Multi-functional fluorinated ionic liquid infused slippery surfaces with dual-responsive wettability switching and self-repairing

Article abstract of DOI:10.1039/c8ta08956f

Stimuli responsive slippery surfaces with controllable wettability have attracted much attention due to their potential application in many fields. However, most of these surfaces only adopted paraffin as the lubricant oil and achieved wettability transformation just by heating. In addition, the self-repairing properties of smart slippery surfaces have rarely been reported. Hence, challenges still remain in fabricating multiple responsive slippery surfaces based on other lubricants possessing self-repairing properties. Here, a smart slippery surface with controllable wettability was fabricated by using solid/liquid fluorinated ionic liquids (FILs) as lubricants and fluoropolymer films as the underlying substrates. The FIL, a thermo-responsive phase-transition material, was synthesized by ionic exchange and the substrate exhibited good self-repairing properties and magnetic-thermal and photo-thermal responses. By tuning the substrate temperature under light irradiation or magnetic field to change the phase state of the FIL, the surface could realize wettability conversion. Besides, the responsive temperature can be adjusted by tuning the melting temperature (T_m) of the FIL. The FIL was first synthesized and there was no report employing FILs as the lubricants to realize the wettability change of slippery surfaces. This work provided a new type of lubricant to prepare intelligent slippery surfaces, thus advancing the application of slippery surfaces in liquid transport devices.

Full text of DOI:10.1039/c8ta08956f

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Materials Chemistry A
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Multi-functional fluorinated ionic liquid infused
slippery surfaces with dual-responsive wettability
switching and self-repairing†
Cite this: DOI: 10.1039/c8ta08956f
*
Qingqing Rao, Ao Li, Jiawen Zhang, Jingxian Jiang, Qinghua Zhang,
and Fengqiu Chen
Xiaoli Zhan
Stimuli responsive slippery surfaces with controllable wettability have attracted much attention due to their
potential application in many fields. However, most of these surfaces only adopted paraffin as the lubricant
oil and achieved wettability transformation just by heating. In addition, the self-repairing properties of smart
slippery surfaces have rarely been reported. Hence, challenges still remain in fabricating multiple responsive
slippery surfaces based on other lubricants possessing self-repairing properties. Here, a smart slippery
surface with controllable wettability was fabricated by using solid/liquid fluorinated ionic liquids (FILs) as
lubricants and fluoropolymer films as the underlying substrates. The FIL, a thermo-responsive phase-
transition material, was synthesized by ionic exchange and the substrate exhibited good self-repairing
properties and magnetic-thermal and photo-thermal responses. By tuning the substrate temperature under
light irradiation or magnetic field to change the phase state of the FIL, the surface could realize wettability
conversion. Besides, the responsive temperature can be adjusted by tuning the melting temperature (Tm) of
the FIL. The FIL was first synthesized and there was no report employing FILs as the lubricants to realize the
wettability change of slippery surfaces. This work provided a new type of lubricant to prepare intelligent
slippery surfaces, thus advancing the application of slippery surfaces in liquid transport devices.
Received 14th September 2018
Accepted 20th December 2018
DOI: 10.1039/c8ta08956f
rsc.li/materials-a
overcome the above defects. For example, Hu's group33 fabri-
cated a silicon oil infused slippery surface based on multilay-
1. Introduction
Inspired by natural biointerfaces like lotus leaves,1,2 rice
leaves3–5 and buttery wings,5,6 surfaces with adjustable wetta-
bility have attracted extensive research. The wettability of these
surfaces will be changed when exposed to external stimuli such
as humidity,7 electric eld,8,9 magnetic eld,10–12 tempera-
ture,13–17 etc. Such surfaces have been applied to no-loss liquid
transportation,15,16,18,19 water collection,17,20–22 oil–water separa-
tion,7,18,23–25 and dropwise condensation.26,27 It is worth noting
that the surface morphology and chemical characteristics are
the two major factors which affect the change of surface
wettability. Briey, the chemical structure is changed with the
migration or contraction of molecular segments. Many studies
about poly(N-isopropyl acrylamide) have realized this conver-
sion.28 However, it's a big challenge to precisely control the
adjustment of the surface microstructure.
