Ultra-Slippery Hydrophilic Surfaces by Hybrid Monolayers

Abstract

Slippery solid surfaces with low droplet contact angle hysteresis (CAH) are crucial for applications in thermal management, energy harvesting, and environmental remediation. Traditionally, reducing CAH has been achieved by enhancing surface homogeneity. This work challenges this conventional approach by developing slippery yet hydrophilic surfaces through hybrid monolayers composed of hydrophilic polyethylene glycol (PEG)-silane and hydrophobic alkyl-silane molecules. These hybrid surfaces exhibited exceptionally low CAH (<2°), outperforming well-established homogeneous slippery surfaces. Molecular structural analyses suggested that the remarkable slipperiness is due to a unique spatially staggered molecular configuration, where longer PEG chains shield shorter alkyl chains, thus creating additional free volume while ensuring surface coverage. This was supported by the observation of decreased CAH with increasing temperature, highlighting the role of grafted chain mobility in enhancing slipperiness by self-smoothing and fluid-like behaviors. Furthermore, condensation experiments demonstrated the exceptional performance of the hydrophilic slippery surfaces in dew harvesting due to superior condensation nucleation, droplet coalescence, and self-sweeping efficiency. These findings offer a novel paradigm for designing advanced slippery surfaces and provide valuable insights into the molecular mechanisms governing dynamic wetting.

Introduction

Slippery or liquid-repellent surfaces¹ are characterized by low contact angle hysteresis (CAH) and have garnered significant interest owing to their diverse applications including self-cleaning,² liquid transport,³ antifouling,⁴ multiphase separations,⁵ and microfluidics.⁶ Existing bioinspired slippery surfaces, including air-mediated superhydrophobic surfaces (SHPS)⁷ and lubricant-infused porous surfaces (SLIPS),⁸ exhibit remarkable liquid repellent and droplet slip performances. Generally, SHPS offer advantages in antifouling and frost resistance, while SLIPS excel in extreme slipperiness and surface tunability. However, SLIPS face challenges related to lubricant evaporation and depletion, which can limit their long-term performance. Moreover, SHPS and SLIPS are typically hydrophobic due to the incorporation of air or lubricant, limiting their range of applications owing to their restricted droplet nucleation capabilities. Recently, the demand for hydrophilic and slippery surfaces has been increasing, particularly for water and energy systems that require both favorable nucleation and easy droplet removal.⁹

Liquid-like surfaces (LLS) have emerged as a novel surface modification technique involving the covalent grafting of highly flexible polymer molecules onto smooth solid surfaces.¹⁰ The flexibility of these grafted species imparts a liquid-like behavior to the coating, resulting in an extremely low CAH, whereas the surface-tethered property ensures stability and durability.¹¹ Unlike SHPS and SLIPS, LLS are free of texture and lubricant, eliminating the risk of texture damage or lubricant depletion.¹² Additionally, the liquid-like character of polymer molecules, which is determined by chain mobility rather than surface energy, enables tunable wettability, facilitating either hydrophobicity [e.g., polydimethylsiloxane (PDMS)]¹³ or hydrophilicity [e.g., polyethylene glycol (PEG)],¹⁴ thus expanding their potential applications. Numerous studies have documented flexible polymers with slippery properties, including PDMS, PEG, and perfluoropolyethers (PFPE).¹⁵

Given surface inhomogeneity is well-known to contribute to CAH, research about slippery LLS has primarily focused on homogeneous monolayer polymers. Recognizing the crucial role of chain mobility in LLS, strategies such as rotation-free molecular structures and low grafting densities have been fundamental in enhancing slipperiness.¹ In chemical engineering, another common approach to improve polymer chain mobility involves the use of “plasticizers”, which are additives that increase flexibility and reduce viscosity by expanding the distance between polymer chains.¹⁶ Inspired by this concept, hybrid surfaces can be designed to enhance the chain mobility and reduce CAH by incorporating small spacer molecules. This approach offers a new dimension in surface design for further improving slipperiness.

This study demonstrates an unexpected reduction in CAH for hybrid monolayer surfaces achieved through the cografting of hydrophilic PEG and hydrophobic alkyl monolayers. These hybrid surfaces exhibited lower CAH compared to surfaces grafted solely with PEG monolayer. X-ray photoelectron spectroscopy (XPS) was utilized to confirm the presence of both components in the hybrid layer, while p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) demonstrated the liquid-like nature of the grafted molecules. Furthermore, the observed decrease in CAH with increasing temperature supports the “fluid-like” hypothesis, suggesting that chain mobility is crucial for achieving low CAH through self-smoothing and rotational dynamics. The condensation experiment demonstrated the superior advantages of hydrophilic slippery surfaces in dew harvesting applications. This study introduces a novel molecular-scale surface design scheme that creates hydrophilic (∼33°) yet ultra-slippery (CAH < 2°) surfaces, with promising applications in fields such as biofluidics¹⁷ and thermal engineering.¹⁸ Moreover, it provides valuable experimental insights into the mechanisms of macroscale dynamic wetting arising from molecular-scale phenomena.

