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. Author manuscript; available in PMC: 2023 Dec 11.
Published in final edited form as: Matter. 2022 Oct 7;5(12):4502–4512. doi: 10.1016/j.matt.2022.09.024

Designing Non-Textured, All-Solid, Slippery Hydrophilic Surfaces

Hamed Vahabi 1,8, Sravanthi Vallabhuneni 1,2,8, Mohammadhasan Hedayati 3, Wei Wang 1,2,4, Diego Krapf 5, Matt J Kipper 3, Nenad Miljkovic 6,7, Arun K Kota 1,2,9,*
PMCID: PMC9784614  NIHMSID: NIHMS1838975  PMID: 36569514

SUMMARY

Slippery surfaces are sought after due to their wide range of applications in self-cleaning, drag reduction, fouling-resistance, enhanced condensation, biomedical implants etc. Recently, non-textured, all-solid, slippery surfaces have gained significant attention because of their advantages over super-repellent surfaces and lubricant-infused surfaces. Currently, almost all non-textured, all-solid, slippery surfaces are hydrophobic. In this work, we elucidate the systematic design of non-textured, all-solid, slippery hydrophilic (SLIC) surfaces by covalently grafting polyethylene glycol (PEG) brushes to smooth substrates. Furthermore, we postulate a plateau in slipperiness above a critical grafting density, which occurs when the tethered brush size is equal to the inter-tether distance. Our SLIC surfaces demonstrate exceptional performance in condensation and fouling-resistance compared to non-slippery hydrophilic surfaces and slippery hydrophobic surfaces. Based on these results, SLIC surfaces constitute an emerging class of surfaces with the potential to benefit multiple technological landscapes ranging from thermofluidics to biofluidics.

Keywords: Slippery, Hydrophilic, Dropwise Condensation, Fouling Resistance, Grafting Density

Graphical Abstract

graphic file with name nihms-1838975-f0001.jpg

eToC blurb

We designed non-textured, all-solid, slippery hydrophilic (SLIC) surfaces by covalently grafting polyethylene glycol brushes to smooth solid substrates above a critical grafting density, which occurs when tethered brush size is equal to inter-tether distance. Our SLIC surfaces demonstrated exceptional performance in condensation and fouling-resistance compared to non-slippery hydrophilic and slippery hydrophobic surfaces. We firmly believe that SLIC surfaces constitute an emerging class of surfaces with the potential to benefit multiple technological landscapes ranging from thermofluidics to biofluidics.

INTRODUCTION

Slippery surfaces (i.e., surfaces enabling high mobility of liquids) have received significant attention due to their wide range of applications in self-cleaning, drag reduction, enhanced condensation, biomedical implants etc.1–6 In the past two decades, most slippery surfaces have been fabricated using two approaches, both of which rely on principles of fluid-film lubrication.7 The first approach employs super-repellent surfaces,8–12 which use texture to trap pockets of air at the solid surface. The trapped air acts as a gaseous lubricant, reducing the solid-liquid interfacial area and inducing slip at the liquid-air interface.13,14 The second approach employs lubricant-infused surfaces,15 16 which use texture to trap pockets of a liquid, immiscible with the contacting liquids, at the solid surface. The trapped immiscible liquid acts as a liquid lubricant, reducing the solid-liquid interfacial area and inducing slip at the liquid-liquid interface.17 Despite the appeal of super-repellent surfaces and lubricant-infused surfaces, they lose slipperiness due to damage of texture, depletion of air via dissolution or external pressure, or depletion of lubricant via evaporation or repeated use.18–22 To circumvent these issues, non-textured, all-solid slippery surfaces have been developed by covalently grafting brushes to substrates (i.e., tethering oligomers or polymers to surfaces).23,24 Since these surfaces are non-textured and all-solid, concerns related to damage of texture and depletion of fluid are greatly mitigated. Nearly all non-textured, all-solid slippery surfaces are hydrophobic.23–27 There are very few reports of non-textured, all-solid slippery hydrophilic surfaces,28–31 and no studies systematically investigated or elucidated the design of such surfaces. In this work, we demonstrate that non-textured, all-solid, slippery hydrophilic surfaces can be designed by covalently binding high surface energy brushes to smooth substrates, only at sufficiently high grafting densities, resulting in both physical and chemical homogeneity. Furthermore, we postulate a plateau in slipperiness above a critical grafting density, which occurs when the tethered brush size (i.e., twice the Flory radius32) is equal to the inter-tether distance. This plateau in slipperiness occurs because, at or above the critical grafting density, water molecules tend to move past the tethered brushes primarily via in-plane motion, with negligible penetration between the brushes, resulting in maximum and constant slipperiness. Building on this understanding, we fabricated slippery hydrophilic (SLIC) surfaces by covalently binding polyethylene glycol (PEG) brushes to smooth substrates with a rapid (< 10 min) grafting-to approach. Our SLIC surfaces demonstrate exceptional performance in condensation and fouling-resistance compared to conventional (i.e., non-slippery) hydrophilic surfaces and slippery hydrophobic surfaces. Based on these results, non-textured, all-solid, slippery hydrophilic surfaces (e.g., SLIC surfaces) constitute an emerging class of surfaces with the potential to benefit multiple technological landscapes ranging from thermofluidics to biofluidics.

