Superhydrophobic Substrates Research Articles

Liquid marble clearance and restoration via gas bubble insertion and bursting.

There is hitherto a lack of a simple way to disrupt the coating of particles from liquid marbles in order to introduce additional reagents. Here, a 40 μL liquid marble, created on a superhydrophobic substrate with a 2 mm hole, forms an overhead and overhanging liquid component from which a single gas bubble of up to 28 μL volume could be introduced via the latter. This caused a localized clearing of the particle shell at the apical region of the overhead component because the particles could not be energetically sustained at the thin film region of the bubble. The subsequent dispensation of 5 μL of an external liquid directly onto the shell-free apex of the liquid marble allowed the coalescence of the two liquid bodies, bubble rupture, and restoration of complete particle shell encapsulation. The addition of the liquid via the overhanging component was alternatively found incapable of increasing the size of the overhead drop component. The localized bubble-actuated transient shell clearance at the apex of the liquid marble to allow the addition of reagents shown here portends new vistas for liquid marbles to be used in biomedical applications.

Evaporation-driven colloidal cluster assembly using droplets on superhydrophobic fractal-like structures.

Microparticles can be considered building units for functional systems, but their assembly into larger structures typically involves complex methods. In this work, we show that a large variety of macro-agglomerate clusters ("supra-particles") can be obtained, by systematically varying the initial particle concentration in an evaporating droplet, spanning more than 3 decades. The key is the use of robust superhydrophobic substrates: in this study we make use of a recently discovered kind of patterned surface with fractal-like microstructures which dramatically reduce the contact of the droplet with the solid substrate. Our results show a clear transition from quasi-2D to 3D clusters as a function of the initial particle concentration, and a clear transition from unstable to stable 3D spheroids as a function of the evaporation rate. The origin of such shape transitions can respectively be found in the dynamic wetting of the fractal-like structure, but also in the enhanced mechanical stability of the particle agglomerate as its particle packing fraction increases.

Fabrication of Transparent and Microstructured Superhydrophobic Substrates Using Additive Manufacturing.

We report facile one- and two-step processes for the fabrication of transparent ultrahydrophobic surfaces and three-dimensional (3D)-printed superhydrophobic (SH) microstructures, respectively. In the one-step method, polydimethylsiloxane (PDMS) solution is treated thermally at 350 °C for 4 h, while PDMS-soot is generated and deposited on a glass slide to obtain a transparent SH surface without further chemical modification. For the two-step approach, SH surfaces are obtained by incorporating a 3D printing technique with a convenient hydrophobic coating method. Herein, we first 3D-print microstructured substrates with particular surface parameters, which are designed to facilitate a stable gas-trapping Cassie-Baxter (CB) wetting state based on a thermodynamic calculation. We subsequently coat the 3D-printed microstructures with candle soot (CS) or octadecyltrichlorosilane (OTS) solution to make superhydrophobic surfaces with mechanical durability. These surfaces exhibit an ultrahigh static water contact angle (CA, θ ≃ 158 ± 2 and 147 ± 2° for the CS and OTS coating, respectively) and a low roll-off angle for water droplets. Both static and dynamic (in terms of the advancing and receding) contact angles of a water droplet on the fabricated SH surfaces are in good agreement with the theoretical prediction of Cassie-Baxter contact angles. Furthermore, after a one-year-long shelf time, the SH substrates fabricated sustain good superhydrophobicity after ultrasonic water treatment and against several chemical droplets. All of these methods are simple, cost-effective, and highly efficient processes. The processes, design principle, and contact angle measurements presented here are useful for preparing transparent and superhydrophobic surfaces using additive manufacturing, which enables large-scale production and promisingly expands the application scope of utilizing self-cleaning superhydrophobic material.

