Regular Article
Porosity-induced mechanically robust superhydrophobicity by the sintering and silanization of hydrophilic porous diatomaceous earth

https://doi.org/10.1016/j.jcis.2020.12.101Get rights and content

Abstract

Hypothesis

Because they have self-similar low-surface-energy microstructures throughout the whole material block, fabricating superhydrophobic monoliths has been currently a promising remedy for the mechanical robustness of non-wetting properties. Noticeably, porous materials have microstructured interfaces throughout the complete volume, and silanization can make surfaces low-surface-energy. Therefore, the porous structure and surface silane-treatment can be combined to render hydrophilic inorganics into mechanically durable superhydrophobic monoliths.

Experiments

Superhydrophobic diatomaceous earth pellets were produced by thermal-sintering, followed by a silanization process with octyltriethoxysilane. The durability of superhydrophobicity was evaluated by changes in wetting properties, surface morphology, and chemistry after a systematic abrasion sliding test.

Findings

The intrinsic porosity of diatomite facilitated surface silanization throughout the whole sintered pellet, thus producing the water-repelling monolith. The abrasion sliding converted multimodal porosity of the volume to hierarchical roughness of the surface comprised of silanized particles, thereby attaining superhydrophobic properties of high contact angles over 150° and sliding angles below 20°. The tribological properties revealed useful information about the superhydrophobicity duration of the non-wetting monolith against friction. The result enables the application of porous structures in the fabrication of the anti-abrasion superhydrophobic materials even though they are originally hydrophilic.

Introduction

Superhydrophobicity is an intriguing interface phenomenon that has attracted worldwide interest. For instance, the surfaces with superhydrophobic properties can perform better in self-cleaning, water-oil separation, corrosion inhibition, and thermal transfer [1], [2], [3]. Unfortunately, a majority of materials used to produce water-repellent coatings are mechanically vulnerable polymers and organics which still make the application of such surfaces problematic under harsh conditions of abrasion [4]. Therefore, it is indeed of significance to find fabrication methods for durable superhydrophobicity.

From the literature, a flat plane of the most hydrophobic polytetrafluoroethylene can only obtain the maximal 110° contact angle (CA) [5]. This CA is far below the benchmark (150°) of the consensus definition of superhydrophobicity [6]. Thus, the designation of superhydrophobic materials/surfaces and their mechanical stability has to base on both material chemistry [7] and morphology [8], [9], [10]. Regarding coatings, it is believed that microscale structures can protect vulnerable surface modifiers and/or nanofeatures from mechanical contacts (sand falling, sand oscillating, and sandpaper abrading) to maintain the non-wetting phenomenon [11]. For example, various substrates were laser-textured (with micropatterns of grooves, pillars, and cones) for the long-lasting superhydrophobicity (CAs > 150°) [12], [13], [14], [15] against mechanical tests. Besides, metallic glasses were embossed with microhole-array patterns for a superior water-repellency [16], [17], [18] with durability against sandpaper rubbing [16]. However, the surface of microstructures is more likely to be wetted when the sub-surface layer is worn-out to expose the hydrophilic substrate [11], [16]. Alternatively, durable non-wetting layers have been prepared from composites of nanoparticles (NPs) with polymer [19], [20], [21], [22], [23], [24] and inorganic [25], [26] binders. Such coatings were rendered hydrophobic throughout the whole thickness, whereas the incorporation of NPs induced nanocavities to maintain a non-wetting Cassie-Baxter state against the abrasion contacts. Similarly, superhydrophobic monoliths with continuous surface-energy-lowered micro/nanostructures have been prepared [27]. Therein, it can be concluded that the abrasion exposes freshly superhydrophobic rough surface from the water-repelling bulk material. The water-repellency will yet vanish when such materials are wholely worn-out. Despite that, the whole volume of water-repellency will be a promising remedy for sustainable superhydrophobicity.

