Wetting behaviour of hydrophilic and hydrophobic nanostructured porous anodic alumina

Surface tension of nanoparticle dispersions unravelled by size-dependent non-occupied sites free energy versus adsorption kinetics

The surface tension of dispersions presents many types of behaviours. Although some models, based on classical surface thermodynamics, allow partial interpretation, fundamental understanding is still lacking. This work develops a single analytical physics-based formulation experimentally validated for the surface tension of various pure nanoparticle dispersions, explaining the underlying mechanisms. Against common belief, surface tension increase of dispersions appears not to occur at low but rather at intermediate surface coverage, owed by the relatively large size of nanoparticles with respect to the fluid molecules. Surprisingly, the closed-form model shows that the main responsible mechanism for the various surface tension behaviours is not the surface chemical potential of adsorbed nanoparticles, but rather that of non-occupied sites, triggered and delicately controlled by the nanoparticles ‘at a distance’, introducing the concept of the ‘non-occupancy’ effect. The model finally invites reconsidering surface thermodynamics of dispersions and provides for criteria that allow in a succinct manner to quantitatively classify the various surface tension behaviours.

The Interrelation of Synthesis Conditions and Wettability Properties of the Porous Anodic Alumina Membranes

The results of studies on the wettability properties and preparation of porous anodic alumina (PAA) membranes with a 3.3 ± 0.2 μm thickness and a variety of pore sizes are presented in this article. The wettability feature results, as well as the fabrication processing characteristics and morphology, are presented. The microstructure effect of these surfaces on wettability properties is analyzed in comparison to outer PAA surfaces. The interfacial contact angle was measured for amorphous PAA membranes as-fabricated and after a modification technique (pore widening), with pore sizes ranging from 20 to 130 nm. Different surface morphologies of such alumina can be obtained by adjusting synthesis conditions, which allows the surface properties to change from hydrophilic (contact angle is approximately 13°) to hydrophobic (contact angle is 100°). This research could propose a new method for designing functional surfaces with tunable wettability. The potential applications of ordinary alumina as multifunctional films are demonstrated.

Role of Surface Topography in the Superhydrophobic Effect-Experimental and Numerical Studies

Within these studies, the effect of surface topography for hydrophobic coatings was studied both numerically and experimentally. Chemically modified polyurethane coating was patterned by application of a laser beam. A set of patterns with variously distant linear peaks and grooves was obtained. The cross section of the pattern showed that the edges of the peaks and grooves were not sharp, instead forming a rounded, rectangle-like shape. For such surfaces, experimental studies were performed, and in particular the static contact angle (SCA), contact angle hysteresis (CAH), and roll-off angle (ROA) were measured. Profilometry was used to create a numerical representation of the surface. Finite volume method was then applied to simulate the behavior of the water droplets. The model developed herewith enabled us to reproduce the experimental results with good accuracy. Based on the verified model, the calculation was extended to study the behavior of the water droplet on the simulated patterns, both spiked and rectangular. These two cases, despite a similar SCA of the water droplet, have shown extremely different ROA. Thus, more detailed studies were dedicated to other geometrical features of such topography, such as the size and distance of the surface elements. Based on the results obtained herewith, the future design of superhydrophobic and/or icephobic topography is discussed.

Superhydrophobic Coating Based on Porous Aluminum Oxide Modified by Polydimethylsiloxane (PDMS)

The aim of this study was to obtain a superhydrophobic coating by modifying anodized aluminum using polydimethylsiloxane (PDMS). In order to obtain a superhydrophobic coating on an aluminum substrate, a multistage treatment was implemented. Specimens of aluminum were treated by abrasive blasting, anodization in sulfuric acid, impregnation by PDMS, rinsing in toluene to remove excess of PDMS, and curing. A rough surface with an additional low free energy layer on it resulted in a superhydrophobic effect. The coating obtained has an average contact angle of 159°. The specimens were tested in terms of durability in natural conditions. Additionally, anti-icing and anti-fouling properties were evaluated. The coating was compared with anodized aluminum obtained by a basic process.

