Droplet Bouncing and Breakup during Impact on a Microgrooved Surface

Interfacial fluid manipulation with bioinspired strategies: special wettability and asymmetric structures

The inspirations from nature always enlighten us to develop advanced science and technology. To survive in complicated and harsh environments, plants and animals have evolved remarkable capabilities to control fluid transfer via sophisticated designs such as wettability contrast, oriented micro-/nano-structures, and geometry gradients. Based on the bioinspired structures, the on-surface fluid manipulation exhibits spontaneous, continuous, smart, and integrated performances, which can promote the applications in the fields of heat transfer, microfluidics, heterogeneous catalysis, water harvesting, etc. Although fluid manipulating interfaces (FMIs) have provided plenty of ideas to optimize the current systems, a comprehensive review of history, classification, fabrication, and integration focusing on their interfacial chemistry and asymmetric structure is highly required. In this review, we systematically introduce development and highlight the state-of-the-art progress of bioinspired FMIs. Firstly, the biological prototype and development timeline are presented, and the underlying mechanism of on-surface fluid control on versatile structures is analyzed. Secondly, the definition and classification of FMIs as well as the strategy for controlling fluid/interface interaction are discussed. Thirdly, emergent applications of FMIs in practical scenarios including fog/vapor collection, fluid diodes, interfacial catalysis, etc. are presented. Furthermore, the challenges and prospects of interfacial liquid manipulation are concluded. We envision that this review should provide guidance for the incorporation of FMIs into suitable situations, which enlightens interdisciplinary research and practical applications in the fields of interface chemistry, materials design, bionic science, fluid dynamics, etc.

Droplets impact on sparse microgrooved non-wetting surfaces

Droplets impinging on sparse microgrooved polydimethylsiloxane (PDMS) surfaces with different solid fractions was experimentally investigated. First, wettability and stability of droplets on these surfaces was analyzed. The advancing and receding contact angles were found to have a large difference between in the longitudinal direction and in the transverse one, which could be attributed to the anisotropy of the micropatterned surfaces. The judgement of whether a droplet on a sparse microgrooved structure is collapsed or suspended is proposed, and it was found that the droplets were in the Cassie-Baxter wetting state when the actual contact line density is greater than the critical contact line density, while they were in the Wenzel wetting state otherwise. Second, for the case of droplets impacting on sparse microgrooved PDMS surfaces, it was found that droplets can bounce off the micro-patterned surface with a solid fraction of 0.158 when the impact velocity was in a certain range. The lower limit of impact velocity for bouncing droplets can be determined by balancing the kinetic energy of the droplets with the energy barrier due to contact angle hysteresis. The upper limit of impact velocity for bouncing droplets was predicted using a theoretical model taking into account the penetration of liquid into the cavities between microstripes.

Robust Rain-Repellency and Droplet Bouncing Properties of Bauhinia Fresh and Aged Leaves Up to 6 Months

Robust rain-repellent surfaces are useful in roofs, solar panels, windshields, etc. Herein, excellent rain-repellency and droplet bouncing properties of Bauhinia Variegata leaves are presented. They possess surface microbumps (l ∼ 13 μm, w ∼ 8 μm, h ∼ 3 μm), which in turn comprise nanoplatelets (l ∼ 741 nm, t ∼ 59 nm) and Wenzel roughness (r w) of ∼2.2. The leaf's surface energy was estimated to be 9.47 ± 0.03 mJ·m-2 by incorporating rw into the van Oss-Good-Chaudhary theory. The leaves exhibited static contact angle of 157 ± 1°, roll-off angle of 9 ± 1°, and contact angle hysteresis of 12 ± 4°, which retained as they aged up to 186 days in the natural weather and laboratory conditions. The water droplets (10 μL, 40 μL) bounced off for free-fall heights from 5 cm to ∼13 m (Weber no. 36 to ∼2990) and displayed robust rain-repellency (Weber no. ∼4500), similar to that of a lotus leaf. Also, Bauhinia leaves survived pressurized water jets (Weber no. ∼4240). Nevertheless, underwater hydrophobicity has been persistent only for up to 3 h when submerged in 20 cm (∼1.96 kPa gauge pressure) deep water, while lotus leaves retained for >7 h. Such robust Bauhinia leaf's nanoplatelets and wax chemistries can be replicated onto glass/metals for preparing rain-repellent surfaces.

