Regular Article
Interfacial tension gradient driven self-assembly of binary colloidal particles for fabrication of superhydrophobic porous films

https://doi.org/10.1016/j.jcis.2019.04.039Get rights and content

Abstract

Hypothesis

Self-assembly of two types of particles into the binary colloidal crystals (BCCs) leads to the peculiar features which the monocomponent colloidal crystals do not possess. However, the self-assembly methods of the BCCs are still limited.

Experiments

A facile, cost-effective and controllable approach was presented to fabricate the BCCs by directly dripping silica particle suspensions containing water and ethanol into the monodisperse polytetrafluoroethylene (PTFE) colloidal solutions. The BCCs composed of the silica and PTFE particles were formed at the air-liquid interface and picked up using the target substrates, followed by the corrosion of silica particles. Using this strategy, various ordered porous PTFE films including homo-pore-size mono/multilayer and hetero-pore-size bilayer films were prepared. The chemical composition of the porous films was well controlled.

Findings

The dissolution of ethanol resulted in the interfacial tension gradient, and thus the Marangoni convection and the relative movement of the liquid and the particles emerged. The silica particles were trapped at the air-liquid interface and the PTFE particles filled the interstitial spaces between the silica particles. After the porous films were sintered at temperatures above the melting point of PTFE, the PTFE particles were transformed into nanofibers, leading to the transferability, excellent superhydrophobicity and environmental durability.

Introduction

Periodic arrays of colloidal particles, usually known as colloidal crystals, have attracted tremendous attention recently because of large-scale applications in photonic crystals [1], patterned surface fabrication [2], surface-enhanced Raman scattering (SERS) [3], superhydrophobic surfaces [4] and so on. However, most of the reported colloidal crystals consist of the particles with uniform size and mono material. They own a limited range of arrangements such as hexagonally close-packed monolayers, face-centered or body-centered cube in crystal forms. They are not the perfect candidates to research the condensed matter physics and to realize the full photonic bandgap [5], [6]. Therefore, the studies have focused on the preparation of binary colloidal crystals (BCCs) that contain two types of particles with different sizes or materials. By changing the sizes and ratios of different particles, the BCCs offer all sorts of arrangements. The binary materials of these colloidal crystals also unlock the gate to a slew of potential applications in sensors and biointerface science [7]. The fabrication of the BCCs is still challenging because the differences in the size and property of binary particles require a tight control over several additional parameters. It is important to develop a general, inexpensive and simple technique to produce the BCCs.

Layer-by-layer (LBL) self-assembly has been utilized to construct the BCCs. A close-packed monolayer of large particles is formed firstly by flow-controlled vertical deposition [8], evaporation-induced assembly [9] or spin-coating [10], and works as a template. Then, small particles settle in the template under the effect of electric-field or capillary forces to finish the BCCs [11], [12]. The LBL self-assembly is complex because at least two self-assembly processes are necessary. Another route that can overcome the above deficiency is the co-self-assembly of the binary colloidal particles. Firstly, two types of particles with a suitable ratio are dispersed in a certain solvent to produce a binary colloidal suspension. Then, the large particles self-assemble into ordered arrays and the small particles co-self-assemble simultaneously in the interstices of large particles, forming the BCCs. Vogel et al. [13] prepared the BCCs at the air-liquid interface in one step using a Langmuir trough, which relied on the particular apparatus. Mukhopadhyay et al. [14] deposited mixed polymer solutions on a substrate, followed by drying in a fume hood. The major limitation of this method was that the substrate must be hydrophobic. In another approach, water was injected around a glass slide at the bottom of a Petri dish until a convex shape appeared, and binary colloidal suspensions were subsequently dripped on the glass slide. As soon as the suspensions got in touch with water, the BCCs formed at the air-water interface [15]. The formation of the convex shape required precise control and the efficiency was limited.

In this article, the co-self-assembly based on interfacial tension gradient was developed to prepare the BCCs, which was facile, cost-effective and flexible. We directly dripped the silica particles in a solvent composed of water and ethanol to the monodisperse polytetrafluoroethylene (PTFE) colloidal solutions. Particle image velocimetry (PIV) observation confirmed that co-self-assembly process of the silica and PTFE particles happened at the air-liquid interface. The BCC was transferred by a substrate. The silica particles were removed through corrosion and the ordered homo-pore-size porous PTFE film was acquired. The thickness and the geometric morphology of the porous films could be easily manipulated. PTFE particles were chosen to constitute the porous films because of their low surface energy, outstanding chemical resistance and excellent thermal stability [16]. After sintering, the porous films exhibited superhydrophobic properties. Compared with other topographies such as nanotubes, microspheres, and coral-like multiscale structures [17], the porous structures are thought to achieve more robust superhydrophobicity because the trapped air is difficult to be expelled from the enclosed spaces formed by the droplet and the pores [18], [19]. It is worth mentioning that the films can be transferred to any desired substrate, which greatly extends their applications.

Section snippets

Materials

The suspensions of 1.5 and 4 μm hydrophobic silica particles as well as 1.5 μm hydrophilic silica particles with a solid content of 2.5% w v-1 were bought from Tianjin Baseline Chromtech Research Center. The solvents were the mixtures of water and ethanol (1:1, v:v). The functional group on the surfaces of the hydrophobic particles was methyl. The contact angles of 1.5 and 4 μm particles at the deionized water surfaces were 90° and 75°, respectively (Fig. S1). They were determined by trapping

Homo-pore-size porous mono/multilayer films

The ordered homo-pore-size porous films with different layers are shown in Fig. 2. All the films were obtained from 4 μm silica particle monolayers and consisted of hexagonally arranged pores. The concentration of the PTFE colloidal solutions was 9 wt%. Obviously, the structures of the porous films are all nearly spherical hollow arrays with truncated tops as openings. The deviations of the pore sizes and the limited dislocations come from the inconsistency of particle size and shape,

Conclusions

A novel co-self-assembly approach of the BCCs has been demonstrated. It is facile and requires no special equipment. Silica suspensions containing water and ethanol were directly dripped into the monodisperse PTFE colloidal solutions without other controls. In the colloidal self-assembly at the air-liquid interface, ethanol was generally considered as the cause of the spreading effect [41] or the evaporation-induced convection [42]. This work revealed that ethanol resulted in the interfacial

Acknowledgements

We thank the financial support from the National Natural Science Foundation of China Project (grant numbers 51775296, 51375253, 51703116 and 20171301886); and the State Key Laboratory of Tribology, Tsinghua University (grant number SKLT2017C06 and SKLT2018C06).

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