3.2.1. Response to UV Irradiation

All fabricated SLIPS were exposed to UV radiation and their wetting behavior was evaluated after 2, 4, 6, and 8 h of irradiation. However, as the changes in contact angles were limited, only the values measured after 8 h are reported for brevity. Figure 2 compares the CAH values of the fabricated SLIPS before and after UV irradiation.

**Figure 2.** Contact angle hysteresis with (**a**) water (CAHW) and (**b**) n-hexadecane drops (CAHH) for SLIPS samples before (blue) and after UV irradiation (red) for 8 h. Standard deviations are reported as error bars.

Krytox-infused SLIPS had almost constant CAH values with both liquids, thus displaying excellent resistance to prolonged UV exposure. These liquids have limited UV absorption [30], therefore they do not undergo radiolysis. Moreover, even though the actual values of vapor pressure for commercial Krytox lubricants are unknown, PFPE usually possesses extremely low vapor pressure, in the order of 10−<sup>8</sup> Torr at 20 ◦C [31].

On the other hand, hexadecane-infused SLIPS behaved differently. After 6 h, contact angle hysteresis with water (CAHW) increased from 2.6◦ to 3.8◦; then, after 8 h, it further increased to 7.6◦ along with standard error (from 0.2◦ to 3.6◦). Both phenomena suggest the formation of defects on the SLIPS, probably due to evaporation of the infused oil which has significant vapor pressure at room temperature (1.4 × <sup>10</sup>−<sup>4</sup> Torr) [32]. The underlying surface might have been exposed, acting as a surface defect point that caused the increase in CAHW. These results suggest that hexadecane-infused SLIPS are not ideal candidates for applications in which the surface is exposed to air for prolonged time, due to the high evaporation rate of the lubricant. Notably, hexadecane is the least volatile among alkanes that are liquid at room temperature; therefore, the same considerations apply to the lower homologues of hexadecane (i.e., tetradecane, dodecane, decane, etc.). On the contrary, PFPE-based SLIPS showed remarkable stability when exposed to UV and represent a good choice for application on surfaces that remain often dry.

### 3.2.2. Response to Chemical Ageing

In order to evaluate their resistance to chemically aggressive environments, SLIPS samples were immersed in either acidic, alkaline, or saline solutions and their wetting properties were evaluated after different immersion times. Figure 3 recaps the behavior of SLIPS samples in terms of CAH with water (left-hand graph) and n-hexadecane drops (right-hand graph).

In both cases, K100 samples proved remarkably less durable than other Krytoxinfused samples. Despite their low initial CAHW and CAHH values, after only 3 days of immersion in the acidic solution their amphiphobicity was lost, with CAHW = 89◦ and CAHH = 41◦. After 14 days of immersion, RCA values with both liquids were rapidly decreasing, therefore data acquisition was interrupted. This behavior indicates that the infused Krytox 100 oil was removed by the acidic solution, thus exposing the underlying coating and degrading it as suggested by the low receding contact angle values with water (RCAW = 39◦) and hexadecane (RCAH = 19◦). On the contrary, the other Krytox-infused SLIPS showed significant durability in the acidic environment. Notably, increasing the viscosity of the infused liquid led to improved retention of the amphiphobic behavior, with K107 sample being the most stable SLIPS. It is worth to highlight the different trends of CAH with water and n-hexadecane. The former increases with time, probably due to the formation of polar -OH hydroxyl groups in PFPE chains which increase the interaction with water molecules via hydrogen bonding. To confirm this hypothesis, we performed FTIR

analysis on the fabricated surfaces. First, the spectrum of each surface was obtained prior to immersion in the acidic solution, then it was acquired after 7 and 60 days of immersion. Figure 4 reports the spectra obtained for K107 as an example.

**Figure 3.** Contact angle hysteresis with (**a**) water (CAHW) and (**b**) n-hexadecane drops (CAHH) for SLIPS samples immersed in an acidic solution (pH = 3) as a function of immersion time t: K100 (black), K103 (blue), K105 (red), K107 (green), and HEX (orange). Standard deviations are reported as error bars. Inset: examples of frames for the measurement of RCAW on K107 (top) and HEX samples (bottom) after 7 days of immersion.