ered biomimetic “tissue” by combining a poroelastic skin with
articial muscle. The morphology of this slippery surface could
be nely tuned by a programmed voltage thus changing the
droplet sliding motion on the surface. Besides, the slippery
surface can be used to manipulate bubbles in an aqueous envi-
ronment. Cao and his co-workers34 prepared a lubricant infused
slippery surface based on super-hydrophobic substrates. The
motion direction of underwater bubbles was nely regulated by
adjusting the track width and sample shape. The porous
substrates and the infused lubricants are two necessary condi-
tions to fabricate a slippery surface. It should be emphasized that
the performance will be altered with different substrates and
diverse lubricants.9,16,30,35,36 However, according to the current
studies, most smart slippery surfaces with controllable wettability
based on the variation of the lubricant phase state only select
paraffin as the lubricant. For instance, Seimei Shiratori and his
co-workers16 prepared a temperature-driven slippery surface with
controllable optical and wettability performances. The droplet
motion and light transmittance on this surface could be regu-
lated with the variation of temperature by altering the solid/liquid
paraffin mixing ratio. Similarly, Heng's group15 fabricated a ther-
mally activated slippery surface with controllable droplet motion
also based on paraffin. However, using paraffin as the lubricant
will weaken the liquid repellency properties of the slippery
Recently, a new kind of slippery liquid infused porous
surface inspired by Nepenthes27,29–32 supplies a new method to
Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture
Technology, College of Chemical and Biochemical Engineering, Zhejiang University,
Hangzhou, 310027, China. E-mail: qhzhang@zju.edu.cn; Fax: +86-571-8795-1227;
Tel: +86-571-8795-3382
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta08956f
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surface due to its higher surface tension than uorinated lubri- pipeline could be repaired. The self-repairing properties of
cant oil, such as peruoroether and peruorocarbon.29,37 In MPRSs also promoted the materials' durability. It is worth
addition, evaporation at high temperatures seems to be mentioning that this is the rst report about FIL-infused slip-
unavoidable for most organic lubricants. Hence, creating a new pery surfaces with reversible wetting behavior and the FILs
type of lubricant which has a suitable phase transfer temperature presented here have never been reported. These ndings in this
and good thermal stability is very signicant to broaden the work provided a new type of lubricant to fabricate smart
application of slippery surfaces.
surfaces with regulatable wettability, which can be used in
Ionic liquids (ILs), as a novel type of solvent besides water micro-uidic and biomedical devices.
and organic solvents, are completely composed of anions and
cations. Owing to the weak interaction force between the large
2. Experimental section
organic anions and cations,27,38 ILs exhibit low melting points.
Moreover, ILs have a very low vapor pressure and superb
thermal stability. All of these are attractive features of ILs for
using them as a new type of lubricant. The excellent high
temperature stability promises a long life for slippery surfaces.
Miranda's group38 veried the stability of IL-infused slippery
surfaces at high temperatures over 200 ꢀC. However, studies
about IL-infused slippery surfaces with reversible wettability,
which was based on lubricant phase transfer to achieve the
change of surface wettability, have never been reported. In
addition, the currently reported smart responsive slippery
surfaces mostly respond to a single stimulus like heating15,16
and electric eld.39,40 Unfortunately, the conditions are more
complex in practical application and the response oen
requires remote implementation. Fe3O4, as a type of magnetic
nanomaterial, is capable of absorbing energy from sunlight and
magnetic eld and converting them to thermal energy.11,41,42
Magnetically responsive switching has attracted widespread
attention due to its remote controllability, excellent biocom-
patibility and double response to light irradiation and magnetic
eld. Furthermore, magnetic nanoparticles are capable of
enhancing the mechanical properties of the obtained polymer.
Hence, the addition of magnetic Fe3O4 will result in a double
responsive and robust smart surface. Besides, few studies have
2.1 Materials
2-Ethylhexyl acrylate (2-EHA), triethylamine, potassium
carbonate (K2CO3), and azobisisobutyronitrile (AIBN, 99%
purity) were purchased from Aladdin Chemical Reagent Co., Ltd
(China). Bis(2-methacryloyl)oxyethyl disulde (BMOD) was ob-
tained from Aldrich. 2-(Peruorohexyl)ethyl acrylate (C6A) was
synthesized as described in our previous work.43 1,3-Dimethy-
limidazolium chloride ([MMIm]Cl) was provided by Yuduo
Biotechnology Co., Ltd (Shanghai, China). 1-Hexyl-3-
methylimidazolium chloride ([HMIm]Cl) was purchased from
Moni Chem. Eng. Sci & Tech. Co., Ltd (Shanghai, China). 1-
Ethylimidazole was provided by Macklin. Peruorobutyl
sulfonyl uoride (C4F9SO2F) was provided by Hengxin Chemical
Co., Ltd (Hubei, China). Ethyl bromide, ammonia solution
(NH3$H2O, 25 wt%), ethanol absolute (C2H5OH, purity $
99.7%), and sulfuric acid (H2SO4, 98 wt%) were purchased from
Sinopharm Chemical Regent Co., Ltd. All these reagents were
used as received. Deionized water was used throughout the
whole experiments.
2.2 Synthesis of uorinated ionic liquids
1,3-Diethylimidazolium bromide ([EEIm]Br). 1-Ethyl-
focused on the self-repairing properties of slippery surfaces, imidazole was reacted with bromoethane in a 25 ml three-
especially smart responsive slippery surfaces, owing to the necked ask at 30 ꢀC for 15 h. Then the excess solvent was
limitation of the fabrication methods of substrates.
