Flexible Superhydrophobic Surface Research Articles

Preparation and applicability of a flexible PDMS superhydrophobic layer

Superhydrophobic surfaces have broad application prospects in the fields of self-cleaning, anti-icing, and fluid drag reduction. In this study, a convenient and environmentally friendly method was used to prepare a flexible PDMS superhydrophobic surface, and the effect of different spraying times on surface wettability was studied. On this basis, the applicability of the PDMS superhydrophobic surface was evaluated by high and low temperature environment experiments. The results show that the better spraying times are 3 times, the contact angle is 165.52°, and the rolling angle is 2.06°. The flexible superhydrophobic surface can be applied to the surface of various materials and structures with good waterproof and antifouling properties. In the high-temperature environment of 100 °C, it can effectively improve the flooding problem of fuel cell bipolar, and it is still effective after one month. In the low-temperature environment of −30 °C, it can effectively inhibits the surface frosting and the surface composite powder is well bonded to the substrate.

Spheroids formation in large drops suspended in superhydrophobic paper cones.

The utilization of 3D cell culture for spheroid formation holds significant implications in cancer research, contributing to a fundamental understanding of the disease and aiding drug development. Conventional methods such as the hanging drop technique and other alternatives encounter limitations due to smaller drop volumes, leading to nutrient starvation and restricted culture duration. In this study, we present a straightforward approach to creating superhydrophobic paper cones capable of accommodating large volumes of culture media drops. These paper cones have sterility, autoclavability, and bacterial repellent properties. Leveraging these attributes, we successfully generate large spheroids of ovarian cancer cells and, as a proof of concept, conduct drug screening to assess the impact of carboplatin. Thus, our method enables the preparation of flexible superhydrophobic surfaces for laboratory applications in an expeditious manner, exemplified here through spheroid formation and drug screening demonstrations.

Ultra-high performance flexible and controllable superhydrophobic films based on microsphere/micro-pyramid hierarchical arrays

Ultra-high performance flexible superhydrophobic surfaces have attracted great research interests, which are widely used in anti-fog, anti-icing, anti-fouling, self-cleaning, and other fields. However, ultra-high hydrophobic properties and complex manufacturing processes are still huge challenges to be solved. In this paper, a facile construction strategy of ultra-high performance flexible and controllable superhydrophobic films was proposed, combining the template transfer and electrospinning methods. The effects of electrospinning parameters on the microstructure and surface wettability were systematically studied, and the controllable preparation of films with different wettability was realized. Under the stable Cassie-Baxter state, the contact angle of prepared superhydrophobic films could reach 172° and the sliding angle could reach 3°. In addition, the stretchability, abrasion resistance, and chemical durability tests were also conducted in simulated harsh real-world environments. The results indicated that the films still exhibited superior performances. For the practical demonstration, the films are expected to be used as protective films for windshields, outdoor electronic screens, and solar cells due to the inherent optical properties and excellent fouling resistance performance.

Fabrication of flexible and stretchable surfaces by modifying SiO2 on waste chewing gum for self-cleaning of fine-dust deposition

With increasingly refined and complex usage scenarios, the use of purely superhydrophobic surfaces/interfaces has become increasingly limited. Flexible and stretchable superhydrophobic surface with self-cleaning fine-dust deposition function is gaining popularity, but their low-cost fabrication faces multiple challenges. Herein, a flexible superhydrophobic surface was fabricated using chewing gum and SiO2, while being can be shapeable, stretchable, self-healable, and reusable. The apparent contact angle (APCA) increases along with the dosage of SiO2 increase, the apparent contact angle (APCA) reaches to the maximum of 156 ± 5°, and the sliding angle (SA) reaches 3 ± 2° at the dosage of SiO2 1.0 g, the adhesive force reaches to minimum of 0.034 mN. The study demonstrated that this composite material exhibits self-cleaning fine-dust deposition surface properties by simply pasting it onto different surfaces of metals, fabrics, paper, and plastics. The chewing gum/SiO2 material shows robust resistance to sandpaper abrasion, tape-peeling, finger wiping, and solutions with different pH values. This material can be used on metals, fabrics, paper, and plastics for a myriad of self-cleaning fine-dust deposition applications.