**Figure 4.** (**a**) Fourier Transform Infrared spectra (FTIR) of K107 sample before (red) and after immersion in acidic solution for 7 (blue) and 60 days (green). (**b**) Detail of the spectra in the 3800–2600 cm−<sup>1</sup> range. The positions of the most relevant peaks are reported.

After 7 days of immersion, a broad band centered at 3309 cm−<sup>1</sup> appeared; it can be assigned to the stretching vibration of -OH groups formed on polyether chains [33,34]. Notably, after 60 days the intensity of the band increased, as highlighted in the inset spectra, suggesting that further hydroxylation of the chain had occurred. Fourier Transform IR (FTIR) spectra for all Krytox-infused surfaces after 7 days are compared in Figure 5.

**Figure 5.** (**a**) Fourier Transform Infrared spectra (FTIR) of K100 (red), K103 (blue), K105 (green), and K107 samples (purple) after immersion in acidic solution for 7 days. (**b**) Detail of the spectra in the 3800–2600 cm−<sup>1</sup> range.

Focusing on the -OH stretching band (Figure 5b), it is clear that K100 suffered the highest degree of hydroxylation, followed by K103, while K105 and K107 showed similar signal. It was not possible to compare FTIR spectra after more prolonged immersion time due to the severe degradation of K100 samples.

On the other hand, CAHH did not change significantly, probably because the aforementioned polar -OH groups do not interact with non-polar hexadecane molecules, thus they do not affect contact angle values. Moreover, the different size of water and n-hexadecane molecules can contribute to explain this phenomenon. According to Wang et al. [35], small water molecules can penetrate the damaged PFPE network, resulting into pinning phenomena and increased CAHW; on the other hand, large n-hexadecane molecules cannot do the same and CAHH remains substantially unaltered.

Regarding HEX samples, they showed a quick increase in CAHW, probably due to n-hexadecane displacement and degradation of the underlying coating as observed for K100. In fact, n-hexadecane is even less viscous than Krytox 100 and can be displaced by water shortly after immersion.

Similar CAH trends were observed for SLIPS immersed in alkaline solution, as displayed in Figure 6.

**Figure 6.** Contact angle hysteresis with (**a**) water (CAHW) and (**b**) n-hexadecane drops (CAHH) for SLIPS samples immersed in a alkaline solution (pH = 11) as a function of immersion time t: K100 (black), K103 (blue), K105 (red), K107 (green), and HEX (orange). Standard deviations are reported as error bars. Inset: examples of frames for the measurement of RCAW on K107 (top) and HEX samples (bottom) after 7 days of immersion.

K100 and HEX samples showed quick degradation of their liquid-repellent behavior, with both CAH values rapidly increasing in few days. The unexpected drop in CAH for K100 after 7–14 days is due to the fact that ACA decreases more than RCA in that period. For other PFPE-infused SLIPS, the increase in CAHW was more evident than in the acidic environment: CAHW value for the K103 sample reached 77◦ after 45 days. Increasing lubricant viscosity led to smaller increase in CAHW, with K107 displaying the best value at 45◦ after 60 days. As described before for the immersion in acidic solution, CAHH increased less than CAHW, also, in alkaline conditions, and the same trend with viscosity was observed, as CAHH for K107 remained constant after 60 days. FTIR spectra were collected after 7 days (Figure 7).

**Figure 7.** Fourier Transform Infrared spectra (FTIR) of K100 (red), K103 (blue), K105 (green), and K107 samples (purple) after immersion in alkaline solution for 7 days.

FTIR spectra confirm that less viscous PFPE oils are more prone to degradation than the more viscous ones: K100 gave no signal, suggesting a complete loss of PFPE; on the other hand, increasing oil viscosity led to more intense signal below 1400 cm<sup>−</sup>1, with K107 being unaltered (see Figure 4 for comparison). From these results, it seems clear that PFPE is more susceptible to alkaline environments than to acidic ones, as already reported in the literature [36]. Indeed, polyether chains are intrinsically hydrophilic and prone to hydrolysis in alkaline conditions [37]; probably, substitution of the polymer backbone with fluorine atoms might only temporarily delay hydrolysis. Judging from FTIR spectra, low-viscosity PFPE oils are totally depleted in alkaline conditions, while in acidic solution they, rather, seem to be hydroxylated but not removed.

