Pattern-dependent tunable adhesion of superhydrophobic MnO2 nanostructured film

Adhesion behaviors on four special wettable surfaces: natural sources, mechanisms, fabrications and applications

The study of adhesion behaviors on solid-liquid surfaces plays an important role in scientific research and development in various fields, such as medicine, biology and agriculture. The contact angle and sliding angle of the liquid on the solid surface are commonly used to characterize and measure the wettability of a particular surface. They have a wide range of values, which results in different wettability. It boils down to the adhesion of solid surfaces to liquids. This feature article is aimed at revealing the essence of the adhesion behavior from the aspects of controlling the chemical composition or changing the geometrical microstructure of the surface, and reviewing the natural sources, wetting models, preparation methods and applications of four kinds of typical solid-liquid surfaces (low-adhesion superhydrophobic surfaces, high-adhesion superhydrophobic surfaces, slippery liquid-infused porous surfaces (SLIPS) and hydrophilic/superhydrophilic surfaces). Last, a summary and outlook on this field are given to point out the current challenges and the potential research directions of surface adhesion in the coming future.

Sessile Droplet Freezing on Hydrophobic Structured Surfaces under Cold Ambient Conditions

There have been conflicting reports as to whether surface wettability is effective in the freezing delay enhancement of attached water droplets. It is an important problem in the development of anti-icing surfaces needed for applications, such as aircraft wings and infrastructures. Here, we prepared precooled ambient conditions and surfaces which included smooth, microstructured, and two nanostructured surfaces with hydrophobic coatings to create an environment closer to the actual environment and to avoid frost formation, which enhances wetting transition and nucleation. Static and dynamic wetting characteristics of each surface were investigated as the fundamental properties and the freezing behavior of precooled water droplets were observed. A distinct elongation of the freezing delay time was observed for droplets on nanostructured surfaces which have static contact angles >150°, in contrast to those on smooth and microstructured surfaces. However, the difference in droplet adhesion induced by nanostructures showed a negligible impact on freezing delay. These results indicated that the reduction of the actual contact area between the solid and liquid phases restricted ice nucleus formation.

Drop Capturing Based on Patterned Substrate in Space

In this work, we introduced a method for capturing aqueous drop based on a patterned substrate in space. Through the manipulation test of a colloidal drop, it could be verified that this patterned substrate had excellent control ability for aqueous drop in microgravity condition. The confinement mechanism of this substrate was clarified, which showed that drops with different volume could be pinned and attracted at a given area on the substrate. The confinement capability was related to the gravity effect, and the patterned substrate could confine aqueous drops with larger volume under microgravity than in normal gravity. With advantages of simple operation and strong capability to control large drops, this technique exhibited the wide application prospect in the fields of fluid management, biosensing, and pharmacy in microgravity condition in the future.

A Facile All-Solution-Processed Surface with High Water Contact Angle and High Water Adhesive Force

A series of sticky superhydrophobicity surfaces with high water contact angle and high water adhesive force is facilely prepared via an all-solution-processed method based on polymerization-induced phase separation between liquid crystals (LCs) and epoxy resin, which produces layers of epoxy microspheres (EMSs) with nanofolds on the surface of a substrate. The morphologies and size distributions of EMSs are confirmed by scanning electron microscopy. Results reveal that the obtained EMS coated-surface exhibits high apparent contact angle of 152.0° and high water adhesive force up to 117.6 μN. By varying the composition of the sample or preparing conditions, the sizes of the produced EMSs can be artificially regulated and, thus, control the wetting properties and water adhesive behaviors. Also, the sticky superhydrophobic surface exhibits excellent chemical stability, as well as long-term durability. Water droplet transportation experiments further prove that the as-made surface can be effectively used as a mechanical hand for water transportation applications. Based on this, it is believed that the simple method proposed in this paper will pave a new way for producing a sticky superhydrophobic surface and obtain a wide range of use.

