PDMS-Infused Poly(High Internal Phase Emulsion) Templates for the

May 31, 2019 - Thus, high internal phase emulsion polymerization is expected to provide an ... The loading content of PDMS was characterized by the we...
0 downloads 0 Views 8MB Size
Download PDF
Article Cite This: Langmuir 2019, 35, 8276−8284

pubs.acs.org/Langmuir

PDMS-Infused Poly(High Internal Phase Emulsion) Templates for the Construction of Slippery Liquid-Infused Porous Surfaces with Selfcleaning and Self-repairing Properties Dong Zhang,† Yuzheng Xia,† Xiaonong Chen,† Shuxian Shi,*,† and Lei Lei*,‡

Downloaded via GUILFORD COLG on July 19, 2019 at 02:28:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Key Laboratory of Carbon Fiber and Functional Polymers (Ministry of Education), Beijing University of Chemical Technology, Beijing 100029, China ‡ Centre for Advanced Macromolecular Design, School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia

ABSTRACT: Advanced liquid-repelling materials that resist both water-based and oil-based contaminants have significant applications in many fields. Herein, a novel protocol for the fabrication of a robust poly(high internal phase emulsion) (polyHIPE)-based slippery liquid-infused porous surface (SLIPS) system with combined self-repairing and self-cleaning properties is developed. Specifically, polystyrene-based polyHIPE (PS-HIPE) membranes with an interconnected porous structure were prepared from polymerization of the continuous oil phase in the water-in-oil HIPE templates. These polyHIPE membranes were used, for the first time, as porous substrates for loading low surface tension silicone oils as lubricating liquids for the fabrication of polyHIPE-based SLIPS membranes. These polyHIPE-based SLIPS membranes could easily repel both water- and oil-based contaminants (e.g., ink, milk, and coffee) with very low sliding angles (3.0 ± 1.3°) and could even repel solid contaminants (e.g., dust) upon washing with water. Meanwhile, such membranes exhibit excellent self-repairing properties so that physical scratching damage, such as cutting a trench, does not affect the liquid-repelling performance. The liquidrepelling ability could be recovered completely within 10 s. More significantly, such a SLIPS membrane shows excellent durability so that the water sliding angle of the SLIPS could be maintained at less than 5.0° for about 80 cycles owing to the regenerated poly(dimethylsiloxane) layer on the surface. This work represents a robust methodology to enrich the development of hydrophobic and oleophobic slippery surfaces, which is promising for many areas, such as biomedical, self-cleaning, antifouling, and self-repairing materials.

■

INTRODUCTION

test droplet and solid surface, yielding a so-called Cassie− Baxter state.8 In such a case, the suspended liquid droplet could readily roll off from the surface at a very low tilt angle. However, once the liquids penetrate these textures and substitute the air voids between protrusions due to an increase in environmental pressure or invasion of low surface tension organic liquids, a Wenzel state will form, and the liquidrepelling properties of these will be lost.9 Such surfaces also suffer from poor oleophobic10 and mechanical damage, since

Advanced liquid-repelling surfaces have shown significant applications in various fields, such as self-cleaning windows,1 anti-icing surfaces,2 and antibiofouling3,4 and anticorrosion coatings,5 which have greatly beautified our world around and facilitated our life. The most well-known “lotus leaf” approach for fabricating both water- and oil-repellent surfaces was pioneered and systematically investigated by Jiang et al.6,7 This strategy often involves rough surface (micro- or nanoscale textures) generation and surface modification with low surface tension chemicals. Under this circumstance, liquid droplets can hang above the roughness surface protrusions and trap “airpockets” to form a stable air−liquid−solid interface, which is greatly benefited from the decreasing contact area between the © 2019 American Chemical Society

Received: April 16, 2019 Revised: May 26, 2019 Published: May 31, 2019 8276

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir

preparation are also a critical limitation to its practical applications.27 A high internal phase emulsion (HIPE), described as an ultraconcentrated emulsion as well, has been extensively used for fabrication of porous materials by polymerizing its continuous monomer phase.28 HIPE represents a class of emulsions with an internal phase or dispersed phase volume fraction of more than 74%, generally containing two types of phases, water-in-oil (W/O) and oil-in-water (O/W), depending on the feature of the continuous and dispersed phases, respectively. Otherwise, some other types of HIPEs have been reported in recent years, such as supercritical-CO2-in-water HIPEs,29 oil-in-oil (O/O) HIPEs,30 and even HIPEs with double emulsion morphology first reported by Lei et al.,31 which makes the fabrication of complicated porous spheres or matrix with high efficiency become possible. Among all of these HIPEs, W/O HIPEs have been mostly investigated.32,33 Through the polymerization of the continuous oil phase, polyHIPE with a continuous hierarchical porous structure could be obtained. Specifically, the oil phase containing polymeric monomers made up the skeleton between the porous structure and dispersed water droplets as the porogen. Additionally, “window” connections could be created between adjacent “cells” because of the volume shrinkage of the oil (monomer) phase during the polymerization, thus generating an “open-cell” porous structure.34 The internal pores can be regulated by adjusting the content of surfactant and crosslinker as confirmed by Lei’s work.35 Their particular hierarchical porous structure endows polyHIPE with various applications, such as catalyst carriers,36 smart separation,35 and absorption.37 All reported polyHIPE works have taken advantage of their 3D porous structure,38,39 but the extension of these hierarchical microstructures as a unique surface material has been rarely well utilized, which provides great potential for constructing novel SLIPS systems for the following reasons: (1) In terms of microstructure fabrication, the polyHIPE strategy is much more convenient and economical compared to the traditional corrosion40,41 or laser etching19 methods. (2) PolyHIPE provides a naturally unique continuous hierarchical porous structure which could facilitate lubricant liquid storage, transportation (continuous porous bulk) and physical restriction (porous-mesh surface) for SLIPS fabrication. Thus, high internal phase emulsion polymerization is expected to provide an uncomplicated and cost-effective method to create a porous substrate that meets the SLIPS conditions. In this work, a robust SLIPS system was fabricated by using a porous polyHIPE membrane with a continuous hierarchical open-cell structure as the substrate for the first time. Specifically, the lubricant liquid, poly(dimethylsiloxane) (PDMS), with varying viscosity was loaded into the polyHIPE substrate, which was both stored within the porous bulk and restricted on the porous-mesh surface. Moreover, the lubricant liquid could pass through (transportation) the continuous porous structure from the polyHIPE bulk to refill the SLIPS substrate surface once the lubricant layer is consumed. PDMS was chosen as the lubricant liquid due to its low surface tension, nontoxicity, low evaporation, environmental friendliness, and cost-effectiveness. Since all previous polyHIPE works have taken advantage of their 3D porous structure, this work represents a valuable exploration that utilizes the continuous hierarchical porous structure as a unique surface substrate material. Thus, such a PDMS-infused PS-HIPE

