Two-tier Roughness Research Articles

Wetting of nanoscale water films on hierarchically structured surfaces

Surfaces with hierarchical structures significantly enhance the hydrophobic properties of solids, proving crucial for diverse applications including self-cleaning, anti-icing, and contamination prevention. In this study, we directly observe the dynamic wetting transitions of nanoscale water films on desirable textured surfaces decorated with dual-scale roughness between various wetting states encompassing Cassie–Cassie, Wenzel–Cassie, Cassie–Wenzel, and Wenzel–Wenzel states. Additionally, detailed information on the wetting of the water film on desirable textured surfaces decorated with dual-scale roughness is obtained using atomistic simulations in conjunction with sampling techniques. Through observation of the dynamic wetting transition, two common types of wetting pathways are directly captured, dubbed the preferential primary intrusion and secondary intrusions. The wetting follows which pathway is dependent on Hs/Ss of the small-scale roughness. The mechanisms behind the wetting transitions are revealed based on corresponding free-energy pathways. Moreover, the effect of aspect ratio and intrinsic contact angle on the wetting behavior has been studied. Subsequently, we construct a wetting phase diagram to exhibit all the possible outcomes and identify different wetting regimes. This work paves the way to understanding the wetting mechanisms on nanoscale textured surfaces with two-tier roughness, which can help to design a hydrophobic surface with superior robustness.

Towards an efficient and durable superhydrophobic mesh coated by PDMS/TiO2 nanocomposites for oil/water separation

Oil spill accidents and industrial oily wastewater are threatening all living species in the ecological system. Development of materials with special wettability for oil/water separation has been the subject of many researches. Herein, superhydrophobic and superoelophilic nanocomposite coatings, based on titanium dioxide (TiO2) nanoparticles and polydimethylsiloxane (PDMS), were applied on metal meshes with different pore sizes (25 and 100 μm). Morphological analysis revealed that adding PDMS, as a binder, significantly changes the surface structure of the coatings. For 100 μm-sized meshes, PDMS causes a two-tier roughness whereas a uniform and finely packed single-scale nanostructure was formed for 25 μm-sized meshes upon PDMS addition. Exceedingly high content of nanoparticles was found to impose a detrimental influence on uniformity of morphology. All the samples exhibited superhydrophobicity; however, their mechanical durability results were significantly diverse. PDMS introduction was found to impose a substantial improvement in abrasion resistance. Oil/water separation experiments revealed that the mesh with smaller pore size exhibited a more efficient separation while a much shorter separation time and higher flux could be achieved by the mesh with larger pore size. In addition, the mesh with smaller pore size exhibited a more abrasion resistant coating which could be more beneficial in real-life applications.

Design and preparation of efficient, stable and superhydrophobic copper foam membrane for selective oil absorption and consecutive oil–water separation

It is extremely important and challenging to develop an effective device for the continuous separation and recovery of large-scale oil–water mixtures due to their contribution to ecological remediation and contamination control. In this work, a superhydrophobic copper foam with high oil–water separation efficiency was successfully fabricated, which could serve both as oil absorption material and oil–water separation membrane, by in situ self-sacrificial template method combining the subsequent steaming modification. The obtained characterization results present that copper foam possesses large pore structure, micro-/nanoscale two-tier surface roughness, high water contact angle (153.6°), and low sliding angle (4.5°). Experimental results show that as-prepared copper foam could not only rapidly absorb residual oil either on the water surface or underwater, but also separate a series of oils from water, like carbon tetrachloride, trichloromethane, methylbenzene, pump oil and diesel. Furthermore, the separation efficiencies of copper foam on trichloromethane-water and diesel-water mixture were maintained above 96.9% and 90.8% after 10 cycles, respectively. Additionally, as-prepared copper foam exhibits excellent corrosion resistant ability and superior hydrophobic stability. The facile, low-cost and controllable strategy presented herein has a bright future in oil–water separation, and also can be further expanded to prepare various anti-corrosion, self-cleaning and water-proof sport materials.

Drop Impact on Oblique Superhydrophobic Surfaces with Two-Tier Roughness.