Intriguingly, the porous structure has also been used to produce robust superhydrophobic materials, such as polymer/gel monoliths [28], [29], [30], organic-inorganic hybrid coatings [31], [32], and metal/ceramic foams [33], [34]. To the best of our knowledge, although the mechanical robustness of superhydrophobicity stems from the self-similar low-surface-energy structure [28], there are rare reports in detail on such the stability mechanism against abrasion, especially for non-polymer-based materials [31], [32], [33], [34]. Therefore, it is worthy of studying the anti-wear super-water-repellency of the non-wetting induced porous inorganics, which are usually hydrophilic materials. Due to the intrinsic porosity, diatomaceous earth (DE) has been applied in the preparation of various coatings with abrasion-enduring superhydrophobicity (CAs > 150°). However, like the aforementioned binder-based composites [21], [22], [23], [24], [25], [26], [27], DE was composed of polymer and cement [35], [36], [37], [38], [39]. There was also a report on the binder-free superhydrophobic modified-DE film, yet without the analysis of mechanical durability [40]. Therefore, diatomites in the form of porous pellets [41] can be used to investigate and understand in detail the robustness mechanism of the porosity-induced stable superhydrophobicity prepared from hydrophilic inorganic materials.

In this study, we report mechanically stable superhydrophobic bulk materials manufactured by a facile and less chemical-processing method, which is free of polymer binders and fluorines. The processes included pressing and sintering intrinsically porous diatomite particles, followed by the alkyl silanization. The resultant water-repellency was durable against the mechanical sliding with contact angles over 150°. This sustainability was studied by observing changes in the contact angle, surface chemistry, and morphological statistical aspects after a pin-on-plate abrasion test. The mechanical robustness of superhydrophobicity was found due to the synergetic effect of silanization-induced low surface energy and porosity-induced hierarchical surface roughness. It is believed that this work will draw attention to the applicability of porous structures in the fabrication of durable superhydrophobicity for various materials, particularly hydrophilic inorganics.

Section snippets

Preparation of the superhydrophobic porous diatomaceous earth

Diatomaceous earth (DE) (Plant Doctor, Australia) had chemical compositions as shown in Table 1. DE (4 g) was pressed into 5 mm thick pellets of a 30 mm diameter. The pressing process was carried out under a pressure of 27.6 MPa and a temperature of 100 °C in 15 min. The as-pressed sample was sintered at 1000 °C in 1 h with a heating rate of 10 °C/min. Then, the sintered pellet was silanized overnight in an ethanol solution of 2% octyltriethoxysilane (OTES) (Sigma-Aldrich, Australia) and

Non-polymer durable superhydrophobic material based on the silanized sintered diatomite

With thermophysical processes, polymer-free bulk materials were prepared in this study by heat-treating the pressed diatomaceous earth (DE) pellets at temperatures of 1000 °C and 1200 °C. They were labeled respectively as DE1000C and DE1200C. The chosen temperatures were based on the work of Farid et al. [41] to compare the impact of sintering-induced structures on the resultant hydrophobicity (Fig. 1a). From the results, as-sintered DE samples were wetted (CAs ~ 0°) due to the combined effect

Conclusions

With the absence of continuous melt within the porous structure, S-DE1000C performed better water-repelling characteristics than S-DE1200C against abrasion. This outcome is due to low-surface-energy silane molecules residing covalently on silica-based interfaces throughout such defect-less structured diatomite forming the non-wetting monolith. The abrasion sliding transforms multiscale porosity to hierarchical roughness, resulting in new superficial micro-structures (comprised of silanized

CRediT authorship contribution statement

Huynh H. Nguyen: Conceptualization, Investigation, Formal analysis, Visualization, Writing - original draft, Writing - review & editing. A. Kiet Tieu: Supervision, Writing - review & editing, Funding acquisition, Project administration, Resources. Bach H. Tran: Writing - review & editing. Shanhong Wan: Writing - review & editing. Hongtao Zhu: Writing - review & editing, Resources. Sang T. Pham: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The first author is grateful for the financial support from the Engineering Mechanics Centre at UOW through a scholarship. The authors acknowledge financial support from the Australian Research Council with the two projects DP190103455 and LP160101871.

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