The immersion tests in saline solution showed similar trends to those observed for the alkaline conditions (Figure 8).

Once again, K100 and HEX quickly lost their amphiphobicity, with steep increase in CAH after only 3 days; the decrease in CAHH for K100 was due to the drop in ACAW. Meanwhile, K103 lost its amphiphobicity more gradually, stabilizing its CAH values after 28–45 days; on the other hand, K105 and K107 retained their wetting properties for the entire testing period. Especially K107 had its CAH values almost completely unaltered after 60 days of immersion in the saline solution. FTIR spectra after 7 days (not reported for brevity) were compared also for these samples; as observed in alkaline solution, lubricant depletion increased with decreasing PFPE viscosity. Such result is remarkable in perspective of an application of these coatings in marine environment and in corrosive conditions in general; the retention of the amphiphobic behavior indirectly suggests anti-corrosion properties for K107 coatings. The anti-corrosion properties of SLIPS have already been reported [38,39], but never in such harsh conditions as usually mild NaCl solutions or seawater are used; in our tests, NaCl concentration was almost three times larger than in seawater, therefore corrosion is expected to be more severe.

**Figure 8.** Contact angle hysteresis with (**a**) water (CAHW) and (**b**) n-hexadecane drops (CAHH) for SLIPS samples immersed in a saline solution (NaCl 100 g L−1) as a function of immersion time t: K100 (black), K103 (blue), K105 (red), K107 (green), and HEX (orange). Standard deviations are reported as error bars. Inset: examples of frames for the measurement of RCAW on K107 (top) and HEX samples (bottom) after 7 days of immersion.

### 3.2.3. Response to Abrasion

Mechanical stresses are the most common cause of performance loss in liquid-repellent coatings; therefore, it is necessary to address their response to such stresses in perspective of future applications. We chose to perform abrasion tests as per the UNI EN 1096-2 standard because it is widely applied on coatings for the building industry; moreover, this test applies compression and shear stress on the surface contemporarily, thus effectively simulating complex operational conditions. CAH values for the tested SLIPS before and after abrasion tests are reported in Figure 9.

**Figure 9.** Contact angle hysteresis with (**a**) water (CAHW) and (**b**) n-hexadecane drops (CAHH) for SLIPS samples before (blue) and after abrasion (red) as per the UNI EN 1096-2 standard. Standard deviations are reported as error bars.

Among PFPE-infused SLIPS, a relationship between lubricant viscosity and increase in CAHW was observed (Figure 10): the sample infused with the least viscous oil (K100) showed the most significant increase in CAH values, eventually losing its amphiphobicity.

**Figure 10.** Increase in contact angle hysteresis with water (ΔCAHW) as a function of oil viscosity for PFPE-infused SLIPS. Standard deviations are reported as error bars.

With increasing lubricant viscosity, the increase in CAH (especially with water) became less evident, with K105 and K107 samples retaining their amphiphobicity after the abrasion tests. In the past years, several papers [40,41] investigated the response of PFPE oils under abrasion because of their application in hard disk drives; it was demonstrated that, in such conditions, these materials can be involved in tribochemical degradation reactions, which can be significantly catalyzed by Lewis acids like Al2O3 [42]. The most important degradation mechanisms include triboelectrical reactions (with creation of radical species) and mechanical cleavage, due to the friction between PTFE and solid surface asperities [43]. The degradation rate depends on the length of polymer chains: short macromolecules like those in K100 have higher mobility (i.e., lower viscosity) which lead to higher degradation rates. Increasing the chain length can slow down degradation reactions (especially mechanical cleavage) [44], thus explaining the retention of amphiphobic properties of K105 and K107. It is also necessary to consider that abrasion tests cause friction and related increase in surface temperature; Krytox 100 is more susceptible to temperature increase than higher Krytox oils, with obvious negative effect on the amphiphobicity of K100.

On the other hand, HEX samples showed limited increase in *CAHW* after the test, although n-hexadecane has lower viscosity than Krytox 100. These results can be explained considering that the C-C bonds in the n-hexadecane molecule are less prone to mechanical cleavage than the C-O bonds in PFPE polymer chain [45]; therefore, HEX SLIPS are less prone than K100 ones to mechanical degradation caused by abrasion.