the complex microstructure is very likely to be abraded during real applications.11 Recently, Aizenberg et al. have reported an alternative method to develop omniphobic surfaces inspired by Nepenthes.12,13 They found that these pitcher plants with special surface microtexture could become very slippery when moistened by condensation or nectar, which could help them trap insects into death.14,15 Based on this interesting phenomenon, they constructed an omniphobic surface by overfilling a lubricant liquid onto a rough surface, which is called a slippery liquid-infused porous surface (SLIPS).16 By changing the traditional solid−liquid interface of the porous surface and test liquid to a smooth liquid (lubricant liquid)− liquid (test liquid) interface after overfilling the surface with the lubricant, SLIPS featured a very low sliding angle and low surface friction coefficients.17 Moreover, SLIPS could be selfrepairing from small surface damage, since the lubricant liquid was able to refill the fissures to minimize the surface energy.18 Due to these excellent performances, researchers tend to extend this research based on the SLIPS construction principle. Yang et al.19 fabricated SLIPS through three steps: (1) preparation of porous surfaces via femtosecond laser direct writing, (2) modification of the surface by fluoroalkylation, and (3) infusion of the porous surface with a lubricating liquid. Their SLIPS system showed excellent liquid-repellent and selfrepairing abilities. Rykaczewski et al.20 used photolithography to fabricate a nanocolumn structure on a silicon wafer surface, which was then infused with a lubricating liquid (Krytox perfluorinated oil, Krytox-1506) to form SLIPS with good slippery and anti-icing properties. SLIPS can slide down complex liquid contaminants easily, such as milk and salt solutions, and protect the substrate from corrosion. Xiang et al.21 prepared a novel Zn−Ni−Co (ZNC) porous coating by electroplating and Co2+ treatment, which was then modified with FAS fluoroalkylation and injected with lubricating oil Krytox100. Such SLIPS exhibited excellent corrosion protection and self-repairing properties. Most reports selected fluoroalkylated fluid or silicone oil with low surface tension as the lubricating oil, since they have low volatility and are immiscible with many kinds of contaminant liquids.16,22 Compared to the selection of lubrication liquids, the fabrication of SLIPS systems is more restricted by porous or textured surface fabrication. In terms of recent research studies, the generation of a porous substrate for SLIPS is usually complicated, and the surface microstructure could be easily destroyed. For example, the Zn−Ni−Co (ZNC) porous coating mentioned above requires a complex process that consists of degreasing, acid pickling, activation, and then electroplating, along with other procedures. Some other porous and textured structure preparation methods may also make the SLIPS unstable, since the layer and the substrate are foreign materials. The substrate created by spraying TiO2 nanoparticles on a low-density polyethylene surface provides an excellent structure and a strategy for constructing SLIPS, but the SLIPS will become invalid if the nanoparticle coating is damaged.23 Likewise, a layer-by-layer method24 and vaporphase deposition25 also have similar problems. The durability of SLIPS constructed by two-dimensional (2D) material surfaces was limited by the scant amount of lubricant, but for the bulk three-dimensional (3D) porous substrate, it can maintain long-standing durability owing to the lubricant regeneration and storage.26 Moreover, high costs of substrate 8277

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir Scheme 1. Schematic Illustration of SLIPS Preparation from a PDMS-Infused PS-HIPE Substratea

a

(a) PDMS infuses into the open-cell porous polyHIPE substrate. (b) PDMS migrates upward and enriches the polyHIPE surface driven by capillary force and surface energy minimization. (c) The PDMS-infused polyHIPE exhibits omniphobic surface properties which could repel both water- and oil-based contaminants.

2.5 cm and thickness of about 0.5 cm, and then polished by sandpaper with 1000 meshes. Preparation of PDMS-Infused PS-HIPE. PDMS-infused polyHIPEs were prepared through “bottom-to-up” capillary migration of PDMS liquid throughout the polyHIPE matrix. Specifically, polyHIPE membranes were placed on top of a certain amount of PDMS droplets, which were taken up by polyHIPE with hierarchical interconnected pores through capillary force automatically. Three kinds of PDMS with a viscosity of 100, 500, and 1000 mPa·s were used. A homogeneous PDMS layer was obtained on the polyHIPE membrane. Characterization. A scanning electron microscope (SEM, S4700, Hitachi, Japan) was used to characterize the microstructure of the PS-HIPE samples. A gold layer (5 nm) was applied on the sample surface before characterization. The loading content of PDMS was characterized by the weight differentiation method based on the weight difference of the PS-HIPE membrane before and after PDMS loading. “Δm = m − m0” represents the PDMS loading content, in which “m0” and “m” represent the PS-HIPE membrane weight before and after PDMS loading, respectively. On the other hand, theoretical PDMS loading capability “mtheory” of the PSHIPE membrane sample was estimated based on the volume difference between a typical PS bulk and the porous PS-HIPE membrane. Accordingly, the percentage of net PDMS loading (L%) can be obtained m − m0 L% = × 100% mtheory (1)

SLIPS system shows excellent antifouling and self-repairing properties.

