Adhesion Of Water Droplets Research Articles

Rice husk-derived self-healing superhydrophobic films using solvent-less approach for drag reduction and oil absorption behaviour

The present work focuses on a sustainable approach to transform rice-husk waste into self-healing superhydrophobic films with potential application for drag-reduction and oil absorption. Rice husk was transformed into amorphous nano silica particles (d50 ∼ 150 nm) exhibiting mesoporosity with a surface area of 400 m2/g. Superhydrophobic films fabricated using a solvent-less approach showed the transition from flattened to nano-globular morphology with increasing silica content in polydimethylsiloxane (PDMS), leading to exceptional superhydrophobicity. The optimized film with a particle fraction of 60 % exhibited high de-wetting (> 150º) and mechanical strength (∼7 MPa). The film showed extremely low water droplet adhesion of ∼19 μN, similar to lotus leaf owing to high negative Laplace pressure. Significantly, the film exhibited persisting de-wettability and endurance under harsh chemical, thermal, and mechanical conditions. The robustness is further fortified with room temperature self-healing and real-time self-regeneration characteristics, persisting even after exposure to strong acids and significant abrasion. The multidimensional aspects of the film embarked with a 70 % drag reduction and effective oil-water separation. The present work not only provides a sustainable pathway for managing agricultural waste but also a practical and easy-to-implement approach to tackle oil spill incidents.

A new way to construct multifunctional superhydrophobic coating and applications in anti-corrosion, self-cleaning, membrane distillation and Water/Oil separation

Superhydrophobic materials have prospective applications in the field of self-cleaning, anti-corrosion and anti-fouling. However, the methods of preparing superhydrophobic surfaces usually have many disadvantages, and the scope of application is limited. Therefore, it is particularly important to adopt a widely applicable method to construct superhydrophobic surfaces. In this work, a general coating method has been used to fabricate superhydrophobic that polyaniline (PANI), titanium dioxide nanoparticles (TiO2-NPs) and 1H,1H,2H,2H-perfluorodecyltrie -thoxysilane (PDTS) has been chosen to modified polyimide nanofibrous membrane (PI NFM) (named as: PI-3). The PI-3 membrane exhibited a superior contact angle of 162˚ and ultralow adhesion of water droplets. Moreover, it not only displayed the properties of self-cleaning and anti-corrosion, but also had the broad application prospects in membrane distillation and oil/water separation. We conducted a series of tests, including contact angle measurement, roughness testing, wettability assessment, and oil-water separation performance evaluation. In the test of direct contact membrane distillation, it showed that the flux was always retained at beyond 10 L m−2 h−1 and the salt rejection was surpassed 99.26 %.

Preparation of Durable Superhydrophobic Coatings Based on Discrete Adhesives

Due to the low adhesion observed at the interface between solid and liquid, superhydrophobic coatings hold significant promise for various applications, such as self-cleaning, anti-corrosion, anti-icing, and drag reduction. However, a notable challenge hindering their widespread adoption in these domains lies in their delicate durability. In this study, we propose a straightforward method for preparation. The fluorosilicone resin is initially discretized through a gradual introduction of nonsolvent into its solution, followed by thorough mixing and stirring with silica nanoparticles. The resulting mixture is then sprayed onto the substrate surface after drying, forming a self-similar, porous, and rough structure extending from top to bottom. This process yields a coating exhibiting excellent chemical and mechanical durability simultaneously. Using this approach, we achieved a superhydrophobic coating with a contact angle of 156° and a roll angle of 2.2°, with water droplet adhesion of only 10.8 ± 0.4 µN. Remarkably, the coating maintained excellent superhydrophobicity even after undergoing sandpaper abrasion (10 m), tape peeling (30 times), and prolonged water impact (60 min), showing its robust mechanical stability. Furthermore, following exposure to acid, alkali, and aqueous solutions (7 days), UV irradiation (10 days), and extreme temperature variations (–20 °C to 80 °C), the coatings retained their superhydrophobic properties and exhibited good chemical durability. This method offers a novel approach to enhance the durability and practicality of superhydrophobic coatings.

