Anti-smudge behavior of facilely fabricated liquid-infused surfaces with extremely low contact angle hysteresis property

Abstract

Slippery liquid-infused porous surfaces (SLIPS) possess excellent liquid-repellency and have been applied for anti-icing and anti-fouling. Following the concept of SLIPS, a liquid-infused surface is facilely fabricated by a stretched polytetrafluoroethylene (PTFE) thread seal tape infused with perfluoropolyether fluorinated lubricant. Owing to the thin thickness of the PTFE film, this surface can be transparent and flexible. The nearly hysteresis-free property of SLIPS is manifested by the absence of hysteresis loops in the plot of contact angle (or base diameter) versus drop volume for both water and hexadecane. The anti-smudge behavior of the surface is first examined by the removal of stains by sliding drops at an inclined plane. Secondly, the anti-smudge behavior of the surface is demonstrated by the wetting competition of a hexadecane drop between SLIPS and fluorinated polyvinyl alcohol surface which is lipophobic and superhydrophobic. Because of the negligible contact angle hysteresis, almost no liquid (only 3.9%) is seized by the SLIPS after rupture. A Surface Evolver simulation is performed to understand the mechanism of the wetting competition and the result is in a good agreement with that of the experiment. Our tests indicate that the liquid-infused surface exhibits excellent anti-smudge properties.

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I. Introduction

The wettability of a drop on a solid is generally determined by the contact angle (CA) between the liquid–gas and solid–liquid interfaces. On a smooth surface, the intrinsic CA (θ*) is depicted by Young’s equation¹ cos θ* = (γ_sg − γ_sl)/γ_lg, where γ_sg, γ_sl, and γ_lg represent the interfacial tensions of solid–gas, solid–liquid, and liquid–gas, respectively. On a real solid, however, the CA is not unique but in a range of θ_r ≤ θ ≤ θ_a. Here θ_a and θ_r are the advancing and receding CAs associated with the angle at which the contact line starts its expansion and contraction, respectively. The advancing and receding CAs are generally obtained by the dynamic sessile drop method. In a cycle of the inflation and deflation of the drop, two CAs can be associated with a specific volume (V) and a hysteresis loop is formed in the plot of θ versus V. The extent of contact angle hysteresis (CAH) is generally defined as the difference between the advancing and receding angle, θ_a − θ_r.^2–6

With the increasing usages of smart phones and tablet PCs, integrated touch panels are indispensable. Since the panels have to be in contact with the human skin, they suffer from contaminations of undesirable sweat, fingerprints, and skin oil when used. Those smudges are often visible and cause aesthetic damage of the product appearance. In order to resolve the problem and conform to the demand of discerning consumers, a great effort has been made to develop anti-smudge/fingerprint surfaces.^7–13 It is found that a self-cleaning surface with very high CAs of liquid drops such as a superomniphobic surface is not necessary. A coating with low CAH and medium CAs can function as an anti-smudge surface as well. That is, the stain on such a surface can be easily removed although the water and oil CAs are not high. For example, an easy-cleaning coating on the touch panel, such as the Daikin OPTOOLTM series,⁷ is commercially provided to achieve this goal. The CAs of water and hexadecane on this surface are medium, about 108° and 60°, respectively but the CAH of them are quite low.

The factors that affect CAH are still subjects of ongoing debates and three mechanisms are generally responsible: inhomogeneity,^1–3,14 adhesion hysteresis,^15–17 and surface roughness.^18,19 In the inhomogeneity mechanism, the localized defects are more lyophilic than the rest of the surface. As a result, the contact line is locally trapped by the chemical defect and forced to deform during retraction. The wettability and size of defects determine the extent of CAH. In the adhesion mechanism, upon wetting, the solid–liquid interface is restructured over a short period of time, resulting in the decrease of the solid–liquid tension from γ_sl to γ′_sl associated with θ_r. In the roughness mechanism, the contact line is pinned at the sharp edge of grooves. Upon inflation or deflation, the apparent CA between θ_r and θ_a is exhibited. When these factors associated with CAH are lessened, the extent of CAH on the surface can be reduced accordingly. For example, on a superomniphobic surface, a liquid drop is supported only by a very small area with CAH. Most of the bottom of the liquid drop is in contact with air trapped within grooves, which posseses extremely low CAH (referred to as hysteresis-free). Unfortunately, the air pockets cannot stand up to pressure and therefore liquid may impregnate the roughness easily under high pressure or even upon impact.²⁰ Moreover, the hierarchical structure associated with superomniphobic surfaces suffers weak mechanical robustness.