Facile fabrication of flexible superhydrophobic surfaces with high durability and good mechanical strength through embedding silica nanoparticle into polymer substrate by spraying method

Flexible superhydrophobic surfaces have been attracting more and more attention due to their foldable property. However, the superhydrophobic structure was easily damaged or destroyed owing to their poor interfacial force between substrate and superhydrophobic coating. Herein, fluorinated silica nanoparticles (F-SiO2) were sprayed onto the wet poly(amide–imide) (PAI) surface, which made F-SiO2 easily went into PAI substrate to enhance the durability of superhydrophobic surface. The PAI/F-SiO2 flexible surface exhibited a water contact angle (CA) of beyond 160° and a sliding angle (SA) of as low as 3°, while the tensile strength (34 MPa) was comparable to that of PAI substrate (35.8 MPa). Encouragingly, the superhydrophobicity and hierarchical porous structure of PAI/F-SiO2 surface were maintained even they were folded by 5000 times or underwent sandpaper abrasion test (500 cm of wear on a 1200 grit sandpaper under 50 g load), while the nanocomposites failed the sandpaper abrasion test when F-SiO2 were sprayed onto completely dried PAI surface. More importantly, the CA and SA just slightly decreased when PAI/F-SiO2 surfaces were heated at 300 °C for 24 h due to the unique surface morphology and high thermal stability of PAI, which is one of the highest thermal stabilities for flexible superhydrophobic films. In addition, PAI/F-SiO2 surfaces maintained superhydrophobic after hot water jet impact, acid–base immersing, UV irradiation, and ultrasonic treatment, indicating the fabricated surfaces could be used for practical application especially under harsh conditions. Furthermore, the superhydrophobic surfaces possessed icephobic properties. It should be noted that PAI/F-SiO2 surfaces showed such excellent properties, although their fabricating procedure did not undergo any optimization. This wet spraying method was also used to fabricate flexible superhydrophobic surfaces with other polymers as substrates such as poly(acrylic acid) (PAA), thermoplastic polyurethane (TPU), polystyrene (PS), and poly(vinyl alcohol) (PVA). Therefore, the simple strategy provided a general route to prepare flexible superhydrophobic films with high durability.

A highly robust, non‐fluorinated, and economical PDMS‐based superhydrophobic flexible surface with repairable and flame‐retardant properties

AbstractNature‐inspired artificial superhydrophobic surfaces have attracted tremendous attention due to their potential applications in various fields. However, the use of toxic chemicals and expensive nanoparticles with limited durability has restricted the practical applications of these surfaces. Here, we proposed a novel and non‐fluorinated technique to fabricate robust and flexible polydimethylsiloxane (PDMS)‐based superhydrophobic surface. Fabrication involved two simple steps: preparation of superhydrophobic nanoparticles by burning PDMS, and fabrication of flexible superhydrophobic PDMS surface (SPS) by dispersing particles in a solvent containing PDMS solution and curing agent. Effect of PDMS particle content on morphology and wettability was examined and 10 wt.% PDMS nanoparticles demonstrated superior superhydrophobicity with a water contact angle of 156° and sliding angle of ~6°. The SPS displayed excellent mechanical robustness in a sandpaper abrasion test and showed no apparent wettability deterioration even when stretched by up to 50%. Additionally, surface maintained outstanding superhydrophobicity even after 300 cycles of mechanical deformation. Impressively, damaged surface can restore its superhydrophobicity after removing the damaged layer using sandpaper abrasion and displayed exceptional chemical, thermal, flame‐retardant, and long‐lasting superhydrophobic properties. The work provides an innovative, low‐cost, and environment‐friendly strategy for designing robust and flexible superhydrophobic surfaces that can be used in real‐life applications.

Bioinspired hierarchical colloidal crystal paper with Janus wettability for oil/water separation and heavy metal ion removal.

Increasing attention has been paid recently to superwettability and its prospective potential applications in various fields. A new approach towards the establishment of flexible, self-assembled superhydrophobic surfaces with self-reported wettability on a variety of substrates has been advanced. The approach involves the fabrication of a dense monolayer of photonic crystal films that possess a layered structure with superior adhesion at the liquid-gas-solid interface. Thus, the resulting hierarchical photonic crystal film with a structurally hydrophobic surface offers a promising addition to the creation of durable and flexible superhydrophobic surfaces across a variety of substrates that exhibit the self-reported wettability. Furthermore, a bifunctional membrane that can effectively remove oil and adsorb heavy metal ions contained in wastewater has been developed for potential use in large-scale industrial wastewater treatment. This research sheds fresh light on the application of bionics and the lotus and mussel functions in oil/water separation.