The influence of temperature on the properties of anti - ice materials

The results of experimental research and practical application of anti-icing surface fully show that its performance is greatly affected by temperature. The anti-icing measures of anti-icing surface can be divided into three stages: before icing, during icing and after icing. In these three stages, the influence brought by temperature change is respectively reflected in: the adhesion of water droplets to the surface, the time it takes for ice nucleation, the adhesion of ice to the surface. Surface anti-ice technology has important application in aviation and aerospace field. The surface temperature of aircraft will change greatly when the aircraft is sailing at different altitudes and passing through different climatic environments. If ice accumulates in some parts of the aircraft, even if it is small, it may lead to a decrease in the climbing force of the aircraft and an increase in flight resistance, thus leading to the deterioration of aerodynamic performance such as the maneuverability and stability of the aircraft. Studying the influence of these temperature changes on the anti-icing performance of aircraft surface can avoid the air disaster caused by surface icing. Starting from these three stages, this paper analyzes the influence of temperature on material surface and the liquid itself successively in these three stages, so as to further explore the influence of temperature on ice suppression performance.

Design Aspects in Vat Photopolymerization Additive Manufacturing of Hydrophobic Surfaces.

Hydrophobic surfaces require finely tuned process chains due to the scale, complexity, and patterning methods. For this purpose, vat photopolymerization (VPP) additive manufacturing is a promising method for surface generation; however, together with the fabrication process, the design phase needs to be optimized to achieve the desired surface property. This work presents the influence of the design features of hydrophobic surfaces through multiple studies on simple pillar structures, intrinsic single-unit geometries, and surface deposition on complex substrates. The results showed that depending on the dimensions of single pillar dimensions, wetting properties can extend between the contact angles (CA) of 83°-115.11°. The hydrophobicity was further increased by applying a re-entrant structure, reaching the CA of 115.24°. The surface deposition on the complex substrates significantly increased water droplet adhesion, preventing it from rolling off, which can be beneficial for manifold device protection from the hazardous influence of the environment. In addition, the influence of the surface on the acoustic properties was examined, which showed that the pattern application in the real-life device does not have a detrimental effect on the intrinsic functionality. This study showed that the design phase should be an essential part of the VPP process chain as it significantly influences the wetting properties of the surfaces.

Facile laser-based process of superwetting zirconia ceramic with adjustable adhesion for self-cleaning and lossless droplet transfer

Zirconia ceramics can be extensively used as biological implant materials because of its excellent mechanical properties and biocompatibility. In order to further enhance the biocompatibility of zirconia ceramics and its application in oral medicine, a nanosecond laser-silicone oil-heat treatment (LSH) composite process was designed to fabricate superhydrophobic zirconia ceramic surfaces with adjustable adhesion to water droplet. The surface topography, chemical composition and wettability of zirconia ceramic surfaces were characterized. The experimental results indicated that the wettability and adhesion of water droplets on zirconia ceramic surfaces were significantly affected by the surface morphology and density of surface micro/nanostructures. The relative content of chemical elements and polar/non-polar functional groups on the surfaces could be changed as the process parameters changed, which would also affect the transition of surface wettability. Meanwhile, the adhesion of water droplets on the zirconia ceramic surfaces could be precisely regulated by changing the laser texturing parameters. Droplet lossless transport and self-cleaning effect were demonstrated, which could provide a key avenue for the application of superhydrophobic zirconia ceramics in biological implants.

Digital Microfluidics─How Magnetically Driven Orientation of Pillars Influences Droplet Positioning.

Interfaces between a water droplet and a network of pillars produce eventually superhydrophobic, self-cleaning properties. Considering the surface fraction of the surface in interaction with water, it is possible to tune precisely the contact angle hysteresis (CAH) to low values, which is at the origin of the poor adhesion of water droplets, inducing their high mobility on such a surface. However, if one wants to move and position a droplet, the lower the CAH, the less precise will be the positioning on the surface. While rigid surfaces limit the possibilities of actuation, smart surfaces have been devised with which a stimulus can be used to trigger the displacement of a droplet. Light, electron beam, mechanical stimulation like vibration, or magnetism can be used to induce a displacement of droplets on surfaces and transfer them from one position to the targeted one. Among these methods, only few are reversible, leading to anisotropy-controlled orientation of the structured interface with water. Magnetically driven superhydrophobic surfaces are the most promising reprogramming surfaces that can lead to the control of wettability and droplet guidance.

