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2020
Fabrication of Transparent Icephobic Surfaces with Self-reparability: Effect of Structuring and Thickness of the Lubricant-elastomer Layer
Cui, Wenjuan
Elsevier BV
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© Elsevier B.V.
CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/
http://dx.doi.org/10.1016/j.apsusc.2019.144061
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Fabrication of Transparent Icephobic Surfaces with Self-reparability: Effect of Structuring and Thickness of the Lubricant-elastomer Layer
Wenjuan Cui, Tapani A. Pakkanen
PII: S0169-4332(19)32877-6
DOI: https://doi.org/10.1016/j.apsusc.2019.144061
Reference: APSUSC 144061
To appear in: Applied Surface Science Received Date: 3 June 2019
Revised Date: 2 September 2019 Accepted Date: 17 September 2019
Please cite this article as: W. Cui, T.A. Pakkanen, Fabrication of Transparent Icephobic Surfaces with Self- reparability: Effect of Structuring and Thickness of the Lubricant-elastomer Layer, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144061
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© 2019 Published by Elsevier B.V.
Fabrication of Transparent Icephobic Surfaces with Self-reparability: Effect of Structuring and Thickness of the Lubricant-elastomer Layer
Wenjuan Cui and Tapani A. Pakkanen*
Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland
*Corresponding author, E-mail: tapani.pakkanen@uef.fi Abstract:
Biomimetic lubricant-infused slippery surface (LISS) has attracted extensive attention as an alternative to a superhydrophobic surface (SHS) for icephobic applications. In this work, the impact of the material parameters of LISS with a lubricant-elastomer layer on icephobic performance is reported. A layer of polydimethylsiloxane elastomer and silicone oil on smooth and structured polypropylene (PP) surfaces was spin-coated by a simple one-step procedure. Results demonstrate that the ultra-smooth lubricant-elastomer layer (LEL) has an extremely low water contact angle hysteresis. Compared to SHS and smooth surface, the LISSs reduced ice adhesion by an order of magnitude due to its molecular homogeneity, low contact area, and strain mismatch. The LISSs exhibited excellent dynamic mobility at -10˚C, shear stability, evaporation resistance, mechanical robustness, and transparency. Physically damaged LISS can self-repair at 80˚C. The change in structures and LEL thickness of structured PP has little effect on ice adhesion.
As the LEL thickness on a smooth PP increases, the ice adhesion decreases. The smooth PP with a thick LEL has very low ice adhesion, indicating that the roughness is not
necessary for maintaining the slippery properties. Transparent, stable, anti-dusting, and self-repairing LELs on smooth materials have potential for icephobic applications.
Keywords: slippery surface, lubricant-elastomer layer, cross-linking network, ice adhesion, stability, self-repairing
1. Introduction
The formation and accumulation of ice on exposed surfaces can result in extensive damage, high maintenance cost, and catastrophic events.[1,2] Therefore, it is important to develop passive icephobic strategies that delay ice nucleation and remove ice effectively without external forces.[3,4] When ice adhesion is low, a strong breeze can blow away the formed ice.[5]
In the past decades, one of the most common icephobic materials is the lotus-inspired superhydrophobic surface (SHS) with liquid-repellency.[4,6–12] Its essence is to entrap air cushions and induce low surface energy in hierarchical micro-nanostructures in the Cassie-Baxter state. Consequently, SHS has a high water contact angle (WCA, > 150˚) and a low contact angle hysteresis (CAH, ≤ 10˚).[13–17] However, several recent studies have shown that SHSs have limitations in icephobic material applications.[18–22] The limitations include weak stability at high pressure, humidity, and impact speed, as well as limited mechanical resistance.[11,18,21,23] The instability causes the wetting transition from the Cassie-Baxter state to the Wenzel state, leading to the increased contact area and ice adhesion compared to a smooth surface. Additionally, SHSs have high production cost, low transparency, and poor self-repairing capacity.[8,11,24,25]
To overcome the limitations of SHSs, Aizenberg’s group proposed an alternative and effective lubricant-infused slippery surface (LISS) with extraordinary non- wettability.[23,26–29] Inspired by nepenthes plants, LISS is a soft matter with a dynamic, molecular-scale smooth liquid interface. The infusion of lubricant can be achieved by roughness and chemical affinity or by electromagnetic interaction. The primary requirement is that the non-volatile lubricant is more susceptible to infusion on the substrate than the immiscible condensate liquid. Differing from the Cassie-Baxter state of SHS, in LISS, the lubricant displaces the air and produces an extremely low CAH.[23,26,30–38] Studies show that the defect-free LISS is effective in frost formation inhibition even in high humidity environments.[39,40]
However, the durability of conventional LISS poses challenges. Reports indicated that the liquid lubricant has a tendency to migrate, deplete or leak over time due to its high mobility.
