively. The beds have been created by depositing a thin layer of particles on a glass substrate. The experimental conditions have allowed the exploration of a large parameter space of Bo0 1.8-8.6, We 0.05-1.40, and Oh 0.001-1.900.

Golden section criterion to achieve droplet trampoline effect on metal-based superhydrophobic surface

Clarifying the consecutive droplet rebound mechanisms can provide scientific inspirations to regulate dynamic wettability of superhydrophobic surface, which facilitates the practical applications on efficient heat control and active anti-icing. Generally, droplet rebound behaviors are directly affected by surface structure and Weber number. Here, we report a novel "golden section" design criterion to regulate the droplet rebound number determined by the structure spacing, subverting conventional knowledge. Especially, the droplet can continuously rebound for 17 times on the metal-based surface, exhibiting an amazing phenomenon of "droplet trampoline". The droplet rebound number has been experimentally revealed to be closely related to Weber number. We propose novel quantitative formulas to predict droplet rebound number and clarify the coupling effect of the structure spacing and the Weber number on the rebound mechanisms, which can be utilized to establish the regulation criteria of rebound numbers and develop novel metal-based superhydrophobic materials.

Compound Droplet Impact on a Thin Hydrophobic Cylinder

The impact of compound droplets on solid surfaces is a ubiquitous phenomenon that pervades both the natural and technological fields. A comprehensive understanding of the dynamics of the droplet impact on solid surfaces is therefore of paramount importance for a broad range of applications. In this study, we investigate the impact of a water-in-oil compound droplet on a thin hydrophobic cylindrical surface, with regard to the Weber number and cylinder dimensions. Owing to the prewetting effect of the oil, the droplet completely engulfs the cylinder during impact. The ensuing breakups of oil and water engender various unique impact outcomes, which are depicted via a phase map. The phase boundaries are described by analyzing the gravitational and drag forces exerted by the cylinder. A threshold value of the Weber number is found beyond which its effect on the azimuthal spreading process becomes less obvious. The distinctive axial spreading processes of oil and water are illustrated through high-speed imaging from both front and side perspectives, revealing that droplet oscillation is critically influenced by the Weber number. Our work elucidates the impact dynamics of compound droplets on curved surfaces, providing pivotal insights into related thermal management, droplet printing, and coating fabrication applications.

Dynamics Behavior of Droplet Impact on a Controllable Curved Micropillar Array Surface Induced by a Magnetic Field

Many fields would greatly benefit from the realization of the manipulation of droplet impact behavior by an asymmetric surface structure, such as self-cleaning, anti-icing, inkjet printing, etc. However, research on the prediction of the impact of the dynamics of small-volume droplets on the asymmetric superhydrophobic surface has been insufficient. In this study, a superhydrophobic curved micropillar array surface with controllable bending angles induced by a magnetic field was prepared. The impact and rebound behaviors of the nanoliter droplets with diameters of 100-300 μm were investigated. The experimental results showed the positive correlation between the threshold Weber number of the impact morphology transition of the droplet and the inclination angle of the micropillar. In addition, the restitution coefficient, which measures the degree of energy loss during the impact process, showed a nonmonotonic dependence on the Weber number. A critical velocity model of the impact morphology transition of the droplet on the curved micropillar array surface and a prediction model of the restitution coefficient of the droplet in different impact morphologies are suggested. Our findings will help in the design of a functional surface for manipulating the impact behavior of the droplet.

Self-Healing of Biocompatible Superhydrophobic Coatings: The Interplay of the Size and Loading of Particles

The broad application potential of superhydrophobic coatings is limited by the usage of environment-threatening materials and poor durability. The nature-inspired design and fabrication of self-healing coatings is a promising approach for addressing these issues. In this study, we report a fluorine-free and biocompatible superhydrophobic coating that can be thermally healed after abrasion. The coating is composed of silica nanoparticles and carnauba wax, and the self-healing is based on surface enrichment of wax in analogy to the wax secretion in plant leaves. The coating not only exhibits fast self-healing, just in 1 min under moderate heating, but also displays increased water repellency and thermal stability after healing. The rapid self-healing ability of the coating is attributed to the relatively low melting point of carnauba wax and its migration to the surface of the hydrophilic silica nanoparticles. The dependence of self-healing on the size and loading of particles provides insights into the process. Furthermore, the coating exhibits high levels of biocompatibility where the viability of fibroblast L929 cells was ∼90%. The presented approach and insights provide valuable guidelines in the design and fabrication of self-healing superhydrophobic coatings.

Energy Loss for Droplets Bouncing Off Superhydrophobic Surfaces

A water droplet can bounce off superhydrophobic surfaces multiple times before coming to a stop. The energy loss for such droplet rebounds can be quantified by the ratio of the rebound speed U_R and the initial impact speed U_I; i.e., its restitution coefficient e = U_R/U_I. Despite much work in this area, a mechanistic explanation for the energy loss for rebounding droplets is still lacking. Here, we measured e for submillimeter- and millimeter-sized droplets impacting two different superhydrophobic surfaces over a wide range of U_I (4-700 cm s^-1). We proposed simple scaling laws to explain the observed nonmonotonic dependence of e on U_I. In the limit of low U_I, energy loss is dominated by contact-line pinning and e is sensitive to the surface wetting properties, in particular to contact angle hysteresis Δ cos θ of the surface. In contrast, e is dominated by inertial-capillary effects and does not depend on Δ cos θ in the limit of high U_I.