Stretchable superhydrophobic fluororubber fabricated by transferring mesh microstructures.

Stretchable flexible superhydrophobic surfaces are in great demand to achieve waterproofing performance in aerospace, electronic industry, and other fields. However, there are still many challenges in developing superhydrophobic surfaces, which maintain their wetting characteristics under high strain conditions with good tensile durability. Here, we propose a simple and efficient method to prepare a stretchable superhydrophobic fluororubber surface composed of hierarchical micro-convexities, which are orderly arranged and interconnected. Its peculiar structure shows excellent superhydrophobicity (155.48 ± 1.97°) and high water sliding angle due to Cassie's impregnating wetting regime. Due to the special structure and high mechanical strength of the surface, it can still maintain its superhydrophobic property after a variety of durability tests, including various stretching tests, sandpaper abrasion, sand impact, and high-temperature treatment. In addition, the surface can still realize the lossless transfer of water droplets even at large stretching strains, which is expected to be applied to microfluidic devices under extreme working conditions.

Facile fabrication of durable and flexible superhydrophobic surface with polydimethylsiloxane and silica nanoparticle coating on a polyethylene terephthalate film by hot-roll lamination

Practical applications of superhydrophobic surfaces have been limited by several disadvantages such as complex fabrication processes, high modification cost, and low mechanical durability. In this study, we present a facile and low-cost fabrication method to make flexible superhydrophobic films by a hot-roll lamination process. The film consists of a hydrophobic polydimethylsiloxane (PDMS) and hydrophobic silica nanoparticle (SNP) composite coating layer on a polyethylene terephthalate (PET) film. The coated film displayed excellent superhydrophobic properties with an apparent water droplet contact angle of 176.3° and a sliding angle of 2.4°. The surface morphology, roughness, and cross-section chemical composition of the coated films were investigated by field emission scanning electron microscopy and confocal laser scanning microscopy. The mechanical durability of the coated films was evaluated using several tests such as scratching with sandpaper, repeated bending deformation, and adhesion tape tests. The fabricated superhydrophobic films still showed superhydrophobic behavior after 20 tape tests. In addition, the fabricated superhydrophobic films exhibited strong mechanical durability, which preserved their very high contact angle >160° and very small sliding angle <10° after a 350 cm abrasion test with 240 grit sandpaper and a pressure of 1.2 kPa. Also, the coated sample exhibited very stable superhydrophobicity after 100 bending cycles due to the elasticity of PDMS. The chemical stability of flexible PDMS/SNP coated PET films was also investigated in acidic, neutral, and alkaline solutions and showed excellent chemical stability in acidic and neutral media in contrast to the alkaline medium. The obtained results demonstrated the outstanding robustness of the fabricated flexible superhydrophobic films that are promising for self-cleaning and water droplet bouncing performance. • Superhydrophobic surfaces were fabricated by a facile method without toxic chemicals. • The durability of the superhydrophobic films was demonstrated by abrasion and tape tests. • The fabricated films exhibited good flexibility in bending tests. • The self-cleaning property of the fabricated coated surfaces was also observed.

A soft gripper with contamination resistance and large friction coefficient

With the development of soft gripper systems, food processing, packaging, and processing technology has seen a significant advancement. The soft gripper must be protected from surface contamination and liquid lubrication; hence, a hydrophobic and contamination-resistant surface is used. There is an urgent need for a soft gripper that meets the functions of hydrophobic performance and contamination resistance, but there has been little study. In this study, the two functions are well achieved by introducing a flexible superhydrophobic surface. A novel design of a flexible superhydrophobic surface-based gripper (SSBG) is proposed by the template method. After a series of testing, it was discovered that the friction coefficient between the SSBG surface and the glass plate, cardboard, and steel plate is larger than that of a standard surface gripper (SSG). At the same time, the contamination resistance of SSBG is particularly important. Because of the existence of the plastron layer, the surface was discovered to be able to repel bacterial colonization. Finally, when the usual gripping force of the SSBG is measured on a 250 ml beaker with contamination and liquid lubrication, it can be shown that the gripping ability is improved by at least 100% and 170% compared to the SSG, respectively. As a result, the SSBG has a high potential for utilization in industrial and therapeutic contexts.