Various crust morphologies of colloidal droplets dried on a super-hydrophobic surface

We have studied the evaporation of water droplets containing silica nanoparticles of various hydrophobicities deposited on a super-hydrophobic substrate. Evaporation induces particle accumulation at the droplet surface and results in the formation of a crust that buckles during further shrinkage. For droplets containing hydrophilic particles, a bowl-shaped crust was observed. For droplets containing hydrophobic particles, the crust develops a multi-buckled shape that could be completely suppressed by increasing the relative humidity. The varied buckling behavior of droplets may be attributed to the different mechanical properties of the gelled layer where particle hydrophobicity plays a role. Our work highlights the important role of particle hydrophobicity and relative humidity in the final crust morphology, thus shedding light on crust shape control and material design via the droplet evaporation approach.

Efficient drop reactor processing of methylene blue degradation with silver nanowire catalysts

A drop-based approach to facilitate the remediation of methylene blue (MB) solution via catalytic degradation with silver nanowires on a superhydrophobic substrate with a 2 mm hole without fouling is presented here. Studies on the dynamical response of drop removal via pressure rate perturbation, showed greater elapsed times at the 7.61 μL/s air flowrate with 25 μL and 30 μL volume drops than at higher air flowrates used. These results are accounted for by the rate of air flow through the plastron, used to equilibrate the lowering of pressure in the transparent receptacle that tends to prevent drop dislodgement, being only able to operate up to a specific upper limit. A method that calculates the absorbance values based on the intensity spectrum of the nanowire solution as reference was found to reveal the time progress degradation of MB solution by spectrometry. It showed that a 25 μL drop of the MB solution could be degraded within 15 s using 0.25 mg of the silver nanowire.

Self-Concentrated Surface-Enhanced Raman Scattering-Active Droplet Sensor with Three-Dimensional Hot Spots for Highly Sensitive Molecular Detection in Complex Liquid Environments.

In this work, a surface-enhanced Raman scattering (SERS)-active droplet with three-dimensional (3D) hot spots prepared from a superhydrophobic SERS substrate, which is inspired by the nut wizard strategy, was developed for ultrasensitive detection in complex liquid environments. The SERS substrate was composed of silver-capped parylene C-coated carbon nanoparticles (Ag-PC@CNPs). Such a SERS substrate was prepared by candle-soot deposition to provide a porous carbon nanoparticle layer followed by deposition of a parylene C film to protect the CNPs and then sputtering of silver nanoparticles. Similar to a nut wizard, a droplet rolling on the Ag-PC@CNP-coated substrate picked up the Ag-PC@CNPs. In this way, a self-concentrated and extremely sensitive SERS-active droplet sensor with 3D hot spots was formed. The sensor did not require precise laser focusing and showed relatively high repeatability and much higher sensitivity than those of a corresponding SERS substrate with two-dimensional hot spots. The sensor also achieved high sensitivity and specificity in complex liquid environments; in addition, bovine serum albumin with a concentration as low as 1 pM can be achieved. Consequently, an extremely simple, flexible, and highly sensitive SERS detection technique applicable to liquid biopsy analysis is anticipated.

Fabrication of superhydrophobic Ag@ZnO@Bi2WO6 membrane disc as flexible and photocatalytic active reusable SERS substrate

Nowadays, there is a great influence of the innovated semiconductor heterojunction on the improvement of photocatalysis in organic pollutants elimination and the Raman signals multiplexing towards quantitative determination. Here, it was an attempt to develop a heterostructure membrane-based platform employing a filtration system to degrade and detect various target molecules at trace level in liquid samples. Herein, the ZnO nanorods/Bi2WO6 nanocomposites (ZBW) were prepared by a moderate temperature hydrothermal method; thereafter ZBW was encrusted with in-situ photoreduced silver nanoparticles (AgNPs) to form AgNPs@ZBW (Ag@ZnO@Bi2WO6) platform. The superhydrophobic AgNPs@ZBW platform encompasses both photocatalytic degradation of rhodamine B (RhB) under UV-irradiation and label-free surface-enhanced Raman scattering (SERS) evaluated for rhodamine 6G (R6G) and RhB. The AgNPs@ZBW exhibited superior photocatalytic and SERS activities for organic dyes degradation. As a result, the photocatalytic efficiency for dyes was 97.95% in 140 min. From the recycling experiment, it was manifested that AgNPs@ZBW maintained superb photocatalytic activity and stability after five cycles of use for SERS analysis. As-prepared platform displayed a scalable SERS response from 100 µM to 1 nM and enhancement factor of 3.2 × 105 and 1.2 × 105 for R6G and RhB, respectively, using an excitation wavelength of 532 nm, laser power of 3.2 mW and time of exposure of 10 s. Displayed trending of superhydrophobic membrane-based SERS substrates is assumed to lead abounds of change in the field of photocatalytic materials, SERS assay, and organic chemical analysis.