Results and Discussion

Chemical grafting of polymers is a common technique for controlling surface wettability and achieving slipperiness. Although it is widely acknowledged that surfaces containing hybrid components always exhibit high CAH due to surface inhomogeneity,¹⁹ this research challenges that notion by highlighting the other side of the coin—showing how hybrid components can reduce hysteresis through enhanced polymer chain mobility in hybrid monolayers.

Hybrid Slippery Surface Achieved by Co-Grafting

We achieved the hybrid slippery surface by cografting hydrophilic PEG-silane and hydrophobic alkyl-silane molecules onto silicon substrates, creating a PEG-based hybrid hydrophilic and slippery surface. Both types of molecules are trimethylsiloxane-terminated (−Si(−O–CH₃)₃) for bonding with the silicon substrate and facilitating the cografting processes (see Figure 1A; additional details are in Methods and Supporting Information). The influence of mixing PEG-silane and alkyl-silane components on the slippery properties of hybrid monolayers was first investigated. Hybrid monolayers were fabricated by cografting PEG-silane with various alkyl-silane chains (chain lengths: 3, 8, 12, 16, and 18 units; see Figure S1 and Table S1 in Supporting Information). Most combinations of hybrid surfaces exhibited increased CAH compared to homogeneous PEG (hereafter indicated as homo-PEG) monolayers, due to increased surface inhomogeneity. However, a surprising result emerged when longer PEG-silane (units 21–25, indicated as Si-PEG23) was cografted with a longer alkyl-silane (octadecyltrimethoxysilane, chain length of 18 carbon units, indicated as Si-C18) in a controlled mixing molar ratio (shown in Figure 1B). When the mixing ratio of PEG-silane in solution was less than 50%, the contact angle of hybrid surfaces rapidly shifted from hydrophobic (as seen in alkyl-silane monolayers) to a hydrophilic range. On these hybrid surfaces, the hydrophobic alkyl-silane components acted as defects on the hydrophilic surfaces, increasing the CAH. As the PEG ratio increased, the receding contact angle (RCA) of the hybrid surface continuously decreased until it stabilized at approximately 30°. This insensitivity of the RCA to the mixing ratio aligns with the general behavior of hydrophilic surfaces that contain varying amounts of hydrophobic defects.²⁰ The advancing contact angle (ACA) decreased more slowly than the RCA and did not reach a steady state. It reached a minimum at a PEG-silane mixing ratio of approximately 50% and then slightly increased until the PEG ratio reached 100%. Therefore, the minimum CAH was observed at a PEG-silane mixing ratio of approximately 50%, exhibiting a remarkably low hysteresis (CAH = 1.7 ± 1.2°), surpassing even homo-PEG monolayers (CAH of approximately 3–7°). The actual grafting ratio of PEG on the surface may differ from the mixing ratio in the solution due to variations in the adsorption and reaction rates of the components with the substrate. This will be analyzed in detail in the next section. The PEG-based hybrid hydrophilic and slippery surface, which achieve exceptionally low CAH, will hereafter be referred to as PHHS (droplet sliding pictures are shown in Figure 1C and Movie S1 in Supporting Information).

Figure 1.

Slipperiness of hybrid surfaces achieved by cografting. (A) Fabrication procedures for hybrid surfaces involve mixing PEG-silane and alkyl-silane in controlled ratios, followed by cografting onto a clean silicon substrate. (B) ACA, RCA, and CAH measurements from surfaces fabricated with different mixing ratios of Si-PEG23 and Si-C18. Note that the mixing ratio may differ from the actual surface grafting ratio. (C) Sliding behavior of a 10 μL water droplet on a PHHS surface with different tilt angle (TA) of the substrate. The scale bar in the images represents 2 mm. (D) Dependence of droplet volume on the sliding angle for three different coating. (E) Summary of typical slippery surfaces, with the left–down direction indicating advantages in hydrophilicity and slippery.

To demonstrate the applicability of PHHS coating, we tested various substrates that can be activated with surface hydroxyl groups for silanization, including silicon, sapphire, and glass. All substrates underwent identical pretreatment, including sonication cleaning, oxygen plasma treatment, and coating in the mixed silane solution. All three substrates exhibited similar trends in CAH depending on the mixing ratio of PEG-silane. The minimum CAH observed at a PEG-silane mixing ratio of around 50% for silicon, glass, and sapphire were approximately 1.7, 2.2, and 4.5°, respectively, surpassing the slippery behavior of homo-PEG monolayer coatings on these substrates, which had CAH values of approximately 4.8, 5.9, and 10.0°, respectively (shown in Figure S2 in Supporting Information). This similarity in mixing behavior is likely due to the comparable hydroxyl group densities on the substrates after oxygen plasma treatment and their undergoing similar silanization procedures. The slight differences across substrates are likely due to the substrate quality, such as defects and roughness. However, our PHHS monolayer coating may not be applicable to all types of substrates, such as metals and certain polymers (shown in Figure S3 in Supporting Information). The HCl catalyst in our recipe can corrode most metal substrate and compromise the oxide layer, and also dense hydroxyl sites and subnanometer smoothness are necessary to achieve optimal slipperiness. Therefore, in this study, we focus on the performance of PHHS coatings on silicon substrates to further explore and understand their slippery behavior.