Therefore, the replenishment of liquid lubricant is required.[11,26,41–45] Wei et al. have found that under high shear conditions, such as 3500 rpm and 1 min, the weight loss of silicone oil at 100 mPa·s viscosity is up to 55%.[27] The improvement of lubricant retention has been the target of many studies. For example, Wong et al. proposed that lubricant with minimal vapour pressure or enhanced viscosity prolonged its lifetime.[26] Vogel et al. showed that close-cell architecture had superior mechanical stability compared to an open-cell micro-pillar sample.[46] Sunny et al. reported that with the increase in surface roughness, a more stable matrix-lubricant interface was produced.[47] Zhang et al.
indicated that grafting a cross-linked network layer of polymer to lock oil extended its lifetime.[41] Liu et al. proposed that the fluorinated hierarchical micro/nano-scaled substrate presented the best icephobicity.[11] However, the aforementioned studies
require two separate steps to lock the lubricant, since the porous solid layer is first formed and then infused with extra lubricant film. A facile one-step strategy to produce LISS with a stable lubricant layer is still rare.
The studies on the fundamentals of the porous LISS also indicate that minimum roughness is required for the maintenance of the lubricant. The liquid lubricant on a smooth surface has been reported to lose the slippery characteristics.[11,36,46,48,49]
Herein, the aim is to enhance the lubricant retention and mechanical durability of LISS for icephobic applications. A coating of a lubricant-elastomer layer was implemented by a simple one-step process. With such a slippery system, icephobic performance on different underlying surface structures and lubricant-elastomer layer thickness will be investigated.
Self-repair by a thermal stimuli-responsive process and transparency of LISS will also be discussed.
2. Materials and Methods
2.1. Fabrication of lubricant-infused slippery surfaces
The process to prepare the LISS is illustrated in Fig. 1. To verify the effect of the underlying structures of LISS on the icephobic performance, smooth polypropylene (SPP), close-cell with micro-pits (CC), and open-cell with micro-pillars (OC) were prepared. The CC and OC used high-aspect-ratio structures on polypropylene (PP) surfaces. The fabrication of ordered and patterned micro-structures has been described in our previous studies.[22,50,51] The SPP surface sample with a 25 mm diameter was fabricated directly from an injection moulding system. The moulding conditions were cylinder temperature of 220˚C,mould temperature of 50˚C, pressure of 600 bar, and mould time of 3 s. The
close-cell structure with regularly arranged barrel-shaped pits on SPP was produced using a micro-working robot with micro-needle. The open-cell structure was fabricated by first using a micro-working robot to make ordered micro-pits on aluminium (Al) foil.
Then, the injection moulding method was employed to replicate and obtain the barrel- shaped micro-pillars on PP. The moulding conditions were cylinder temperature of 265˚C, mould temperature of 70˚C, pressure of 750 bar, and mould time of 3 s.
Fig. 1. (a) Schematic of the fabrication of LISS. (b) Curing effect of PDMS at a molecular level. The scheme is not drawn to scale.
All PP substrates were modified by silanization to enhance the chemical affinity between the PP surfaces and the silicone oil as the lubricant.[48] The first step in silanization is to treat the clean substrates with oxygen (O2) plasma. An O2 gas flow of 20 sccm was used for two minutes under 80 mTorr pressure and 30 W of power.[52] The O -plasma
activated substrates were placed in a desiccator at 70˚C for four hours. An ethanol solution containing 0.5w% of 1H, 1H, 2H, 2H-Perfluorododecyltrichlorosilane (97%, Sigma-Aldrich) was introduced to the desiccator, facilitating silanization by chemical vapour deposition (CVD) to the surface.
Table 1. Abbreviations and descriptions of surface materials.