Recently, a remarkable idea inspired by the pitcher plant was offered to diminish CAH.^21–30 The surface of a pitcher plant is covered with homogenous liquid films under humid conditions. Such a surface is slippery for insects because of the vanishing capillary force associated with weak CAH. Accordingly, slippery liquid-infused porous surfaces (SLIPS) which contain a thin film of lubricant caught in a micro/nanoporous structure are developed.²¹ On SLIPS, a thin film of the lubricant (e.g. lubricating liquid Krytox) in contact with immiscible liquids (e.g. water) is intrinsically smooth and defectless. Thus, contact line pinning associated with CAH on such a surface is essentially absent. Three steps are typically involved in the preparation of SLIPS: (i) a flat substrate should be first roughened and chemically functionalized, (ii) the functionalized substrate is infused with the compatible lubricating liquid, (iii) the excess lubricating liquid is removed.²⁵ Note that the substrate should prefer the lubricating liquid rather than the test drops. The advantages of SLIPS include liquid-repellency, pressure stability, optical transparency, and thermal stability.²⁷ They have been applied to the applications of anti-icing²⁶ and anti-biofouling.²⁵

In this work, following the same idea of SLIPS, facile and low-cost fabrication of transparent liquid-infused surfaces is achieved by a polytetrafluoroethylene (PTFE) thread seal tape infused with fluorinated lubricant. Because of the flexibility of the seal tape, curved or flexible SLIPS can be easily obtained as well. First, the hysteresis-free property of this surface is examined. Then, the anti-smudge property of this surface is explored. The anti-smudge performance of the test surface includes the removal of stains by sliding drops at an inclined plane and examination of the amount of the liquid residue after the breakup of a liquid bridge between two surfaces. The variation of the sliding angle with the drop volume is determined and the ability for stain removal is demonstrated. The wetting competition of a liquid drop between the liquid-infused surface and the surface with CAH is emaxined both experimentally and theoretically. A comparison between the experimental result and simulation outcome is made.

II. Experimental and simulation methods

A. Materials and experimental method

Fabrication of liquid-infused surfaces. A strip of polytetrafluoroethylene (PTFE) thread seal tape, which is purchased from Yeu Ming Tai Chemical Industrial Co., Ltd, is stretched and covers solid substrates, such as polymethyl methacrylate (PMMA) slides, polyethylene terephthalate (PET) sheets, and glass tubes which are obtained from Kwo-Yi Co., Taiwan. The PTFE film used has a thickness of approximately 100 μm. The porous structure giving space for lubricant infusion is clearly illustrated in Fig. Sl of the ESI.† A small drop of perfluoropolyether fluorinated lubricant (Fomblin® YR1800) is added onto the PTFE porous film. The density and kinematic viscosity of the lubricant are 1850 cSt and 1.92 g cm⁻³, respectively. Then, an over-coated liquid layer is formed using a spin coater.

Plots of hysteresis loops. The measurements of CA, base diameter, and drop volume of a liquid drop on PMMA or the liquid-infused surface are conducted by the inflation–deflation method, using the drop shape analyzer DSA10-MK2 (Krűss, Germany). Water is deionized and n-hexadecane (98%) is purchased from Tokyo Chemical Industry Co., Ltd.