RESULTS AND DISCUSSION

Hydrophilic surfaces can be fabricated by covalently attaching high surface energy oligomeric brushes (e.g., PEG brushes) to a solid surface.33 Such hydrophilic surfaces can display slipperiness when contact angle hysteresis Δθ (i.e., the difference between advancing contact angle θadv and receding contact angle θrec) is low. Low contact angle hysteresis can be achieved when a surface has low physical and chemical inhomogeneities.34,35 Non-textured (i.e., smooth) surfaces with very low surface roughness display low physical inhomogeneity. High grafting density σ leads to low chemical inhomogeneity. When hydrophilic PEG brushes are grafted to smooth surfaces, slipperiness increases with increasing grafting density. This is because, at lower grafting densities, water molecules tend to penetrate the space between the PEG brushes (see Figure 1A), resulting in pinning and higher hindrance to lateral mobility of water droplets, which in turn leads to lower slipperiness (i.e., higher Δθ). As grafting density increases, the penetration of water molecules between the PEG brushes decreases, resulting in lower pinning and lower hindrance to lateral mobility of water droplets, which in turn leads to increasing slipperiness (i.e., decreasing Δθ). We postulate that at or above a critical grafting density (i.e., σ ≥ σcrit), where the brush size (i.e., twice the Flory radius, 2RF) is equal to the inter-tether distance, slipperiness is maximum (i.e., Δθ is minimum) and constant. This is because, at or above the critical grafting density, water molecules tend to move past the oligomeric brushes primarily via in-plane motion with negligible penetration (see Figure 1B), possibly due to an ice-like hydration layer.36–38 These physical insights can be conveniently expressed in terms of the non-dimensional slipperiness factor σ∗ = σ/σcrit. When σ∗ < 1, it signifies a regime where slipperiness increases with increasing grafting density, and when σ∗ ≥ 1, it signifies a regime of maximum and constant slipperiness. So, σ∗ > 1 is a prudent criterion for designing non-textured, all-solid, slippery hydrophilic surfaces.

Figure 1. Design and fabrication of SLIC surfaces.

Figure 1.

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(A) Schematic depicting significant penetration of water molecules between PEG brushes at low grafting densities.

(B) Schematic depicting negligible penetration of water molecules between PEG brushes at the critical grafting density.

(C) Schematic depicting the fabrication of SLIC surfaces via hydroxylation and silanization; hydroxyl groups (yellow), PEG brushes (green). Schematics are not to scale and do not depict the precise molecular conformation of PEG brushes.