Materials Description
SPP Smooth polypropylene
CC PP with high-aspect-ratio micro-pits OC PP with high-aspect-ratio micro-pillars
LESPP Lubricant-elastomer infused SPP
LECC Lubricant-elastomer infused CC
LEOC Lubricant-elastomer infused OC
IEOC Impregnated emerging OC
LEL Lubricant-elastomer layer
LICLE solution Lubricant infused cross-linked elastomer solution
Silicone oil with a viscosity of 100 cP from Brookfield was used as a lubricant.
Polydimethylsiloxane (PDMS) from Dow Corning (Sylgard 184) was used to obtain a lubricant-elastomer layer. To achieve the low ice adhesion and good mechanical durability, 40w% PDMS prepolymer and 60w% silicone oil were mixed evenly.[53] After that, a curing agent of PDMS (10:1 by weight) was added. The lubricant-infused cross- linked elastomer (LICLE) solution was homogenized by blending. Finally, different thicknesses of lubricant-elastomer layers (LELs) were introduced by spin coating onto each substrate. During the spin coating, the LICLE solution wetted and filled into the
substrates due to the capillary force and similar chemical affinity. A spin coater from Laurell Technologies Corporation (WS-400-6NPP-LITE model) was used. The specific parameters of the spin coating are given in Table 2. After curing in an oven at 80˚C overnight, slippery surfaces were achieved. The obtained slippery surfaces are the SPP, CC, and OC with LELs, named as LESPP, LECC, and LEOC.
Table 2. Parameters of the spin coating to control the LEL thickness.
300-µm LEL 600-µm LEL
Specimen
Volume Steps Volume Steps
LESPP 200 µL
50 rpm, 20 s 100 rpm, 2 min
350 rpm, 15 s
300 µL
LECC 600 µL 750 µL
LEOC 350 µL
50 rpm, 20 s 100 rpm, 2 min
200 rpm, 40 s
700 µL
50 rpm, 20 s 100 rpm, 2 min
200 rpm, 20 s
In addition, to examine the importance of the complete encapsulation of the LEL on OC, an impregnated emerging OC surface was prepared with the LEL, designated as IEOC.
A smooth reference PDMS (SPDMS) disc was also prepared by curing the PDMS with its curing agent in the oven.
2.2. Surface characterization
Wettability was measured with a contact angle goniometer (KSV Cam 200). A 5 µL boiled water droplet was released at 2 µL/s for the measurement of static water contact angle (WCA, θW). The average WCA was taken from eighteen independent measurements. The
advancing contact angle (ACA, θadv) was determined by increasing the droplet volume from 4 µL to 8 µL. The receding contact angle (RCA, θrec) was determined by decreasing the droplet volume from 8 µL to 4 µL. The CAH is the difference between the ACA and RCA. The dynamic contact angle was calculated from nine independent measurements.
The surface morphology and LEL thickness were examined by scanning electron microscopy (SEM, Hitachi S-4800, Japan). A 3 nm gold layer was deposited with a sputter coater (Cressington 208HR) on the specimens to increase conductivity.
2.3. Optical transmittance
The optical transmittance of specimens was determined with a UV-Vis-NIR spectrometer (Perkin-Elmer Lambda 900) in the spectral range of 400 – 900 nm.
2.4. Ice adhesion shear strength test
Ice adhesion shear strength (τice) was examined with a method described previously.[22]
Briefly, specimens and glass columns with an inner diameter of 18 mm were fixed on a cooling plate under ambient conditions. The average relative humidity of the environment was 60%. One millilitre of tap water was added into the glass column. The temperature of the system was then set to -10˚C. After 30 min of stabilization, another 2 mL of tap water was added to the glass column. The water column with a total of 3 mL was completely frozen at -10˚C in three hours. A force gauge at 1 mm above the surface was used to detach ice columns vertically and to record the maximum shear off force. The fracture modes of the ice columns consisted of visual adhesive, mixed, and cohesive modes. The equation for evaluating τice (kPa) is τice = Fmax A, where Fmax (N) is the peak force during the fracture and A (mm2) is the apparent area of the ice-surface contact.[54]
The averages of τice on the original PP, SPDMS, and IEOC were calculated from 24
independent measurements. The average τice of the LISS was taken from 48 parallel measurements.
2.5. Assessment of dynamic mobility and stability
The dynamic mobility of aqueous droplets on original PP and LISSs was examined at - 10˚C. The sample was tilted at a 16˚ angle. A 100 µL aqueous droplet was dripped on the surface edge 1 cm from the surface. An HD camera with an Exmor CMOS-sensor (CX560, SONY) recorded the dynamic mobility and the sliding time (St). The aqueous droplet was composed of a deionized water solution containing 10-w% synthesized diamond nanoparticles (SDNPs). The SDNPs (4-25 nm) were obtained from Nanostructured &
Amorphous Materials Inc. The relative humidity of the environment was 60%.