Formation of stains on substrates. Stains are developed by the evaporation of sessile liquid drops of 10 μL containing various solutes on the PMMA slide or the liquid-infused surface. Polyvinyl alcohol (PVA, MW = 130k), titanium dioxide (TiO₂, 21 nm), and PMMA (MW = 550k) powders are purchased from Fluka Chemika, UniRegion Bio-Tech, and Alfa Aesar, respectively. Graphene particles are acquired via sonication of a graphite sheet immersed in hexadecane. The high purity graphite sheet is obtained from Nippon Techno-Carbon Co., Japan.

Removal of stains on surfaces. A water drop with a specific volume is deposited on the PMMA sheet or the liquid-infused surface. Then, the surface is inclined gradually by a manually tilted platform. The sliding angle α is determined as the tilted angle where the drop starts to move along the surface. The removal test involves the process of a sliding drop passing through the stain to see whether it can carry the stain away from the surface or not. During the removal process, the images of top view are captured by a charge-coupled device, STC-620 (SENTECH, Japan).

Fabrication of fluorinated PVA substrates. The fluorinated PVA substrate is fabricated by the vapor deposition of perfluorooctyl-trichlorosilane (PFOTS). PFOTS is purchased from Alfa Aesar. The solution of 20 ml hexane dissolving 8 μL PFOTS is heated to 60 °C. Then the vapor-phase deposition of PFOTS on the PVA substrate is conducted in an inverted Petri dish. After the vapor-phase deposition is complete, the substrates are rinsed by hexane to eliminate the residue of PFOTS and then heated at 80 °C for a day to remove the solvent. This substrate is superhydrophobic and lipophobic.

Wetting competition between two surfaces. The compressing and separating processes are conducted by using DSA10-MK2. A droplet is placed on the lower substrate that lies on the stage of the goniometer. The upper substrate is fixed above the stage. When the stage is elevated, the droplet on the lower substrate is compressed by the upper surface. On the contrary, the droplet is stretched by the upper surface as the stage is lowered. The movement of the stage is as slow as possible to keep a quasi-equilibrium condition. During the processes, the shape evolution of the liquid bridge is recorded and transformed into an enlarged image. CA, base diameter, and drop volume are determined by the built-in scale of MultiCam Easy 2007 (Shengtek). It takes about 2 min to complete one cycle of compressing/stretching. The influences of evaporation and volume change can be neglected, because less than 1% of the droplet volume is lost after one cycle.

B. Simulation method

Surface Evolver^31,32 (SE) is used to simulate the evolution of the shape of liquid bridge between two surfaces during the separation process. The basic concept of SE is to minimize the energy of a surface subject to some constraints by evolving the surfaces as unions of triangles with vertices. For a liquid bridge system, the total free energy F_t can be expressed as

The first term on the right hand side of eqn (1) represents the liquid–gas interfacial energy and A_lg denotes the liquid–gas interfacial area. The second term is the solid–liquid interfacial energy and A_sl depicts the solid–liquid interfacial area. According to Young’s equation, eqn (1) can be expressed in terms of θ_a. The third term represents the gravitational energy of the liquid bridge with volume V_w and density ρ. The gravitational acceleration is g.

During iterations, the vertices are moved from an initial shape to a final equilibrium one corresponding to a global minimum of the system free energy. Two constraints are imposed: the contact line has to lie on the surface and the liquid volume is conserved. The calculation ends when the total free energy difference converges in the acceptable tolerance of 10⁻⁵. Typically, CAH is not accounted for in SE simulations. However, in our simulations the hysteresis effect is considered.^5,14,16 The solid–liquid tension within the wetted region is lowered to γ′_sl while that in the exterior region remains at γ_sl. These two solid–liquid tensions correspond to the two CAs associated with droplet wetting: the advancing CA (θ_a) via cos θ_a = (γ_sg − γ_sl)/γ_lg and the receding CA (θ_r) via cos θ_r = (γ_sg − γ′_sl)/γ_lg.