Building on this understanding, in this work, we fabricated SLIC surfaces by covalently grafting PEG brushes (molecular weight, M ≈ 330 Da) to silicon wafers using a rapid grafting-to approach (see Figure 1C). We chose silicon wafers as substrates to minimize physical inhomogeneities (i.e., surface roughness, Rrms). We chose PEG functional groups to render high solid surface energy, which makes the surfaces hydrophilic. We used a simple and rapid liquid phase silanization39 (see experimental procedures) to covalently graft PEG brushes (see XPS spectra, Figure 2A) to hydroxylated silicon wafers, while maintaining low surface roughness (Rrms < 1 nm; see Figure 2B). To understand the influence of grafting density on wettability and slipperiness, we systematically tuned the non-dimensional slipperiness factor σ∗ by tailoring the silanization time tsil (see Section S1 and Table S1). At different tsil, we estimated the grafting density (in chains nm−2), σ = (hρNA × 10−21)/M; here, h is the thickness of the PEG layer (in nm) estimated from ellipsometry (see experimental procedures), ρ is the density of PEG (in g cm−3), NA is Avogadro’s number, and M is the number averaged molecular weight of PEG (in Da). We estimated the critical grafting density σcrit=1/(23RF2), assuming hexagonal packing with a grafting density σ. At tsil = 0 min, water droplets completely spread (static contact angle, θ = 0°) and did not slide on hydroxylated silicon wafers. As tsil increased, the dynamic contact angles (i.e., θadv and θrec) of water increased (see Figure 2C), indicating increasing covalent grafting of PEG brushes, resulting in increasing σ∗. In addition, as tsil increased, Δθ decreased (see Figure 2D), indicating a decrease in inhomogeneities on the surface as σ∗ → 1. At tsil = 7 min, we estimated that σ∗ > 1 for PEG brushes, indicating the onset of maximum and constant slipperiness regime with θadv = 39°, θrec = 36°, and Δθ = 3°. For tsil ≥ 7 min, we obtained SLIC surfaces (see Figure 2E and Movie S1) with nearly constant contact angles and contact angle hysteresis, confirming that σ∗ > 1 signifies a regime of maximum and constant slipperiness. In a similar manner, we also fabricated SLIC surfaces using longer PEG chains (M ≈ 462 Da; see Section S2 and Table S2) at σ∗ > 1 reaffirming the criterion for designing non-textured, all-solid, slippery hydrophilic surfaces.

Figure 2. Characterization of SLIC surfaces.

Figure 2.

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(A) High resolution C1s XPS spectra of untreated silicon and SLIC surfaces. The C-C peak (at 285 eV) on untreated silicon is due to the presence of the adventitious carbon. The C-O peak (at 286.5 eV) on SLIC surface indicates the presence of PEG brushes.

(B) AFM image depicting the topography of SLIC surface with surface roughness Rrms < 1 nm, indicating low physical inhomogeneity.

(C) and (D) Advancing and receding contact angles, and contact angle hysteresis, respectively, of water on PEG functionalized surfaces at different silanization times, tsil. Data are represented as mean ± standard deviation.

(E) Time-lapse images showing a water droplet (20 μL) sliding easily on a tilted SLIC surface. Scale bar represents 1 mm.