Herein, we present a magnetic-thermal and photo-thermal drying for 24 h to obtain pure [EEIm]Br.
removed by rotary evaporation at 25 ꢀC followed by freeze-
responsive solid/liquid FIL infused slippery surface (MPRS)
Bis-peruorobutyl sulfonimide potassium ((C4F9SO2)2NK).
with reversible wettability, on which the droplet motion can be First, 150 ml liquid ammonia was obtained in a three-necked
smartly controlled by adjusting the substrate temperature. ask by condensation. Then 40 g C4F9SO2F was slowly injec-
Briey, when the temperature exceeded the Tm of the FIL under ted into the ask via a peristaltic pump and reacted with the
light irradiation or magnetic eld, the water droplets easily slid liquid ammonia for 3 h. It is noteworthy that liquid nitrogen
on the surface. Conversely, the droplets pinned on the surfaces was used for the whole process to cool down the temperature.
if the temperature decreased below the Tm. The Tm of the FIL Subsequently, the prepared intermediate was acidied with
could be easily revised by using different cation groups. Besides, 50% H2SO4 to form peruorobutylsulfonamide (C4F9SO2NH2)
the surface showed good self-repairing performance, excellent in anhydrous ether solution and then washed three times with
ꢀ
stability and liquid repellency properties. The freezing time of distilled water and sublimated for purication at 80 C.
the FIL-1H3M infused slippery surface was extended to 3605 s.
Then, C4F9SO2NH2, triethylamine and CH3CN were mixed in
These multi-functional intelligent materials could be applied in a 250 ml three-necked ask and stirred at 45 ꢀC for 0.5 h.
transport pipeline equipment for liquid transmission. The C4F9SO2F was slowly dropped into the mixed solution and the
superb liquid repellency and anti-icing properties of MPRSs temperature was raised to 80 ꢀC for 24 h immediately, followed
were capable of preventing the pipeline from blocking and by heating the resulting orange solution to remove the solvent.
freezing. Besides, once the hydrates and other substances were Then, the products were dissolved in CH2Cl2 solution and
frozen in the pipeline under harsh situations, they could be washed with distilled water three times. Finally, the dried
melted under light irradiation or magnetic eld through remote outputs were acidied with 98% H2SO4 and sublimed at 120 ꢀC
control and then the original transport performance of the to yield diperuorobutyl sulfonimide ((C4F9SO2)2NH). Then
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aqueous K2CO3 was slowly added to the (C4F9SO2)2NH aqueous 80 ꢀC for 10 days. The CA of the sample surface was then tested
solution. During this process, a kind of white oc appeared, daily to evaluate the loss of the lubricant.
which was (C4F9SO2)2NK.
Anti-icing properties. An OCA instrument was used to
FILs. 1.86 g (C4F9SO2)2NK and 20 ml CH3CN were blended in observe the icing delay time (DT) of the water droplets (2 ml) on
a 100 ml single-necked ask, followed by dropping [MMIm]Cl the sample surface. These specimens were placed on a Peltier
(or [EEIm]Br or [HMIm]Cl) which was dissolved with CH3CN in cooling controller (PCC) to cool down the sample surface
ꢀ
ꢀ
the mixture and reacted at 30 C for 3 h. Then the FILs (FIL- temperature and keep it at À15 C, and the relative humidity
1M3M, FIL-1E3E and FIL-1H3M) were synthesized by elimi- (RH) was 45 Æ 5%. Then by photographing the shape of the
nating the residual solvent.
water droplets at intervals, the DT was dened by the time at
which the shape of the droplets changed. The anti-frosting
performance was judged by the time when frost began to
2.3 Preparation of magnetic-thermal and photo-thermal
responsive slippery surfaces
ꢀ
appear. The sample was putted on a PCC at À17 C, RH 80 Æ
3%. The accumulation of water droplets on the surface of the
The underlying substrate of the MPRSs was prepared by bulk sample was then recorded using a camera.
polymerization. A typical copolymerization process is described
as follows. Briey, different contents of Fe3O4 nanoparticles
(0 wt%, 10 wt%, 30 wt%, and 50 wt%) were mixed with C6A, 2-
EHA and BMOD to obtain the mixed solution, and then
3. Results and discussion
3.1 Preparation and characterization of the FILs
dispersed by ultrasonic treatment for 10 min. Aerwards, the Scheme S1† illustrates the related reaction equation during the
prepared solution was poured into a rectangular silicone mold synthetic procedures of the FILs. The anion of the FILs was
and polymerized at 72 ꢀC for 2 h to obtain the hybrid lms (HFs- prepared by our previous work through the reaction of
0, HFs-5, HFs-10, HFs-30, and HFs-50). Then, the as-prepared C4F9SO2NH and K2CO3. The chemical structure of
FILs were dropped onto the HFs to form slippery surfaces: (C4F9SO2)2NH was veried by 19F-NMR (Fig. S2a†). Then, the K+
MPRSs-1M3M, MPRSs-1E3E and MPRSs-1H3M ascribed to the on (C4F9SO2)2NK was substituted by three kinds of cationic salts
FIL-1M3M, FIL-1E3E and FIL-1H3M, respectively. The Fe3O4 ([MMIm]Cl, [EEIm]Br, and [HMIm]Cl) and uorinated ionic
nanoparticles were synthesized by previously reported liquids were formed: FIL-1M3M, FIL-1E3E and FIL-1H3M,
methods.11,42 Unless otherwise specied, the MPRSs used for respectively. The corresponding 19F-NMR spectra of the FILs
the following measurements were based on HFs-30.