■

EXPERIMENT Materials. Styrene, purchased from Tianjin DaMao Chemical Reagent Factory in China (St, AR), was purified through reduced pressure distillation. Divinylbenzene (DVB, Aladdin, containing 80% divinylbenzene, 20% ethyl styrene, and 1000 ppm TBC stabilizer) was purified by removing the inhibitor via a column packed with basic alumina. Span80 (Aladdin), ethanol (EtOH, AR, Beijing Chemical, China), deionized water, anhydrous calcium chloride (CaCl2, Tianjin Fuchen Chemical Reagent Factory, China), potassium peroxydisulfate (K2S2O8, XiLong Chemical, China), and poly(dimethylsiloxane) (PDMS, Aladdin) with three different viscosities of 100 mPa·s (Mn = 8500 g/mol, named PDMS0.1k), 500 mPa·s (Mn = 22 000 g/mol, named PDMS-0.5k), and 1000 mPa·s (Mn = 26 200 g/mol, named PDMS-1.0k) were used as received without any further treatment. Sandpaper of 1000 meshes was used to polish the surface of materials. Milk (milk was from Yili, China), coffee, and ink were those normally used in daily life. Dust was simulated by lignin with size in the range from 5 to 100 μm. Kimwipes were purchased from Kimberly-Clark. Preparation of Polystyrene-HIPE (PS-HIPE). All polystyrene high internal phase emulsion (PS-HIPE) samples were fabricated via polymerizing the continuous oil phase (styrene). HIPEs with a water-to-oil ratio of 9:1 were used as emulsion templates. The oil phase was prepared by adding 0.032 mol of styrene (St) monomer, 2.5 × 10−3 mol of DVB as a crosslinker, and 2.5 × 10−3 mol of Span-80 as an emulsifier into a 50 mL centrifuge tube. The aqueous phase contained 36 mL of deionized water, 3.0 × 10−4 mol K2S2O8 as an initiator, and 3.6 × 10−3 mol CaCl2 as a stabilizer. HIPE samples were prepared by adding the aqueous solution by stirring the oil phase with a homogenizer at 18 000 rpm. Aqueous phase addition lasted for about 3 min, and the homogenization continued for another 2 min to make sure the homogeneity of the emulsion. The freshly prepared HIPE was finely sealed in the centrifuge tube with curing in an oven at 60 °C for 48 h to finish the polymerization. Due to the presence of residual monomer and surfactants, the polyHIPE products were purified in hot ethanol through Soxhlet extraction for 12 h and dried at 25 °C under vacuum overnight. PolyHIPE samples were sliced into membranes with a diameter of about

An OCA20 machine (Data Physics, Germany) was used to measure the static contact angle (CA) and sliding angle (SA) of all samples at 25 °C. The water contact angle (WCA) is tested by applying a 3 μL water droplet onto the test surface of PS-HIPE and then analyzing the data of CA by taking a picture. The water sliding angle (WSA) was measured by taking a video with the tilt angle increasing from 0° to a certain degree, where the liquid droplet begins to slide down the sample surface, and then the value of WSA was further confirmed by analyzing the video. The self-cleaning property of the repelling liquids was investigated by testing whether the simulated aqueous contaminant (e.g., milk, coffee, or ink) can slide down the SLIPS without leaving stains. The solid self-cleaning performance of the SLIPS was simulated by removing dust on the SLIPS surface by washing with water at a tilt angle of 10°. The self-repairing behavior of the PDMS-infused PS-HIPE SLIPS 8278

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir

generated due to the volume contraction during continuous monomer phase polymerization.35 As shown in Figure 1 and the statistics in Table 1, the diameters of both cell and window decrease as the DVB content of PS-HIPE increases. Specifically, as the DVB content increased from 5 to 10 and 20 wt %, the cell size decreased from 3.15 to 2.83 and 2.40 μm, and the window size decreased from 0.55 to 0.50 and 0.42 μm, respectively. Moreover, the openness (O), defined as the area ratio of window (So) and cell (Sc), was calculated. The average number of windows (N) was identified to be 11, 11, and 9 for PS-HIPE containing 5, 10, and 20 wt % of DVB, respectively. An increase in DVB content decreased the cell and window size, since the more hydrophobic DVB than styrene weakens the tendency of dispersed droplets to coalesce. At the same time, as seen from the SEM, more DVB content causes a collapse of internal structures due to greater volumetric shrinkage. In addition, since the open-cell provides a channel to transfer PDMS, its openness plays an important role in PDMS loading efficiency. The corresponding openness was then calculated, suggesting that the PS-HIPE with 10 wt % of DVB exhibits the highest openness value of 19.8. PDMS-Infused PolyHIPE Preparation: Porosity vs PDMS Loading. The porous PS-HIPE substrates with three different porous morphologies were used for PDMS-0.1k loading. Figure 2a illustrates the PDMS loading procedure. Specifically, when the PS-HIPE10 substrate was placed on the PDMS source, it was found that PDMS could automatically overfill this substrate with the open-cell porous structure driven by capillary force, which was inversely proportional to the internal aperture (the capillary radius) and minimized the system surface energy. After about 10 min, PDMS started to overflow from the PS-HIPE10 substrate, from the bottom to the top surface until the top surface was completely covered with a layer of PDMS, as shown in Figure 2a. PDMS loading is defined as the ratio of PDMS content practically filling the substrate to the content required for theoretically filling the substrate. PDMS content was also recorded as the loading time was extended. It was found that PDMS loading reached about 87% (w/w) within 30 min, and a loading equilibrium could be reached in about 3 h, after which no obvious PDMS loading increase could be detected. As shown in Figure 2b, PS-HIPE5 exhibits the highest loading speed, which reaches its equilibrium PDMS loading content of about 94% within the first 1.5 h. This could be attributed to the fact that the PS-HIPE5 substrate possesses the highest cell and window size compared to PS-HIPE10 and PSHIPE20, as listed in Table 1. However, as the loading process was extended to 5 h, PS-HIPE10 presented the highest PDMS loading of nearly 98%, which was consistent with its highest “openness” of 19.8% that donated the total vacancy inside the polyHIPE substrate. Thus, by taking PS-HIPE10, which possessed the highest PDMS loading, as the sample, the infusion of PDMS-0.1k, PDMS-0.5k, and PDMS-1.0k, which are named SLIPS-0.1k, SLIPS-0.5k, and SLIPS-1.0k, respectively, was investigated and is presented in Figure 2c. It could be found that SLIPS-0.1k and SLIPS-0.5k loading could reach about 95% within 1 and 3 h, respectively, whereas SLIPS-1.0k loading can only reach about 90% after 9 h. Considering that PDMS viscosity is proportional to its molecular weight, PDMS loading decelerates due to the increased viscosity, suggesting that as PDMS molecular weight increases, the increased