Three-Dimensional Superhydrophobic Hollow Hemispherical MXene for Efficient Water-in-Oil Emulsions Separation

A superhydrophobic macroporous material composed of hollow hemispherical MXene (HSMX) was synthesized by the thermal annealing of MXene-wrapped cationic polystyrene spheres (CPS@MXene). Notably, the spherical MXene shells exhibited highly efficient catalysis of the carbonization of CPS into carbon nanoparticles. Their insertion into the interlayer of MXene increased the d-spacing and created hollow hemispheres. The as-prepared HSMX with nanoscale walls had a lower packing density than MXene, but higher porosity, total pore volume, and total pore area. Moreover, the stacking of hollow hemispheres promoted the formation of a highly undulating macroporous surface and significantly improved the surface roughness of the HSMX-based 3D membrane, resulting in superhydrophobicity with a water contact angle of 156.4° and a rolling angle of 6°. As a result, the membrane exhibited good separation efficiency and Flux for emulsifier-stabilized water-in-paraffin liquid emulsions, which was dependent on its superhydrophobic performance and strong demulsification ability derived from the razor effect originating from the ultrathin walls of HSMX. This work provides a facile approach for the transformation of highly hydrophilic 2D MXene into superhydrophobic 3D HSMX, and opens a new pathway for the development of advanced MXene-based materials for environmental remediation applications.

Investigation of Start-Up Characteristics of Thermosyphons Modified with Different Hydrophilic and Hydrophobic Inner Surfaces

In this paper, the influence of wettability properties on the start-up characteristics of two-phase closed thermosyphons (TPCTs) is investigated. Chemical coating and etching techniques are performed to prepare the surfaces with different wettabilities that is quantified in the form of the contact angle (CA). The 12 TPCTs are processed including the same CA and a different CA combination on the inner surfaces inside both the evaporator and the condenser sections. For TPCTs with the same wettability properties, the introduction of hydrophilic properties inside the evaporator section not only significantly reduces the start-up time but also decreases the start-up temperature. For example, the start-up time of a TPCT with CA = 28° at 40 W, 60 W and 80 W is 46%, 50% and 55% shorter than that of a TPCT with a smooth surface and the wall superheat degrees is 55%, 39% and 28% lower, respectively. For TPCTs with combined hydrophilic and hydrophobic properties, the start-up time spent on the evaporator section with hydrophilic properties is shorter than that of the hydrophobic evaporator section and the smaller CA on the condenser section shows better results. The start-up time of a TPCT with CA = 28° on the evaporator section and CA = 105° on the condenser section has the best start-up process at 40 W, 60 W and 80 W which is 14%, 22% and 26% shorter than that of a TPCT with smooth surface. Thus, the hydrophilic and hydrophobic modifications play a significant role in promoting the start-up process of a TPCT.

Wettability of nanostructured hexagonal boron nitride surfaces: molecular dynamics insights on the effect of wetting anisotropy

Nanostructured van der Waals (vdW) layered materials hold great potential for achieving smart surfaces with controllable wettability.

Preparation and morphology-dependent wettability of porous alumina membranes

This article presents the preparation and study of the wetting properties of porous alumina membranes (PAMs) with a thickness of 25 to 75 μm and with a different pore sizes. The fabrication process features, scanning electron microscopy and atomic force microscopy characterization results are presented. The comparative analysis of PAM surfaces (outer and inner) and the effect of morphology of these surfaces on the wetting properties are discussed. Both alumina surfaces show significant morphology-dependent wettability. Measurements of the interfacial contact angle were made on the as-fabricated amorphous membrane and after pore widening with a range of pore diameters from 25 to 100 nm. The possible applications of PAMs for various membrane technologies is shown.