This paper investigates the complex physical phenomenon of oblique drop impact on superhydrophobic substrates with two-tier roughness (patterned with varied submillimeter-scale posts and coated with nanoparticles). Experimental results show that the impact Weber number of drops and the solid fraction of the submillimeter-scale post structures are crucial for the outcomes of oblique drop collisions. Water droplets with 10 < Wen < 245 are found to rebound on all the test substrates. As the surface solid fraction decreases, four possible bouncing patterns occur in sequence: sliding rebound, stretched rebound, penetration rebound, and breakup rebound. We demonstrate that the stretched rebound, in which the drops bounce off the surface rapidly in an elongated shape without tangential retraction, allows a 10%∼30% reduction of contact time compared with conventional sliding rebound on oblique surfaces. Three types of stretched rebound are observed on substrates with moderate solid fraction (0.1 < φ < 0.5), in which the liquid detachment starts from front, center, and tail, respectively. A simple analytical argument is presented to explain the occurrence of distinct patterns. These findings are believed to provide valuable guidance to the design of self-cleaning and anti-icing surfaces under oblique liquid impacts where rapid drop shedding is profitable.

Hierarchical patterning of hydrogels by replica molding of impregnated breath figures leads to superoleophobicity.

The surface chemistry and topography govern the spreading of liquids on a solid. When an oil drop makes a contact angle, θ > 90° on a solid surface, the solid is termed as oleophobic. Adding roughness to an inherently oleophobic surface enhances its oil dewetting and can lead to superoleophobicity when θ > 150°. In this study, we introduce the concept of a two-tier hierarchical roughness on the surface of soft materials such as hydrogels by forming the patterned inverse replica of breath figure polymer films impregnated with nanoparticles. The directed deposition of nanoparticles in the breath figure pores is accomplished by an aerosol assisted technique that exclusively leads to deposition within the pores and filling of the pores. The inverse replica of such impregnated films exhibits a close packed hexagonally structured second tier of surface roughness which directly leads to a superoleophobic surface. Since these structures have well defined geometries, it is possible to estimate the contact angle by assuming a partial wetting of the oil drop in a 'fakir' state on the rough surface. The estimation is in good agreement with the experimental contact angle value. While the work demonstrates a facile method to impart superoleophobicity to a hydrogel surface, it also demonstrates new methods to imbue breath figure pores with functional materials that can be easily transferred to the pores of the inverse replica.

Dewetting Transitions of Dropwise Condensation on Nanotexture-Enhanced Superhydrophobic Surfaces.

Although realizing dewetting transitions of droplets spontaneously on solid textured surfaces is quite challenging, it has become a key research topic in many practical applications that require highly efficient removal of liquid. Despite intensive efforts over the past few decades, due to impalement of vapor pockets inducing strong pinning of the contact lines, how to realize the self-removal of small droplets trapped in the textures remains an urgent problem. We report an in situ spontaneous dewetting transition of condensed droplets occurring on pillared surfaces with two-tier roughness, from the valleys to the tops of the pillars, owing to the nanotexture-enhanced superhydrophobicity, as well as the topology of the micropillars. Three wetting transition modes are observed. It is found that a further decreased Laplace pressure on the top side of the individual droplets accounts for such a surprising transition and self-removal of condensed water. An explicit model is constructed, which quite effectively predicts the Laplace pressure of droplets trapped by the textures. Our model also reveals that the critical size of the droplet for transition scales as the spacing of the micropillars. These findings are expected to be crucial to a fundamental understanding, as well as a remarkable strategy to guide the fabrication, of optimum super-water-repellant surfaces.

Contaminant adhesion (aerial/ground biofouling) on the skin of a gecko.

In this study, we have investigated the micro- and nano-structuring and contaminant adhesional forces of the outer skin layer of the ground dwelling gecko--Lucasium steindachneri. The lizard's skin displayed a high density of hairs with lengths up to 4 μm which were spherically capped with a radius of curvature typically less than 30 nm. The adhesion of artificial hydrophilic (silica) and hydrophobic (C18) spherical particles and natural pollen grains were measured by atomic force microscopy and demonstrated extremely low values comparable to those recorded on superhydrophobic insects. The lizard scales which exhibited a three-tier hierarchical architecture demonstrated higher adhesion than the trough regions between scales. The two-tier roughness of the troughs comprising folding of the skin (wrinkling) limits the number of contacting hairs with particles of the dimensions used in our study. The gecko skin architecture on both the dorsal and trough regions demonstrates an optimized topography for minimizing solid-solid and solid-liquid particle contact area, as well as facilitating a variety of particulate removal mechanisms including water-assisted processes. These contrasting skin topographies may also be optimized for other functions such as increased structural integrity, levels of wear protection and flexibility of skin for movement and growth. While single hair adhesion is low, contributions of many thousands of individual hairs (especially on the abdominal scale surface and if deformation occurs) may potentially aid in providing additional adhesional capabilities (sticking ability) for some gecko species when interacting with environmental substrates such as rocks, foliage and even man-made structuring.