The exceptionally low CAH of PHHS surfaces on silicon was confirmed through droplet sliding experiments. When a water droplet is placed on a gradually tilted surface, the downslope gravitational force increases until it overcomes the surface resistance, causing the droplet to slide. The critical tilt angle at which sliding occurs, known as the sliding angle, serves as a general measure of surface slipperiness. A lower angle indicates a more slippery surface. As shown in Figure 1D, water droplets with a volume of 10 μL exhibited significantly lower sliding angles on PHHS (3.8°) compared to homo-PEG surfaces (11.0°). Notably, droplet size can strongly influence the sliding angle. For a droplet with volume V, the force balance during sliding on a tilted plane with angle α can be described as

1

where ρ denotes the liquid density, g denotes the gravity acceleration, a denotes the sectional length of droplet, γ denotes the surface tension of the liquid, and θ_a and θ_r denote the ACA and RCA, respectively. For a given liquid and contact angle, a geometric relationship reveals that droplet volume is proportional to the cube of the droplet contact length. Combined with eq 1, it results in an inverse relationship with droplet size: sin α ∝ V^–2/3 for sliding angle α. This equation indicates that smaller droplets have larger sliding angles and may even be unable to slide on vertical surfaces. As demonstrated with 1 μL droplets, the PHHS can still facilitate sliding at a tilt of approximately 47°, whereas homo-PEG surfaces, renowned for their exceptional slipperiness, inhibit spontaneous sliding even at a vertical orientation (90° tilt).

A wide range of slippery surfaces have been developed, including SHPS inspired by lotus leaves,^21,22 SLIPS inspired by pitcher plants,⁹ and LLS.^11,13,22 However, these slippery surfaces often rely on minimizing water–solid interactions through the introduction of hydrophobic media such as oil or air, resulting in a trade-off between slipperiness and hydrophilicity. Here, we introduced PHHS surfaces, which exhibit superior performance in both metrics. This unique combination offers significant advantages for applications such as water condensation and harvesting. First, surface hydrophilicity provides a larger number of nucleation sites compared to hydrophobic surfaces during condensation.²³ Additionally, the slippery nature of the surface facilitates easier droplet coalescence and removal as droplet volume increases over time. Figure 1E summarizes the reported contact angle and CAH values of various slippery surfaces, highlighting the potential superior performance of PHHS surfaces in terms of both nucleation favorability and ease of slipperiness.

Molecular Configuration of the Hybrid Surface

Common knowledge suggests that hybrid monolayers typically decrease surface slipperiness by introducing physical or chemical inhomogeneity.¹⁹ Understanding the exceptionally low CAH of PHHS surfaces requires elucidating the molecular configuration of the hybrid monolayers. A crucial aspect is determining the actual surface grafting ratio of PEG-silane to alkyl-silane on the monolayer surfaces, which may differ from the initial mixing ratio due to variations in substrate affinity and adsorption kinetics. XPS was employed to provide insights into the elemental composition and chemical state of the thin layers. Carbon peaks were detected from all surfaces made with Si-PEG23 and Si-C18 at different mixing ratios. After charge correction, the C 1s peak (corresponding to the 1s core level electrons of carbon atoms) was deconvoluted into three peaks at 284.8, 286.5, and 288.4 eV, corresponding to C–C/C–H, C–O, and C=O components, respectively (shown in Figure 2A). The C–C component was observed to dominate the C peak on the pure alkyl-silane surface, whereas the C–O component dominates the C peak on the pure PEG-silane surface. The relative abundance of each component was determined by calculating the ratio of the corresponding subpeak area to the total C 1s peak area. These ratios were used to estimate the PEG grafting ratio on the hybrid surfaces based on a linear combination principle, with 100% PEG content for pure PEG surfaces and 0% PEG content for pure alkyl-silane surfaces. This analysis was performed for both C–C and C–O peaks, and the average value was used to represent the PEG content in the hybrid surface (see Figure 2B, with additional data shown in Table S2 in Supporting Information). The results showed that the hybrid surfaces fabricated with a PEG mixing ratio of 50% in the chemical solution actually contained approximately 86% PEG. This discrepancy can be attributed to the faster adsorption and reaction rates of PEG-silane molecules on the clean silicon substrate. Experiments involved exposing the substrate to the silane solutions for increasing time intervals. For a solution concentration of 0.5 mM/L, the silicon substrate grafted with Si-C18 reached saturation (indicated by a constant contact angle) after approximately 50 min, while the substrate grafted with Si-PEG23 reached saturation in only 20 min (see Figure S4 in Supporting Information).