Silver nanoparticles decorated ZnO–CuO core–shell nanowire arrays with low water adhesion and high antibacterial activity

Nanostructured surfaces based on silver nanoparticles decorated ZnO–CuO core–shell nanowire arrays, which can assure protection against various environmental factors such as water and bacteria were developed by combining dry preparation techniques namely thermal oxidation in air, radio frequency (RF) magnetron sputtering and thermal vacuum evaporation. Thus, high-aspect-ratio ZnO nanowire arrays were grown directly on zinc foils by thermal oxidation in air. Further ZnO nanowires were coated with a CuO layer by RF magnetron sputtering, the obtained ZnO–CuO core–shell nanowires being decorated with Ag nanoparticles by thermal vacuum evaporation. The prepared samples were comprehensively assessed from morphological, compositional, structural, optical, surface chemistry, wetting and antibacterial activity point of view. The wettability studies show that native Zn foil and ZnO nanowire arrays grown on it are featured by a high water droplet adhesion while ZnO–CuO core–shell nanowire arrays (before and after decoration with Ag nanoparticles) reveal a low water droplet adhesion. The antibacterial tests carried on Escherichia coli (a Gram-negative bacterium) and Staphylococcus aureus (a Gram-positive bacterium) emphasize that the nanostructured surfaces based on nanowire arrays present excellent antibacterial activity against both type of bacteria. This study proves that functional surfaces obtained by relatively simple and highly reproducible preparation techniques that can be easily scaled to large area are very attractive in the field of water repellent coatings with enhanced antibacterial function.

Comment on “Vapor Lubrication for Reducing Water and Ice Adhesion on Poly(dimethylsiloxane) Brushes”: Vapor Alteration Alone Reduces Water Droplet Adhesion

A reduction in lateral adhesion of water droplets on poly(dimethylsiloxane) (PDMS) brush surfaces exposed to various vapor conditionswasrecently reported. Itwassuggested that the mobility of droplets is due to swelling of the PDMS brushes. When changing the vapor surrounding sliding droplets on bare surfaces, a similar phenomenon is observed, presenting a much simpler explanation of the observed results.

Flexible Electrothermal Hydrophobic Self-Lubricating Tape for Controllable Anti-icing and De-icing

In winter, the freezing phenomenon threatens the safety of people both in the work and home environments. At present, electrothermal de-icing technology is widely used, but how to improve the portability and anti-icing/de-icing efficiency of electrothermal de-icing technology has attracted extensive attention. Here, we proposed a simple yet universal flexible electrothermal hydrophobic self-lubricating tape (EHT) with a multilayer structure including an electrothermal layer and hydrophobic self-lubricating layer on a polyimide base. The resulting tape surface exhibits a hydrophobic/electrothermal synergistic effect on anti-icing and de-icing performance. Compared with the electrothermal tape (ET) without a hydrophobic self-lubricating layer, the friction coefficient and ice adhesion strength on the EHT surface were reduced by 29% and 57%, respectively. In addition, the hydrophobic self-lubrication effect of EHT can reduce the adhesion of supercooled water droplets on its surface in an extremely harsh environment to allow more de-icing easily under joule heat, and the different electrothermal power demand in diverse surroundings can be controlled by adjustable voltage. More importantly, the uniform EHT material can be prepared in a large area using a traditional blade-coating process that gives a convenient route to anti-icing in the engineering protection field.