Fundamentals and Applications of Surface Wetting

In an era defined by an insatiable thirst for sustainable energy solutions, responsible water management, and cutting-edge lab-on-a-chip diagnostics, surface wettability plays a pivotal role in these fields. The seamless integration of fundamental research and the following demonstration of applications on these groundbreaking technologies hinges on manipulating fluid through surface wettability, significantly optimizing performance, enhancing efficiency, and advancing overall sustainability. This Review explores the behavior of liquids when they engage with engineered surfaces, delving into the far-reaching implications of these interactions in various applications. Specifically, we explore surface wetting, dissecting it into three distinctive facets. First, we delve into the fundamental principles that underpin surface wetting. Next, we navigate the intricate liquid-surface interactions, unraveling the complex interplay of various fluid dynamics, as well as heat- and mass-transport mechanisms. Finally, we report on the practical realm, where we scrutinize the myriad applications of these principles in everyday processes and real-world scenarios.

Hydrophilic and Hydrophobic Surfaces: Features of Interaction with Liquid Drops

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.

A Study of the Critical Velocity of the Droplet Transition from the Cassie to Wenzel State on the Symmetric Pillared Surface

A droplet hitting a superhydrophobic surface will undergo the Cassie to Wenzel transition when the wetting force exceeds the anti-wetting force. The critical velocity of the droplet’s Cassie to Wenzel state transition can reflect the wettability of the surface. However, the critical velocity research is still at the microscale and has not been extended to the nanoscale mechanism. A cross-scale critical velocity prediction model for superhydrophobic surfaces with symmetric structures is proposed here based on a mechanical equilibrium system. The model’s applicability is verified by experimental data. It demonstrates that the mechanical equilibrium system of droplet impact with capillary pressure and Laplace pressure as anti-wetting forces is more comprehensive, and the model proposed in this study predicts the critical velocity more precisely with a maximum error of 12% compared to the simulation results. Furthermore, the correlation between the simulation at the nanoscale and the evaluation of the macroscopic symmetrical protrusion surface properties is established. Combined with the model and the correlation, the relationship between the microscopic mechanism and the macroscopic examination of droplet dynamics on the superhydrophobic surface be presented, and the wettability evaluation method of macroscopic surfaces based on the molecular simulation mechanism can be realized.

Droplet motion on a wrinkled PDMS surface with a gradient structural length scale shorter than the droplet diameter

Droplet transportation using a wettability gradient surface has attracted much attention owing to applications such as in microfluidic devices. A surface with a spatial structural gradient was prepared through a simple and cost-effective process even though understanding of droplet behavior on the structure was still limited. Here, we report impinging droplet motion on a gradient wrinkled surface. Surfaces were prepared through hard film deposition on soft pre-strained polydimethylsiloxane (PDMS) with a mask installed with a slit to control the amount of deposition, which is related to the wavelength of the wrinkles. Droplets were impinged with varying position with respect to the structure, and the droplet motion was observed in the direction away from the region under the slit. We found an asymmetric contact angle and alternate motion on both sides of the three-phase contact line during the motion according to the gradient of the wrinkle wavelength. These results may help not only to understand the behavior of droplet impingement on a gradient structural surface but also to further develop applications using directional droplet transfer.

Nanowall Textured Hydrophobic Surfaces and Liquid Droplet Impact

Water droplet impact on nanowires/nanowalls' textured hydrophobic silicon surfaces was examined by assessing the influence of texture on the droplet impact dynamics. Silicon wafer surfaces were treated, resulting in closely packed nanowire/nanowall textures with an average spacing and height of 130 nm and 10.45 μm, respectively. The top surfaces of the nanowires/nanowalls were hydrophobized through the deposition of functionalized silica nanoparticles, resulting in a droplet contact angle of 158° ± 2° with a hysteresis of 4° ± 1°. A high-speed camera was utilized to monitor the impacting droplets on hydrophobized nanowires/nanowalls' textured surfaces. The nanowires/nanowalls texturing of the surface enhances the pinning of the droplet on the impacted surface and lowers the droplet spreading. The maximum spreading diameter of the impacting droplet on the hydrophobized nanowires/nanowalls surfaces becomes smaller than that of the hydrophobized as-received silicon, hydrophobized graphite, micro-grooved, and nano-springs surfaces. Penetration of the impacted droplet fluid into the nanowall-cell structures increases trapped air pressure in the cells, acting as an air cushion at the interface of the droplet fluid and nanowalls' top surface. This lowers the droplet pinning and reduces the work of droplet volume deformation while enhancing the droplet rebound height.