Figure 2.

Composition and thickness of hybrid surfaces created using Si-PEG23 and Si-C18 with varying mixing ratios. (A) XPS peaks of C 1s and the deconvoluted peaks for hybrid surfaces. (B) Component ratios of C 1s subpeaks and calculated grafting ratios in the hybrid surfaces. (C) Thickness of hybrid surfaces with different PEG grafting ratios. The thickness was measured by ellipsometry, and the surface grafting ratios were estimated from XPS results.

As indicated by the estimated surface grafting ratios, the PHHS surfaces are predominantly composed of PEG molecules, which explain the surface hydrophilicity. To further understand the surface configuration, the thickness of the hybrid surfaces was measured using ellipsometer. The average thickness of the hybrid monolayers increased with the grafting ratio of PEG-silane (shown in Figure 2C). A densely packed monolayer of Si-C18 exhibits a thickness of approximately 1.9 nm, which aligns with the theoretical length of a Si-C18 chain (approximately 2.2 nm) tilted at an angle of approximately 70°. In contrast, the measured thickness of a homo-PEG monolayer or a PEG-dominant hybrid surface was approximately 2.5 nm. Although this result is consistent with previously reported experimental results,^24,25 it is significantly lower than the theoretical length of Si-PEG23, estimated to be approximately 10 nm.²⁶ This discrepancy can be attributed to two main factors: first, the homo-PEG monolayer and hybrid PEG monolayer may be loosely packed, causing the ellipsometer to underestimate the physical height by using bulk PEG parameters. Second, the flexible nature of the PEG chain enables easy fold and entanglement, resulting in a smaller effective height compared to a straight configuration.

To verify the molecular state and orientation of the grafted chains,²⁷ we used the pMAIRS technique. This method surpasses traditional infrared (IR) spectroscopy by enabling the simultaneous extraction of both in-plane (IP) and out-of-plane (OP) vibrational components from a single beam spectrum collected at various incident angles. Representative pMAIRS spectra for alkyl-silane, PEG, and hybrid PEG samples are shown in Figure 3. The C–H stretching vibrations of the alkyl-silane monolayer appear as absorption bands at around 2860 and 2970 cm^–1. In contrast, the PEG and hybrid PEG spectra display broad peaks around the similar wavenumber range, indicative of their amorphous nature. The orientation angle (ϕ) of functional groups within the grafted chains was calculated using the dichroic ratio (A_IP/A_OP) as follows²⁸

2

Figure 3.

Molecular topography of monolayer surfaces. pMAIRS spectroscopy, schematic illustration of molecular configuration, and atomic force microscopy (AFM) topography of the surfaces: (A) alkyl-silane monolayer, (B) homo-PEG monolayer, (C) nonslippery hybrid surface (PEG grafting ratio around 69%), and (D) PHHS surface (PEG grafting ratio around 86%). The “RMS” in the AFM images indicate the root mean squared roughness of each surface.

The anisotropic peak signals of the alkyl-silane monolayer reveal a chain orientation angle of approximately 68.2°, indicating a more ordered arrangement consistent with the thickness estimation. In contrast, the isotropic and wide peak signals observed for PEG yield a “magic angle” orientation of 54.7° obtained by equating A_IP and A_OP in eq 2, indicating an average of the random and flexible chain conformation.²⁸ This is consistent with the overlap of in-plane (IP) and out-of-plane (OP) bands across the measured spectral range. The inherent flexibility and entanglement of PEG chains provide direct evidence for the “liquid-like” nature of both pure PEG and hybrid PEG monolayers.

The extended length, high flexibility, and loose packing of PEG chains in the PHHS surfaces suggest a unique spatially staggered molecular configuration. This configuration likely positions PEG chains at the outermost surface, creating a water-facing layer that shields the underlying alkyl chains (see Figure 3). The arrangement can explain the observed high slipperiness of the PHHS surfaces from the perspective of “topmost homogeneity” despite the “underlying heterogeneity” in the monolayer composition. A key question remains whether the inserted alkyl-silane molecules can be effectively shielded by the topmost PEG layer. To examine this hypothesis, we estimated the packing or grafting density of the surfaces using the equation σ = (hρN_A)/M, where h is polymer layer thickness, ρ is polymer density, N_A is Avogadro’s number, and M is the molecular weight of the polymer.