Phase-Change-Enabled, Rapid, High-Resolution Direct Ink Writing of Soft Silicone.

Soft silicone is an ideal flexible material for application, e.g., in soft robotics, flexible electronics, bionics, or implantable biomedical devices. However, gravity-driven sagging, filament stretching, and deformation can cause inevitable defects during rapid manufacturing, making it hard to obtain complex, high-resolution 3D silicone structures with direct ink writing (DIW) technology. Here, rapid DIW of soft silicone enabled by a phase-change-induced, reversible change of the ink's hierarchical microstructure is presented. During printing, the silicone-based ink, containing silica nanoparticles and wax microparticles, is extruded from a heated nozzle into a cold environment under controlled stress. The wax phase change (solid-liquid-solid) during printing rapidly destroys and rebuilds the particle networks, realizing fast control of the ink flow behavior and printability. This high-operating-temperature DIW method is fast (maximum speed ≈3100mm min-1 ) and extends the DIW scale range of soft silicone. The extruded filaments have small diameters (50± 5µm), and allow for large spans (≈13-fold filament diameter) and high aspect ratios (≈1), setting a new benchmark in the DIW of soft silicone. Printed silicone structures exhibit excellent performance as flexible sensors, superhydrophobic surfaces, and shape-memory bionic devices, illustrating the potential of the new 3D printing strategy.

Realization of integrative hierarchy by in-situ solidification of ‘semi-cured’ microcilia array in candle flame for robust and flexible superhydrophobicity

Flexible superhydrophobic surfaces have recently attracted extensive interest owing to the potential for diverse curvatures and emerging flexible electronics. However, the simultaneous realization of mechanical robustness, chemical resistance, and maintained superhydrophobicity during deformation is still challenging. Herein, we introduced an environmental-friendly method to produce the integrative multi-level structures by generating the micro-cilia array in a ‘template-free’ manner, followed with in-situ thermal curing in the candle flame before the complete solidification of the micro-structured surface. Instead of weak physical adhesion (van der Waals forces), the candle soot nanoparticles penetrated into the ‘semi-cured’ micro-cilia surface, along with the high temperature to rapidly cure and generate the PDMS-connected integrative architectures, which effectively avoid the interface between the micro/nano-scaled components to eliminate the mechanical fragility. Comparative investigations confirm that the elevated robustness is attributed by the synergistic effect from the ‘semi-cured’ surface and the formation mechanism of subsequent hierarchical structures. The flexible film thus exhibits significantly improved stability against mechanical damage (ultrasonic processing, adhesive pressing, and linear abrasion), chemical corrosion, and water-jet impalement (∼10.4 m s−1 at ∼ 0.17 MPa). With the realization of co-existent features and real-life applications, the green methodology should be promising to open up an avenue that moves superhydrophobicity further to our world.

Non-Fluorinated Flexible Superhydrophobic Surface with Excellent Mechanical Durability and Self-Cleaning Performance.

Although plenty of superhydrophobic surfaces have been developed owing to their tremendous potential applications, it is still a great challenge for the superhydrophobic surfaces to possess environmental friendliness, biocompatibility, and mechanical durability simultaneously. Herein, a non-fluorinated flexible superhydrophobic surface was designed by constructing a film-substrate system with labyrinth-like wrinkles combining an intrinsically hydrophobic Zn film and a polydimethylsiloxane (PDMS) substrate. Excellent superhydrophobicity with a contact angle up to 168.5° and a slide angle as low as 0° has been achieved on the Zn/PDMS surface, which is attributed to the micro-/nano-textured structures of the labyrinth-like wrinkles, providing sufficient air pockets to form a stable Cassie-Baxter state. Furthermore, the Zn/PDMS surface retains excellent superhydrophobicity under stretching, bending, and twisting mechanical deformation up to 500 cycles due to the stability of the micro-/nano-textured structures of the labyrinth-like wrinkles protected by the fantastic self-healing ability of the micro-cracks. Additionally, the Zn/PDMS superhydrophobic surface possesses an outstanding self-cleaning performance for various contaminants. The present work provides a valuable routine to design non-fluorinated flexible superhydrophobic surfaces with superb mechanical durability and self-cleaning property as promising functional layers for flexible electronics, wearable devices, biomedical engineering, and so forth.