system was characterized by slashing the surface of the SLIPS with a scalpel and then observing whether the water droplet could smoothly slide across the dent. The repulsion of various liquids and the process of self-repair can be directly characterized by taking photos. The mechanical properties of PS-HIPE and SLIPS were tested using a microcomputer-controlled electronic universal testing machine 4304 (Shenzhen SUNS Technology, China). The stress−strain curve of a cylindrical sample with a diameter of 2.5 cm and a height of 0.6 cm was measured at a compressive rate of 5 mm/min.

■

RESULTS AND DISCUSSION A novel system of slippery liquid-infused porous surface (SLIPS) was built up using the PS-HIPE membrane as a porous substrate and silicone oil as a slippery lubricating phase. As illustrated in Scheme 1, PS-HIPE with an interconnected porous structure was first prepared from the polymerization of the W/O HIPE. After purification to remove the residual monomers and surfactants, PDMS-0.1k, PDMS-0.5k, and PDMS-1.0k were infused into the porous PS-HIPE substrate through capillary, osmatic, and surface tension routes to overcome the gravity and reach equilibrium. The as-prepared PDMS-infused polyHIPE membranes were then used as a typical SLIPS surface for testing their surface wettability, selfcleaning, and self-repairing properties. Preparation and Characterization of PS-HIPE. PSHIPE membranes with various microporous structures were prepared through adjusting the relative content of the crosslinker, DVB, during the polymerization. Table 1 Table 1. Preparation and Characterization of PS-HIPE with Various Porous Structures oil phase recipeb

PS-HIPE morphologyc

samplea

St/%

DVB/%

D/μm

d/μm

n

O/%

PS-HIPE5 PS-HIPE10 PS-HIPE20

95 90 80

5 10 20

3.15 2.83 2.40

0.55 0.50 0.42

11 11 9

19.4 19.8 15.9

PS-HIPE; the numbers “5”, “10”, and “20” represent the DVB content in PS-HIPE preparation, respectively. bComponents of the monomer and crosslinker used for PS-HIPE preparation. cCharacterization of PS-HIPE morphology: “D” and “d” represent the cell and window diameter, respectively. And “n” is the average of windows in

a

each cell; openness “O” is defined as O =

So Sc

=

N 4

×

d2 D2

,

2 3

N=2× n in which “ So” denotes the area of the open surface and “Sc” represents the area of the cavity.

summarizes both the formulation and characterization of three PS-HIPE membranes. For all HIPE preparation, St and DVB were used as the oil phase and water as the internal phase. Typically, 30 wt % of Span-80 was used to stabilize 90.0 vol % of the internal water phase. DVB as the crosslinker was increased from 5 to 10 and 20 wt %, to adjust the porous structure. SEM images in Figure 1 present the microstructures of the PS-HIPE substrates with various crosslinker DVB contents. The continuous hierarchical porous structure could be identified from all polyHIPE samples. The primary void sphere named “cell” was produced from the removal of dispersed porogen droplets (aqueous phase), whereas the open connection “windows” created between adjacent cells were 8279

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir

Figure 1. SEM images of PS-HIPE samples prepared with different DVB contents: (a-1,2) 5 wt %, (b-1,2) 10 wt %, and (d-1,2) 20 wt %.

Figure 2. (a) Schematic illustration and photo image demonstration of PDMS loading into the PS-HIPE10 substrate with the open-cell porous structure. The effects of PS-HIPE porosity (b) and PDMS viscosity (c) on PDMS loading into polyHIPE substrates.