Our calculations revealed a grafting density of approximately 3.0 molecules/nm² for the pure alkyl monolayer and 1.3 molecules/nm² for the pure PEG monolayer. The hybrid slippery surface exhibited a density of approximately 1.35 molecules/nm², with an estimated composition of 1.1 molecules/nm² for the PEG-silane component and 0.25 molecules/nm² for the alkyl-silane component based on averaged molecular weight. A more indicative parameter considering the configuration of different molecules is the dimensionless grafting density, which is defined as considering hexagonal closest packing.²⁴ Here, R_g is the radius of gyration, which describes the typical size of the polymer chain. Using Flory’s approach, we calculated the radius of gyration for PEG chains to be approximately 1.0 and 0.4 nm for alkyl chains.²⁹ The resulting dimensionless grafting densities were 4.5 for pure PEG chains and 1.6 for pure alkyl chains. Notably, even in the hybrid case of PHHS, the PEG component alone maintained a high grafting density Γ* of 3.8. This value, which exceeds 1, indicates complete coverage of the monolayer, effectively shielding the short alkyl molecules underneath.

These analyses support the proposed spatially staggered molecular configuration within the PHHS surfaces, where alkyl chains occupy the lower part of the film, while long and flexible PEG chains extend into the outermost part, thereby contributing to surface self-smoothness and “topmost homogeneity”. To investigate this further, we used AFM to confirm the uniformity of the hybrid monolayer on the silicon substrate (see Figure 3A). The surface roughness of both homogeneous alkyl-silane and PEG-silane surfaces remained very low, within a range of approximately 0.1 nm. Interestingly, the roughness of the nonslippery hybrid surface (PEG grafting ratio around 69%) exhibits a slight increase. This suggests the presence of nanoscale heterogeneity, which can explain the increased CAH compared with that of homo-PEG. In contrast, the surface of PHHS (PEG grafting ratio around 86%) suggests a uniform mixture of components, lacking any detectable phase separation at the AFM tip’s resolution limit. Phase separation would indicate inhomogeneities, potentially leading to high CAH, which is not observed in this case. This observation is in good agreement with our expectations.

For most hybrid monolayers, however, achieving this optimal molecular stacking configuration is highly dependent on adsorption/reaction rates and molecular configuration. Introducing an excessive amount of spacer molecules on the surface could lead to isolated patches that anchor the droplet contact line, thereby increasing CAH.²⁷ This explains why other hybrid surfaces with different chain lengths or mixing ratios might not exhibit the same decrease in CAH.

Temperature Dependence and Slippery Mechanism

The spatially staggered molecular configuration can avoid introducing heterogeneous defects into the hybrid monolayer; however, this only ensures that the slipperiness of PHHS is not worse than that of the homo-PEG surface. To understand the superior slipperiness of PHHS compared to its homogeneous counterpart, the concept of “fluid-like” behavior needs to be accepted. This concept originates from polymer glasses, where chain mobility at the surface is significantly higher than that in the bulk material.¹ Similar to the glass-to-liquid transition observed in bulk polymers, polymer molecules with low glass transition temperatures grafted onto solid surfaces exhibit more freedom to rotate, bend, and stretch, thus showing “liquid-like” properties. Although “liquid-like” behavior has been used to explain the slipperiness of certain polymer-grafted surfaces, such as homo-PEG surfaces,¹ the behaviors and underlying principles lack clear description. To demonstrate the “liquid-like” behavior of the monolayer surfaces, we highlighted it with a temperature-dependent effect.

A water droplet and the surfaces under investigation were placed on a hot plate, and temperature was controlled in the range 25–90 °C. A syringe connected to the droplet through an inserted small needle controlled the droplet volume by pumping water in and out at a controlled flow rate of 20 μL/min. The slow flow rate was chosen to maintain near-equilibrium temperature conditions. This setup enabled us to obtain the dynamic contact angle change with the contact line velocity at different temperatures (Figure 4A). The dynamic contact angle change with velocity mainly originates from the viscosity dissipation near the contact line,³⁰ whereas the vertical gap of the contact angle at zero velocity in these curves represents the CAH. For both homo-PEG and PHHS monolayers, the static contact angle increased with increasing temperature (Figure 4B). This behavior contrasts with most solid surfaces, which exhibit a slight decrease with increasing temperature, considering the decrease in the solid–liquid interaction.³¹ This difference can be attributed to the stronger temperature dependence of surface tension of “liquid-like” monolayer surfaces.

Figure 4.

Temperature effect on slipperiness and slippery mechanism. (A) Typical contact angle variation curves with contact line velocity at different temperatures for the homo-PEG surface. (B) Contact angle and CAH for both homo-PEG and hybrid PEG surfaces at various temperatures. The square and circle symbols represent the advancing and receding contact angles, respectively. (C) Schematic illustration of the effect of temperature on polymer configuration and its impact on CAH.

Young’s equation can be used to estimate the contact angle (θ) by accepting an approximation of interface energy combination rule, , which only consider van der Waals interaction and may underestimate the exact value when hydrogen bonding is involved.³² This gives the expression: . The surface energies of both water and the PEG monolayer decreased with increasing temperature. However, liquid PEG may exhibit more pronounced temperature dependence. Here, we consider the reported value of water [γ_l (mN/m) = 76.1–0.17T (°C)] and a linear extrapolation of PEG molecules at liquid phase [γ_s (mN/m) = 50.9–0.16T (°C)].³³ A theoretical increase of approximately 5° in the water contact angle on the PEG monolayer was predicted over the temperature range 25–90 °C, which was roughly consistent with the experimental results.