III. Results and discussion

If a surface is anti-smudge, it would be difficult for dirty water or oil drops to stay on such a surface. The drops on the anti-smudge surface can slide away easily and thus carry dust along with themselves. In other words, the capillary force is generally too weak to sustain drops on an anti-smudge surface. In addition, when a drop is in contact with an anti-smudge surface and a typical surface such as fingers, it prefers not to stay on the anti-smudge one after the separation of the two surfaces. That is, wetting competition between two surfaces is also an indicator of the anti-smudge performance. From the viewpoint of capillary force and wetting competition, the anti-smudge performance of a surface is closely related to the wetting properties, particularly CAH.

An anti-smudge transparent surface can be easily acquired by covering a transparent substrate such as PMMA sheet with a stretched PTFE film. Due to the mismatch of the refractive index n between PTFE (n = 1.35) and air (n = 1.00), the PTFE film is opaque but becomes translucent by stretching, as demonstrated in Fig. 1a. When the stretched porous PTFE film is infused with Fomblin lubricant, the thin film becomes transparent because of the match of n between PTFE and Fomblin (n = 1.30), as depicted in Fig. 1b. Since the PTFE film which is often used in sealing pipe threads is readily shaped, it can be exploited to cover a solid substrate of an arbitrary shape. As shown in Fig. 1c, if the solid substrate such as PET is elastic, the liquid-infused surface can exhibit flexibility as well. Fig. 2a illustrates a cylindrical liquid-infused surface which is obtained simply by covering the glass tube with the PTFE film. A water drop of 2 μL can stand still on the glass tube of radius 0.2 cm due to CAH, as depicted in Fig. 2b. However, this small drop is unable to stay on the upper side of the tube covered with the liquid-infused surface because of weak CAH. The drop is always driven downward by gravity and sustained at the lower side of the tube by the capillary force, as shown in Fig. 2c. The experiment is repeated three times and the process takes about 33, 26, and 31 s, respectively.

Fig. 1 (a) PTFE film is opaque. (b) Liquid-infused surfaces become transparent. (c) Liquid-infused surfaces can be curved or flexible.

Fig. 2 (a) Transparent cylindrical liquid-infused surface made by covering the glass tube with the liquid-infused PTFE film. (b) A small water drop can stand on the upper side of a glass tube because of CAH. (c) A small water drop always moves downward to the lower side of a tube covered with the liquid-infused surface due to negligible CAH.

A. Wetting property: hysteresis-free behavior

Most of the surfaces exhibit CAH and the typical hysteresis loops are illustrated in Fig. 3a by varying the volume (V) of a water drop on PMMA. The characteristics associated with the change of the base diameter (BD) are advancing and receding pinning (1 and 3). The characteristics associated with the variation of the CA (θ) are the upper and lower limits of the CA (2 and 4). As V is increased in step 1, the contact line is pinned until the CA reaches θ_a. If V is further increased in step 2, the contact line starts to move outward but the CA remains at θ_a. At some point, as the drop volume is decreased in step 3, one can observe that the contact line is also pinned until the CA is lowered to θ_r. Further decrease of V in step 4 will lead to the inward movement of the contact line with θ = θ_r. However, as shown below, for a liquid drop on a Fomblin-infused surface, a distinct phenomenon occurs.

Fig. 3 (a) The variation of base diameter (BD) and contact angle (θ) with drop volume (V) for a water drop on PMMA during inflation and deflation. (b) The variation of base diameter (BD) and contact angle (θ) with drop volume (V) for a water drop on the liquid-infused surface during inflation and deflation. (c) The variation of base diameter (BD) and contact angle (θ) with drop volume (V) for a hexadecane drop on the liquid-infused surface during inflation and deflation.