are shown in Fig. S2b–d.† [EEIm]Br was synthesized by one-step
replacement between 1-ethylimidazole and bromoethane. Its
chemical structure is presented in Fig. S1.†
2.4 Characterization
Fig. 1a shows the chemical composition of FIL-1H3M, FIL-
Chemical structure and physical properties. Fourier trans- 1E3E and FIL-1M3M. The absorption peak appearing at about
form infrared (FT-IR) spectroscopy (Nicolet 5700) and 19F-NMR 3100 cmÀ1 was assigned to the C–H stretching vibrations in the
(Bruker Advance DMX500, 25 ꢀC) were used to analyze the imidazole ring and the broad band at 746 cmÀ1 corresponded to
chemical composition of the samples. A differential scanning the C–H rocking bending vibrations in the imidazole ring. In
calorimeter (DSC, TA-Q100, USA) was employed to test the Tm of addition, the signal of the carbon skeleton in the imidazole ring
the FILs. The thermal stability of the FILs was measured using was observed around 1570 cmÀ1 with little shi in different FILs
a thermogravimetriÀc1analyzer (TGA, TA-Q500, USA). The heating due to the difference of alkyl groups in the imidazole ring. The
absorption peaks at 1470 cmÀ1, 1454 cmÀ1 and 1469 cmÀ1 were
ꢀ
rate was 10 C min
.
Surface topography and wettability. The surface topography primarily ascribed to the C–N stretching vibration in the imid-
and surface roughness of the samples were characterized by azole ring and these peaks were overlapped with the C–H rocking
eld emission scanning microscopy (SEM, SU-8010, Japan) at an bending vibrations in the alkyl group. All of these absorption
accelerating voltage of 20 kV and atomic force microscopy peaks mentioned above conrmed that the imidazolyl cation salt
(AFM, TT2-AFM, USA) using a multimode in tapping mode, successfully substituted the K+ on (C4F9SO2)2NK. Meanwhile, the
respectively. The scanning range was 5 Â 5 mm2. The contact characteristic groups in (C4F9SO2)2NÀ could also be observed
angle (CA) was measured using an SDC-100 optical contact from the FT-IR spectra. Specically, the stretching vibration of
angle goniometer (SINDIN Co., Ltd., China). The average value the –SO2 group appeared at approximately 1573 cmÀ1. The broad
was obtained by measuring at least ve different positions on band at 1139–1236 cmÀ1 was assigned to the C–F and the
the same sample.
absorption peak of the C–N stretching vibration in the alkyl group
Magnetic properties and magneto-thermal effects. Magnetic was discovered around 1077 cmÀ1. These results indicated that
studies of the samples were performed using a multi-functional a uorinated ionic liquid has been successfully synthesized.
vibrating sample magnetometer (J3426, UK) at 293 K. A home-
The thermal stability of the FILs is shown in Fig. 1b. The
made radio frequency (RF) induction heater44 (power: 450 W) initial decomposition temperatures (Td) of the three FILs were
was employed for magneto-thermal experiments. The sample all above 250 ꢀC. Especially, the Td of FIL-1H3M reached above
was fabricated in a Petri dish (diameter: 54 mm, height: 5 mm). 350 ꢀC, thus demonstrating that the uorinated ionic liquids
Thermal stability measurement of lubricants. The samples prepared in this work possess outstanding thermal stability.
(2 Â 3 cm2) with three kinds of FILs were placed in an oven at The Tm of the FILs was measured by DSC to further prove the
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Fig. 1 FT-IR spectra (a) and TGA curves (b) of FIL-1H3M, FIL-1E3E and FIL-1M3M. DSC curves of FIL-1M3M (c) and FIL-1H3M (d).
transition temperature of the FILs from the solid to the liquid
The enhancement effect of nanoparticles on the mechanical
state. As shown in Fig. 1c, d and S3,† the Tm of the FILs was properties and surface roughness has been veried in many
61.7 ꢀC, À7.3 ꢀC, À16.4 ꢀC corresponding to FIL-1M3M, FIL- systems. This can also be found in this work. As shown in
1E3E and FIL-1H3M, respectively. The wide difference in Tm Fig. 2b and S6,† the compressive strength and surface CA
was caused by the different alkyl substituents on the imidazole increased with increasing content of Fe3O4 nanoparticles.
ring.
Besides, due to the photo-thermal effect and magneto-thermal
To achieve smart control of droplet motion on these effect of magnetic Fe3O4 nanoparticles, the surface tempera-
surfaces, FIL-1M3M with Tm around 61.7 ꢀC was selected as the ture could respond to light irradiation or magnetic eld.
lubricant to fabricate magneto-thermal and photo-thermal Considering the mechanical properties of the underlying
driven slippery surfaces. As shown in Scheme 1, FIL-1M3M substrate and the photo-thermal and magneto-thermal
could be reversibly transformed between the solid phase and conversion efficiency of the nanoparticles, HFs-30 was
the liquid phase by changing the temperature under light irra- selected for fabricating a slippery surface. The reversible
diation or magnetic eld. Then the surface changed to be disulde bond endowed the substrates with good self-repairing
slippery from the hydrophobic state. FIL-1M3M was solid at properties. As shown in Fig. 2c, the cutting marks on the
ꢀ
room temperature and did not require a high temperature to repaired matrix disappeared aer healing at 72 C for 4 h. The
melt, thus making the process exible to control.
substrates showed good healing capability with 70% healing
efficiency for fracture stress and 80% healing efficiency for
fracture strain (Fig. S4†). In addition, the water droplet could
move along the slippery surface aer the surface was healed
(Fig. S5†). It is believed that the reversible disulde bonds were
capable of re-connecting themselves together and then forming
a new crosslinked network to repair the damaged section.45
Besides, the cut lm could also be repaired under illumination
or magnetic eld ascribed to the magnetic and light heating
efficiency of Fe3O4 nanoparticles. The good self-repairing
properties of the underlying substrates contributed to keeping
the slippery surface stability for practical application.