Corrosion-resistance, robust and wear-durable highly amphiphobic polymer based composite coating via a simple spraying approach

Abstract This study successfully developed a simple spray approach to fabricate a robust highly amphiphobic poly(phenylene sulfide) (PPS)/fluorinated ethylene propylene (FEP)/poly(dimethylsiloxane) (PDMS) composite coating with high-performance in corrosion-resistance, wear-durable through designing the nano/micro two-tier roughness and fluorinating with materials of the low surface free energy. The highly amphiphobic and tribological properties of the coatings were measured by the contact angle meter and the pin-on-disc tribometer, respectively. It was interested to observe that the composite coating showed superhydrophobic and highly oleophobic simultaneously, with the highest contact angles of water, glycerine and ethylene glycol up to 173 ± 2.1°, 142 ± 2.2° and 139 ± 2.1°, respectively. Moreover, the surfaces of the PPS/FEP composite coatings were investigated by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD) and energy-dispersive X-ray spectroscopic (EDS). The robust highly amphiphobic coating also showed remarkable durability against strong acid and strong alkali in the pH range from 1 to 14. After 47 h sliding wear test, no failure sign on the PPS/45%FEP/PDMS composite coating was observed. Such unique characteristics were attributed to the synergistic effect of the nano/micro two-tier roughness and fluorinating with low surface free energy groups (–CF 2 –, –CF 3 –).

Wicking nanopillar arrays with dual roughness for selective transport and fluorescence measurements.

Silicon nanopillars are important building elements for innovative nanoscale systems with unique optical, wetting, and chemical separation functionalities. However, technologies for creating expansive pillars arrays on the submicron scale are often complex and with practical time, cost, and method limitations. Herein we demonstrate the rapid fabrication of nanopillar arrays using the thermal dewetting of Pt films with thicknesses in the range from 5 to 19 nm followed by anisotropic reactive ion etching (RIE) of the substrate materials. A second level of roughness on the sub-30 nm scale is added by overcoating the silicon nanopillars with a conformal layer of porous silicon oxide (PSO) using room temperature plasma enhanced chemical vapor deposition (PECVD). This technique produced environmentally conscious, economically feasible, expansive nanopillar arrays with a production pathway scalable to industrial demands. The arrays were systematically analyzed for size, density, and variability of the pillar dimensions. We show that these stochastic arrays exhibit rapid wicking of various fluids and, when functionalized with a physiosorbed layer of silicone oil, act as a superhydrophobic surface. We also demonstrate high brightness fluorescence and selective transport of model dye compounds on surfaces of the implemented nanopillar arrays with two-tier roughness. The demonstrated combination of functionalities creates a platform with attributes inherently important for advanced separations and chemical analysis.

Facile preparation of super durable superhydrophobic materials

The low stability, complicated and expensive fabrication procedures seriously hinder practical applications of superhydrophobic materials. Here we report an extremely simple method for preparing super durable superhydrophobic materials, e.g., textiles and sponges, by dip coating in fluoropolymers (FPs). The morphology, surface chemical composition, mechanical, chemical and environmental stabilities of the superhydrophobic textiles were investigated. The results show how simple the preparation of super durable superhydrophobic textiles can be! The superhydrophobic textiles outperform their natural counterparts and most of the state-of-the-art synthetic superhydrophobic materials in stability. The intensive mechanical abrasion, long time immersion in various liquids and repeated washing have no obvious influence on the superhydrophobicity. Water drops are spherical in shape on the samples and could easily roll off after these harsh stability tests. In addition, this simple dip coating approach is applicable to various synthetic and natural textiles and can be easily scaled up. Furthermore, the results prove that a two-tier roughness is helpful but not essential with regard to the creation of super durable superhydrophobic textiles. The combination of microscale roughness of textiles and materials with very low surface tension is enough to form super durable superhydrophobic textiles. According to the same procedure, superhydrophobic polyurethane sponges can be prepared, which show high oil absorbency, oil/water separation efficiency and stability.

Frost and Ice Transport on Superhydrophobic Copper Surfaces with Patterned Micro- and Nano-Structures

Texture and wettability have an important influence on fogging, frosting, and icing on a metal surface. We fabricated micro- and nano-structure patterns on a copper surface by wire electrical discharge machining and subsequent chemical oxidation. By controlling the manufacturing process, three types of microstructure were machined: gratings, pillars, and pyramids. We then studied the wetting performance of the superhydrophobic surfaces with one-tier texture or two-tier texture and the corresponding transport of water in different phase states including fog, frost and icing and their melting processes. Two-tier roughness on the copper effectively improved the superhydrophobicity and retarded the formation and growth of frost. More importantly, these surfaces showed a long delayed icing time, even after several heating and cooling cycles, displaying good resistance to frost and icing. This can be well explained by an understanding of classical nucleation theory, Brown coalescence, and one-dimensional heat and mass transport.