In this work, the wetting properties of water and hexadecane on the liquid-infused surface are examined by the inflation-and-deflation method. Fig. 3b shows the variations of BD and θ with V of the sessile water drop. During inflation, BD always grows and θ remains at θ_a ≈ 115°. In contrast, during deflation, BD declines monotonically and θ is kept at θ_r ≈ 115°. In fact, at a specified V, BD or θ is independent of the pathway and it is essentially the same in the inflation and deflation processes. Compared to the typical CAH, regions of constant BD do not exist and one has a unique CA, θ_a ≈ θ_r ≈ 115°. Some snapshots for water drops on PMMA and the liquid-infused surface during their inflation and deflation are also demonstrated in Fig. 4. Evidently, for PMMA, the contact line pinning associated with volume expansion or shrinkage occurs. In contrast, for the liquid-infused surface, the contact line pinning behavior disappears. This outcome clearly indicates that the hysteresis loops vanish in the plots of BD vs. V and θ vs. V and the liquid-infused surface shows the hysteresis-free property for water. As illustrated in Fig. 3c, a similar behavior is also observed for a hexadecane drop on the liquid-infused surface. During inflation and deflation of the oil drop, θ stays at θ ≈ 64° and no contact line pinning and depinning are observed. Therefore, this liquid-infused surface possesses a hysteresis-free property for hexadecane as well.

Fig. 4 Some of the snapshots during inflation and deflation processes for water drops on PMMA and liquid-infused surface.

The hysteresis-free property of the liquid-infused surface reveals that such a surface is essentially covered with a thin layer of Fomblin lubricant. Fomblin is captured inside the porous PTFE structure and becomes immobilized. As a result, the water or oil drop is directly in contact with Fomblin lubricant instead of the PTFE thin film. For a water drop, since the PTFE solid surface is replaced by the Fomblin liquid surface, the solid–gas interfacial tension becomes γ_sg = γ_{Fomblin–air} = 24 dyne cm⁻¹ and the solid–liquid interfacial tension γ_sl = γ_{Fomblin–water} = 59 dyne cm⁻¹ determined by the pendant drop method. By using γ_water–air = 72.8 dyne cm⁻¹, the apparent CA on this liquid-infused surface is evaluated as θ = 119° based on the Young’s equation. The calculated value is consistent with our experimental measurement, θ = 115°. This consequence reveals that the water drop sits basically on the Fomblin liquid film. Similarly, for a hexadecane drop, γ_sl = γ_{Fomblin–hex} = 9.6 dyne cm⁻¹ and γ_hex–air = 27.2 dyne cm⁻¹ can be measured. The calculated CA θ = 58° is also in agreement with our experimental outcome, θ = 64°. Similar to superhydrophobic surfaces with a lot of air pockets, this surface possesses a plenty of lubricant pockets. Therefore, the negligible CAH is obtained due to the molecularly smooth interface between the lubricant and liquid phases. Since the liquid drop has to be supported by the solid substrate, the small value of CAH (<2°) is attributed to the contact between a very small part of the bottom of the liquid drop and the PTFE islands.

B. Anti-smudge performance: easy removal of stains

The drying of droplets containing nonvolatile solutes or particles frequently leaves a ring of solutes or particles on a surface rather than a uniform spot. This well-known coffee-ring pattern developed after the liquid drop evaporates has been explained by radial outward flow and contact line pinning.^33,34 Recently, it was found that the coffee-ring effect can be suppressed by various approaches^35–38 and a concentrated stain can be formed on a substrate with weak CAH.³⁸ The formation of the ring-like evaporation stain can be demonstrated by water or oil drops containing polymers and nanoparticles on PMMA, as shown in Fig. 5a. For a water drop of 10 μL having 1 wt% PVA, a circular evaporation stain with diameter about 4 mm is observed and nearly all of the solutes deposit around the contact line. For a water drop having 1 wt% TiO₂, the circular wetted region is covered with a layer of TiO₂ deposit after drying. Nonetheless, the layer thickness near the rim is significantly greater than that in the center region, revealing the characteristic of the ring-like stain. For a decane drop of 10 μL having graphene nanoparticles, the ring-like pattern is also shown after drying. Compared to water drops, its wetted area is large and pattern shape is irregular because CA of decane (θ_a < 5°) is much smaller than that of water (θ_a = 75°).