Ultra Water Repellent Polypropylene Surfaces with Tunable Water Adhesion

Polypropylene (PP), including isotactic PP (i-PP) and atactic PP (a-PP) with distinct tacticity, is one of the most widely used general plastics. Herein, ultra water repellent PP coatings with tunable adhesion to water were prepared via a simple casting method. The pure i-PP coating shows a hierarchical morphology with micro/nanobinary structures, exhibiting a water contact angle (CA) larger than 150° and a sliding angle less than 5° (for 5 μL water droplet). In contrast, the pure a-PP coating has a less rough morphology with a water contact angle of about 130°, and the water droplets stick on the coating at any tilted angles. For the composite i-PP/a-PP coatings, however, ultra water repellency with CA > 150° but water adhesion tailorable from slippery to sticky can be realized, depending on the contents of a-PP and i-PP. The different wetting behaviors are due to the various microstructures of the composite coatings resulting from the distinct crystallization ability of a-PP and i-PP. Furthermore, the existence of a-PP in the composite coatings enhances the mechanical properties compared to the i-PP coating. The proposed method is feasible to modify various substrates and potential applications in no-loss liquid transportation, slippery surfaces, and patterned superhydrophobic surfaces are demonstrated.

Fabrication of Superhydrophobic Surfaces with Controllable Electrical Conductivity and Water Adhesion

A facile and versatile strategy for fabricating superhydrophobic surfaces with controllable electrical conductivity and water adhesion is reported. "Vine-on-fence"-structured and cerebral cortex-like superhydrophobic surfaces are constructed by filtering a suspension of multiwalled carbon nanotubes (MWCNTs), using polyoxymethylene nonwovens as the filter paper. The nonwovens with micro- and nanoporous two-tier structures act as the skeleton, introducing a microscale structure. The MWCNTs act as nanoscale structures, creating hierarchical surface roughness. The surface topography and the electrical conductivity of the superhydrophobic surfaces are controlled by varying the MWCNT loading. The vine-on-fence-structured surfaces exhibit "sticky" superhydrophobicity with high water adhesion. The cerebral cortex-like surfaces exhibit self-cleaning properties with low water adhesion. The as-prepared superhydrophobic surfaces are chemically resistant to acidic and alkaline environments of pH 2-12. They therefore have potential in applications such as droplet-based microreactors and thin-film microextraction. These findings aid our understanding of the role that surface topography plays in the design and fabrication of superhydrophobic surfaces with different water-adhesion properties.

Superoleophobic surfaces

This review systematically summarizes the recent developments of superoleophobic surfaces, focusing on their design, fabrication, characteristics, functions, and important applications.

Enhancement of Water Evaporation on Solid Surfaces with Nanoscale Hydrophobic-Hydrophilic Patterns

Using molecular dynamics simulations, we show that the evaporation of nanoscale water on hydrophobic-hydrophilic patterned surfaces is unexpectedly faster than that on any surfaces with uniform wettability. The key to this phenomenon is that, on the patterned surface, the evaporation rate from the hydrophilic region only slightly decreases due to the correspondingly increased water thickness; meanwhile, a considerable number of water molecules evaporate from the hydrophobic region despite the lack of water film. Most of the evaporated water from the hydrophobic region originates from the hydrophilic region by diffusing across the contact lines. Further analysis shows that the evaporation rate from the hydrophobic region is approximately proportional to the total length of the contact lines.

Bio-inspired design of hierarchical PDMS microstructures with tunable adhesive superhydrophobicity

In this paper, bio-inspired PDMS films with different hierarchical microstructures were designed and tunable adhesive super-hydrophobicity was achieved on these films. The adhesive forces between a water droplet and the PDMS film can be adjusted from extremely low (about 8.3 μN) to very high (about 57 μN), and the tunable effect can be ascribed to different wetting states for the water droplets that result from different microstructures on the films. Noticeably, the obtained super-hydrophobic surfaces are acid/alkali-resisting, and water droplets with different pH values have similar contact angles and adhesive forces on the same surface. Finally, the application of the obtained surfaces for microdroplet transportation and self-cleaning are also discussed. The results reported herein provide a new method to obtain super-hydrophobic surfaces with controlled adhesion, and significantly improve our understanding of the relationship between surface adhesion, surface microstructures and the fabrication principle of tunable adhesive super-hydrophobic surfaces.

Correlation between hierarchical structure of crystal networks and macroscopic performance of mesoscopic soft materials and engineering principles

The performance of soft materials is correlated with the hierarchical crystal network structure by topology, correlation length, symmetry/ordering, and strength.

Synthesis of birnessite structure layers at the solution-air interface and the formation of microtubules from them

H(x)MnO2·nH2O layers have been successfully produced through a facile low-temperature process at the solution-air interface without using any templates. The crystalline structures of layers can be tuned by the compositions and the pH of the growth solutions. The analysis of birnessite-like layers indicates that they are formed by nanosheets approximately 4-6 nm thick that are oriented for the most part normally to the interface. Our results demonstrated that 1-3-μm-thick layers can roll up into microtubules 20 to 100 μm in diameter and up to 10 mm long. The hypothesis explaining the formation of the microtubular structures is assumed.