To examine the lubricant retention of the LISS, a lubricant shear stability test and a thermal stability test were applied. For the shear stability test, the specimens were spun at 3500 rpm for 1 min with the spin coater. For the thermal stability test, the specimens were heated for 168 hours at 70˚C in an oven. The weight loss (%) of LISS was obtained by measuring the mass of LISS before and after the tests. The average weight loss was calculated from six independent runs. An abrasion test was used to verify the mechanical and wear resistance. The test was performed on the specimens by sandpaper (1000 mesh, Mirka) loaded with a weight of 100 g. Each abrasion cycle included a linear back- and-forth motion on the specimen disc. The total distance of each abrasion cycle was 5 cm. After 5, 10, 20, 40, and 50 cycles, the CAH of specimens was measured. The average CAH after the abrasion cycles were taken from nine parallel measurements.
3. Results and Discussion
3.1. Morphology
The topographies of structured PP surfaces with high-aspect ratios, LISS, and IEOC are shown in Fig. 2. The OC structure consists of a regular array of barrel-shaped micro- pillars, as shown in the top and side views (Fig. 2a). Fig. 2c shows the side geometry of OC where ao is the diameter at the top of the micro-pillar, bo is the diameter at the bottom of the micro-pillar, co is the distance between the upper necks of the two adjacent micro- pillars, do is the distance between the bottoms of the two adjacent micro-pillars, and Ho is the height of the micro-pillar. The dimensions of OC are ao = 30 µm, bo = 75 µm, co = 65 µm, do = 30 µm, and Ho = 120 µm. The pattern of CC is periodically arranged high-aspect- ratio micro-pits. Fig. 2b shows a top view of the CC, with an inset displaying its side view.
The side geometry of CC is presented in Fig. 2c. The diameter ac at the top of the micro-pit is 85 µm. The diameter bc at the bottom of the micro-pit is 40 µm. The distance cc between the tops of the two adjacent micro-pits is 55 µm. The distance dc between thebottom necks of the two adjacent micro-pits is 115 µm. The height Hc of the micro-pit is 350 µm.
Slippery LISSs were obtained by LEL infiltrations with thicknesses of 300 µm and 600 µm on SPP, CC, and OC surfaces. The samples are designated as LESPP-300, LECC-300, LEOC-300, LESPP-600, LECC-600, and LEOC-600, respectively. The verification of LEL thickness was done by SEM, as shown in the side view of LEOC-600 in Fig. 2d. The thickness of the LEL is defined as the distance between the top of the structures and the top of the LEL. Coady et al. have reported that lubricant infused polymer surfaces display wave-like topography.[55] Our LISSs, however, have a smooth topography, as shown in
Fig. 2e for LESPP-600. Fig. 2f reveals the morphology of the top and side views of the IEOC. The impregnated LEL just properly covers the OC pattern, leaving it unexposed.
Fig. 2. SEM images of the surface morphology. (a) OC with micro-pillars, (b) CC with micro-pits, (c) lateral geometries of high-aspect-ratio CC and OC, (d) side view of LEOC- 600, (e) LESPP-600, and (f) IEOC.
3.2. Wettability
Fig. 3 shows the wettability of all specimens at room temperature (RT). All original polymer surfaces are hydrophobic, having a WCA value of 102˚-160˚. The WCA of SPP is 102±2˚. The introduction of high-aspect-ratio micro-pits and micro-pillars increases the hydrophobicity. The WCA of CC increases to 119±4˚. The OC is superhydrophobic with a WCA of 160±3˚. The CAH of OC is 14˚, clearly indicating that the OC is a petal surface and in a Cassie-Wenzel transition state.[15] To examine the effect of a cold environment, we recorded the WCA of smooth and structured PP surfaces at -10˚C as well. There was no obvious change for the WCA of SPP at -10˚C compared to the SPP at room temperature. This shows that cold has little effect on the smooth surface. However, the WCA of CC decreased to 89±3˚. The WCA of OC dropped to 122±8˚, losing its superhydrophobicity. The reduction of WCA of the structured PP surfaces demonstrates that the condensed water penetrates and propagates in the structures. This supports the water condensation phenomenon seen by optical microscopy in previous work.[22]
Fig. 3. (a) Static water contact angle (WCA) and (b) contact angle hysteresis (CAH) on the original polymer surfaces and LISSs at room temperature. Uncertainties were determined by Student’s t-distribution at 95% confidence intervals.