Fig. 5 Different stain patterns formed on (a) PMMA and (b) the liquid-infused surface.

The contact line of a water or oil drop is strongly pinned on PMMA. During evaporation, the evaporative flux grows as the position approaches the contact line. In order to replenish the liquid evaporating from the edge, the outward flow from the drop interior carries the dispersed nonvolatile solutes or nanoparticles toward the edge. Eventually, a ring-like stain is formed by the solute or nanoparticle deposition in the vicinity of the contact line. In contrast, for a hysteresis-free surface, the contact line pinning is absent for an evaporating drop on such a surface. As a result, the contact line retreats immediately and the CA of the evaporating drop remains unchanged.³⁹ Eventually, a concentrated stain is anticipated to develop. Since the Fomblin-infused surface possesses a nearly hysteresis-free property, the coffee ring effect must be insignificant. As shown in Fig. 5b, the diameter of the concentrated stains formed by water and decane drops of 10 μL shrink to less than 1.4 mm. Because of a very low CA of a decane drop on PMMA, the stain pattern on PMMA is momentously larger than that on the liquid-infused surface.

Once a stain is unfortunately formed on a substrate, the requirement of easy removal of the stain becomes important for an anti-smudge surface. The stain can be removed by an adhesive tape or a liquid drop. In the latter case, a small drop sliding across the stain on a slightly tilted surface is able to carry it away. That is, the drop must move readily along the substrate by a small driving force such as gravity. The comparison of the motility of a drop on an inclined plane is made between PMMA and the Fomblin-infused surface. Fig. 6 shows the sliding angle (α) at which water drops with various volumes (V) start to slide along the PMMA substrate. Evidently, for V ≤ 10 μL, water drops always stick on a vertical PMMA substrate (90°) due to large enough CAH (θ_a = 75° and θ_r = 56°). However, for V ≥ 15 μL, the sliding angle of PMMA declines gradually as V is increased. The sliding angle is α ≈ 50° for V = 15 μL. Even for V = 50 μL, the movement of the water drop on inclined PMMA still requires α ≈ 20°. In contrast, water drops can move easily along the liquid-infused surface due to the absence of CAH. As depicted in Fig. 6, the sliding angle generally decreases with increasing V. Nonetheless, compared to PMMA, water drops on the liquid-infused surface have much lower sliding angles. For V = 5 μL, the drop can slide on the surface at α ≈ 4°. For V ≥ 15 μL, very small inclination (α ≈ 1°) can lead to the movement of water drops on the liquid-infused surface. This outcome reveals that water drops can move readily on the liquid-infused surface due to the large θ_a and negligible CAH. Note that a sessile drop system (liquid in air) is in principle equivalent to a pendant bubble system (air in liquid). It is found that the air bubble of V = 15 μL surrounded by water also can slide along the liquid-infused surface at very small inclination (e.g. α = 2°).

Fig. 6 The variation of sliding angle (α) with drop volume (V) for a water drop on PMMA and the liquid-infused surface.

The anti-smudge ability of PMMA and the Fomblin-infused surface is examined by placing a water drop on the inclined surface to slide across the stain. The adhesion of the evaporation stain on the typical substrate is so strong that the stain cannot be removed by the sliding drop. First, consider TiO₂ stain on PMMA in Fig. 5a. For a water drop with V = 50 μL at the sliding angle 20° ≤ α ≤ 35°, its sliding behavior will be halted by the stain which acts as a hydrophilic defect, as depicted in Fig. 7a. The removal of the TiO₂ stain cannot be achieved. For α ≥ 35°, the drop is able to move across the stain but it still fails to carry the TiO₂ stain away from PMMA, as demonstrated in Fig. 7b. Note that most of the TiO₂ stain is still left on PMMA even when an adhesive tape is used. In contrast, for the concentrated TiO₂ stain on the liquid-infused surface, the adhesion of the deposit is weak so that the stain on it can be easily carried away. As illustrated in Fig. 8a, a water drop with V = 50 μL can carry most of the stain away from the surface at α ≥ 10°. Nonetheless, for smaller α, the stain defect is strong enough to trap the droplet. In addition to the hydrophilic stain, the hydrophobic stain such as PMMA powders on the liquid-infused surface also can be removed by a sliding drop. The PMMA stain is developed by the evaporation of a toluene drop containing 0.5 wt% PMMA. The stain is carried away from the surface at only α = 5° by a toluene drop with V = 50 μL. Some of the snapshots during the removal process of the PMMA stain are shown in Fig. 8b. The experimental outcomes reveal that due to the weak adhesion associated with negligible CAH, the concentrated stains on the liquid-infused surface can be easily cleaned by a liquid drop which can dissolve the residue.