3.2 Fabrication and characterization of MPRSs
The hybrid lm used as the underlying substrate was fabricated
by one-step solvent-free polymerization of C6A, 2-EHA and
Fe3O4 nanoparticles, and BMOD was used as a cross-linking
agent. The chemical structure of this polymer was character-
ized by FT-IR spectra presented in Fig. 2a. The broad band
appearing at 1189–1079 cmÀ1 was ascribed to the C–F stretching
vibrations. The absorption peak of C]O was observed at
1731 cmÀ1 instead at 1630 cmÀ1 which indicated that the –CH]
CH– group was completely polymerized. Besides, the charac-
Hereaer, the as-prepared FILs, as the lubricant oils, were
dropped onto the hybrid lms to form slippery surfaces. The
specic fabrication processes and the change of the liquid
teristic peak of the Fe–O group could also be found at 808 cmÀ1
This revealed that the Fe3O4 nanoparticles existed in the bulk of
the copolymer.
.
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Scheme 1 Preparation procedure of MPRSs and the magnetic-thermal and photo-thermal responsive processes of the liquid drop motion.
droplet motion are illustrated in Scheme 1. In detail, when the the surface was in the Wenzel state and strong. Under light
temperature was lower than the Tm of the FIL, the FIL was in the irradiation or magnetic eld, the surface temperature rose
solid state and droplets would pin on the surface. At this time, above the Tm of the FIL and the hybrid lm was covered by
Fig. 2 (a) FT-IR spectra of HFs-30. (b) Compressive stress–strain curve of the hybrid films with different contents of Fe3O4 nanoparticles. (c) Self-
healing process of HFs-30.
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liquid lubricants. Hence, the water droplet would slide down hydrophilic imidazole groups (Fig. 3g and h). And the surface
the surface.
CA further decreased to 39.56ꢀ when the temperature was above
The surface topography and surface roughness of HFs-30 the Tm of the FIL. Because the roughness of the FIL-infused
and LMSs-30 were further investigated by SEM and AFM. As surface decreased with the change of the FIL state, the contact
shown in Fig. 3a and b, the nanoparticles were evenly dispersed area between the droplet and the surface increased.
on the substrate and several microscopic bumps are present on
the surface resulting in the micro-scale roughness with a RMS of
261 nm (Fig. 3d). These rough structures boosted the capillary
action of the surface which promoted the storage capacity of the
lubricants and contributed to keeping the high stability of the
slippery surface. Moreover, the large amount of uorinated
groups contained in the FILs and hybrid lms also increased
the affinity between the lubricants and substrates. However, the
compatibility between the FILs and HF substrates would be
limited ascribed to the strong intermolecular force in the FILs.
As shown in Fig. 3e, the surface RMS roughness decreased to
8.78 nm and the structure of the hybrid lm remained faintly
visible (Fig. 3c) aer the surface was covered by the solid FIL.
Then the surface became highly smooth and defect-free, with
a RMS roughness of 0.08 nm (Fig. 3f) at 65 ꢀC. The surface
topography observed from both SEM and AFM images was in
good agreement with each other, thus demonstrating that the
FIL permeated into the HF matrix and formed a molecular-scale
smooth lubricant layer over the substrates. In addition, the CA
decreased with the cover of the FIL from 119.81ꢀ to 87.88ꢀ owing
to the reduction of the roughness and the introduction of
3.3 Liquid repellency and self-cleaning properties
The sliding motion of water droplets on the slippery surfaces
recorded by the digital pictures is shown in Fig. S7.† A water
droplet slipped on the surfaces covered by different FILs with
a low tilting angle (ꢁ5ꢀ) and le no traces. These results indi-
cated that there was no replacement of the lubricants by water
and the FIL was compatible with the substrates. More speci-
cally, it took 3 s, 18 s, and 23 s for the droplets to slide down the
surface of MPRSs-1M3M, MPRSs-1H3M and MPRSs-1E3E,
respectively, which increased with increasing viscosity of the
FILs. In addition, the sliding velocity of MPRSs-1M3M was
tested at 65ꢀ, which reduced the viscosity of FIL-1M3M and then
resulted in a faster sliding velocity.