Ice freezing for hydrotechnical structures

In the domain of hydrotechnical structures, novel methods are emerging to increase ice thickness, notably in the context of ice islands. The layer-by-layer freezing process serves as a cornerstone, where freezing time determines efficacy. The article explores diverse freezing techniques, their stages, and implications. Layer-by-layer freezing entails water pouring, followed by freezing after each layer. Augmentations involve introducing ice rubble and inclined plane freezing. Sprinkling accelerates freezing through water droplet adhesion. The block method uses extracted ice blocks, creating vertical surfaces without formwork. Volumetric freezing involves pipelines and refrigerants. One method blends ice rubble with additives for versatility. Combined methods expedite construction, especially for substantial projects. These techniques cater not only to ice island creation but also to temporally reviving deteriorating structures such as moorage contractions.

CFD simulation of water droplet adhesion on the GDL of a low temperature PEM FC in air cross-flow conditions

Water management is a critical challenge in low temperature (LT) Proton Exchange Membrane Fuel Cells (PEM FC); condensed liquid appears mainly at the cathode side, where water from the reduction reaction is generated. Differences in concentration may result in the transfer of water to the anode side across the membrane electrode assembly (MEA). Excessive liquid can negatively affect fuel cell performance, causing low efficiency and instability. This occurs due to water movement through porous layers and channels that are the essential pathways for the reactant gas to reach the MEA. However, water is necessary in the PEM FC for enhancing ion conductivity of the membrane. The present study can contribute to the optimization of LT PEM FCs by analysing the water behaviour under flow conditions. The widely used Volume Of Fluid (VOF) method is adopted for simulating multiphase flow. CFD simulation of droplet adhesion on the gas diffusion layer (GDL) is performed to describe the interaction between water and gas flow. Deformation and oscillations of droplets with diameters in the 0.3-1.0 mm range are investigated by considering airflow rates up to 15.0 m/s. CFD analysis is validated by optical data from digital imaging with high spatial (up to 5.8 μm/pixel) and temporal (up to 1.0 ms) resolution.

(Invited) Surface Modification to Control Wetting and Adhesion of Aqueous Solutions

rf plasmas and electrochemical treatments have been used extensively for thin film deposition and etching in the fabrication of microelectronic and photonic devices. These methods can be applied to non-traditional substrates (e.g., paper and stainless steel) to alter surface and bulk properties to create (i) paper-based analytical devices with application to microfluidic devices, and (ii) antibacterial structures on stainless steel. Two examples will be described. First, the effects of plasma-based oxygen etching of cellulose surfaces followed by plasma-enhanced fluorocarbon film deposition will be described and shown to control wetting and adhesion of water droplets. These process steps can also be used to form fully enclosed hydrophilic channels in bulk paper and thereby allow microfluidic device fabrication on inexpensive and renewable substrates. Electrochemical etching of stainless steel creates micro- and nano-structures on stainless steel surfaces that repel water and bacteria. These surfaces have potential applications in orthopedic implants and consumer metal products prone to bacterial contamination.

Hierarchically Structured, All-Aqueous-Coated Hydrophobic Surfaces with pH-Selective Droplet Transfer Capability.

Often inspired by nature, techniques for precise droplet manipulation have found applications in microfluidics, microreactors, and water harvesting. However, a widely applicable strategy for surface modification combining simultaneous hydrophobicity and pH-sensitivity has not yet been achieved by employing environmentally friendly assembly conditions. The introduction of pH-responsive groups to an otherwise fluorinated polyphosphazene (PPZ) unlocks pH-selective droplet capture and transfer. Here, an all-aqueous layer-by-layer (LbL) deposition of polyelectrolytes is used to create unique hydrophobic coatings, endowing surfaces with the ability to sense environmental pH. The high hydrophobicity of these coatings (ultimately reaching a contact angle >120° on flat surfaces) is enabled by the formation of hydrophobic nanoscale domains and controllable by the degree of fluorination of PPZs, polyamine-binding partners, deposition pH, and coating thickness. Inspired by the hierarchical structure of rose petals, these versatile coatings reach a contact angle >150° when deposited on structured surfaces while introducing a tunable adhesivity that enables precise droplet manipulation. The films exhibited a strongly pronounced parahydrophobic rose petal behavior characterized through the contact angle hysteresis. Depositing as few as five bilayers (∼25 nm) on microstructured rather than smooth substrates resulted in superhydrophobicity with water contact angles >150° and the attenuation of the contact angle hysteresis, enabling highly controlled transfer of aqueous droplets. The pH-selective droplet transfer was achieved between surfaces with either the same microstructure and LbL film building blocks, which were assembled at different pH, or between surfaces with different microstructures coated with identical films. The demonstrated capability of these hydrophobic LbL films to endow surfaces with controlled hydrophobicity through adsorption from aqueous solutions and control the adhesion and transfer of water droplets between surfaces can be used in droplet-based microfluidics applications and water collection/harvesting.