Evaluating a transparent coating on a face shield for repelling airborne respiratory droplets

A face shield is an important personal protective equipment to avoid the airborne transmission of COVID-19. We assess a transparent coating on a face shield that repels airborne respiratory droplets to mitigate the spread of COVID-19. The surface of the available face shield is hydrophilic and exhibits high contact angle hysteresis. The impacting droplets stick on it, resulting in an enhanced risk of fomite transmission of the disease. Further, it may get wetted in the rain, and moisture may condense on it in the presence of large humidity, which may blur the user's vision. Therefore, the present study aims to improve the effectiveness of a face shield. Our measurements demonstrate that the face shield, coated by silica nanoparticles solution, becomes superhydrophobic and results in a nominal hysteresis to the underlying surface. We employ high-speed visualization to record the impact dynamics of microliter droplets with a varying impact velocity and angle of attack on coated and non-coated surfaces. While the droplet on non-coated surface sticks to it, in the coated surface the droplets bounce off and roll down the surface, for a wide range of Weber number. We develop an analytical model and present a regime map of the bouncing and non-bouncing events, parametrized with respect to the wettability, hysteresis of the surface, and the Weber number. The present measurements provide the fundamental insights of the bouncing droplet impact dynamics and show that the coated face shield is potentially more effective in suppressing the airborne and fomite transmission.

Droplet impact on pillar-arrayed non-wetting surfaces

Droplet impact on pillar-arrayed polydimethylsiloxane (PDMS) surfaces with different solid fractions was studied. The lower and upper limits of Weber number, We, for complete rebound of impacting droplets decreased with decreasing solid fractions. Gaps were visible during the spreading and retraction processes of bouncing droplets on the surface with a solid fraction of 0.06 while no gaps were observed during the retraction process when We was greater than its upper limit, indicating that there existed a transition from the Cassie-Baxter wetting state to the Wenzel wetting state. Therefore, a novel model accounting for the penetration of a liquid into the cavities between the pillars was developed to predict the upper limit of the impact velocity of bouncing droplets. At high We, partial rebound was observed for surfaces with solid fractions of 0.50 and 0.20 while a sticky state was observed for the surface with a solid fraction of 0.06. Moreover, surface roughness has a great influence on the contact time of bouncing droplets. Besides, the maximum spreading parameter was found to follow a scaling law of We^1/4.

Droplet Impact on Suspended Metallic Meshes: Effects of Wettability, Reynolds and Weber Numbers

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult to measure experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) numerical simulation framework is utilised, previously developed in the general context of the OpenFOAM CFD Toolbox. Droplet impacts on metallic meshes are performed both experimentally and numerically with satisfactory degree of agreement. From the experimental investigation three main outcomes are observed—deposition, partial imbibition, and penetration. The penetration into suspended meshes leads to spectacular multiple jetting below the mesh. A higher amount of liquid penetration is linked to higher impact velocity, lower viscosity and larger pore size dimension. An estimation of the liquid penetration is given in order to evaluate the impregnation properties of the meshes. From the parametric analysis it is shown that liquid viscosity affects the adhesion characteristics of the drops significantly, whereas droplet break-up after the impact is mostly controlled by surface tension. Additionally, wettability characteristics are found to play an important role in both liquid penetration and droplet break-up below the mesh.

Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof

We investigate wetting and water repellency characteristics of Colocasia esculenta (taro) leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf. Scanning electron microscopic images of the leaf surface reveal a two-tier honeycomb-like microstructures, as compared to previously-reported two-tier micropillars on a Nelumbo nucifera (lotus) leaf. We measured static, advancing, and receding angle on the taro leaf and these values are around 10% lesser than those for the lotus leaf. Using standard photolithography techniques, we manufactured bioinspired surfaces with hexagonal cavities of different sizes. The ratio of inner to the outer radius of the circumscribed circle to the hexagon (b/a) was varied. We found that the measured static contact angle on the bioinspired surface varies with b/a and this variation is consistent with a free-energy based model for a droplet in Cassie-Baxter state. The static contact angle on the bioinspired surface is closer to that for the leaf for b/a ≈ 1. However, the contact angle hysteresis is much larger on these surfaces as compared to that on the leaf and the droplet sticks to the surfaces. We explain this behavior using a first-order model based on force balance on the contact line. Finally, the droplet impact dynamics was recorded on the leaf and different bioinspired surfaces. The droplets bounce on the leaf beyond a critical Weber number (We ~  1.1), exhibiting remarkable water-repellency characteristics. However, the droplet sticks to the bioinspired surfaces in all cases of We. At larger We, we recorded droplet breakup on the surface with larger b/a and droplet assumes full or partial Wenzel state. The breakup is found to be a function of We and b/a and the measured angles in full Wenzel state are closer to the predictions of the free-energy based model. The sticky bioinspired surfaces are potentially useful in applications such as water-harvesting.