Moreover, both homo-PEG and hybrid-PEG monolayers exhibited a significant decrease in CAH as the surface temperature increased from 25 to 90 °C. Homo-PEG showed a 48% reduction, whereas hybrid-PEG showed a 54% reduction. PHHS surfaces achieved an exceptionally low CAH of approximately 0.8° ± 0.4° at 90 °C. Based on the concept of “fluid-like” behaviors, the temperature dependence of CAH can be attributed to the dynamic mobility of the grafted polymer chains, which contributes to a reduction in CAH through two main mechanisms: surface self-smoothing and chain rotation-free dynamics.¹ First, the high mobility of the chains, due to the “free volume” available for polymer chain movement, enables them to mask inherent surface imperfections on the solid substrate (as shown in Figure 4C). The observed increase in film thickness measured by ellipsometry with rising surface temperature (shown in Figure S5 in Supporting Information) suggests a corresponding increase in free volume. Additionally, the gyration radius of the polymer chains also increases with temperature, further enhancing their ability to mask surface defects. This effectively creates a smoother and chemically homogeneous surface, reducing surface energy corrugation. Second, the dynamic nature of the grafted chains enables free rotation. Higher temperatures provide more thermal energy, further enhancing these thermodynamic processes. This facilitates overcoming the energy barriers associated with the movement of a droplet’s contact line and reduces CAH.

Notably, homo-PEG exhibits comparably low CAH at elevated temperatures as PHHS shows at room temperature. This suggests a potential correlation between temperature enhanced slipperiness of homo-PEG and the hybrid structures enhanced slipperiness of PHHS. As previously discussed, polymer chains with high mobility exhibit a liquid-like behavior, enabling them to cover defects and facilitate the motion of the contact line, leading to significantly low CAH. Generally, strategies such as incorporating branched molecular structures, reducing grafting density, and increasing temperature are effective in lowering CAH,³⁴ as they increase free volume and enhance chain mobility. However, for a given set of grafting molecules at a fixed temperature, reducing the grafting density may decrease surface coverage, potentially introducing more surface imperfections and increasing the risk of liquid penetration into the substrate, which could anchor the droplet contact line.²⁷ Our hybrid slippery monolayers, PHHS, address this challenge by incorporating small hydrophobic molecule “spacers” to create a spatially staggered molecular configuration. Long, flexible PEG chains deform and fold to occupy the topmost surface exposure to droplets, while the small alkyl chains remain shielded underneath. This arrangement provides sufficient free volume in the upper region for optimal PEG chain mobility without compromising surface coverage, producing effects like those observed in homogeneous structures at elevated temperatures. The “spacer” effect in the monolayer can be analogous to the “plasticizer” effect in polymer fabrication. Plasticizers are small molecules added to polymers that reduce intermolecular forces between polymer chains, thereby increasing chain mobility, enhancing flexibility, and reducing viscosity.

Water Condensation Applications

The proposed PHHS with its excellent hydrophilic and slippery surfaces properties, offer several advantages for water condensation:^35,36 First, hydrophilicity promote droplet nucleation due to lower free energy, which can be described by Young–Dupré equation, −G_sl = γ_sl (1 + cos θ). The free energy of a droplet G_sl on a hydrophilic surface is lower than on a hydrophobic one. Second, droplets of the same volume on a hydrophilic surface exhibit a larger contact area. This promotes faster coalescence during droplet growth due to the closer proximity of droplets. Third, the slippery nature of the surface enables droplet sliding and collection easier, leading to higher water collection efficiency (shown in Figure 5A).

Figure 5.

Superior performance of PHHS in dew harvesting experiments. (A) Schematic illustration of the advantages of hydrophilic and slippery surfaces in dew condensation and collection. (B) Schematic illustration of condensation and harvesting experiment setup. (C) Optical images of water droplet nucleation and mobility on typical LLS: PHHS, homo-PEG, and homo-PDMS. Inset squares provides magnified microscopy views of local regions. Arrows indicate droplet movement during sweeping. (D) The mass change of collected water over condensation time. (E) Measured dew collection rate of the PHHS compared with homo-PEG and other surfaces. Data from other works are cited from ref (39).