When the impregnated LEL just covered the micro-pillars in OC, the WCA dropped to approximately 111˚. The CAH of IEOC with a value of 31˚ suggests that the surface has a certain roughness and is sticky. It is difficult for the droplets to glide down from the IEOC. The thick LEL was then infused on all of theoriginal PP surfaces, producing the surfaces having thicknesses of 300 µm and 600 µm. The complete encapsulation of the LEL on the PP surfaces dramatically affects the WCA and CAH. The WCA decreased to 105˚-110˚ and all CAHs were below 10˚, even reaching an extremely low value of 2˚.
There was no significant change in the wettability of the LISS with different LEL thicknesses and surface structures. The low CAH confirms the presence of an ultra- smooth surface and homogeneous liquid-liquid interface and the complete elimination of the pinning point.[46]
3.3. Transparency
Fig. 4 shows the UV-vis transmittance spectra of SPP, OC, LEOC-300, SPDMS, and LEOC-300 without silanization. The transmittance of OC is ~ 73% in the visible and near- infrared light regions. The infusion of the 300-µm LEL on the OC (LEOC-300) increases the transmittance to ~ 91.5% at a wavelength of 800 nm. Its transmittance is close to that of SPP. The infusion of the LEL makes the translucent OC transparent. Its mechanism is that the superhydrophobic OC has a solid-air interface that induces irregular reflections and high light scattering. The difference in the optical path on the rough OC increases and thus reduces the transmittance. After the addition of LEL, the LICLE solution fills the air pockets of the OC; the LEL and PP have similar reflection indices. This reduces the Rayleigh scattering and dramatically increases the transmittance.[49,56]
Fig. 4. UV-Vis transmittance spectra of SPP, OC, LEOC-300, LEOC-300 without silanization, and SPDMS.
However, for the fabrication of LEOC-300 without silanization, the transmittance with a value of ~ 87.5% is lower than that of the LEOC-300 at 800 nm. This proves that silanization allows the LICLE solution to completely infuse the structured PP during the fabrication of LISS. The high transmittance of LISS indicates the strong adhesion and no air gap between the LEL and matrix. It also verifies the smooth and homogeneous LISS observed in the SEM images, demonstrating that there is no phase separation on the LISS. The mechanism of silanization is that O2 plasma converts the methyl groups of clean PP substrates into hydroxyl groups.[57] During CVD, it produces strong covalent bonds between silane and hydroxyl groups and enhances the chemical affinity for the LICLE solution.[58]
3.4. Icephobicity
We adopted the ice adhesion shear strength test, which is an approach commonly used to characterize the icephobicity. The fracture mode of ice columns was judged by the naked eye. If no ice debris remained on the surfaces, ice-surface fracture occurred
(adhesive mode). If partial ice debris was observed, an ice-surface and ice-ice fracture (mixed mode) occurred. When the entire section had ice debris, an ice-ice fracture occurred (cohesive mode).[22,54] The actual cross-sectional contact area of the ice- surface was π × 92 mm2.