Precise Controlling of Friction and Adhesion on Reprogrammable Shape Memory Micropillars.

Microstructured surfaces with stimuli-responsive performances have aroused great attention in recent years, but it still remains a significant challenge to endow surfaces with precisely controlled morphological changes in microstructures, so as to get the precise control of regional properties (e.g., friction, adhesion). Herein, a kind of carbonyl iron particle-doped shape memory polyurethane micropillar with precisely controllable morphological changes is realized, upon remote near-infrared light (NIR) irradiation. Owing to the reversible transition of micropillars between bent and upright states, the micro-structured surface exhibits precisely controllable low-to-high friction transitions, together with the changes of friction coefficient ranging from ∼0.8 to ∼1.2. Hence, the changes of the surface friction even within an extremely small area can be precisely targeted, under local NIR laser irradiation. Moreover, the water droplet adhesion force of the surface can be reversibly switched between ∼160 and ∼760 μN, demonstrating the application potential in precisely controllable wettability. These features indicate that the smart stimuli-responsive micropillar arrays would be amenable to a variety of applications that require remote, selective, and on-demand responses, such as a refreshable Braille display system, micro-particle motion control, lab-on-a-chip, and microfluidics.

Superhydrophobic micro-nanofibers from PHBV-SiO2 biopolymer composites produced by electrospinning

Electrospinning is a relatively simple technique for producing continuous fibers of various sizes and morphologies. In this study, an intrinsically hydrophilic poly(3-hydroxybutarate-co-3-hydroxyvalerate) (PHBV) biopolymer strain was electrospun from a solution under optimal processing conditions to produce bilayers of beadless micro-fibers and beaded nano-fibers. The fibrous mats produced from the pure PHBV solution exhibited hydrophilicity with complete wetting. Incorporation of polydimethylsiloxane (PDMS) treated silica into the electrospinning solutions resulted in a non-wetting state with increased fiber roughness and enhanced porosity; however, the fiber mats displayed high water droplet-adhesion. The SiO2–incorporated fibrous mats were then treated with stearic acid at an activation temperature of 80 °C. This treatment caused fiber surface plasticization, creating a tertiary hierarchical roughness owing to the interaction of PHBV chains with the polar carboxyl groups of the stearic acid. Scanning electron microscopy was used to assess the influence of the electrospinning process parameters and the incorporation of nanoparticles on surface morphology of the fibers; energy dispersive X-ray spectroscopy confirmed the presence of SiO2 nanoparticles. Fourier transform infrared spectroscopy was performed to study the incorporation of SiO2 and the interaction of stearic acid with PHBV at various concentrations. The chemical interaction between stearic acid and PHBV was confirmed, while SiO2 nanoparticles were successfully incorporated into the PHBV fibers at concentrations up to 4.5% by weight. The incorporation of nanoparticles and plasticization altered the thermal properties of PHBV and a decrease in crystalline fraction was observed. The stearic acid modified bilayers produced from the micro-nano-fibrous composites showed very low water droplet sticking, a roll off angle of approximately 4° and a high static contact angle of approximately 155° were achieved.Graphical

A novel biomimetic nanocomposite exhibiting petal wetting phenomenon: fabrication and experimental investigations