PDMS film is generated on the surface, which is the same as the typical WCA of PDMS.42 However, the PDMS infusion greatly improved the water slippery performance of PS-HIPE10 (SLIPS-0.1k) against all of the water-based contaminants, even though the WCA on SLIPS-0.1k decreased. As evidenced from Figure 3b,d, the water droplet is still pinned onto the PSHIPE10 surface at a tilt angle of 90°, instead of sliding down due to high contact angle hysteresis, which is the critical factor to ensure whether a droplet can slide down a surface easily or not.19 The larger the contact angle hysteresis, the harder the water droplet slips off from the surface. Surprisingly, compared to PS-HIPE10, the water droplet (about 3.5 μL) could easily slide off the SLIPS-0.1k surface at a very small tilt angle of about 3.0°. This is because the layer of PDMS film covered on the PS-HIPE10 surface acts as a lubricant layer, which facilitates the repulsion and sliding off of the test liquid droplets. Thus,

viscosity limits the progression of PDMS molecular throughout the polyHIPE channels. Surface Wettability and Sliding Properties of PDMSInfused PS-HIPE SLIPS. Effect of Loading PDMS on PSHIPE. PS-HIPE, as a super-hydrophobic material by itself, was compared with PDMS-infused PS-HIPE from wettability performance to investigate the effects of PDMS loading on PS-HIPE. The wettability of the material surface was studied by static water contact angle (WCA) and water sliding angle (WSA). As seen from Figure 3a, PS-HIPE10 is inherently a superhydrophobic material with a WCA of 147.9 ± 0.3°, which could be readily wetted by the oil-based test liquid that is compatible with polystyrene. After infusing the lubricant liquid, PDMS, and generating a typical SLIPS, the WCA of SLIPS0.1k decreases to 100.8 ± 1.2° (Figure 3c), since a layer of 8280

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir

Figure 3. Sliding property of water on PS-HIPE10 and SLIPS-0.1k: (a) WCA and (b) the 90° tilt angle of PS-HIPE10, (c) WCA and (d) WSA of SLIPS-0.1k.

the infusion of the lubricant liquid could greatly increase the slippery properties of the polyHIPE surface. Effect of the Viscosity of PDMS on the Critical Sliding Angle (SA) of SLIPS. The lubricant liquid PDMS loading has greatly improved the repellent properties of the PS-HIPE surface against water- and oil-based test liquids. The viscosity of the lubricant PDMS is also crucial to the slippery properties. The critical sliding angle is determined by the gravity of PDMS, the tilt angle, and the surface’s adhesion force, which is determined by the viscosity of PDMS.42 And for the water droplets to slide smoothly, the component of gravity that is parallel to the surface must be larger than the adhesion force, which is compatible with previous works.43 Therefore, under the condition of the same weight of water droplets in this experiment, the sliding angle is associated with the viscosity of PDMS for increasing the adhesion force. As shown in Figure 4, PDMS-0.1k, PDMS-0.5k, and PDMS1.0k were loaded in PS-HIPE10 substrates. The WSA of these SLIPS was investigated by sliding down 3.5 μL water droplets. As the viscosity of the infused PDMS increased from 100 to 500 and 1000 mPa·s, the SA of the SLIPS increased from 3.0 ± 1.3 to 7.2 ± 1.7 and 11.1 ± 2.4°, respectively. On comparing the sliding distances of the samples at the same sliding time, it is worth noting that the sliding speed of the water droplet is related to the viscosity of PDMS. The water droplet slides faster from SLIPS-0.1k, which also proved that the low viscosity of PDMS (low molecular weight) is good for sliding. Self-cleaning and Self-repairing. Self-cleaning. As a typical omniphobic surface, the self-cleaning properties of the PDMS-infused PS-HIPE SLIPS were verified by their repelling ability against various smudge liquids. As seen from Figure 5a, milk, coffee, and ink could readily slide off from the SLIPS-0.1k surface at an inclined angle of 20°, and they left no stain on the surface because all the simulated contaminates are water-based liquids. Although ink exhibits relatively slower sliding behavior, which needs about 60 s to slide down the surface tilted at 20°, it could hardly slide down at 10°. However, it could be readily washed off from the surface with water droplets without leaving any stain, as shown in Figure 5b. However, the ink

Figure 4. Water sliding angle (WSA) of SLIPS fabricated from PSHIPE10 infused by PDMS with viscosities of (a) 100 mPa·s (WSA = 3.0 ± 1.3°), (b) 500 mPa·s (WSA = 7.2 ± 1.7°), and (c) 1000 mPa·s (WSA = 11.1 ± 2.4°).

droplet left obvious stain even after washing with water at PSHIPE and the glass substrate (Figure 5b). Self-cleaning is not only effective for liquid contaminants but also for solid contaminants. As shown in Figure 5c, simulated dust deposited on the SLIPS surface can be easily removed upon washing with water. The self-cleaning property of the SLIPS is benefited from the lubricant layer protection, which isolates the polyHIPE substrate from the test liquid and solid. In addition, the ultralow fraction between PDMS and the test liquid enables easy sliding off even at a very small tilt angle. Self-repairing. SLIPS possess superior self-repairing ability that restores their surface repellent property over other omniphobic surfaces due to refilling of the gap area, resulting from minor damage, with the liquid lubricant. Thus, the selfrepairing behavior of the PDMS-infused PS-HIPE SLIPS was also investigated by surface scratching and recording the restoration of its repellent property against water, which is shown in Figure 6. 8281

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir

Figure 5. (a) Sliding properties of simulated aqueous contaminants (e.g., milk, coffee, and ink) from the SLIPS-0.1k surface at a tilt angle of 20°. (b) Self-cleaning of ink (liquid contaminant) from the SLIPS-0.1k surface with water washing, compared to PS-HIPE10 and a glass surface at a tilt angle of 10°. (c) Self-cleaning of dust (solid contaminant) from the SLIPS-0.1k surface at a tilt angle of 10° by water washing.