Facile spray-coating process for the fabrication of tunable adhesive superhydrophobic surfaces with heterogeneous chemical compositions used for selective transportation of microdroplets with different volumes

In this paper, tunable adhesive superhydrophobic ZnO surfaces have been fabricated successfully by spraying ZnO nanoparticle (NP) suspensions onto desired substrates. We regulate the spray-coating process by changing the mass percentage of hydrophobic ZnO NPs (which were achieved by modifying hydrophilic ZnO NPs with stearic acid) in the hydrophobic/hydrophilic ZnO NP mixtures to control heterogeneous chemical composition of the ZnO surfaces. Thus, the water adhesion on the same superhydrophobic ZnO surface could be effectively tuned by controlling the surface chemical composition without altering the surface morphology. Compared with the conventional tunable adhesive superhydrophobic surfaces, on which there were only three different water sliding angle values: lower than 10°, 90° (the water droplet is firmly pinned on the surface at any tilted angles), and the value between the two ones, the water adhesion on the superhydrophobic ZnO surfaces has been tuned effectively, on which the sliding angle is controlled from 2 ± 1° to 9 ± 1°, 21 ± 2°, 39 ± 3°, and 90°. Accordingly, the adhesive force can be adjusted from extremely low (∼2.5 μN) to very high (∼111.6 μN). On the basis of the different adhesive forces of the tunable adhesive superhydrophobic surfaces, the selective transportation of microdroplets with different volumes was achieved, which has never been reported before. In addition, we demonstrated a proof of selective transportation of microdroplets with different volumes for application in the droplet-based microreactors via our tunable adhesive superhydrophobic surfaces for the quantitative detection of AgNO3 and NaOH. The results reported herein realize the selective transportation of microdroplets with different volumes and we believe that this method would potentially be used in many important applications, such as selective water droplet transportation, biomolecular quantitative detection and droplet-based biodetection.

Spatially controlled surface energy traps on superhydrophobic surfaces

Water wetting and adhesion control on polymeric patterns are achieved by tuning the configuration of their surface's structural characteristics from single to dual and triple length-scale. In particular, surfaces with combined micro-, submicrometer-,and nanoroughness are developed, using photolithographically structured SU-8 micro-pillars as substrates for the consecutive spray deposition of polytetrafluoroethylene (PTFE) submicrometer particles and hydrophobically capped iron oxide colloidal nanoparticles. The PTFE particles alone or in combination with the nanoparticles render the SU-8 micropillars superhydrophobic. The water adhesion behaviour of the sprayed pillars is more complex since they can be tuned gradually from totally adhesive to completely non adhesive. The influence of the hierarchical geometrical features of the functionalized surfaces on this behaviour is discussed within the frame of the theory. Specially designed surfaces using the described technique are presented for selective drop deposition and evaporation. This simple method for liquid adhesion control on superhydrophobic surfaces can find various applications in the field of microfluidics, sensors, biotechnology, antifouling materials, etc.

Designing heterogeneous chemical composition on hierarchical structured copper substrates for the fabrication of superhydrophobic surfaces with controlled adhesion

Controlling water adhesion is important for superhydrophobic surfaces in many applications. Compared with numerous researches about the effect of microstructures on the surface adhesion, research relating to the influence of surface chemical composition on the surface adhesion is extremely rare. Herein, a new strategy for preparation of tunable adhesive superhydrophobic surfaces through designing heterogeneous chemical composition (hydrophobic/hydrophilic) on the rough substrate is reported, and the influence of surface chemical composition on the surface adhesion are examined. The surfaces were prepared through self-assembling of mixed thiol (containing both HS(CH2)9CH3 and HS(CH2)11OH) on the hierarchical structured copper substrates. By simply controlling the concentration of HS(CH2)11OH in the modified solution, tunable adhesive superhydrophobic surfaces can be obtained. The adhesive force of the surfaces can be increased from extreme low (about 8 μN) to very high (about 65 μN). The following two reasons can be used to explain the tunable effect: one is the number of hydrogen bond for the variation of surface chemical composition; and the other is the variation of contact area between the water droplet and surface because of the capillary effect that results from the combined effect of hydrophilic hydroxyl groups and microstructures on the surface. Noticeably, water droplets with different pH (2-12) have similar contact angles and adhesive forces on the surfaces, indicating that these surfaces are chemical resistant to acid and alkali. Moreover, the as-prepared surfaces were also used as the reaction substrates and applied in the droplet-based microreactor for the detection of vitamin C. This report provides a new method for preparation of superhydrophobic surfaces with tunable adhesion, which could not only help us further understand the principle for the fabrication of tunable adhesive superhydrophobic surfaces, but also potentially be used in many important applications, such as microfluidic devices and chemical microreactors.