To demonstrate the exceptional performance of the slippery surface for condensation applications, dew condensation and harvesting experiments were conducted using a microscope system within an environmental chamber (shown in Figure 5B). This setup enabled the microscopic observation of droplet nucleation, coalescence dynamics, and droplet mobility, as well as recording the dew collection rate. The samples were mounted on a thermostat tilted at 90° inside a humidity chamber. The substrate temperature was maintained at 0.0 °C ± 0.5 °C, while the relative humidity was controlled at 70% ± 2% to ensure continuous condensation from vapor to water droplets. An absorbent cotton pad was placed on a scale near the sample to measure the mass of collected droplets. We compared droplet dynamics on three different LLS: homo-PDMS brushes, homo-PEG surfaces, and PHHS surfaces. PDMS brushes with a molecular weight of approximately 2000 have been reported to exhibit an optimized slippery performance (contact angle of approximately 110° and CAH of less than 8°).¹³ Homo-PEG monolayers are known for their exceptionally low hysteresis properties, rendering them valuable in diverse applications such as water transport,⁶ dropwise condensation,³⁷ and antifouling.³⁸ Homo-PEG showed a contact angle of approximately 32° and a CAH of approximately 5°, whereas PHHS showed a contact angle of approximately 33° and a CAH of approximately 2°, as shown in Figure 1B.

Although all surfaces exhibited effective slipperiness, a significant difference in initial droplet nucleation was observed between the hydrophobic PDMS and the hydrophilic homo-PEG and PHHS surfaces (see Figure 5C). During the first minute of condensation, the homo-PDMS surface showed a considerably lower droplet density compared to homo-PEG and PHHS surfaces. As condensation progressed (approximately 9 min), droplet volume on the hydrophilic slippery surfaces increased more significantly. This can be attributed to both the higher initial nucleation density and larger droplet contact area on these surfaces, which promotes faster coalescence due to the closer proximity of droplets. Furthermore, PHHS demonstrated superior performance in droplet coalescence and growth by maintaining a more circular shape of the contact line compared to the homo-PEG surface. On the homo-PEG surface, droplets tended to become partially corrugated or elongated after coalescence, which limited the refreshed surface area. In contrast, on the PHHS surfaces, droplets smoothly recovered their circular shape after coalescence, effectively offering refreshed nucleation sites (see Figure 5C from 9 min for PHHS and homo-PEG, and Movie S2).

Both homo-PEG and PHHS surfaces demonstrated advantages over PDMS in terms of nucleation rate and coalescence owing to their similar hydrophilicity. However, for long-term applications, efficient removal of large droplets by gravity becomes crucial for effective droplet harvesting and continuous condensation. This removal, known as “droplet sweep”, occurs when the gravitational force acting on a growing droplet overcomes the resistance from the surface. The sweeping efficiency on the hydrophobic PDMS surface was considerably lower than on the homo-PEG and PHHS surfaces. Consequently, no droplets were drained by gravity from the PDMS surface during the 20 min test period. The sweeping frequency of condensate droplets on the PHHS surface (0.40/min/cm²) was approximately twice that of the homo-PEG surface (0.19 min/cm²) during steady condensation. This higher sweeping frequency resulted in a greater water collection rate, as indicated by the change in collected mass over condensation time (shown in Figure 5D). The water collection rate was calculated from the slope of the mass increase over time. This value was then corrected to account for the evaporation rate of the collected water from the absorbent cotton pad, which was independently measured at approximately 0.0008 g/min (shown in Figure S6 in Supporting Information). The PHHS surface achieved a high droplet harvest rate of 6.9 ± 1.2 g/m²/min, surpassing that of homo-PEG surface about 4.9 ± 0.3 g/m²/min. This value is also comparable to that of other studies utilizing hydrophilic SLIPS (shown in Figure 5E),³⁹ which are already known for their superior droplet collection performance. This result highlights the significant advantage of PHHS for collected water compared to other LLS. Additionally, PHHS surfaces benefit from their nontextured and lubricant-free nature, eliminating concerns related to texture damage and the depletion or contamination of lubricants faced by SHPS and SLIPS surfaces.

Conclusions

This study introduces a novel approach for creating slippery yet hydrophilic surfaces, termed PHHS, by combining hydrophilic PEG-silane polymers with hydrophobic alkyl-silane polymers in a hybrid monolayer. Surprisingly, for choosing specific mixing ratios and chain lengths combinations, this method resulted in a significant decrease in CAH rather than the expected high hysteresis. XPS and AFM confirmed the formation of a smooth hybrid monolayer surface, while pMAIRS measurements indicated a more flexible structure compared to the rigid alkyl-silane chains. As the surface temperature increased, the hybrid monolayer exhibited a reduction in wetting hysteresis and an increase in thickness. At 90 °C, PHHS surfaces achieved an exceptionally low CAH of approximately 0.8° ± 0.4°, whereas the homogeneous PEG monolayer at 90 °C had a CAH as low as 1.7° ± 1.2°, comparable to PHHS at room temperature. These results suggest a spatially staggered configuration in the PHHS, where long hydrophilic PEG chains shield the shorter, distributed hydrophobic alkyl-silane chains. This arrangement creates additional free volume in the uppermost region of the hybrid monolayer, enhancing PEG chain mobility without compromising overall surface coverage. The increased chain mobility likely contributes to a liquid-like self-smoothing effect and enhances rotational dynamics, reducing the energy barrier at the triple-phase contact line, especially at elevated temperatures. This ultimately improves the slipperiness of water droplets on the hybrid monolayer surface. Water condensation experiments further demonstrated the significant advantage of PHHS for water harvesting and energy applications, due to their superior droplet nucleation, faster coalescence dynamics, and higher droplet sweep frequency. Overall, this study presents a novel method for fabricating hydrophilic yet ultra-slippery surfaces and provides valuable insights into the molecular mechanisms underlying wetting hysteresis. This knowledge can pave the way for the development of advanced materials with tailored surface properties for a variety of applications.