Simultaneous Effect of Droplet Temperature and Surface Wettability on Single Drop Impact Dynamics

In this paper, the influence of the liquid droplet temperature on thermo–hydrodynamics of a single droplet impinging on surfaces having different hydrophobicities is experimentally investigated. Variation in the liquid temperature typically results in alteration of properties such as the density, the viscosity, the surface tension, and the enthalpy, consequently, the droplet dynamics gets to be modified. Employing high-speed imaging technique, the morphology and spreading pattern are investigated for water droplet collision on hydrophilic, hydrophobic and super-hydrophobic surfaces. The droplet deformation is monitored qualitatively and quantitatively for drops in the temperature range from 5 to 85°C and the Weber number between 14.5 and 160. It is observed that with an increase in the liquid temperature the spreading factor increases owing to the combined effect of reduction in the density, the surface tension, the viscosity and the contact angle of the solid surface. The differences in extension of droplets under the extreme temperatures for hydrophilic, hydrophobic and super-hydrophobic surfaces are noted to be 62.7, 27.76, and 20.52%, respectively. At the low temperature, the surface tension force dominates and the Cassie–Baxter state prevails on a textured super-hydrophobic surface and the droplets bounce off. In contrast at elevated temperatures, the liquid–solid interface ruptures and liquid penetrates into the cavities and results in the Wenzel state. Furthermore, the drop which exhibits multiple bounces in the low temperature regime is found sticking on a super-hydrophobic substrate at the high droplet temperature irrespective of the Weber number.

A droplet reactor on a super-hydrophobic surface allows control and characterization of amyloid fibril growth

Methods to produce protein amyloid fibrils, in vitro, and in situ structure characterization, are of primary importance in biology, medicine, and pharmacology. We first demonstrated the droplet on a super-hydrophobic substrate as the reactor to produce protein amyloid fibrils with real-time monitoring of the growth process by using combined light-sheet microscopy and thermal imaging. The molecular structures were characterized by Raman spectroscopy, X-ray diffraction and X-ray scattering. We demonstrated that the convective flow induced by the temperature gradient of the sample is the main driving force in the growth of well-ordered protein fibrils. Particular attention was devoted to PHF6 peptide and full-length Tau441 protein to form amyloid fibrils. By a combined experimental with the molecular dynamics simulations, the conformational polymorphism of these amyloid fibrils were characterized. The study provided a feasible procedure to optimize the amyloid fibrils formation and characterizations of other types of proteins in future studies.

Bioinspired superwetting surfaces for biosensing

AbstractInspired by nature, scientists and researchers have studied the wetting behaviors on various creatures and mimicked their structures to fabricate diverse functional superwetting materials. As one kind of emerging application, the bioinspired superwettable surfaces used for biosensing have been aroused wide interests. In this review, we summarized the recent developments of bioinspired superwettable surfaces in the field of biosensing. In the first part, superwettable creatures in nature, namely, superhydrophobic self‐cleaning lotus leaf, high‐adhesion superhydrophobic rose petal, amphiphobic springtail, patterned wetting desert beetles, slippery pitcher plant, were introduced. In sequence, we successively described the special wetting models of superhydrophobicity, superamphiphobicity, responsive wettability, patterned wettability, and slipperiness. Then, biosensing applications based on the respective patterned wettable, superhydrophobic, responsive wettable, and slippery substrates that were combined with the common detection approaches (colorimetry, fluorescence, surface‐enhanced Raman scattering (SERS), electrochemistry) were shown in detailed. At last, an insight of remaining challenges and future development for bioinspired superwetting materials applied in biosensing was provided.