restored within seconds after damage. The dents resulted from physical cutting could be readily filled with PDMS, generating the liquid lubricant layer and restoring the superior slippery properties of the surface. Furthermore, Figure 6b with a simple simulated scheme shows that PDMS on the surface of SLIPS0.1k in a red circular area disappears upon using Kimwipes to absorb the water on the surface, which will be refilled with PDMS within 10 s owing to capillary force and surface energy minimization, with PDMS stored within adjacent vacancies acting as reservoirs. Thus, the self-repairing process of the PDMS-infused polyHIPE SLIPS was verified, and it was suggested that such a SLIPS system is indomitable in maintaining its slippery properties after external damage, which is of great significance for real applications. Durability and Stability. The durability of the SLIPS is significant for its applications. Specifically, the durability involves the durability of the slippery feature, which relies on the lasting of the lubricant liquid during real applications, and the mechanical durability of the microstructure, which restores and restricts the lubricant layer.44,45 Taking SLIPS-0.1k for example, the durability of slippery performance was studied by the consumption of PDMS. After loading sufficient PDMS-0.1k, the silicone oil at the bottom of SLIPS-0.1k was wiped off with Kimwipes. Then, the PDMS consumption, which is proportional to the SLIPS’ durability during the slippery test, was reflected by recording the SA changes of a 5 μL water droplet at the same spot of the surface every 2 min. As shown in Figure 7a, the WSA could be maintained at lower than 3.0° for about 30 cycles, which slightly increased to 5.0° after 80 cycles. This suggested that the PDMS consumption in polyHIPE-based SLIPS systems could still be maintained at affordable levels even after multiple usage cycles. It should be noted that if sufficient PDMS was stored below the polyHIPE substrate, the SLIPS could retain the smaller WSA of about 3.0°, since the consumed lubricant

Figure 6. (a) Self-repairing behavior with the recording of SLIPS0.1k. The water was dyed with methyl orange. The platform is tilted at 10°. (b) Disappearance and restoration of PDMS.

As seen from Figure 6a, the surface of SLIPS-0.1k was scribed twice with a scalpel resulting in two indentations, as highlighted by blue lines, for destroying its surface structure and repellent properties. Immediately, a water droplet, dyed with methyl orange for clear viewing, was dripped over the upper dent. As demonstrated, water droplets can easily slide through the two dents without any obstruction, which suggested that the repellent property of such SLIPS could be 8282

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir

Figure 7. Durability test of (a) the slippery property characterized by the changes of WSA due to PDMS consumption and (b) the mechanical strength of PS-HIPE and SLIPS-0.1k measured with the compressive test.

■

PDMS layer could be continually regenerated by PDMS transported through the PS-HIPE substrate. The mechanical strength of PS-HIPE and SLIPS-0.k were determined for comparison to investigate the stability before and after PDMS loading through a compression test. The stress−strain curve in Figure 7b suggests that PS-HIPE and SLIPS-0.1k exhibit similar stress at about 0.35 MPa before the strain reaches 60%. This result indicates that physical PDMS loading does not have any negative effect on the mechanical properties of PS-HIPE. After 60% strain, the stress of SLIPS0.1k was slightly lower than that of PS-HIPE, which might be attributed to the swelling of PS-HIPE by PDMS. The mechanical property of SLIPS mainly depends on the PSHIPE substrate.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.S.). *E-mail: [email protected] (L.L.). ORCID

Shuxian Shi: 0000-0003-2048-4519 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the Fundamental Research Funds for the Central Universities and Research projects on biomedical transformation of China−Japan Friendship Hospital (No. PYBZ1840).

■

■

CONCLUSIONS In conclusion, a novel SLIPS system constructed from PDMSinfused polyHIPE was introduced. Specifically, the porous polyHIPE substrate with an interconnecting open-cell structure was infused with PDMS as a lubricant liquid for the construction of a slippery surface. PDMS loading endows polyHIPE with superior repellent properties to various contaminant liquids, such as water, milk, coffee, and ink. Furthermore, the liquid-repellent process could simultaneously endow the SLIPS with self-cleaning properties to remove solid and liquid contaminants, such as dust and smudge. Moreover, compared to other SLIPS systems that require complicated, expensive, and wearable textured substrate preparation, the PDMS-infused PS-HIPE SLIPS shows the following advantages. The inherent open-cell porous structure, which is manufactured throughout the polyHIPE matrix, allows polyHIPE to be a superior, robust, and durable storage, transposition, and physical restriction substrate for loading PDMS for SLIPS construction. Due to the hierarchical porous structure, which endues the surface PDMS layer regeneration for SLIPS, surface layer abrasion does not restrict the repellent application of such SLIPS. The surface repelling property could be restored within minutes, even though the PDMSinfused polyHIPE SLIPS suffered from mechanical damage. Last but not least, the loss of lubricant liquid during repellent application could be remedied by intermittent PDMS resupply. Thus, such PDMS-infused polyHIPE SLIPS with superior contaminant repellent properties are promising for various areas, such as antifouling, self-cleaning, and selfrepairing.