From petal effect to lotus effect: a facile solution immersion process for the fabrication of super-hydrophobic surfaces with controlled adhesion

In this paper, a convenient approach based on the reaction between an alkyl thiol and hierarchical structured Cu(OH)2 substrates is reported for the fabrication of super-hydrophobic surfaces with controlled adhesion. This reaction can etch the Cu(OH)2 microstructures and simultaneously introduce a coating with low surface energy. By simply controlling the reaction time or the chain length of the thiol, super-hydrophobic surfaces with controlled adhesion can be achieved, and the adhesive force between the surface and the water droplet can be adjusted from extreme low (∼14 μN) to very high (∼65 μN). The tunable effect of the adhesion is ascribed to the different wetting states for the droplet on the surface that results from the change of the morphology and microstructure scale after the thiolate reaction. Noticeably, the as-prepared surfaces are acid/alkali-resisting; the acidic and basic water droplets have similar contact angles and adhesive forces to that of the neutral water droplet. Moreover, we demonstrate a proof of water droplet transportation for application in droplet-based microreactors via our surfaces. We believe that the results reported here would be helpful for the further understanding of the effect of wetting states on the surface adhesion and the fabrication principle for a super-hydrophobic surface with controlled adhesion.

Controllable adhesive superhydrophobic surfaces based on PDMS microwell arrays

This paper presents a one-step method to fabricate superhydrophobic surfaces with extremely controllable adhesion based on PDMS microwell arrays. The microwell array structures are rapidly produced on PDMS films by a point-by-point femtosecond laser scanning process. The as-prepared superhydrophobic surfaces show water controllable adhesion that ranges from ultrahigh to ultralow by adjusting the extent of overlap of the adjacent microwells, on which the sliding angle can be controlled from 180° (a water droplet can not slide down even when the as-prepared surface is turned upside down) to 3°. A "micro-airbag effect" is introduced to explain the adhesion transition phenomenon of the microwell array structures. This work provides a facile and promising strategy to fabricate superhydrophobic surfaces with controllable adhesion.

Single-step process for the deposition of high water contact angle and high water sliding angle surfaces by atmospheric pressure dielectric barrier discharge

Fluorine-free surfaces with high water contact angle (WCA) and high adhesion force to water are prepared by the atmospheric pressure dielectric barrier discharge (AP-DBD) of hexamethyldisiloxane on cold rolled aluminum foil. Water droplets, which remained on the plasma-polymerized hexamethyldisiloxane (ppHMDSO) surface with contact angle of 155°, do not slide even when the surface is tilted vertically or turned upside down. Scanning electron microscopy, atomic force microscopy and confocal microscopy highlight the importance of the dual-scale roughness of the ppHMDSO surface. The "sticky" high WCA property is achieved only when the nanometer scale particles generated during the AP-DBD process are present at the surface of the film and combine to the micrometer scale rolling lines of the aluminum substrate.

Anomalous contact angle hysteresis of a captive bubble: advancing contact line pinning

Contact angle hysteresis of a sessile drop on a substrate consists of continuous invasion of liquid phase with the advancing angle (θ(a)) and contact line pinning of liquid phase retreat until the receding angle (θ(r)) is reached. Receding pinning is generally attributed to localized defects that are more wettable than the rest of the surface. However, the defect model cannot explain advancing pinning of liquid phase invasion driven by a deflating bubble and continuous retreat of liquid phase driven by the inflating bubble. A simple thermodynamic model based on adhesion hysteresis is proposed to explain anomalous contact angle hysteresis of a captive bubble quantitatively. The adhesion model involves two solid–liquid interfacial tensions (γ(sl) > γ(sl)′). Young’s equation with γ(sl) gives the advancing angle θ(a) while that with γ(sl)′ due to surface rearrangement yields the receding angle θ(r). Our analytical analysis indicates that contact line pinning represents frustration in surface free energy, and the equilibrium shape corresponds to a nondifferential minimum instead of a local minimum. On the basis of our thermodynamic model, Surface Evolver simulations are performed to reproduce both advancing and receding behavior associated with a captive bubble on the acrylic glass.