Evaporation-Crystallization Method to Promote Coalescence-Induced Jumping on Superhydrophobic Surfaces.

Biphilic surfaces exhibit outstanding condensation efficiency compared to surfaces having homogeneous wettability. Especially, hydrophilic patterns on a superhydrophobic substrate significantly promote the coalescence-induced jumping of condensed droplets by increasing the nucleation rate of condensation, thus enhancing the condensation efficiency drastically. However, the application of biphilic surfaces in practical industries remains challenging because controlling the size and spacing of the hydrophilic spots on large and complex surfaces is difficult. In this study, we have achieved heterogeneous wettability using the evaporation-crystallization method, which can be applied to various surfaces as required by industries. The crystals generated using the evaporation-crystallization process drastically increased the number density of condensed droplets on a superhydrophobic surface (SHS), so the developed biphilic surface increased the cumulative volume of jumping droplets by up to 63% compared to that on a conventional superhydrophobic surface. Furthermore, the condensation dynamics on the biphilic surface were analyzed with the classical nucleation theory and the Ohnesorge number. The analysis results indicated that the generated hydrophilic crystals can reduce the nucleation energy barrier and decrease the available excessive surface energy of coalesced droplets on the biphilic surface; this implies that the size distribution of the crystals determines the condensation dynamics. In sum, this study not only introduced an effective surface tailoring approach for enhancing condensation but also provided insights into the design of optimum biphilic surfaces for various conditions, creating new opportunities to widen the applicability of biphilic surfaces in practical industries that exploit condensation.

Self Assembly of Super-hydrophobic Nanotextured Methyl Functionalized Silica on Copper and Aluminium Surfaces for Moist Air Condensation

The super-hydrophobic surfaces enhance the transport coefficients of the condensation process and therefore it is very useful in water harvesting devices. In the present study, super-hydrophobic substrates are fabricated by growing self-assembled silica nanostructures on copper (Cu) and aluminum (Al) surfaces by a sol-gel based solution immersion method at room temperature. The contact angles measured were 155 ± 2° and 156 ± 2° on coated Cu and Al surfaces respectively. The water condensation rate on the coated Cu surface significantly increased from 188 ml/m2-hr to 630 ml/m2-hr, while that of the coated Al surface increased from 190 ml/m2-hr to 650 ml/m2-hr. The present work shows that the metallic substrates with super-hydrophobic coatings have excellent potential for water harvesting from moist air, and they were effectively used for more than 120 days.

Nonwet Kingfisher Flying in the Rain: The Tumble of Droplets on Moving Oriented Anisotropic Superhydrophobic Substrates.

Extensive studies of antiwetting have been restricted to stationary substrates, while dewetting mechanisms on moving interfaces are still poorly understood. Due to the hydrophobic and anisotropic surface characteristics of kingfishers, they are able to easily change flight direction even under high-intensity precipitation. The present study aims to mechanistically analyze how the synergy of interfacial movement, anisotropy, and superhydrophobicity affects rapid dehydration. We have designed a droplet-conveyor system to simulate the bouncing of droplets on moving anisotropic superhydrophobic targets and performed simulations via the lattice Boltzmann algorithm. The moving interface can induce a directional tumbling behavior of the droplet and effectively avoid continuous wetting in the same region. We found that droplet tumbling is essentially caused by transformed depinning velocity vectors at the interface downstream. Also, the hang time of a tumbling droplet is positively related to the angle between the motion vector and the texture. The oriented anisotropic motion facilitates the tumbling of droplets and decreases their hang time by up to 23% as compared to that on a stationary inclined superhydrophobic surface. Similar interfacial process dehydration also occurs on a nonwet kingfisher flying in the rain, and we believe that these findings provide valuable new insights for high-efficiency water repellency of surfaces.