Considering the practical application, several droplets with
different polarities and a series of solutions constantly used in
our daily life were selected to evaluate the liquid repellency
performance of this slippery surface. As shown in Fig. 4a and e,
the hexadecane and toluene droplets easily slid away the
MPRSs-1H3M surface. The sliding rate of water droplets (7 ml)
could reach 5.78 mm sÀ1. However, honey droplets were
Fig. 3 SEM images of the HFs-30 cross section (a), HFs-30 surface (b) and MPRSs-1M3M surface (c) at 25 ꢀC. 3D AFM height images of HFs-30
(d), and MPRSs-1M3M at 25 ꢀC (e) and 65 ꢀC (f). Contact angle photographs of HFs-30 (g), and MPRSs-1M3M at 25 ꢀC (h) and 65 ꢀC (i).
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Fig. 4 (a) Sliding velocity of several droplets (7 ml) on MPRSs-1H3M, tilt angle (TA): 25ꢀ and (b) evaporation stability of slippery surfaces at 80 ꢀC for
10 days. (c) Snapshots showing the sliding of honey (i), soy sauce (ii) and coffee (iii) on MPRSs-1H3M, TA: 25ꢀ. (d) Self-cleaning tests by using silica
particles (i) and copper(II) chloride dehydrate particles (CuCl2$2H2O) (ii) on MPRSs-1H3M, TA: 25ꢀ. (e) Snapshots of water (i), toluene (ii), dodecane
(iii) and hexadecane (iv) droplets sliding down MPRSs-1H3M, TA: 25ꢀ.
difficult to move on most surfaces due to their high viscosity. In These results suggested that the MPRSs were provided with
this work, the honey droplet le the surface in 10.34 s without superb self-cleaning properties.
no traces. And the coffee droplet slid down the surface in 3.77 s,
Longevity is another important factor in practical applica-
thus indicating that the MPRSs were enough to prevent surface tion. In this work, the MPRSs exhibited excellent evaporation
ꢀ
contamination by reagents. Besides, the liquid repellency of stability. At 80 C for 10 days, there was no obvious change in
MPRSs-1M3M and MPRSs-1E3E was also measured adopting the surface CA (Fig. 4b). Besides, the slipping velocity increased
the same target pollutant. As shown in Fig. S8a and S9,† all with increasing cycle times due to the reduction of lubricant oil
liquid droplets could slip off MPRSs-1M3M and MPRSs-1E3E, viscous resistance. As shown in Fig. S10,† the slipping rate of
and no track was le on their path. However, the sliding MPRSs remained high and showed no trend of decline aer 30
velocity of the droplets was very different on these three surfaces cycles. In addition, the MPRSs were capable of maintaining the
(Videos S1–S3†) caused by the discrepancy in the viscosity of the surface slippery properties when the samples were placed
FILs, which increased with larger alkyl chains.
vertically (Fig. S11†). The sliding velocity of the water droplet
In addition, the self-cleaning properties were estimated by and mass of the MPRSs showed no changes aer 1400 min. The
dirt-removal tests, using hydrophilic CuCl2$2H2O and hydro- signicant stability of this slippery surface benetted from the
phobic silica particles as model pollutants. Fig. 4d, S8b and c† FILs' thermal stability and compatibility with the substrate. In
show that some amounts of powders were pinned on the general, the superb thermal stability of FILs and the self-
MPRSs. Aer water impact, SiO2 particles oated on the water repairing properties of substrates ensure a long lifespan of
droplets and then slid down the slippery surface with the MPRSs, thus promoting the practical application of smart
droplets. No trace was le and no obvious powder residue could slippery surfaces.
be monitored on the impacted place. Besides, the CuCl2$2H2O
particles would be dissolved once they come into contact with
water and then wiped away with water droplets because of their
excellent solubility. Hence, the dirt could be completely
removed by oating on the droplets or dissolution in water.
3.4 Magnetic-thermal and photo-thermal responses of
MPRSs
To smartly control the droplet motion on the MPRSs, magnetic
Fe3O4 nanoparticles were mixed with the hybrid lms to endow
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their surfaces with magnetic-thermal and photo-thermal the HFs were obtained aer putting the samples under
responses. As shown in Fig. 5a, the magnetization curves of a magnetic eld for 20 min. As shown in Fig. 5b, there was no
the HFs were in an S-shape and without hysteresis, thus signi- heating effect generated for HFs-0, as no temperature change
fying that the MPRS wettability could be exibly controlled due was observed within a reasonable error range. However, the
to the excellent paramagnetic properties of Fe3O4 nanoparticles. temperature of the HFs-30 surface and HFs-50 surface increased
ꢀ
ꢀ
Briey, the magnetization saturation values were 56.5, 31.0, by 17.8 C and 22 C. It was worth emphasizing that the oper-
13.0, and 0 emu gÀ1 corresponding to HFs-50, HFs-30, HFs-10 ating power was 450 W, which was much lower than others'
and HFs-0, which increased with increasing loading of Fe3O4 work.11 The results indicated that the addition of magnetic
nanoparticles. These results suggested that the hybrid nanoparticles could provide obvious magnetic heating perfor-
lms were easily manipulated under a magnetic eld, thus mance for the hybrid lms, which was benecial for the smart
promoting the conversion of magnetic energy to thermal control of the droplet motion on the slippery surfaces.
energy.