Condensation and jumping relay of droplets on lotus leaf

Dynamic behavior of micro water droplet condensed on a lotus leaf with two-tier roughness is studied. Under laboratory environment, the contact angle of the micro droplet on single micro papilla increases smoothly from 80° to 160° during the growth of condensed water. The best-known “self-cleaning” phenomenon will be lost. A striking observation is the out-of-plane jumping relay of condensed droplets triggered by falling droplets, as well as its sustained speed obtained in continuous jumping relays. The underlying mechanism can be used to enhance the automatic removal of dropwise condensation without the help from any external force. The surface tension energy dissipation is the main reason controlling the critical size of jumping droplet and its onset velocity of rebounding.

Characterization of ultrahydrophobic hierarchical surfaces fabricated using a single-step fabrication methodology

Hydrophobic surfaces with microscale roughness can be rendered ultrahydrophobic by the addition of sub-micron-scale roughness. A simple yet highly effective concept of fabricating hierarchical structured surfaces using a single-step deep reactive ion etch process is proposed. Using this method the complexities generally associated with the fabrication of two-tier roughness structures are eliminated. Three two-tier roughness surfaces with different roughness parameters are fabricated and tested. The surfaces are characterized in terms of the static contact angle and roll-off angle, and are compared with surfaces consisting of only single-tier microscale roughness. The evaporation characteristics of a sessile droplet on the hierarchical surfaces is also assessed relative to comparable single-roughness (SR) surfaces. The robustness of the new hierarchical roughness surfaces is verified through droplet impingement tests. The hierarchical surfaces exhibit very high contact angle and lower contact angle hysteresis compared to the SR surfaces and are more resistant to wetting. The energy loss during impact on the surfaces is quantified in terms of the coefficient of restitution for droplets bouncing off the surface.

Wetting and Dewetting Transitions on Hierarchical Superhydrophobic Surfaces

Many natural superhydrophobic structures have hierarchical two-tier roughness which is empirically known to promote robust superhydrophobicity. We report the wetting and dewetting properties of two-tier roughness as a function of the wettability of the working fluid, where the surface tension of water/ethanol drops is tuned by the mixing ratio, and compare the results to one-tier roughness. When the ethanol concentration of deposited drops is gradually increased on one-tier control samples, the impalement of the microtier-only surface occurs at a lower ethanol concentration compared to the nanotier-only surface. The corresponding two-tier surface exhibits a two-stage wetting transition, first for the impalement of the microscale texture and then for the nanoscale one. The impaled drops are subsequently subjected to vibration-induced dewetting. Drops impaling one-tier surfaces could not be dewetted; neither could drops impaling both tiers of the two-tier roughness. However, on the two-tier surface, drops impaling only the microscale roughness exhibited a full dewetting transition upon vibration. Our work suggests that two-tier roughness is essential for preventing catastrophic, irreversible wetting of superhydrophobic surfaces.

Fabrication of self-cleaning poly(vinylidene fluoride) membrane with micro/nanoscaled two-tier roughness

Based on the “lotus effect” principle, smooth microreliefs of polyvinylidene fluoride (PVDF) membrane were prepared via thermally induced phase separation process. Hydroxyl groups were introduced into PVDF membrane by pretreatment with KOH/alcohol solution. Subsequently, these hydroxyl groups grafted with (CH3)2SiCl2/CH3SiCl3 to form nano-clusters, which were decorated on the microreliefs of PVDF membrane. Scanning Electronic Microscopy (SEM) and Atomic Force Microscope (AFM) analysis showed the micro- and nano-scale structures, similar to lotus leaf, were successfully fabricated on the PVDF membrane surface. The water contact angle and sliding angle on the fabricated lotus-leaf-like PVDF membrane surface were 154 and 4°, respectively. Self-cleaning test indicated that the lotus-leaf-like surface of PVDF membrane has excellent superhydrophobic and self-cleaning properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Superhydrophobicity and superoleophobicity from hierarchical silica sphere stacking layers

The present study demonstrates the creation of a stable superhydrophobic and superoleophobic surface using a hierarchical nano/submicron silica sphere stacking layer with a thin fluorination coating on the surface of the spheres. The super repellent surface not only repels the liquids with surface tension, ranging from 23.4 to 73.2 mN m −1, but also shows a stable superoleophobicity toward sunflower oil at least for 3 days. The deposition of nanospheres provides the nanoscale cavity to repel the oil penetration, referring to the Cassie state. Thus, there is an existing thin air film that is capable of protecting each submicron spheres from oil contamination, subsequently leading to the superoleophobicity. The significant enhancement of superoleophobicity can be attributed to the following facts: (i) surface fluorination (low surface tension), (ii) surface topography (roughness), and (iii) creation of bionic surface hierarchically combined with nano/submicron architecture. Accordingly, designing of the surface geometry with two-tier roughness acts as the key factor in inducing superoleophobicity.

