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An experimental visualization is undertaken to investigate the impact dynamic behaviors of water, absolute ethanol, and low surface energy droplets with different viscosities impacting on hydrophobic surfaces. Droplets’ impacting behaviors, including spreading, rebounding, and oscillation retraction, are observed and quantitatively characterized by transient spreading factor and maximum spreading diameter. Effects of droplet impact velocity, surface wettability, and droplet viscosity on the impact dynamics are explored and analyzed. As the droplet impact velocity increases, the droplet kinetic energy increases, resulting in an increase in the spreading factor and spreading velocity simultaneously. Hydrophobic surfaces are not easy to be wetted by water droplets due to their low surface energy, leading to the partial rebound of water droplets when impacting on the hydrophobic surfaces. However, this phenomenon does not occur when low surface energy droplets, such as absolute ethanol and simethicone, impact on hydrophobic surfaces at the same velocity. The increasing droplet viscosity enhances the viscous dissipation, slowing down the impact process and inhibiting the droplet spreading, oscillation, and retraction behaviors. Based on the energy conservation method, a universal model for the maximum spreading factor of low surface energy droplets with different viscosities impacting on hydrophobic surface was established. According to the experimental results, a new spreading time model tm = 2D0/U0 was proposed to enhance applicability of the model for low surface energy droplets with high viscosity, reducing the calculation error to less than 10%.

The processes of interaction of liquid droplets with solid surfaces have become of interest to many researchers. The achievements of world science should be used for the development of technologies for spray cooling, metal hardening, inkjet printing, anti-icing surfaces, fire extinguishing, fuel spraying, etc. Collisions of drops with surfaces significantly affect the conditions and characteristics of heat transfer. One of the main areas of research into the interaction of drops with solid surfaces is the modification of the latter. Changes in the hydrophilic and hydrophobic properties of surfaces give the materials various functional properties—increased heat transfer, resistance to corrosion and biofouling, anti-icing, etc. This review paper describes methods for obtaining hydrophilic and hydrophobic surfaces. The features of the interaction of liquid droplets with such surfaces are considered. The existing and possible applications of modified surfaces are discussed, as well as topical areas of research.

By using molecular dynamics simulation, we investigate the wettability of a surface texturized with a periodic array of hierarchical pillars. By varying the height and spacing of the minor pillars on top of major pillars, we investigate the wetting transition from the Cassie–Baxter (CB) to Wenzel (WZ) states. We uncover the molecular structures and free energies of the transition and meta-stable states existing between the CB and WZ states. The relatively tall and dense minor pillars greatly enhance the hydrophobicity of a pillared surface, in that, the CB-to-WZ transition requires an increased activation energy and the contact angle of a water droplet on such a surface is significantly larger.

In this study, we developed a one-step method for fabricating hydrophobic surfaces on copper (Cu) substrates. Cuprous oxide (Cu2O) with low free energy was successfully formed after low-fluence laser direct irradiation. The formation of Cu2O enhanced the hydrophobicity of the Cu substrate surface, and the contact angle linearly increased with the proportion of Cu2O. The Cu2O fabricated by low-fluence laser treatment showed the same crystal plane orientation as the pristine Cu substrate, implying an epitaxial growth of Cu2O on a Cu substrate.

The motion of ferrofluid droplet on hydrophobic surface has attracted significant research interests due to its potential applications in biomedicine, sensor technologies, and oil–water separation processes. This study investigates ferrofluid droplet impact on a hydrophobic surface and analyzes how ferromagnetic particles in the droplet affect the interaction characteristics of droplet in terms of spreading, retraction, and the formation of secondary droplets during rebound cycles. The results reveal that introducing a magnetic field reduces the droplet spreading factor by approximately 20%. Additionally, increasing ferro particles concentrations to 0.05% (wt%) gives rise to a significant reduction in the spreading factor. The effect on the droplet rebound height is also noteworthy: for a lower particle concentration (0.005%, wt%), the restitution coefficient decreases by 15.5%. However, at high weight concentration (0.05%), this decrease is as much as 69%, which indicates a strong dependency on particle concentration. The analysis shows that the force generated under magnetic field, which is around 2.28 × 10 −5  N, is substantially greater than the capillary force, which is ~ 2.69 × 10 −6  N, leading to the attachment of droplets at magnet when the spacing from the magnet to droplet becomes small. During the rebound, the droplet experiences momentum dissipation, which results in the break-off of smaller “newborn” droplets. This phenomenon is primarily governed by the interfacial force (∼ 1.47 × 10 −4  N) due to the particles and the surrounding liquid that is larger than the magnetic force itself. The newborn droplets tend to have a higher concentration of particles and are prone to sticking on the magnet.