3.4.1. Mechanism of the LISS
Based on Kendall and Chaudhury’s equation, the large difference in moduli between ice and the soft elastomer causes a mismatch of strain under stress. The mismatch can result in low ice adhesion.[8,33,59,60] To obtain a one-step fabrication of a durable LISS on substrates, a low-surface-energy PDMS elastomer was chosen to lock the silicone oil and act as a reservoir. The liquid linear PDMS and silicone oil were mixed to form a homogeneous LICLE solution. During the curing, linear PDMS formed covalently bridging bonds. Finally, a 3D cross-linking network with rubber-like elasticity was generated to immobilize the silicone oil, as shown in Fig. 1b.[55,59,61]
Van der Waals and/or capillary forces immobilized the LICLE solution on the fluorosilane- modified substrates. There is an affinity between the substrate and solution due to the van der Waals interaction. The structured PP surfaces induce the capillary force.[34] The possible diffusion mechanism of silicone oil in PDMS is that the liquid PDMS and silicone oil are miscible. The free energy, G, of the mixing process is negative (G = H - TS
< 0). The G is positive after the cross-linking of the curing process. The positive G results in demixing of PDMS and silicone oil, allowing the silicone oil to transfer to the surface spontaneously.[62] In addition, silicone oil gradually migrates and performs self- regulated secretion through the continuously trapped micro-droplets formed in the cross-
linked PDMS matrix. The slowly released silicone oil on the surfaces maintains an extremely low CAH and slippery characteristics.[63]
3.4.2. Effect of LISS
Fig. 5 exhibits the τice of the original polymer surfaces and the LISS. The structured PP surfaces with high aspect ratios have the highest ice adhesions of all of the surfaces. The τice of CC is 111 kPa in mixed mode and 311 kPa in cohesive mode. The high ice adhesion is because it is slightly hydrophobic and in a Wenzel state. The complete penetration of water into the structure creates a larger contact area and results in mechanical interlocking. The τice of OC is higher thanthat of the CC. The physical mechanism is that the OC is SHS at RT, but it goes to the Wenzel state because of the water condensation at -10˚C. Therefore, it produces more contact area and greater mechanical interlocking than the CC.[19,21,22] Obviously, the SPP has a lower τice than the structured PP surfaces, which is in line with previous reports.[22,64]
Fig. 5. Ice adhesion shear strength (τice) of original PP surfaces, SPDMS, and LISS.
Uncertainties were determined by Student’s t-distribution at 95% confidence intervals.
The threshold of self-removal of accreted ice is ~55 kPa by a strong breeze.[5] After the addition of LEL, the τice of all LISSs is dramatically reduced. On all LISSs with the LEL, the accreted ice can be removed by a strong breeze. The τice of the LISSs is an order of magnitude lower than on the original PP surfaces, even reaching an ultra-low value of 5 kPa. The LISSs exhibit extraordinary humidity tolerance. The reasons for the low τice
values of skin-like LISS are 1) the molecular homogeneity, ultra-smoothness, and low CAH of the surfaces; 2) the mismatch of the strain between ice and the soft PDMS elastomer matrix; 3) the low surface energy of PDMS and silicone oil as well as the presence of the oil layer significantly reducing the ice-substrate contact area.[8,23,65]
However, for the IEOC that just covered the micro-pillars and had roughness, the τice
reaches up to 54 kPa (Fig. 6a). After three repeated icing/ice-detachment cycles (RIIDC), the τice of IEOC is similar to that of the SPDMS. This phenomenon makes the addition of silicone oil useless for the IEOC. Fig. 6b shows the SEM images of the IEOC after three RIIDC in top and side views. This demonstrates that the thin LEL of IEOC was worn away.
The tips of the micro-pillars are exposed. This demonstrates that it is not advisable to use impregnated emerging surfaces and highlights the importance of complete encapsulation for anti-icing applications.
Fig. 6. (a) Three repeated icing/ice-detachment cycles (RIIDC) of IEOC in adhesive mode.
Uncertainties were determined by Student’s t-distribution at 95% confidence intervals. (b) SEM images of IEOC after three RIIDC.
3.4.3. Effect of surface structuring and LEL thickness
The studies on conventional LISS with two separate steps indicate that the LISS with structures improves and extends the slippery behaviour.[11,36,46,48,49] The close-cell structure has an advantage over the open-cell structure in the immobilization of lubricant.
However, for the LISS containing LEL, whether the underlying structures can effectively improve the icephobic performance remains to be verified. Here, the underlying structures, including SPP, CC, and OC, were used to study icephobic properties. In addition, previous studies on lubricant-infused PDMS focus on the effect of the molecular weight and cross-linking density of PDMS as well as the oil content and viscosity on slippery behaviour.[53,66–68] Here, the LEL thickness was controlled to detect the effect of thickness on the ice adhesion. Optimal silicone oil content and viscosity were directly adopted.[53]
Wang et al. reported that when the thickness of dry PDMS elastomer increased from 18 µm to 533 µm, the τice decreased, being consistent with Kendall’ theory. When the thickness of dry PDMS was 533 µm, the τice was reduced to ~ 120 kPa.[69] Even though the application of low LEL thickness is interesting, the control of the low LEL thickness was difficult due to the strong attraction of the high-aspect-ratio structures to the LICLE solution. Here, based on the study of Wang et al., the 300-µm and 600-µm LELs were used.