REFERENCES

(1) Urata, C.; Dunderdale, G. J.; England, M. W.; Hozumi, A. Selflubricating organogels (SLUGs) with exceptional syneresis-induced anti-sticking properties against viscous emulsions and ices. J. Mater. Chem. A 2015, 3, 12626−12630. (2) Zhang, J.; Gu, C.; Tu, J. Robust Slippery Coating with Superior Corrosion Resistance and Anti-Icing Performance for AZ31B Mg Alloy Protection. ACS Appl. Mater. Interfaces 2017, 9, 11247−11257. (3) Epstein, A. K.; Wong, T. S.; Belisle, R. A.; Boggs, E. M.; Aizenberg, J. Liquid-infused structured surfaces with exceptional antibiofouling performance. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 13182−13187. (4) Xiao, L.; Li, J.; Mieszkin, S.; Di Fino, A.; Clare, A. S.; Callow, M. E.; Callow, J. A.; Grunze, M.; Rosenhahn, A.; Levkin, P. A. Slippery liquid-infused porous surfaces showing marine antibiofouling properties. ACS Appl. Mater. Interfaces 2013, 5, 10074−10080. (5) Tuo, Y.; Zhang, H.; Chen, W.; Liu, X. Corrosion protection application of slippery liquid-infused porous surface based on aluminium foil. Appl. Surf. Sci. 2017, 423, 365−374. (6) Jiang, L.; Zhao, Y.; Zhai, J. A lotus-leaf-like superhydrophobic surface: a porous microsphere/nanofiber composite film prepared by electrohydrodynamics. Angew. Chem., Int. Ed. 2004, 43, 4338−4341. (7) Shi, X.; Dou, R.; Ma, T.; Liu, W.; Lu, X.; Shea, K. J.; Song, Y.; Jiang, L. Bioinspired Lotus-like Self-Illuminous Coating. ACS Appl. Mater. Interfaces 2015, 7, 18424−18428. (8) Vrancken, R. J.; Kusumaatmaja, H.; Hermans, K.; Prenen, A. M.; Pierre-Louis, O.; Bastiaansen, C. W.; Broer, D. J. Fully reversible transition from Wenzel to Cassie-Baxter states on corrugated superhydrophobic surfaces. Langmuir 2010, 26, 3335−3341. (9) Murakami, D.; Jinnai, H.; Takahara, A. Wetting transition from the Cassie-Baxter state to the Wenzel state on textured polymer surfaces. Langmuir 2014, 30, 2061−2067. 8283

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284

Article

Langmuir (10) Quéré, D. Wetting and Roughness. Annu. Rev. Mater. Res. 2008, 38, 71−99. (11) Wang, Y.; Zhao, W.; Wu, W.; Wang, C.; Wu, X.; Xue, Q. Fabricating Bionic Ultraslippery Surface on Titanium Alloys with Excellent Fouling-Resistant Performance. ACS Appl. Bio Mater. 2018, 2, 155−162. (12) Luo, H.; Yin, S.; Huang, S.; Chen, F.; Tang, Q.; Li, X. Fabrication of slippery Zn surface with improved water-impellent, condensation and anti-icing properties. Appl. Surf. Sci. 2019, 470, 1139−1147. (13) Wilson, P. W.; Lu, W.; Xu, H.; Kim, P.; Kreder, M. J.; Alvarenga, J.; Aizenberg, J. Inhibition of ice nucleation by slippery liquid-infused porous surfaces (SLIPS). Phys. Chem. Chem. Phys. 2013, 15, 581−585. (14) Zhang, J.; Wu, L.; Li, B.; Li, L.; Seeger, S.; Wang, A. Evaporation-induced transition from Nepenthes pitcher-inspired slippery surfaces to lotus leaf-inspired superoleophobic surfaces. Langmuir 2014, 30, 14292−14299. (15) Wang, L.; Zhou, Q. Nepenthes pitchers: surface structure, physical property, anti-attachment function and potential application in mechanical controlling plague locust. Chin. Sci. Bull. 2014, 59, 2513−2523. (16) Wong, T. S.; Kang, S. H.; Tang, S. K.; Smythe, E. J.; Hatton, B. D.; Grinthal, A.; Aizenberg, J. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 2011, 477, 443− 447. (17) Li, Q.; Guo, Z. Lubricant-infused slippery surfaces: Facile fabrication, unique liquid repellence and antireflective properties. J. Colloid Interface Sci. 2019, 536, 507−515. (18) Wang, Z.; Heng, L.; Jiang, L. Effect of lubricant viscosity on the self-healing properties and electrically driven sliding of droplets on anisotropic slippery surfaces. J. Mater. Chem. A 2018, 6, 3414−3421. (19) Yong, J.; Chen, F.; Yang, Q.; Fang, Y.; Huo, J.; Zhang, J.; Hou, X. Nepenthes Inspired Design of Self-Repairing Omniphobic Slippery Liquid Infused Porous Surface (SLIPS) by Femtosecond Laser Direct Writing. Adv. Mater. Interfaces 2017, 4, No. 1700552. (20) Rykaczewski, K.; Paxson, A. T.; Staymates, M.; Walker, M. L.; Sun, X.; Anand, S.; Srinivasan, S.; McKinley, G. H.; Chinn, J.; Scott, J. H.; Varanasi, K. K. Dropwise condensation of low surface tension fluids on omniphobic surfaces. Sci. Rep. 2014, 4, No. 4158. (21) Xiang, T.; Zhang, M.; Sadig, H. R.; Li, Z.; Zhang, M.; Dong, C.; Yang, L.; Chan, W.; Li, C. Slippery liquid-infused porous surface for corrosion protection with self-healing property. Chem. Eng. J. 2018, 345, 147−155. (22) Wei, C.; Zhang, G.; Zhang, Q.; Zhan, X.; Chen, F. Silicone OilInfused Slippery Surfaces Based on Sol-Gel Process-Induced Nanocomposite Coatings: A Facile Approach to Highly Stable Bioinspired Surface for Biofouling Resistance. ACS Appl. Mater. Interfaces 2016, 8, 34810−34819. (23) Juuti, P.; Haapanen, J.; Stenroos, C.; Niemelä-Anttonen, H.; Harra, J.; Koivuluoto, H.; Teisala, H.; Lahti, J.; Tuominen, M.; Kuusipalo, J.; Vuoristo, P.; Mäkelä, J. M. Achieving a slippery, liquidinfused porous surface with anti-icing properties by direct deposition of flame synthesized aerosol nanoparticles on a thermally fragile substrate. Appl. Phys. Lett. 2017, 110, No. 161603. (24) Zhou, X.; Lee, Y.-Y.; Chong, K. S. L.; He, C. Superhydrophobic and slippery liquid-infused porous surfaces formed by the selfassembly of a hybrid ABC triblock copolymer and their antifouling performance. J. Mater. Chem. B 2018, 6, 440−448. (25) Miranda, D. F.; Urata, C.; Masheder, B.; Dunderdale, G. J.; Yagihashi, M.; Hozumi, A. Physically and chemically stable ionic liquid-infused textured surfaces showing excellent dynamic omniphobicity. APL Mater. 2014, 2, No. 056108. (26) Li, J.; Ueda, E.; Paulssen, D.; Levkin, P. A. Slippery LubricantInfused Surfaces: Properties and Emerging Applications. Adv. Funct. Mater. 2019, 29, No. 1802317. (27) Tian, X.; Shaw, S.; Lind, K. R.; Cademartiri, L. Thermal Processing of Silicones for Green, Scalable, and Healable Superhydrophobic Coatings. Adv. Mater. 2016, 28, 3677−3682.