Fig. 7 The TiO₂ stain on PMMA cannot be removed by a sliding water drop of 50 μL. The drop will (a) be pinned at α = 30° but (b) pass through at α = 35°.

Fig. 8 Removal of (a) TiO₂ and (b) PMMA stains on the liquid-infused surface by water and toluene drops of 50 μL, respectively. The sliding angles are small.

C. Anti-smudge performance: wetting competition

When a surface is in contact with a liquid contaminant on another surface, a liquid bridge is formed. When the two surfaces are separated, the liquid bridge ruptures and the liquid residues stay on both surfaces. More liquid is left on the surface with good wettability. That is, the outcome of the wetting competition depends on the wetting properties of the two surfaces. For example, touch panels in contact with human skin suffer from contaminations of undesirable sweat, fingerprints, skin oil and cosmetics when used. It is anticipated that the anti-smudge surface always loses in a wetting competition and almost no liquid residue is left on it. Obviously, the process of contamination involves the wetting competition of a liquid bridge (liquid contaminant) between separating, asymmetric surfaces (e.g., human skin vs. anti-smudge surface). The anti-smudge performance of our liquid-infused surface is also examined through a wetting competition test against a fluorinated PVA surface which is more hydrophobic and lipophobic than SLIPS.

A liquid bridge is formed by a hexadecane drop of 4.87 μL between the fluorinated PVA (upper) and liquid-infused (lower) surfaces. For the fluorinated PVA surface, the advancing and receding CAs are θ_a = 93° and θ_r = 29°, respectively. For the liquid-infused surface (θ_a ≈ θ_r ≈ 64°), its receding CA of oil is quite high compared to typical surfaces. Once the drop is placed on the lower surface, the compressing process proceeds until the CAs on both surfaces reach their θ_a. Then, the separation process is conducted until the liquid bridge is ruptured. Some of the snapshots during the separation process are shown in Fig. 9. During the separation process, the upper contact line will be pinned at first and it begins to retreat with the receding CA as the upper CA decreases to θ_u = θ_r.⁵ If the separation distance is continuously increased, the break-up of the neck of the bridge leads to the formation of two droplets. In the last stage before the rupture, hexadecane in the droplet is gradually partitioned into the two surfaces and the contact line motion depends on the variation of the CA on two surfaces. In this case, the lower contact line continuously retreats and the lower CA stays at θ_l = θ_r until the bridge is pinched off. On the contrary, the upper contact line withdraws at first and then remains pinned till the bridge break-up, leading to θ_r ≤ θ_u ≤ θ_a.

Fig. 9 Some typical snapshots for the comparison between experiment and simulation during the separation process in the wetting competition.

The variations of the CAs (θ_u and θ_l) and base diameters (BD_u and BD_l) on both surfaces with separation distances (h) are shown in Fig. 10a and b, respectively. The quantitative results of the experiment agree well with those of simulation. When the separation process begins, θ_u is reduced from θ_a to θ_r immediately. Because of the highly compressive deformation of the liquid bridge, a slight increase of h leads to the rapid retreat of the contact lines on both surfaces. In the following separation, both the contact lines (base diameters) withdraw with θ = θ_r. As the separation distance increases to h ≈ 1.15 mm, the upper contact line becomes pinned while the lower contact line continues receding. As a result, when h is further increased, θ_u begins to rise from θ_r. Eventually, the liquid bridge is pinched off as the separation distance reaches h ≈ 2.23 mm. Before the liquid bridge ruptures, θ_u rises to about 49° which is smaller than θ_a. After rupture, the CA of the pendant drop (67°) is still not close to the θ_a of the upper surface while the CA of the sessile drop (64°) remains at θ_r of the lower surface. The volume of the pendant drop is about 96.1% of the liquid bridge, which is close to the simulation result (94.1%).