Hydrophobic surfaces have a wide range of applications, such as water harvesting, self-cleaning, and anti-biofouling. However, traditional methods of achieving hydrophobicity often involve the use of toxic materials such as fluoropolymers. This study aims to create controllable wettability surfaces with a three-dimensional geometry using a laser base powder bed fusion (PBF) process with commercially pure titanium (CP-Ti) and silicone oil as non-toxic materials. The optimal PBF process parameters for fabricating micropillar structures, which are critical for obtaining the surface roughness necessary for achieving hydrophobic properties, were investigated experimentally. After fabricating the micropillar structures using PBF, their surface energy was reduced by treatment with silicone oil. Silicone oil provides a low-surface-energy coating that contributes to the water-repellent nature of hydrophobic surfaces. The wettability of the treated CP-Ti surfaces was evaluated based on the diameter of the pillars and the space between them. The structure with the optimal diameter and spacing of micropillars exhibited a high contact angle (156.15°). A pronounced petal effect (sliding angle of 25.9°) was achieved because of the morphology of the pillars, indicating the controllability of wetting. The micropillar diameter, spacing, and silicone oil played crucial roles in determining the water contact and sliding angle, which are key metrics for surface wettability.

In the fields of agriculture, medical treatment, food, and packaging, polymers are required to have the characteristics of self-cleaning, anti-icing, and anti-corrosion. The traditional preparation method of hydrophobic coatings is costly and the process is complex, which has special requirements on the surface of the part. In this study, fused deposition modeling (FDM) 3D printing technology with design and processing flexibility was applied to the preparation of hydrophobic coatings on polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) parts, and the relationship between the printing process parameters and the surface roughness and wettability of the printed test parts was discussed. The experimental results show that the layer thickness and filling method have a significant effect on the surface roughness of the 3D-printed parts, while the printing speed has no effect on the surface roughness. The orthogonal experiment analysis method was used to perform the wettability experiment analysis, and the optimal preparation process parameters were found to be a layer thickness of 0.25 mm, the Grid filling method, and a printing speed of 150 mm/s.

The motion of a water droplet in a hydrophobic wedge fixture was examined to assess droplet rolling and spinning for improved dust mitigation from surfaces. A wedge fixture composed of two inclined hydrophobic plates had different wetting states on surfaces. Droplet rolling and spinning velocities were analyzed and findings were compared with the experiments. A wedge fixture was designed and realized using a 3D printing facility and a high speed recording system was adopted to evaluate droplet motion in the wedge fixture. Polycarbonate sheets were used as plates in the fixture, and solution crystallization and functionalized silica particles coating were adopted separately on plate surfaces, which provided different wetting states on each plate surface while generating different droplet pinning forces on each hydrophobic plate surface. This arrangement also gave rise to the spinning of rolling droplets in the wedge fixture. Experiments were extended to include dust mitigation from inclined hydrophobic surfaces while incorporating spinning- and rolling droplet and rolling droplet-only cases. The findings revealed the wedge fixture arrangement resulted in spinning and rolling droplets and spinning velocity became almost 25% of the droplet rolling velocity, which agrees well with both predictions and experiments. Rolling and spinning droplet gave rise to parallel edges droplet paths on dusty hydrophobic surfaces while striations in droplet paths were observed for rolling droplet-only cases. Spinning and rolling droplets mitigated a relatively larger area of dust on inclined hydrophobic surfaces as compared to their counterparts corresponding to rolling droplet-only cases.

Self-cleaning surfaces revolutionizing the technology world due to their novel property of cleaning themselves, and its multi-functional self-cleaning surfaces exhibit at least one or more functional properties (transparent, conducting, anti-bacterial, anti-corrosion, etc.) This review article focuses on the fundamentals of wettability, material parameters controlling surface wettability and three different paths to realization of self-cleaning surfaces, i.e., (i) super-hydrophobic, (ii) super-hydrophilic and (iii) photocatalytic. The subsequent part of the article mostly focuses on the super-hydrophobic path towards realizing self-cleaning surfaces. In the super-hydrophobic path, the objective is to make the surface extremely repellent to water so that water droplets slide and ‘roll off’ from the surface. The next section of the review article focuses on the role of additive manufacturing in the fabrication of super-hydrophobic micro-structures. Amidst the different fabrication processes of self-cleaning surfaces, additive manufacturing stays ahead as it has the manufacturing capacity to create complex micro-structures in a scalable and cost-effective manner. A few prominent types of additive manufacturing processes were strategically chosen which are based on powder bed fusion, vat photopolymerization, material extrusion and material jetting techniques. All these additive manufacturing techniques have been extensively reviewed, and the relative advantages and challenges faced by each during the scalable and affordable fabrication of super-hydrophobic self-cleaning surfaces have been discussed. The article concludes with the latest developments in this field of research and future potential. These surfaces are key to answer sustainable development goals in manufacturing industries. Graphical abstract

In inverted perovskite solar cells (PSCs), high-quality perovskite film grown on hole-transporting material (HTM) with pinhole-free coverage and a large grain size is crucial for high efficiency. Here, we report on the growth of pinhole-free and large grain CH3NH3PbI3 crystals favored by a hydrophobic small molecular HTM, namely, 4,4′-Bis(4-(di-p-toyl)aminostyryl)biphenyl (TPASBP). The hydrophobic surface induced by TPASBP suppressed the density of the perovskite nuclei and heterogeneous nucleation, thus promoting the perovskite to grow into a dense and homogeneous film with a large grain size. The CH3NH3PbI3 deposited on the TPASBP exhibited better crystallization and a lower trap density than that on the hydrophilic surface of indium tin oxide (ITO), resulting in a significant reduction in carrier recombination. Combined with the efficient hole extraction ability of TPASBP, a high efficiency of 18.72% in the inverted PSCs fabricated on TPASBP was achieved.