In addition to elucidating the underlying design principles and fabricating SLIC surfaces, we evaluated the performance of our SLIC surfaces in relevant thermofluidic and biofluidic applications. Specifically, our SLIC surfaces are ideal for enhancing condensation heat transfer of aqueous liquids because they simultaneously offer low static contact angle θ and low sliding angle ω (i.e., minimum tilt angle for a droplet to slide on a surface) for aqueous liquids. Low contact angle θ promotes high nucleation rates and low conduction resistance, both of which enhance condensation heat transfer.31,40,41 Low sliding angle ω (due to low contact angle hysteresis Δθ) promotes rapid and efficient removal of the condensate droplets, thereby enabling sustained dropwise condensation with significantly higher heat transfer coefficient compared to filmwise condensation.31,40,41 To demonstrate such enhanced condensation heat transfer resulting, we qualitatively compared the condensation of water (see experimental procedures) on our SLIC surfaces (θ ≈ 37°, ω ≈ 3° for 20 μL water droplets) with condensation on non-slippery hydrophilic surfaces (having low contact angle, but higher sliding angle) and slippery hydrophobic surfaces (having low sliding angle, but higher contact angle). We used untreated silicon wafers as non-slippery hydrophilic surfaces (θ ≈ 45°, ω ≈ 20° for 20 μL water droplets) and 1,3-dichlorotetramethyldisiloxane-treated silicon wafers as slippery hydrophobic surfaces (θ ≈ 103°, θadv ≈ 104°, θrec ≈ 101°, ω ≈ 3° for 20 μL water droplets; see experimental procedures). We exposed vertically mounted surfaces at ambient temperature (≈ 20°C) to steam at approximately 100°C (see Figures 3A–3C and Movie S2). The non-slippery hydrophilic surface and our SLIC surface displayed faster droplet coalescence and growth due to their higher nucleation rate compared to the hydrophobic slippery surface. Condensate droplets attained a critical size (≈ 1.4 mm; see Figure 3C) and slid down our SLIC surface at a faster rate compared to the hydrophobic slippery surface and the non-slippery hydrophilic surface. This rapid droplet shedding indicates that our SLIC surfaces not only facilitate high nucleation rates due to their hydrophilicity, but also facilitate rapid condensate removal due to their high slipperiness (i.e., low ω). After removal of a condensate droplet, the nucleation, coalescence, growth, and removal of additional condensate droplets continued on our SLIC surface indicating the potential for sustained dropwise condensation and enhanced condensation heat transfer. In practice, condensation can occur at different relative humidities for a wide range of aqueous liquids. To ensure that our SLIC surfaces retain their hydrophilicity and slipperiness, we characterized their contact angles and sliding angles for water at different relative humidities and for aqueous liquids (solutions of sodium dodecyl sulfate in water) with a wide range of surface tensions (γlv = 40 mN m−1 to 72 mN m−1; see experimental procedures). Our SLIC surfaces displayed negligible change in static contact angles and sliding angles of water at relative humidities ranging from 10% to 100% (see Figure 3D). As the surface tension of aqueous liquids decreased from 72 mN m−1 to 40 mN m−1, our SLIC surfaces displayed an expected decrease in static contact angles and sliding angles (see Figure 3E, Section S3 and Table S3). These results demonstrate the retention of hydrophilicity and slipperiness of our SLIC surfaces and their potential for condensation applications. In addition, our SLIC surfaces also demonstrated the retention of hydrophilicity and slipperiness even after exposure to air for 20 days, immersion in water for 20 days, exposure to steam (at 100°C and 1 atm) for a cumulative time of 48 hours and 100,000 water droplets sliding past the surface (see Section S4).

Figure 3. Condensation on SLIC surfaces.

Figure 3.

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(A), (B) and (C) Water droplet nucleation, growth and coalescence on vertically oriented non-slippery hydrophilic surface, slippery hydrophobic surface and SLIC surface, respectively. Upon coalescence, water droplets attained a critical size (≈ 1.4 mm) and slid down the SLIC surface at a faster rate. Scale bar represents 2 mm for all images.

(D) Static contact angles and sliding angles of water droplets on SLIC surfaces for different relative humidities.

(E) Static contact angles and sliding angles of aqueous liquid droplets with different surface tensions on SLIC surfaces. Data are represented as mean ± standard deviation.