Contact time of impacting droplets on a superhydrophobic surface with tunable curvature and groove orientation

Regulating the impact dynamics of water droplets on a solid surface is of great significance for some practical applications. In this study, the droplet impingement on a flexible superhydrophobic surface arrayed with micro-scale grooves was investigated experimentally. The surface was curved into cylindrical shapes with certain curvatures from two orthogonal directions, where axial and circumferential grooves were formed, respectively. The effects of curvature diameter and Weber number, as well as the orientation of grooves on droplet spreading and retracting dynamics were analyzed and explained. Results show that the circumferential grooves promote the spreading of a droplet in the azimuthal direction, where the droplet rebounds from the surface with a stretched shape. This mechanism further reduces the contact time of impacting droplets on the superhydrophobic surface compared to the other curving mode.

Designing superhydrophobic robotic surfaces: Self-cleaning, high-grip impact, and bacterial repelling

Biomimetic surface has attracted a lot of interest in designing the high grip and superhydrophobic surfaces. In particular, adjusting the load on a flexible surface to increase the solid-solid contact area can effectively increase its overall solid-solid static friction. Here we utilized laser processing and hydrochloric acid etching methods on the aluminum alloy substrate surface to generate different textures as templates to produce polydimethylsiloxane (PDMS) films with flexible superhydrophobic surfaces. Four versatile PDMS films with surface morphologies were produced, which gave superhydrophobic properties at varying angles. Scanning electron microscopy revealed that a micron- and nano-scale structure is produced by the etched methods applied here. The static friction and its impact on grip using these flexible superhydrophobic surfaces were further studied under different loads and various conditions. We found that the flexible superhydrophobic surface increased grip under all conditions, using a critical threshold load. The high surface area of superhydrophobic surfaces can increase surface contact and friction. Also, the concave-convex structure can act as a suction cup, increasing the static friction. The designed surfaces were further applied to soft robotic hands, which significantly improved its gripping impact and bacteria-repelling properties after washing.

Preparation of a Flexible Superhydrophobic Surface and Its Wetting Mechanism Based on Fractal Theory

Substrates of superhydrophobic surface are important for their application. Preparation of a flexible superhydrophobic surface has drawn more and more attention. In this work, a flexible substrate was made using a semi-curing spray method to obtain a flexible superhydrophobic surface with excellent abrasion resistance on the surface of a room temperature vulcanized silicone rubber. Results show that under a bending condition, excellent superhydrophobic properties are still maintained. The Cassie-Baxter model and Wenzel model can be used to estimate the static water contact angle for regular roughness surfaces. There are few numerical theoretical models to predict contact angle or wetting mode for irregular micro-nanostructures superhydrophobic surfaces. The fractal theory can be used to transform the equation of Wenzel model and obtained the fractal wetting theory suitable for fractal structures on irregular rough surfaces. However, this fractal-wetting model cannot be applied to Cassie-Baxter state, which is always suitable for superhydrophobic surfaces. A new method was developed to calculate the static water contact angle of water droplets in the Cassie-Baxter model state. Using image identification and split surface method, a new model is constructed based on the fractal theory. Experimental data for water contact angles on flexible superhydrophobic surface with SiC/CNTs micro-nanostructures is agreement with the simulated values.