Collective behavior of evaporating droplets on superhydrophobic surfaces

AbstractWe study the evaporation dynamics of multiple water droplets deposited in ordered arrays or randomly distributed (sprayed) on superhydrophobic substrates (SHP) and smooth silicon wafers (SW). The evaluation of mass of the droplets as a function of time shows a power‐law behavior with exponent 3/2, and from the prefactor of the power‐law an evaporation rate can be determined. We find that the evaporation rate on a SHP surface is slower than a normal surface for both single droplet and collection of droplets. By dividing a large droplet into more smaller ones, the evaporation rate increases and the difference between the evaporation rates on SHP and SW surfaces becomes higher. The evaporation rates depend also on the distance of the droplets which increase with increasing this distance.

Liquid Marbles: Exceptional Robustness and Self‐Reconfigurability of Liquid Marbles on Superhydrophobic Substrate (Adv. Mater. Interfaces 11/2020)

Liquid Marbles: Exceptional Robustness and Self‐Reconfigurability of Liquid Marbles on Superhydrophobic Substrate (Adv. Mater. Interfaces 11/2020)

Active effect of super-hydrophobicity on droplet nucleate boiling

It is well known that a surface with microstructures can be used to promote droplet nucleate boiling. However, the effect of the microstructure wettability on the droplet nucleate boiling is unknown. In this article, a test surface consists of two regions, one square region with microstructures is fabricated in the central area of the test surface, and another region keeps smooth. The droplets can maintain a similar shape before boiling so that the influence of droplet morphological changes on the boiling of the droplets is avoided. Surfaces with more hydrophilic and super-hydrophobic microstructures are called heterogeneous hydrophilic and heterogeneous hydrophobic surfaces, respectively. The result shows that the super-hydrophobic microstructure can promote the droplet to enter the nucleate boiling state at a lower superheat. The reason is that when the bubbles break up at the surface of the droplet, the super-hydrophobic substrate can attract part of the vapor to be trapped under the droplet. This part of the trapped bubble will continue to grow up and break up, leading the droplet entering the nucleate boiling state. Compared with the smooth hydrophilic surface, the heat transfer rate of the droplet on the heterogeneous hydrophobic surface can increase 381.25% for a superheat of 20 K.

Optimal Design of a Fog Collector: Unidirectional Water Transport on a System Integrated by Conical Copper Needles with Gradient Wettability and Hydrophilic Slippery Rough Surfaces.

Inspired by a cactus spine and pitcher plant slippery surface, a strategy is proposed to design a superhydrophobic-hydrophilic conical copper needle (SHB-HL CCN) and hydrophilic slippery rough surface (SRS) integrative system. In this strategy, the SHB-HL CCN was inserted vertically on the hydrophilic SRS, and such a hydrophilic SRS + SHB-HL CCN system exhibited a high-efficiency cycle in droplet capture-coalescence (supply)-transport during the fog collection process. Even with a single SHB-HL CCN or hydrophilic SRS, the water collection rate is much higher than that of the usual materials (original copper needle, superhydrophobic substrate, hydrophobic SRS, etc.). It is demonstrated that a newly enhanced fog harvesting mechanism and higher fog collection rate can be realized due to the synergy between the Laplace pressure difference from the conical needle, wettability force of wettability difference in the conical copper needles, and released surface energy in droplet coalescence in addition to the attracting force from water bridges formed between needles and substrate. Compared with a single SHB-HL CCN and hydrophilic SRS, the water collection rate of the hydrophilic SRS + SHB-HL CCN system increased by approximately 328 and 152%, respectively. This fog collector provides direction to design water harvesting systems, which has important promotion significance for water collection application engineering in industry, aerospace, and other fields.