Incorporation of Nanoalumina Improves Mechanical Properties and Osteogenesis of Hydroxyapatite Bioceramics

A handful of work focused on improving the intrinsic low mechanical properties of hydroxyapatite (HA) by various reinforcing agents. However, the big challenge regarding improving mechanical properties is maintaining bioactivity. To address this issue, we report fabrication of apatite-based composites by incorporation of alumina nanoparticles (n-Al₂O₃). Although numerous studies have used micron or submicron alumina for reinforcing hydroxyapatite, only few reports are available about the use of n-Al₂O₃. In this study, spark plasma sintering (SPS) method was utilized to develop HA-nAl₂O₃ dense bodies. Compared to the conventional sintering, decomposition of HA and formation of calcium aluminates phases are restricted using SPS. Moreover, n-Al₂O₃ acts as a bioactive agent while its conventional form is an inert bioceramics. The addition of n-Al₂O₃ resulted in 40% improvement in hardness along with a 110% increase in fracture toughness, while attaining nearly full dense bodies. The in vitro characterization of nanocomposite demonstrated improved bone-specific cell function markers as evidenced by cell attachment and proliferation, alkaline phosphatase activity, calcium and collagen detection and nitric oxide production. Specifically, gene expression analysis demonstrated that introduction of n-Al₂O₃ in HA matrix resulted in accelerated osteogenic differentiation of osteoblast and mesenchymal stem cells, as expression of Runx-2 and OSP showed 2.5 and 19.6 fold increase after 2 weeks (p < 0.05). Moreover, protein adsorption analysis showed enhanced adsorption of plasma proteins to HA-nAl₂O₃ sample compared to HA. These findings suggest that HA-nAl₂O₃ could be a prospective candidate for orthopedic applications due to its improved mechanical and osteogenic properties.

Flourishing Bioinspired Antifogging Materials with Superwettability: Progresses and Challenges

Antifogging (AF) structure materials found in nature have great potential for enabling novel and emerging products and technologies to facilitate the daily life of human societies, attracting enormous research interests owing to their potential applications in display devices, traffics, agricultural greenhouse, food packaging, solar products, and other fields. The outstanding performance of biological AF surfaces encourages the rapid development and wide application of new AF materials. In fact, AF properties are inextricably associated with their surface superwettability. Generally, the superwettability of AF materials depends on a combination of their surface geometrical structures and surface chemical compositions. To explore their general design principles, recent progresses in the investigation of bioinspired AF materials are summarized herein. Recent developments of the mechanism, fabrication, and applications of bioinspired AF materials with superwettability are also a focus. This includes information on constructing superwetting AF materials based on designing the topographical structure and regulating the surface chemical composition. Finally, the remaining challenges and promising breakthroughs in this field are also briefly discussed.

Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates

The resolution of inkjet printing technology is determined by wetting and evaporation processes after the jet drop contacts the substrate. Here, the wetting of different picoliter solubilized polymer droplets jetting onto one-end-closed porous alumina was investigated. The selected polymers are commonly used in inkjet ink. The synergistic effects of the hierarchical structure and substrate surface modification were used to control the behavior of polymer-based ink drops. A model that invokes the effect of surface tension was applied to calculate the amount of polymer solution penetrating into the pores. The calculation corroborates experimental observations and shows that the volume of polymer solution in the pores increases with an increase in pore radius and depth, resulting in less solution remaining on the substrate surface. The structure of the porous substrate coupled with intrinsic polymer properties and surface modifications all contribute to the resolution that can be achieved via inkjet printing.

Controlled hydrothermal pore reduction in anodic alumina membranes

Porous anodic aluminium oxide nanostructures are popular templates for the fabrication of a wide range of nanomaterials. When open at both ends, they are now being used as model membranes, called anodic alumina membranes (AAM). In both cases, their appeal resides in the possibility of accurately controlling pore size via the anodization voltage, with a narrow size distribution. This characteristic, though, is maintained only in specific pore size ranges, reflecting specific ordering regimes in the material. Outside these domains, less ordered structures are obtained. Furthermore, the smallest pores currently achieved by anodization are about ∼10 nm in diameter, using sulphuric acid, which yields very thin and fragile nanostructured membranes. In this work we address these limitations by decoupling the control of pore size from the anodization stage. We achieve this by subjecting AAMs produced under a high order regime (40 V, 0.3 M oxalic acid) to a post-anodization hydrothermal treatment using steam. With this process we were able to decrease the pore size by 80% down to ∼10 nm. The membranes retain their integrity and are more robust than AAMs with the same pore structure produced via anodization in sulphuric acid.