Fig. 5c presents the reversible switching of droplet motion
Fig. S12† shows the typical device used to trigger heating of on the MPRSs-1M3M surface, using water and hexadecane as
the hybrid lms under a magnetic eld. The heating effects of the targets. The water droplet (3 ml) was pinned on the surface
Fig. 5 (a) Hysteresis curves of HFs with different contents of Fe3O4 nanoparticles. (b) The heating effects of HF surfaces under an alternative
magnetic field for 20 min (power: 450 W), and the inset images show the water droplet motion on MPRSs-1M3M before (above) and after (below)
applying the magnetic field. The thermo-controllable sliding motion of the water droplet (c) and hexadecane droplet (d) on the MPRSs-1M3M
surface, TA: 9ꢀ, drop volume: 3 ml.
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when T < Tm. Because FIL-1M3M was solid, the low surface CA FIL-1M3M state from solid to liquid. Meanwhile, the surface
was not enough to make the droplet roll down the surface. Aer transformed to a slippery surface from the hydrophobic surface
heating for a few seconds, the droplets started to slide on the and resulted in the droplets being slipping off the surface
surface, at this time, T > Tm. Then the surface was cooled down, instead of pinning. The controllable variation of surface CA and
and the water droplet further slipped for a while and stopped droplet motion on these surfaces promoted the application of
sliding because time was needed for the liquid FIL-1M3M to smart slippery surfaces in medical equipment used for liquid
completely solidify. The pinned droplet started to move again transfer pipes. Besides, the reversible switching of the surface
aer reheating. The same sliding motion of the hexadecane CA and the sliding speed of water droplets on the MPRSs-1M3M
droplet is presented in Fig. 5d. However, the CA and sliding rate are demonstrated in Fig. 6c and d. At 25 ꢀC, the water droplets
of the hexadecane droplet were smaller than those of the water pinned on the surface with a surface CA of 88ꢀ. Aer the surface
droplet on this slippery surface. This was caused by the differ- temperature was increased to 65 ꢀC, the water droplet began to
ence in polarity and surface tension.
slide on the surface with a speed of 35 mm sÀ1 and the surface
In addition, this smart surface could respond to light irra- CA decreased to 38.5ꢀ. Interestingly, no signicant change of
diation. The common experimental equipment for the photo- sliding velocity and surface CA was detected aer 10 cycles, thus
thermal response is presented in Fig. 6a. A sunlamp was demonstrating that the slippery surface has excellent endur-
selected as the heating source because it was more convenient ance properties.
than other sources and could be used in an actual environment.
As shown in Fig. 6b, the surface temperature of HFs-30
increased by 20 ꢀC under sunlamp irradiation for about
20 min and was unchanged for further irradiation. However,
there was no signicant change of the HFs-0 surface tempera-
ture aer illumination for 60 min, thus revealing that the
surface endowed with magnetic nanoparticles had good light
heating efficiency. When the surfaces temperature was up to Tm
of the FIL under sunlamp irradiation, the CA of the MPRSs-
1M3M surface changed to 39.56ꢀ from 88.78ꢀ as shown in the
insets of Fig. 6b, which was caused by the change of the
3.5 Anti-icing and anti-frosting properties
In this section, the anti-icing properties of MPRSs-1H3M were
tested to evaluate its practical value under extreme conditions.
The specimen was placed on a PCC device which was used to
adjust the ambient temperature. At a constant temperature,
a water droplet (2 ml) was then dropped on the surface and the
icing delay time was recorded. The DT of blank glass, rough
solid HFs-30, at solid MPRSs-1M3M and the HFs covered with
silicone oil (MPRSs-Si) was also measured to compare anti-icing
Fig. 6 (a) Photo-thermal test device for absorbing light energy from a sunlamp, and the surface temperature was monitored using a non-contact
infrared thermometer. (b) The heating effect of HFs-0 and HFs-30 under sunlamp irradiation (75 W). The CA (c) and sliding rate (d) changes on the
MPRSs-1M3M at 25 ꢀC and 65 ꢀC for 10 cycles.
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properties with those of MPRSs-1H3M, as silicone oil was difference in the DT of the slippery surfaces. The icing delay
constantly selected as the model lubricant to prepare slippery time of the MPRSs-Si surface was only extended to 662 s, which
surfaces. Fig. 7c shows the variation of the individual water was much shorter than that of MPRSs-1H3M. Besides, the DT of
droplet shape on these samples during the whole freezing the Teon template was prolonged to 363 s (Fig. S13†), which
process. As we can see, the DT of uncoated glass was prolonged was longer than the DT of MPRSs-1M3M, but shorter than the
to 23 s, HFs-30 to 563 s, MPRSs-1M3M to 330 s, and MPRSs- DT of HFs-30, because the roughness of the Teon template
1H3M to 3605 s (Fig. 7a), thus indicating that FIL-1H3M was surface was between that of MPRSs-1M3M and HFs-30. More-
prominent in delaying icing due to the high energy barrier of over, the surface chemical composition was similar to that of
nucleation for water droplets. However, there was a huge the uorinated surface. The presence of Fe3O4 was capable of
Fig. 7 (a) Icing delay time of different samples. (b) The time taken for frosting of four samples. (c) Snapshots of individual water droplets on
samples during the whole freezing process. (d) Snapshots of the frosting experiment on the surfaces of glass, HFs-30, MPRSs-1M3M, and MPRSs-
1H3M.