Patterned Superhydrophobic SERS Substrates for Sample Pre-Concentration and Demonstration of Its Utility through Monitoring of Inhibitory Effects of Paraoxon and Carbaryl on AChE.

We describe a patterned surface-enhanced Raman spectroscopy (SERS) substrate with the ability to pre-concentrate target molecules. A surface-adsorbed nanosphere monolayer can serve two different functions. First, it can be made into a SERS platform when covered by silver. Alternatively, it can be fashioned into a superhydrophobic surface when coated with a hydrophobic molecular species such as decyltrimethoxy silane (DCTMS). Thus, if silver is patterned onto a latter type of substrate, a SERS spot surrounded by a superhydrophobic surface can be prepared. When an aqueous sample is placed on it and allowed to dry, target molecules in the sample become pre-concentrated. We demonstrate the utility of the patterned SERS substrate by evaluating the effects of inhibitors to acetylcholinesterase (AChE). AChE is a popular target for drugs and pesticides because it plays a critical role in nerve signal transduction. We monitored the enzymatic activity of AChE through the SERS spectrum of thiocholine (TC), the end product from acetylthiocholine (ATC). Inhibitory effects of paraoxon and carbaryl on AChE were evaluated from the TC peak intensity. We show that the patterned SERS substrate can reduce both the necessary volumes and concentrations of the enzyme and substrate by a few orders of magnitude in comparison to a non-patterned SERS substrate and the conventional colorimetric method.

Ultra-sensitive, reusable, and superhydrophobic Ag/ZnO/Ag 3D hybrid surface enhanced Raman scattering substrate for hemoglobin detection

The small Raman scattering cross section of hemoglobin (Hb) molecules limits its application using a Raman spectroscopy based optical biosensor. Label-free surface enhanced Raman scattering (SERS) detection and degradation of Hb have been achieved using 3D reusable superhydrophobic SERS substrates based on a Ag/ZnO/Ag hybrid structure. The fabrication process follows the decoration of thermally evaporated non-spherical like Ag nanoparticles on hydrothermally grown ZnO nanorods on a catalytic ultra-thin Ag film. From SERS point of view, these 3D SERS substrates exhibit four important characteristics such as a higher surface to volume ratio, surface plasmon resonance in the broad wavelength region of the visible spectrum, a strong electric field at the Ag–ZnO interface due to the formation of a Schottky barrier, and the superhydrophobic surface. The SERS substrates not only performed an outstanding Raman enhancement effect due to the above factors but also displayed multiple recyclabilities owing to their excellent self-cleaning property via a UV light assisted photocatalytic degradation process. The quantitative SERS analysis has been performed by a linear regression method and resulted in 10−13.42M and 10−7.24M limit of detection for Rhodamine 6G (Rh6G) and Hb molecules, respectively, with an enhancement factor of 6 × 1011. The effect of the 3D hybrid structure toward higher SERS activity has been compared with that of 2D SERS substrates, and the SERS mapping of Rh6G molecules proves good homogeneity of the 3D SERS substrates. These ultra-sensitive 3D SERS substrates with reusable capability open the possibility of their use toward biosensors for the early detection of diseases.

Exceptional Robustness and Self‐Reconfigurability of Liquid Marbles on Superhydrophobic Substrate

AbstractNonstick transport of droplets on a solid surface is observed when they are covered with jammed hydrophobic particles. However, these “liquid marbles” (LMs) have low mechanical stability because, in controlling the jammed particles, exposure of the droplet surface often occurs with deformation that irreversibly causes the droplet to adhere to the solid surface in a funicular state. Here, it is shown that LMs on superhydrophobic surfaces are mechanically stable and self‐reconfigurable following mechanical impact or exposure of the droplet surface. Observation by a high‐speed bottom‐view camera revealed that this is because of reversible capillary‐partial Cassie transitions. Thus, superhydrophobic surfaces are scalable and low‐cost platforms for LM synthesis and manipulation, and enable reversible tuning of interfacial packing on LMs with mechanical stability.