Fig. 7 shows the τice of LESPP, LECC, and LEOC with 300-µm and 600-µm LELs, respectively. In the case of a 300-µm LEL, structured PP surfaces with a high aspect ratio reduce the τice compared to that of the SPP. The percentages of reduction are 48% and 24% for OC and CC, respectively. The CC and OC structures for the LISS have no significant difference in the reduction of ice adhesion based on the overlap of standard deviations. However, in the case of a 600-µm LEL, SPP has the lowest ice adhesion. The structured surfaces with high aspect ratios do not reduce the τice in LISS.
Fig. 7. Ice adhesion shear strength (τice) as a function of underlying structures and LEL thickness of the LISS.
The change in the LEL thickness of structured PP has little effect on the τice for the LISS.
This is due to the overlap of τice ranges of different LEL thicknesses. For the SPP on the LISS, the effect of LEL thickness on τice is consistent with the trends of Wang et al. and Beemer et al.[33,69] The ice adhesion decreases as the LEL thickness increases. The LESPP with a 600-µm LEL exhibits an extremely low ice adhesion of ~5 kPa. This is in contrast with previous studies that the lubricant-infused smooth surfaces were sticky, and the accommodation of lubricants required the minimum roughness.[11,36,46,48,49] In addition, the PDMS of LEL is transparent (Fig. 4). The LEL can be coated directly onto smooth surfaces with a simple one-step process, creating a transparent slippery surface and obtaining an ultralow ice adhesion.
3.5. Dynamic mobility, stability, and self-repairing behavior
Fig. 8 indicates that the 100-µL aqueous droplets settle at first on the edge of the original PP surfaces with a 16˚ slope at -10˚C. After 5 min, the droplets still stick to the surfaces.
However, all the LISSs with LELs have outstanding dynamic mobility of aqueous droplets at -10˚C. The sliding times, St, of aqueous droplets with 10-w% SDNPs, as anti-dust indicators, are from 3 s-11 s on the LISSs. (Fig. 9) The LISSs after the aqueous droplets slid were clean, indicating that the surfaces have an anti-dust property. The dynamic mobility behaviour of aqueous droplets containing SDNPs on LESPP-600 is shown in Movie 1. The low St of the LISSs confirms the low CAH of the LISSs.
Fig. 8. Views of the dynamic mobility behavior of 100-µL aqueous droplets on SPP, CC, and OC at a 16˚ slope. The aqueous droplets contain 10-w% synthesized diamond nanoparticles (SDNPs).
In most of the previous studies, the liquid lubricant was directly immobilized by capillary force and chemical affinity. The liquid lubricants easily evaporate, and the surface loses its slippery properties.[41–45,70] The stability of the materials in practical industrial applications is important.[35] For the verification of the shear stability, the weight loss was tested for one minute at 3500 rpm. For most of the LISSs with LELs, the weight loss was less than 3%. (Fig. 9) Even though the weight loss of LESPP-300 reached 10%, its CAH still had a low value of 6±3˚. Low CAH on the surfaces indicates the slippery properties, low ice-matrix contact area, and ice repellency.[71] The shear stability of the LISSs with LELs was found to have an improvement over the previously reported weight loss of 55%.[27] This shows that the LEL significantly improves the shear stability of the LISS.
For the thermal stability, the weight loss of all LISSs with LELs is less than 3% after evaporation for 168 hours at 70˚C. The CAH of LISSs remained below 10˚ after the thermal test. In addition, the LISSs with LELs were still slippery in the ambient environment for more than 360 days. This indicates that the LEL has excellent evaporation resistance.
Fig. 9. Weight loss after the shear stability test (SST) and thermal stability test (TST) as well as the sliding times (St) in the dynamic mobility test.
After 50 abrasion cycles with sandpaper, the CAH values of LESPP-300, LECC-300, LEOC-300, and LESPP-600 were still below 10˚. (Fig. 10a) The τice of the surfaces remained almost unchanged, shown in Fig. 10b. This demonstrates that the LISSs with LELs are robust. The high stability of these LISSs makes it unnecessary to replenish the lubricant after abrasion. The mechanism of mechanical and wear stability is that the silicone oil in the cross-linked PDMS matrix has regenerative properties. Based on the Young-Laplace equation, the pressure difference between the inside of the LEL and air produces a driving force for the regenerative performance. The internal pressure of the LEL (Pi) is greater than the air pressure (Pa). Under Laplace pressure, the micro-droplets discharge to the surface from the continuous silicone oil droplets in the LEL. The surface reaches a metastable state.[8] The equation of Laplace pressure is:
= (1) Pi ― Pa 2γR
Here, γ is the surface tension of silicone oil. R is the mean curvature of the droplet.[72]
After the abrasion, the Laplace pressure of the LEL regenerates the silicone oil to achieve the metastable balance again.