Methods

Fabrication of Polymer Grafted Surfaces

Silicon wafers (⟨100⟩ orientation) were cleaned by sequential sonication in acetone, ethanol, and water for 5 min each. The cleaned substrates were then exposed to oxygen plasma (Harrick Plasma, PDC-32G) for 10 min to achieve hydroxylation. For the hybrid-PEG grafted surfaces, the hydroxylated samples were immersed in a solution of anhydrous toluene, HCl, and monolayer chemicals mixed in various ratios (details provided in the Supporting Information). After sealing the containers, the samples underwent silanization for 18 h. They were then thoroughly rinsed with anhydrous toluene, ethanol, and deionized water sequentially, and finally dried with a flow of nitrogen.

Contact Angle and Sliding Angle Measurements

Contact angles and sliding angles were measured using sessile droplets ranging from 1 to 10 μL with a contact angle analysis setup (Kyowa, DropMaster DMo-602). For sliding angle measurements, the tilt angle of the substrates was adjusted from 0 to 90° at a controlled velocity of 1°/s. The sliding angle and hysteresis angle were recorded when both sides of the contact line moved more than 0.15 mm. At least three measurements were conducted at different locations on each surface.

Surface Component Measurement by XPS

XPS was used to characterize the chemical composition of the PHHS surfaces. This analysis was performed with a Physical Electronics PHI 5000 VersaProbe III spectrometer equipped with a monochromatic Al Kα X-ray source operating at 15 kV. Photoelectrons were collected at a 45° takeoff angle relative to the sample surface, enabling the analysis of the top few nanometers. The collected spectra were analyzed using CasaXPS software for detailed elemental identification and quantification.⁴⁰

Film Thickness Measurement by Ellipsometry

Ellipsometry was used to determine the thickness of the homo-PEG and PHHS surfaces. This technique utilized variable angle spectroscopic ellipsometry (J.A. Woollam, M-2000U) to measure changes in light polarization upon reflection from the sample. A spectral scan was performed between 500 and 900 nm at incident angles in the range 55–75° in increments of 10°. The collected spectra were fitted using a three-layer planar model (air/PEG/silica) to determine the thickness of the molecular layer, with an assumed refractive index of 1.45 at 900 nm. To ensure data reliability, at least three measurements were conducted at different locations on each surface.

FT-IR and pMAIRS Analysis

Both standard Fourier transform infrared (FT-IR) spectroscopy and pMAIRS were performed using an automated instrument (Thermo Fisher Scientific, Nicolet iS50) equipped with a liquid nitrogen-cooled mercury–cadmium–telluride detector. The test chamber was continuously purged with dry air to minimize moisture interference. For pMAIRS measurements, the incident angle of p-polarized IR light was varied from 9 to 44° in increments of 5°. At each angle, 400 scans were collected with a wavenumber resolution of 0.4 cm^–1. The pMAIRS software (Thermo Fisher Scientific) automatically calculated the out-of-plane and in-plane spectra from the collected data.⁴¹ A double-polished silicon substrate was used as the background for all the measurements. The schematic diagram of the FT-IR pMAIRS measurement procedures is provided in Figure S7 in Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c15331.

• Additional experimental details on: (S1) fabrication of PHHS, (S2) XPS analysis and PEG component ratio, (S3) reaction rate of silanization process, (S4) thickness of monolayer with temperature, and (S5) principles of pMAIRS measurement are included; supporting tables (Tables S1–S2) and supporting figures (Figures S1–S7) are mentioned in the text (PDF)

• Additional video about tilt angle measurement (Movie S1) are included (MP4)

• Additional video about condensation experiment (Movie S2) are included (MP4)

Author Contributions

^⊥ Y(Z).L. and Y(R).L. contributed equally. Y(R).L. and J(I).S. conceived and supervised the study. Y(Z).L., Y(R).L. and J(I).S. designed the experiments and Y(Z).L., F.S., J(X).S. performed the experiments. S. S., M. M., and N. M. examined the experiments and provided suggestions for condensation experiments. T.H. and T.Y advised pMAIRS measurement and analysis. Y(Z).L., Y(R).L. analyzed experimental results. Y(Z).L., Y(R).L. and J(I).S. wrote and revised the manuscript. All authors have given approval to the final version of the manuscript.

This work was funded by the Japan Society for the Promotion of Science Postdoctoral Fellowship (no. 23KF0022) and the grants-in-aid for Scientific Research KAKENHI (nos. 22K14490 and 22H04950).

The authors declare no competing financial interest.