(28) Cameron, N. R. High internal phase emulsion templating as a route to well-defined porous polymers. Polymer 2005, 46, 1439−1449. (29) Boyère, C.; Leonard, A. F.; Grignard, B.; Favrelle, A.; Pirard, J. P.; Paquot, M.; Jerome, C.; Debuigne, A. Synthesis of microsphereloaded porous polymers by combining emulsion and dispersion polymerisations in supercritical carbon dioxide. Chem. Commun. 2012, 48, 8356−8358. (30) Cai, D.; Thijssen, J. H.; Clegg, P. S. Making non-aqueous high internal phase pickering emulsions: influence of added polymer and selective drying. ACS Appl. Mater. Interfaces 2014, 6, 9214−9219. (31) Lei, L.; Zhang, Q.; Shi, S.; Zhu, S. High internal phase emulsion with double emulsion morphology and their templated porous polymer systems. J. Colloid Interface Sci. 2016, 483, 232−240. (32) Silverstein, M. S. PolyHIPEs: Recent advances in emulsiontemplated porous polymers. Prog. Polym. Sci. 2014, 39, 199−234. (33) Cameron, N. R.; Sherrington, D. C. High internal phase emulsions (HIPEs)-Structure, properties and use in polymer preparation. Adv. Polym. Sci. 1996, 126, 163−214. (34) Zhu, H.; Lei, L.; Li, B.-G.; Zhu, S. Development of Novel Materials from Polymerization of Pickering Emulsion Templates. Adv. Polym. Sci. 2017, 101−119. (35) Lei, L.; Zhang, Q.; Shi, S.; Zhu, S. Highly Porous Poly(high internal phase emulsion) Membranes with “Open-Cell” Structure and CO2-Switchable Wettability Used for Controlled Oil/Water Separation. Langmuir 2017, 33, 11936−11944. (36) Liu, H.; Wan, D.; Du, J.; Jin, M. Dendritic Amphiphile Mediated One-Pot Preparation of 3D Pt Nanoparticles-Decorated PolyHIPE as a Durable and Well-Recyclable Catalyst(Article)[J]. ACS Appl. Mater. Interfaces 2015, 7, 20885−20892. (37) Khodabandeh, A.; Arrua, R. D.; Mansour, F. R.; Thickett, S. C.; Hilder, E. F. PEO-based brush-type amphiphilic macro-RAFT agents and their assembled polyHIPE monolithic structures for applications in separation science. Sci. Rep. 2017, 7, No. 7847. (38) Zhang, M.; Liu, Q.; Chen, R.; Chen, H.; Song, D.; Liu, J.; Zhang, H.; Li, R.; Wang, Y.; Wang, J. Lubricant-infused coating by double-layer ZnO on aluminium and its anti-corrosion performance. J. Alloys Compd. 2018, 764, 730−737. (39) Loh, Q. L.; Choong, C. Three-dimensional scaffolds for tissue engineering applications: role of porosity and pore size. Tissue Eng., Part B 2013, 19, 485−502. (40) Zhang, P.; Chen, H.; Zhang, L.; Zhang, D. Anti-adhesion effects of liquid-infused textured surfaces on high-temperature stainless steel for soft tissue. Appl. Surf. Sci. 2016, 385, 249−256. (41) Wang, N.; Xiong, D.; Pan, S.; Deng, Y.; Shi, Y. Fabrication of superhydrophobic and lyophobic slippery surface on steel substrate. Appl. Surf. Sci. 2016, 387, 1219−1224. (42) Gee, E.; Liu, G.; Hu, H.; Wang, J. Effect of Varying Chain Length and Content of Poly(dimethylsiloxane) on Dynamic Dewetting Performance of NP-GLIDE Polyurethane Coatings. Langmuir 2018, 34, 10102−10113. (43) Liu, H.; Zhang, P.; Liu, M.; Wang, S.; Jiang, L. Organogel-based thin films for self-cleaning on various surfaces. Adv. Mater. 2013, 25, 4477−4481. (44) Howell, C.; Vu, T. L.; Lin, J. J.; Kolle, S.; Juthani, N.; Watson, E.; Weaver, J. C.; Alvarenga, J.; Aizenberg, J. Self-replenishing vascularized fouling-release surfaces. ACS Appl. Mater. Interfaces 2014, 6, 13299−13307. (45) Damle, V. G.; Tummala, A.; Chandrashekar, S.; Kido, C.; Roopesh, A.; Sun, X.; Doudrick, K.; Chinn, J.; Lee, J. R.; Burgin, T. P.; Rykaczewski, K. “Insensitive” to touch: fabric-supported lubricantswollen polymeric films for omniphobic personal protective gear. ACS Appl. Mater. Interfaces 2015, 7, 4224−4232.

8284

DOI: 10.1021/acs.langmuir.9b01115 Langmuir 2019, 35, 8276−8284