Fig. 10. (a) Contact angle hysteresis (CAH) of LISS after abrasion cycles and self-repair.
(b) Ice adhesion shear strength (τice) of LISSs with robust mechanical stability after abrasion cycles. (c) SEM image of LEOC-600 after 10 abrasion cycles. (d) SEM image of LEOC-600 after self-repair.
However, the CAH values of LECC-600 and LEOC-600 exceeded 10˚ even after 5 abrasion cycles. (Fig. 10a) The reason for the large CAH of the LECC-600 and LEOC- 600 may be that the underlying surfaces have high-aspect-ratio structures. The attraction between silicone oil and structures was stronger than oil molecule cohesion.[73]
Therefore, the oil molecules sunk into the structures and fewer oil molecules were distributed on the surfaces. The increase in CAH indicates the loss of lubricant or damage
to the ultra-smooth surface. The damage to the LEOC-600 surface was visualized with an SEM, shown in Fig. 10c. After 10 abrasion cycles, some scratches are seen on the LEOC-600 surface.
The damaged LECC-600 regains water repellency after 30 min under 80˚C with a CAH of 8±2˚. The damaged LEOC-600 recovers the slippery characteristics after 5 min under 80˚C as well with a CAH of 6±4˚. This demonstrates that after the physical damage of the LISS with an LEL, it will self-repair under the thermal stimulus. This self-repairing effect was visualized through the SEM, shown in Fig. 10d. After the self-repair, the LEOC-600 surface healed the scratches and restored smoothness. The mechanism is that the trapped continuous droplets in the cross-linked PDMS matrix will gradually migrate and refill the damaged area. The process isstimulated by temperature. The liquid lubricant as an adaptive component induces the dynamic feedback behaviour.[35,63] The self-repair behaviour is the redistribution of lubricants to the damaged areas and recovery of the slippery property.
4. Conclusions
We have developed ultra-smooth and durable lubricant-infused slippery surfaces (LISSs) with a simple one-step production method. The surfaces were completely encapsulated by the lubricant-elastomer layers (LEL) consisting of low-surface energy PDMS elastomer and silicone oil. Compared to SHS and smooth surface, the LISS with an LEL has significant advantages. It not only has exceptional anti-dust properties and low ice adhesion at -10˚C, but is also transparent. The LELs of LISSs have very low CAH values and reduces the ice adhesion by an order of magnitude. The LISSs also had outstanding
shear stability, evaporation resistance, and mechanical stability. Physically damaged LISSs self-repaired under thermal stimulus (80˚C).
The structured PP surfaces can reduce ice adhesion by 24-48% compared to that of the smooth PP only with a 300-µm LEL. The close-cell and open-cell structures on the LISS with an LEL have no significant effect on the reduction of ice adhesion. As the LEL thickness increases, the structured PP surfaces under the LISS no longer give an advantage. For the smooth PP with an LEL, as its LEL thickness increases, the ice adhesion decreases. The smooth PP with a 600-µm LEL exhibits an ultra-low ice adhesion of 5 kPa, demonstrating that roughness is not necessary for maintaining slippery properties. The strategy to produce extremely low ice adhesion on smooth materials provides excellent stability, transparency, anti-dusting, and self-reparability.
The one-step production method is simple, inexpensive, and fully scalable on a variety of smooth materials.
Acknowledgements
Financial support from the University of Eastern Finland is gratefully acknowledged. We thank Prof. Mika Suvanto for useful discussions, and Dr. Leila Alvila, Dr. Lena Ammosova, and Dr. Kati Mielonen for practical help.
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Highlights
LISSs have an order of magnitude lower ice adhesion than smooth and superhydrophobic surfaces.
LISSs is very stable, transparent, anti-dusting, and self-repairing.
The effects of structuring and thickness of the LEL in LISSs on ice adhesion were investigated.
Roughness for maintaining the icephobicity is not necessary when there is an LEL.
The smooth PP with a thick LEL has a very low ice adhesion.
Graphical abstract