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endowing the surface with good photo-thermal responsive
deicing properties and creating a low-energy de-icing mecha-
nism, which has been demonstrated in our previous work.11
These results suggested that the FIL-1H3M infused slippery
surfaces have excellent anti-icing properties.
2 L. Feng, S. H. Li, Y. S. Li, H. J. Li, L. J. Zhang, J. Zhai,
Y. L. Song, B. Q. Liu, L. Jiang and D. B. Zhu, Adv. Mater.,
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3 S. G. Lee, H. S. Lim, D. Y. Lee, D. Kwak and K. Cho, Adv.
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In addition, the anti-frosting performance of this surface was
also measured. As shown in Fig. 7b and d, there was a signi-
cant difference of the frosting state between these samples. For
HFs-30, frost was very easy to deposit on it, because the rough
surface provided many nucleation sites for frosting. The anti-
frosting time (AFT) was less than 10 min at À17 ꢀC and the
AFT of MPRSs-1M3M was extended to 30 min. This resulted
from the reduction in the number of nucleation sites on the
smooth surface of MPRSs-1M3M. But both surfaces were
covered by a large amount of frost aer 60 min, since the heat
4 J. K. Oh, X. Lu, Y. Min, L. Cisneros-Zevallos and M. Akbulut,
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of the membrane aer cooling for 30 min. Then the droplets 11 T. Cheng, R. He, Q. Zhang, X. Zhan and F. Chen, J. Mater.
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liquids and the high energy barrier of nucleation for water
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13 T. Hessberger, L. B. Braun and R. Zentel, Adv. Funct. Mater.,
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14 X. Yao, J. Ju, S. Yang, J. Wang and L. Jiang, Adv. Mater., 2014,
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4. Conclusions
In this work, we successfully synthesized a magnetic-thermal 15 L. Wang, L. Heng and L. Jiang, ACS Appl. Mater. Interfaces,
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under light irradiation or magnetic eld, the reversible wetta-
Y. Tsuge, K. H. Kyung and S. Shiratori, ACS Nano, 2016, 10,
9387–9396.
bility, from hydrophobic to slippery, and droplet motion, from 17 H. Yang, H. Zhu, M. M. R. M. Hendrix,
pinning to sliding, were achieved. Besides, the responsive
N. J. H. G. M. Lousberg, G. de With, A. C. C. Esteves and
temperature could be controlled by using different substituent
J. H. Xin, Adv. Mater., 2013, 25, 1150–1154.
groups to synthesize FILs. In addition, the slippery surfaces 18 F. R. Li, Z. R. Wang, S. C. Huang, Y. L. Pan and X. Z. Zhao,
presented good liquid repellency, self-cleaning properties and Adv. Funct. Mater., 2018, 28, 1706867.
high stability. More interestingly, the freezing time of FIL-1H3M 19 X. Dai, N. Sun, S. O. Nielsen, B. B. Stogin, J. Wang, S. Yang
infused slippery surfaces was prolonged to 3605 s at À15 ꢀC,
and T.-S. Wong, Sci. Adv., 2018, 4, eaaq0919.
which was 156 times that of the DT of glass. This work provides 20 X. J. Zeng, L. Qian, X. X. Yuan, C. L. Zhou, Z. W. Li, J. Cheng,
a new type of lubricant oil to fabricate a stable slippery surface
with controllable wettability, thus promoting the application of
S. P. Xu, S. F. Wang, P. H. Pi and X. F. Wen, ACS Nano, 2017,
11, 760–769.
slippery surfaces in microuidic devices and biomedical 21 H. Bai, L. Wang, J. Ju, R. Z. Sun, Y. M. Zheng and L. Jiang,
systems.
Adv. Mater., 2014, 26, 5025–5030.
22 Y. Zheng, H. Bai, Z. Huang, X. Tian, F. Q. Nie, Y. Zhao, J. Zhai
and L. Jiang, Nature, 2010, 463, 640–643.
23 Y. Si, Q. Fu, X. Wang, J. Zhu, J. Yu, G. Sun and B. Ding, ACS
Nano, 2015, 9, 3791–3799.
Conflicts of interest
None.
24 Z. L. Chu, Y. J. Feng and S. Seeger, Angew. Chem., Int. Ed.,
2015, 54, 2328–2338.
25 C. L. Xu and Y. Z. Wang, J. Mater. Chem. A, 2018, 6, 2935–
2941.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21676248, 21878267 and 21576236) 26 Y. M. Hou, M. Yu, X. M. Chen, Z. K. Wang and S. H. Yao, ACS
and Zhejiang Provincial Major Project of Science Technology for
Award (No. 2014C13SAA10006).
Nano, 2015, 9, 71–81.
27 X. Q. Wang, C. D. Gu, L. Y. Wang, J. L. Zhang and J. P. Tu,
Chem. Eng. J., 2018, 343, 561–571.
28 J. C. Liu, N. Wang, L. J. Yu, A. Karton, W. Li, W. X. Zhang,
F. Y. Guo, L. L. Hou, Q. F. Cheng, L. Jiang, D. A. Weitz and
Y. Zhao, Nat. Commun., 2017, 8, 2011.
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