A functional composite material that simultaneously exhibits hydrophobicity and water droplet adhesion has monumental potential in controlling fluid flow, studying phase separation, and biological research. This article reports the fabrication of a petal wetting biomimetic Boron Nitride Nanotubes (BNNTs) -Polydimethylsiloxane (PDMS) nanocomposite achieved by drop casting. The petal effect was investigated by non-destructive techniques. The nanotubes were synthesized by chemical vapor deposition at 1150 °C and were characterized by x-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The mean diameter of the nanotubes was found to be 70 nm. The nanocomposites had BNNT fillers ranging from 0.5 wt% to 2 wt%. Water contact angles for pure PDMS polymer was 94.7° and for the 2 wt% BNNT-PDMS nanocomposite was 132.4°. The petal wetting nanocomposite displayed a characteristic trait of high contact angle hysteresis. The surface roughness parameters of the nanocomposites were determined by atomic force microscopy. Laser scanning confocal microscopy aided in analyzing the droplet penetration and in observing the trapped air between the water droplet and the nanocomposite surface. Based on surface observations, roughness parameters, and the extent of droplet penetration by the surface, we shed light on the Cassie impregnating wetting regime followed by the biomimetic nanocomposite. Such a surface would be beneficial in the study of the embryogenesis of cells and aid in moisture collection.

Strain-Engineered Adhesion and Reversible Transfer Printing of Water Droplets and Nanoparticles.

Transfer printing has been playing a crucial role in the fabrication of various functional devices. In spite of the extensive progress in technology, challenges are remaining, in the aspects of accuracy, efficiency, and adaptivity. Here, we propose a reversible transfer printing technique of tailoring adhesion by selectively stretching the surfaces. Through molecular dynamics simulations, we demonstrate the transfer of nanoscale substances such as water droplets, colloids, and nanoparticles between two graphene surfaces with strains switched on and off. We reveal the mechanism of the dynamic behaviors by analyzing the energies and driving forces of the substances during the process of transfer. The work not only advances the fundamental understanding of adhesion but also can inspire applications in the design of next-generation electronic and biomedical devices.

Wetting, Adhesion, and Droplet Impact on Face Masks.

In the present pandemic time, face masks are found to be the most effective strategy against the spread of the virus within the community. As aerosol-based spreading of the virus is considered as the primary mode of transmission, the interaction of masks with incoming droplets needs to be understood thoroughly for an effective usage among the public. In the present work, we explore the interactions of the droplets over the most commonly used 3-ply surgical masks. A detailed study of the wetting signature, adhesion, and impact dynamics of water droplets and microbe-laden droplets is carried out for both sides of the mask. We found that the interfacial characteristics of the incoming droplets with the mask are very similar for the front and the back side of the mask. Further, in an anticipated attempt to reduce the adhesion, we have tested masks with a superhydrophobic coating. It is found that a superhydrophobic coating may not be the best choice for a regular mask as it can give rise to a number of smaller daughter droplets that can linger in air for longer times and can contribute to the transmission of potential viral loads.

Molecularly grafted PVDF membranes with in-air superamphiphilicity and underwater superoleophobicity for oil/water separation

Microporous polyvinylidene fluoride (PVDF) based membranes are widely used in separation and purification applications, including oil/water separation. However, due to hydrophobic nature, these membranes are prone to high fouling and wetting by low surface tension liquids and substances, thereby limiting its full potential in many important applications such as oil/water separation. Herein, we present a simple and straight forward method to modify and produce in-air superamphiphilic and under-water superoleophobic PVDF membranes by molecular grafting using three different amino silanes. The molecular grafting imparted novel properties due to the combined effect of chemistry, nanostructuring, and roughness. After grafting, the membranes exhibited in-air superamphiphilicity in both dry and wet conditions. In water, the membrane does not allow the attachment of oil droplets, making it superoleophobic. However, when placed under oil, the membrane showed superhydrophobicity but still allowed the adhesion of water droplets. After thorough characterization for structure, modification, and physicochemical properties, the membranes were tested for oil/water mixture separation. The functionalized membranes showed > 99% oil rejection and flux recovery. These results showed great promise for molecular grafting of membranes for separation and purification applications.