In addition to thermofluidic applications, we also evaluated the performance of our SLIC surfaces in biofluidic applications. Specifically, our SLIC surfaces are ideal for fouling-resistant lab-on-a-chip miniature platforms because they simultaneously offer hydrophilicity and slipperiness for aqueous liquids. Hydrophilicity imparted by the PEG brushes allows improved resistance to fouling by proteins because of the hydration layer and the steric hindrance.42–45 To demonstrate the fouling-resistance of our SLIC surfaces, we compared them with that of non-slippery hydrophilic surfaces and slippery hydrophobic surfaces. We used untreated glass coverslips as non-slippery hydrophilic surfaces (θ ≈ 53°, ω ≈ 47° for 20 μL water droplets), 1,3-dichlorotetramethyldisiloxane-treated glass coverslips as slippery hydrophobic surfaces (θ ≈ 103°, ω ≈ 4° for 20 μL water droplets) and glass coverslips with covalently grafted PEG brushes as our SLIC surfaces (θ ≈ 37°, ω ≈ 5° for 20 μL water droplets). We used glass coverslips because they are smooth (allowing low physical inhomogeneity) and transparent (allowing light microscopy). We exposed the surfaces to fibrinogen solution and studied protein adsorption in-situ using total internal reflection fluorescence (TIRF) microscopy (see experimental procedures). Fibrinogen adsorbed within 1 min on both the non-slippery hydrophilic surfaces and slippery hydrophobic surfaces (see Figures 4A and 4B, and Movie S3). In contrast, our SLIC surfaces displayed exceptional fouling-resistance by preventing adsorption of fibrinogen even at 1800 min (see Figure 4C and Movie S3).46,47 The exceptional fouling-resistance is possibly due to an ice-like hydration layer,37,38,43 but a comprehensive study is needed to develop a mechanistic understanding. In addition to the outstanding fouling-resistance, the slipperiness of our SLIC surfaces allows aqueous droplets to slide easily (see Figure 4D), allowing manipulation of aqueous droplets. To demonstrate the utility of slipperiness in lab-on-a-chip miniature platforms, we fabricated SLIC surfaces patterned with an array of non-slippery domains (see experimental procedures). When an aqueous droplet (e.g., protein solution) slides past such patterned SLIC surfaces, tiny volumes of the droplet are trapped in the non-slippery domains due to higher adhesion (see Figure 4D and Movie S4). In this manner, on patterned SLIC surfaces, biological analytes (e.g., proteins, nucleic acids, cells, microorganisms etc.) can be trapped in targeted domains, while ensuring that the remaining areas fouling-free (see Figure 4D). Furthermore, while aqueous droplets slide easily, oil droplets adhere to our SLIC surfaces. When a compound droplet, consisting of water and oil, is placed on a tilted SLIC surface, water displaces oil and slides away while the oil remains adhered (see Figures 4E–4G, Movie S5, and Section S5), indicating the potential for biofluidic separations.

Figure 4. Fouling resistance and oil-water separation on SLIC surfaces.

Figure 4.

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(A) and (B) Fluorescent microscopy images showing significant adsorption of fibrinogen (red) on non-slippery hydrophilic surface (surface coverage ≈ 96%) and slippery hydrophobic surface (surface coverage ≈ 93%), respectively, within 1 min.

(C) Fluorescent microscopy image showing negligible fibrinogen adsorption on SLIC surfaces (surface coverage ≈ 0.2%), even at 1800 min. Scale bar represents 5 μm in all images.

(D) Time-lapse images showing tiny volumes of water (pink) trapped in non-slippery domains (star shaped) when a water droplet slid past a patterned SLIC surface. Inset images showing fibrinogen trapped in the star shaped domains on the SLIC surface. Scale bar represents 1 mm.

(E), (F) and (G) Time-lapse images showing the separation of water (colorless) from hexadecane (red) on a SLIC surface.

Conclusions

In this work, we elucidated the design of non-textured, all-solid, slippery hydrophilic surfaces. We demonstrated two distinct regimes of slipperiness: σ∗ < 1, where slipperiness increases with increasing grafting density, and σ∗ ≥ 1, where slipperiness is maximum and constant. We postulate that σ∗ > 1 is a prudent criterion for the design of non-textured, all-solid, slippery hydrophilic surfaces. Building on this design criterion, we fabricated slippery hydrophilic (SLIC) surfaces by covalently grafting PEG brushes to silicon wafers and glass coverslips. We demonstrated the exceptional performance of our SLIC surfaces in condensation and fouling-resistance compared to non-slippery hydrophilic and slippery hydrophobic surfaces. While a comprehensive study (on processing-structure relationships with a wide variety of chemistries and substrates and the role of ice-like hydration layers) is needed to develop a thorough understanding of this emerging class of surfaces, our results indicate that SLIC surfaces have the potential to positively transform the technological landscape of thermofluidic and biofluidic devices.

EXPERIMENTAL PROCEDURES

Resource availability

Lead contact

Further information and requests for resources and materials should be directed to and will be fulfilled by the lead contact, Arun K. Kota (akota2@ncsu.edu).

Materials availability

This study did not generate new unique reagents.

Data and code availability

The data generated during this study are available within the article and its Supplementary Materials files and also are available from the corresponding authors upon reasonable request.