Hierarchically organized soft-materials based on fullerenes

Simple chemical modifications of fullerene (C60) with long aliphatic chains provide novel type amphiphilic molecules playing in organic solvents due to the two different intermolecular interactions, namely π-π on C60 and van der Waals interactions on aliphatic chain moieties, respectively, and open a door developing supramolecular soft-materials having hierarchically organized architectures, various morphologies and functions based on fullerenes. By tuning the length and number of aliphatic chains on the derivatives as well as experimental conditions such as solvents, temperature, substrates for preparation of the assemblies, the assembled fullerenes showed various faces such as creating of many unique-shaped objects with controlled their dimensionality. For instance, nanowires and thin disks with single bilayer thickness in nanometer size, globular, fibrous, conical objects in mesoscopic (sub-micrometer) scale and flower-shaped and direction-controlled spiral objects in micrometer scale are obtained. As bulk states, thermotropic liquid crystals and room temperature (isotropic) liquid fullerenes are interestingly prepared from this molecular designs and showed not only their fluid natures and comparably high carrier mobility as fullerene-based organic-semiconductor phenomena. In addition, nano-carbon superhydrophobic surface with fractal morphology of the two-tier roughness on a nano- and microscopic scale was created from one of the supramolecular objects. The all of hierarchical supramolecular assemblies describing in this review is derived from fine-tuning intermolecular interactions of fullerene derivatives bearing long aliphatic chains.

Fabrication and superhydrophobicity of fluorinated carbon fabrics with micro/nanoscaled two-tier roughness

Superhydrophobic carbon fabric with micro/nanoscaled two-tier roughness was fabricated by decorating carbon nanotubes (CNTs) onto microsized carbon fibers, using a catalytic chemical vapor deposition and subsequent fluorination surface treatment. The superhydrophobic surfaces are based on the regularly ordered carbon fibers (8–10 μm in diameter) that are decorated by CNTs with an average size of 20–40 nm. The contact angle of water significantly increases from 148.2 ± 2.1° to 169.7 ± 2.2° through the introduction of CNTs. This confirms that the wettability of carbon fabric changes from hydrophobicity to superhydrophobicity due to structural transformation. This finding sheds light on how the two-tier roughness surface induces superhydrophobicity of rough surfaces, and how the presence of CNTs reduces the area fraction of a water droplet in contact with the carbon surface with two-tier roughness.

Dropwise condensation on superhydrophobic surfaces with two-tier roughness

Dropwise condensation can enhance heat transfer by an order of magnitude compared to film condensation. Superhydrophobicity appears ideal to promote continued dropwise condensation which requires rapid removal of condensate drops; however, such promotion has not been reported on engineered surfaces. This letter reports continuous dropwise condensation on a superhydrophobic surface with short carbon nanotubes deposited on micromachined posts, a two-tier texture mimicking lotus leaves. On such micro-/nanostructured surfaces, the condensate drops prefer the Cassie state which is thermodynamically more stable than the Wenzel state. With a hexadecanethiol coating, superhydrophobicity is retained during and after condensation and rapid drop removal is enabled.

Superhydrophobicity on Two-Tier Rough Surfaces Fabricated by Controlled Growth of Aligned Carbon Nanotube Arrays Coated with Fluorocarbon

Considerable effort has been expended on theoretical studies of superhydrophobic surfaces with two-tier (micro and nano) roughness, but experimental studies are few due to the difficulties in fabricating such surfaces in a controllable way. The objective of this work is to experimentally study the wetting and hydrophobicity of water droplets on two-tier rough surfaces for comparison with theoretical analyses. To compare wetting on micropatterned silicon surfaces with wetting on nanoscale roughness surfaces, two model systems are fabricated: carbon nanotube arrays on silicon wafers and carbon nanotube arrays on carbon nanotube films. All surfaces are coated with 20 nm thick fluorocarbon films to obtain low surface energies. The results show that the microstructural characteristics must be optimized to achieve stable superhydrophobicity on microscale rough surfaces. However, the presence of nanoscale roughness allows a much broader range of surface design criteria, decreases the contact angle hysteresis to less than 1 degrees , and establishes stable and robust superhydrophobicity, although nanoscale roughness could not increase the apparent contact angle significantly if the microscale roughness dominates.