Theoretical and Experimental Studies on the Controllable Pancake Bouncing Behavior of Droplets

A droplet that impacts on a superhydrophobic surface will undergo a process of unfolding, contracting, and finally rebounding from the surface. With regards to the pancake bouncing behavior of a droplet, since the retraction process of the droplet is omitted, the contact time is greatly shortened compared to the normal type of bouncing. However, the quantitative prediction to the range of droplet pancake bouncing and the adjustment of pancake bouncing state have yet to be probed into. In this paper, we reported the controllable pancake bouncing of droplets by adjusting the size of the superhydrophobic surface with microstructures. In addition, we also discovered a dimensional effect with regards to pancake bouncing, namely, the pancake bouncing would be more likely to happen on the surfaces with large post spacing for the droplet with the larger radius. The contact time could be reduced to 2 ms by adjusting the size of the microstructures and the radius of the droplets. Based on the relationship between the droplet bouncing state and the surface microstructure size, we are able to propose reasonable dimensions for the surfaces in order to control pancake bouncing.

Droplet Impact on the Super-Hydrophobic Surface with Micro-Pillar Arrays Fabricated by Hybrid Laser Ablation and Silanization Process

A super-hydrophobic aluminum alloy surface with decorated pillar arrays was obtained by hybrid laser ablation and further silanization process. The as-prepared surface showed a high apparent contact angle of 158.2 ± 2.0° and low sliding angle of 3 ± 1°. Surface morphologies and surface chemistry were explored to obtain insights into the generation process of super-hydrophobicity. The main objective of this current work is to investigate the maximum spreading factor of water droplets impacting on the pillar-patterned super-hydrophobic surface based on the energy conservation concept. Although many previous studies have investigated the droplet impacting behavior on flat solid surfaces, the empirical models were proposed based on a few parameters including the Reynolds number (Re), Weber number (We), as well as the Ohnesorge number (Oh). This resulted in limitations for the super-hydrophobic surfaces due to the ignorance of the geometrical parameters of the pillars and viscous energy dissipation for liquid flow within the pillar arrays. In this paper, the maximum spreading factor was deduced from the perspective of energy balance, and the predicted results were in good agreement with our experimental results with a mean error of 4.99% and standard deviation of 0.10.

Motion of a Droplet on an Anisotropic Microgrooved Surface

We experimentally characterize the sliding angle of water droplets (volume 3.1-22.2 μL) migrating on inclined microgrooved surfaces along the longitudinal and transverse directions of the grooves. The rectangular microgrooves are manufactured on silicon wafers using standard photolithography techniques. We tilt the surface gradually using a rotating stage mechanism until the incipience of the sliding. The droplet migration in the longitudinal and transverse directions to the grooves is recorded using a high-speed camera. For the droplets migrating downward in the transverse direction, the contact line exhibits a "stick-slip" type motion, that is, the advancing contact line is attached to the surface, whereas the receding contact line is detached from the surface. However, no significant change in the relative position of the advancing and receding contact lines is observed in the case of the longitudinal migration of the droplets. The sliding behavior of the droplet in the longitudinal direction is similar to that observed in the case of a smooth surface. The sliding angle in the longitudinal direction of motion is found to be smaller as compared to that in the transverse motion of the droplet. In both longitudinal and transverse migrations, increasing the pitch of the grooves increases the contact angle, which in turn decreases the sliding angle. As the droplet volume is increased, the component of the gravitational force in the direction of inclination increases, which acts to decrease the sliding angle. A theoretical analysis is also conducted to predict the sliding angle of a droplet on microgrooved surfaces. The model predictions agree with the trends observed in our experiments and thus validate the proposed sliding mechanisms in the longitudinal and transverse migrations of the droplet.

3D mossy structures of zinc filaments: A facile strategy for superamphiphobic surface design

The superamphiphobic surfaces with extreme repellency to liquids are very attractive in many fields, but their fabrication processes are always low effective and expensive. So it is still a challenge to create the superamphiphobic surfaces by simple, time saving and universal method. In this work, the mossy zinc (Zn) filaments, a promising re-entrant structure, was rapidly constructed on various metal surfaces by electrochemical deposition approach. After modification by 1H,1H,2H,2H- perfluorodecyltrichlorosilane (PFDTCS), the Zn@PFDTCS coating exhibited superamphiphobicity in air. The correlation between the morphology of Zn filaments and electrochemical deposition parameters has been studied. The superamphiphobic surface with contact angle higher than 154°, sliding angle lower than 5° and adhesive force lower than 0.043 mN to water and hexadecane was obtained, when the current density was 1.78 A ·dm^-2, the mass fraction of zinc was 0.71 wt% and the deposition time was 40 min. Furthermore, the Zn@PFDTCS 2D-meshes were used to collect oil droplets under water and cut water droplet in oil due to their superoleophilicity under water and superhydrophobicity under oil. We anticipated that the simple and rapid method guides the design of perfect artificial superamphiphobic surfaces in practical application.