Fabrication of hydrophilic and hydrophobic surfaces

Silicon wafers (<1 0 0> orientation; University Wafers) and glass cover slips (Fisher) were cleaned by rinsing thoroughly with acetone (Fisher) and DI water, and then dried with nitrogen. The cleaned substrates were exposed to oxygen plasma (Plasma Etch PE-25) for 10 min for hydroxylation. To prepare hydrophilic surfaces, the hydroxylated samples were immersed in a solution consisting of 2 μL of 2-methoxy polyethyleneoxy (6–9) propyl trimethoxysilane (Gelest) and 12 μL hydrochloric acid (Fisher) in 45 mL of anhydrous toluene (Fisher) for the desired time at room temperature. We chose low molecular weight PEG silane to reduce steric hindrance and obtain higher grafting densities. To prepare hydrophobic surfaces, the hydroxylated samples were exposed to the vapors of 150 μL of 1,3-dichlorotetramethyldisiloxane (Gelest) in an enclosed chamber for 15 min at room temperature. Finally, the silanized samples were cleaned by rinsing thoroughly with anhydrous toluene, DI water and ethanol (Fisher) sequentially, and then dried with nitrogen.

Fabrication of SLIC surfaces with non-slippery domains

Star-shaped non-slippery domains were fabricated on SLIC surfaces via laser ablation with a commercially available, quasi-continuous CO2 laser system with a central wavelength of 10.6 μm (Epilog Legend 36EXT). Laser ablation increased the roughness (i.e., physical inhomogeneity) resulting in loss of slipperiness in the domains.

Atomic force microscopy (AFM)

Surface roughness on SLIC surfaces was characterized using AFM (Bruker MultiMode 8-HR) with silicon nitride probes in the ScanAsyst mode. At least 30 images were acquired by scanning 4 μm × 4 μm areas at a scan rate of 1 Hz and analyzed with Nanoscope Analysis software 1.8 to obtain the root mean square roughness Rrms.

X-ray photoelectron spectroscopy (XPS)

The surface chemistry on SLIC surfaces was characterized using XPS (Physical Electronics PHI-5800 spectrometer). XPS was conducted using a monochromatic Al Kα x-ray source operated at 15 kV, and photoelectrons were collected at a takeoff angle of 45° relative to the sample surface. XPS data was acquired from at least 5 spatially different locations on the surface and the spectral analysis was conducted using PHI Multipak software.

Ellipsometry

Ellipsometry was conducted to measure the thickness of PEG layer on SLIC surfaces at different tsil using a variable angle spectroscopic ellipsometer (VASE-VB-250). A spectral scan of the surface was collected between 500 nm and 900 nm for an incident angle between 55° and 75° with an increment of 5°. The thickness of PEG layer (refractive index = 1.45) was determined using a three-layer planar model (air/PEG/silica) of the solid surface from the collected spectra. At least five measurements were conducted at different locations on each surface.

Contact angle, sliding angle and surface tension measurements

Contact angles and sliding angles were measured with 20 μL sessile droplets and surface tension was measured with pendant droplets, using a contact angle goniometer/tensiometer (Rame-Hart 260). A custom-built humidity chamber was used to control the humidity. At least six measurements were performed on each surface at spatially distinct locations. The standard deviation is reported as error with all measurements.

Condensation

Water condensation experiments were conducted in a custom-built insulated enclosed chamber equipped with an inlet for steam and a transparent glass window. Steam (at 100°C and 1 atm) was generated by boiling water in a conical flask and was directed to the inlet of enclosed chamber through insulated pipes for condensation. The substrate was vertically mounted on a Peltier plate set to ambient temperature (≈ 20°C), opposite to the glass window and was exposed to steam inside the enclosed chamber. The condensation process was monitored and recorded through the glass window using a camera.