One-step fabrication of flexible superhydrophobic surfaces to enhance water repellency

This paper investigates a one-step fabrication method of flexible superhydrophobic surfaces on silicone rubber substrates, which enhance the repellency to both static and impacting water droplets. Grooved structures with hierarchical roughness were constructed directly on intrinsically hydrophobic silicone rubber surfaces using nanosecond ultraviolet laser. The effects of laser parameters, laser scanning speed and number of scans, on surface morphology and wettability were explored. Results show that the scanning speed and the number of scans jointly affect the roughness of the processed surfaces and further the contact angle of static droplets. Due to the anisotropic wettability, the contact angle perpendicular to grooves is generally larger than that in the parallel direction. When the scanning speed is 540 mm/s and the number of scans is 75, the maximum contact angles in the perpendicular and parallel directions are 156.5°, 155.0°, respectively. One of the silicone rubber based superhydrophobic surfaces was then bent into convex shapes with various curvatures and a series of droplet impact experiments were performed on it. Compared to that on a flat superhydrophobic surface, the contact time of impacting droplets on the curved surface is reduced dramatically. It demonstrates that the flexible surfaces can also have improved repellency to dynamic droplets. This work provides an effective approach for repelling water through fabricating flexible superhydrophobic surfaces.

Flexible superhydrophobic surfaces with condensate microdrop self-propelling functionality based on carbon nanotube films.

With the development of flexible electronics and wearable devices, there is strong demand for flexible, superhydrophobic, and multifunctional coatings. Motivated by the promise of attractive multifaceted functionality, various techniques have been developed to fabricate flexible surfaces with non-wetting properties. However, until now, there have been few reports on superhydrophobic surfaces with condensate microdrop self-propelling (CMDSP) functionality on a carbon nanotube film. Here, we used a facile electrodeposition method to develop for the first time a new type of flexible superhydrophobic surface with CMDSP functionality based on carbon nanotube films. These flexible CMDSP surfaces are robust after multiple cycles of bending of the film-coated substrate, i.e., without impacting the surface superhydrophobicity and CMDSP performance. The proposed light and flexible surface, combined with CMDSP, will support a novel generation of coatings that are multifunctional, flexible, smart, and energy saving. This new type of functional flexible interface not only opens new avenues in research into the fundamental structure–property relationships of materials, but also exhibits significant application potential for advanced technologies.

Mechanical Stability of PDMS-Based Micro/Nanotextured Flexible Superhydrophobic Surfaces under External Loading.

Superhydrophobicity based on the micro- and nanostructures is frangible to external loading. It is a challenging issue for flexible superhydrophobic surfaces to maintain superhydrophobicity while deforming under external loading. Herein, PDMS-based micro/nanotextured flexible surfaces with robust superhydrophobicity were fabricated by an effective and environmentally friendly method. The formation mechanism of the micro/nanotextured structures and the formation reason for superhydrophobicity were investigated. The effects of reaction time on wettability were also explored. Besides, the changes of morphology and superhydrophobicity under external loading were studied. It is found that micro/nanotextured structures remained unchanged, and only the bottom layer generated cracks under external loading. There were no obvious changes of superhydrophobicity under a tensile strain up to breakage, 500 cycles of bending, and 100 cm height of water droplets and NaCl particle impact. The self-cleaning property was also verified in cleaning the surfaces covered with various contaminants by water droplets. The fabricated PDMS-based micro/nanotextured flexible surfaces with good mechanical stability and self-cleaning property are expected to be applied in flexible electronic devices.

Dynamics of a flexible superhydrophobic surface during a drop impact

In this study, coupled dynamic responses of flexible superhydrophobic surfaces during a drop impact were investigated with position sensing and high-speed imaging. A smooth polydimethylsiloxane surface was spray coated with commercially available superhydrophobic paint particles. The influence of initial and subsequent impacts of a water droplet on the surface dynamics was studied at various natural frequencies of the surface (50 &amp;lt; fs &amp;lt; 230 Hz) and Weber numbers (2 &amp;lt; We &amp;lt; 90). We discovered that the flexible superhydrophobic surface was deflected twice during contact of the droplet by an impact force of the droplet as well as its reaction force during recoil. The magnitude of the droplet reaction force was estimated to be comparable to the droplet impact force. As the Weber number increased, however, the influence of the droplet reaction force on the surface displacement was attenuated because of the instability of the droplet rim. The contact time of the droplet and surface dynamics were found to be dependent on the phase of the surface. The contact time was reduced as much as 7% when a completion of the droplet spreading matched to the upward motion of the surface. One of the two local minima of the surface position observed during the contact of the droplet was diminished by matching the instance of the droplet reaction force to the downward motion of the surface. This study provides new insight into the effect of the droplet reaction force on dynamics of flexible superhydrophobic surfaces.