Antifogging and icing-delay properties of composite micro- and nanostructured surfaces

A composite micro/nanostrucutred (MN) surface was designed using poly(vinylidene difluoride) (PVDF) polymer in combination with ZnO materials via heat-pattern-transfer and crystal-growth techniques. The surface, composed of ZnO nanohairs over PVDF microratchets (i.e., ZP-MN), displays excellent antifogging and icing-delay properties. Condensed water droplets can be easily shed from the ZP-MN surface at -5 °C for ∼1600 s via a slight wind or tilting. The droplets do not completely freeze on the ZP-MN surface at -10 °C until ∼7360 s. This investigation offers a way to design a structured surface that possesses anti-icing ability, which is significant because it can be extended to fields such as microdevices, engineering systems, and engines that operate in a cold or humid environment.

Sustained frictional instabilities on nanodomed surfaces: stick-slip amplitude coefficient

Understanding the frictional properties of nanostructured surfaces is important because of their increasing application in modern miniaturized devices. In this work, lateral force microscopy was used to study the frictional properties between an AFM nanotip and surfaces bearing well-defined nanodomes comprising densely packed prolate spheroids, of diameters ranging from tens to hundreds of nanometers. Our results show that the average lateral force varied linearly with applied load, as described by Amontons' first law of friction, although no direct correlation between the sample topographic properties and their measured friction coefficients was identified. Furthermore, all the nanodomed textures exhibited pronounced oscillations in the shear traces, similar to the classic stick-slip behavior, under all the shear velocities and load regimes studied. That is, the nanotextured topography led to sustained frictional instabilities, effectively with no contact frictional sliding. The amplitude of the stick-slip oscillations, σf, was found to correlate with the topographic properties of the surfaces and scale linearly with the applied load. In line with the friction coefficient, we define the slope of this linear plot as the stick-slip amplitude coefficient (SSAC). We suggest that such stick-slip behaviors are characteristics of surfaces with nanotextures and that such local frictional instabilities have important implications to surface damage and wear. We thus propose that the shear characteristics of the nanodomed surfaces cannot be fully described by the framework of Amontons' laws of friction and that additional parameters (e.g., σf and SSAC) are required, when their friction, lubrication, and wear properties are important considerations in related nanodevices.

Study of fluid and transport properties of porous anodic aluminum membranes by dynamic atomic force microscopy

Recent work on carbon nanotubes (CNT) has focused on their potential application in water treatment as a result of their predicted and observed enhanced flow rates. Recent work on the lesser-known porous anodic alumina membranes (PAAMs) has also shown flow enhancement, albeit at only a fraction of what has been observed in CNTs. Despite their potential applications, little research has been conducted on PAAMs' hydrodynamic properties, and in this Article we present experimental results and theoretical models that explore the fluid flow behavior around and through these membranes. The experiments were conducted using an atomic force microscope (AFM) that pushed a solid silica particle against PAAMs that were characterized with different pore diameters. Furthermore, the PAAMs were classified as either closed or open, with the latter allowing fluid to pass through. The theoretical model developed to describe the experimental data incorporates Derjaguin-Landau-Verwey-Overbeek (DLVO) effects, cantilever drag, and hydrodynamic forces. By using the slip boundary condition for the hydrodynamic forces, we were able to fit the model to experimental findings and also demonstrate that the difference between closed and open PAAMs was negligible. The slip lengths did not correspond to any physical feature of the PAAMs, but our model does provide a simple yet effective means of describing the hydrodynamics for not only PAAMs but for membranes in general.