Total Internal Reflection Fluorescence (TIRF) microscopy

TIRF microscopy was conducted to study protein adsorption using time-lapse imaging with fibrinogen from human plasma conjugated to Alexa Fluor 647 (Fisher). The time-lapse imaging was conducted using a custom-built microscope48 equipped with Olympus IX71 body, 100x objective and a CRISP ASI autofocus system. The excitation was conducted using a 638 nm laser (DL638–328 050, CrystaLaser, Reno, NV). Emission was collected using the appropriate Semrock bandpass filters and the images were acquired in a water-cooled, back-illuminated EMCCD camera (iXon DU-888, Andor, Belfast, UK) liquid-cooled to −70°C with an electronic gain of 60. An enzymatic oxygen scavenging system was used in the imaging buffer to reduce photobleaching.49 Imaging buffer consisted of 50 mM Tris-HCl (pH 8.0), 10 mM NaCl, 0.15 mg ml−1 glucose oxidase, 34 μg ml−1 catalase, 0.8% (w/v) glucose and 1% (v/v) β-mercaptoethanol. On SLIC surfaces, time-lapse imaging was conducted at a frame rate of 30 frames hr−1. On other surfaces, the time-lapse imaging was conducted at a frame rate of 10 frames s−1. During imaging, the concentration of fibrinogen was maintained at 5 nM for all experiments.

Supplementary Material

1

Movie S1. Water droplet (20 μL) sliding past a SLIC surface at a tilt angle of 10°. Scale bar represents 1 mm.

Download video file (1.5MB, mp4)
2

Movie S2. Water condensation on non-slippery hydrophilic, slippery hydrophobic and SLIC surfaces for ≈ 20 mins. Scale bar represents 2 mm.

Download video file (7.2MB, mp4)
3

Movie S3. Fibrinogen adsorption on non-slippery hydrophilic, slippery hydrophobic and SLIC surfaces. Fading contrast with time is due to photobleaching. Scale bar represents 5 μm.

Download video file (9.5MB, mp4)
4

Movie S4. Trapping tiny quantities of water by patterning SLIC surfaces with non-slippery hydrophilic domains. Scale bar represents 5 mm.

Download video file (656.7KB, mp4)
5

Movie S5. Oil-water separation using compound droplet (water and hexadecane) on our SLIC surface. Scale bar represents 5 mm.

Download video file (1.8MB, mp4)
6

Highlights.

  1. Elucidated design of non-textured, all-solid, slippery hydrophilic (SLIC) surfaces.

  2. SLIC surfaces are ideal for sustained dropwise condensation.

  3. SLIC surfaces display outstanding fouling resistance.

Progress and Potential.

Water droplets tend to move and slide easily on non-stick coatings. Such slipperiness is intuitive because almost all non-stick coatings have hydrophobic (i.e., water-repellent) surfaces, which do not stick much to water. On the contrary, water spreads and sticks easily to hydrophilic (i.e., water-loving) surfaces. So, hydrophilic surfaces, which allow water droplets to move and slide easily, are counter-intuitive and rare. In this work, we divulge the design principles for making such slippery hydrophilic (SLIC) surfaces. Such SLIC surfaces constitute an emerging class of surfaces with significant potential to benefit multiple technological landscapes ranging from thermofluidics to biofluidics.

ACKNOWLEDGMENTS

A.K.K. gratefully acknowledges financial support under award 1751628 from the National Science Foundation and under awards R01HL135505 from the National Institutes of Health.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

SUPPLEMENTAL INFORMATION

Supplemental information is available online or from the author. Sections S1–S5. Figures S1–S4. Tables S1–S3. Movies S1–S5.

DECLARATION OF INTERESTS

A.K.K. and H.V. are inventors on a patent filed by Colorado State University.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

1

Movie S1. Water droplet (20 μL) sliding past a SLIC surface at a tilt angle of 10°. Scale bar represents 1 mm.

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2

Movie S2. Water condensation on non-slippery hydrophilic, slippery hydrophobic and SLIC surfaces for ≈ 20 mins. Scale bar represents 2 mm.

Download video file (7.2MB, mp4)
3

Movie S3. Fibrinogen adsorption on non-slippery hydrophilic, slippery hydrophobic and SLIC surfaces. Fading contrast with time is due to photobleaching. Scale bar represents 5 μm.

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4

Movie S4. Trapping tiny quantities of water by patterning SLIC surfaces with non-slippery hydrophilic domains. Scale bar represents 5 mm.

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5

Movie S5. Oil-water separation using compound droplet (water and hexadecane) on our SLIC surface. Scale bar represents 5 mm.

Download video file (1.8MB, mp4)
6
NIHMS1838975-supplement-6.pdf (1.3MB, pdf)

Data Availability Statement

The data generated during this study are available within the article and its Supplementary Materials files and also are available from the corresponding authors upon reasonable request.

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