Fig. 10 (a) The variation of contact angle (θ) with separating distance (h) during the separation process. (b) The variation of base diameter (BD) with separating distance (h) during the separation process.

θ_a on the upper surface is higher than that of the lower one. However, θ_r on the upper surface is lower than that of the lower one due to the large extent of CAH. The aforementioned analysis indicates that during the separation process the liquid bridge tends to flow from the surface where the contact line still recedes toward the surface where the contact line is already pinned. The latter corresponds to the advancing pinning,⁵ in which the CA must grow to reach the advancing CA to expand the contact line. Therefore, after rupture, more liquid is seen on the surface upon which the contact line is pinned earlier. The shapes of liquid bridge for the separation process are also acquired from SE simulations. The comparison between simulation results and experimental outcomes shows a good agreement. Wetting competition is closely related to the wettability of surfaces. If the CAH is extremely low, the CA is unique and also an indication of the wettability. The liquid prefers to wet the surface with a low CA rather than that with high CA and thus more liquid is expected on the former surface after rupture. However, when the CAH is present, the solid–liquid interfacial tension is lowered from γ_sl to γ′_sl after wetting and thus the wettability has to be represented by the receding CA associated with γ′_sl after the compressing process. As a result, one anticipates that the liquid prefers to stay on the surface with lower θ_r as the bridge is pinched off. Because the liquid volume is increased on the surface with lower θ_r, the CA has to rise from θ_r toward θ_a. Simultaneously, the contact line has to pin before expanding.

IV. Conclusions

A slippery liquid-infused porous surface is easily acquired by covering an arbitrary substrate with the PTFE seal tape infused with the lubricating liquid Fomblin. Since the thickness of the PTFE film is very thin, these surfaces can be flexible or curved, depending on the solid substrates. In addition, the match of the refractive index between PTFE and Fomblin leads to the surface transparency. The inflation–deflation experiment of liquid drops (water and hexadecane) shows that the hysteresis loop is absent for our liquid-infused surface but is present for a typical substrate such as PMMA. That is, the contact line pinning associated with CAH is essentially eliminated on this surface due to the liquid–liquid interface.

The anti-smudge performances of the surfaces are examined by the removal of stains by sliding drops at an inclined plane and the wetting competition of a liquid bridge between the SLIPS and a surface with CAH. On a typical substrate, the evaporation of a drop containing particles leads to the formation of the ring-like stain due to CAH. However, on the liquid-infused surface, the coffee-ring effect is suppressed by negligible CAH and a concentrated stain is developed. The size of the concentrated stain is always significantly smaller than that of the ring-like stain. In addition, the removal experiment shows that the concentrated stain can be carried away from the surface by a liquid drop which can dissolve the stain particles and move readily at small inclination. The wetting competition of a hexadecane drop between SLIPS and a fluorinated PVA surface is shown. Because of the weak CAH, the receding CA of the SLIPS is about 64° which is generally greater than those of typical substrates. During the separation process, contact line pinning occurs only on the fluorinated PVA surface which possesses a high θ_a but low θ_r. As a result, it seizes almost all the liquid after rupture. A SE simulation of the wetting competition has been performed and the agreement between experiment and simulation is quite good. This consequence reveals that the outcome of the wetting competition is associated with the competition of the receding CAs between the two surfaces.

Acknowledgements

Y.-J. S. and H.-K. T. thank the Ministry of Science and Technology of Taiwan for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27699c

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