The Characterization of Sliding/Pinning Behaviors of Water Droplets on Highly Adhesive Hydrophobic Surfaces

Abstract

Sliding angle (SA) has been widely employed to describe the sliding behaviors of water droplets on substrates. However, it does not work on highly adhesive surfaces since droplets cannot slide along the substrate even at a 90° tilt. In this work, a novel strategy has been developed to characterize the sliding/pinning behaviors based on the collision between a tilted substrate and falling droplets released from a certain height. The critical tilted angle of substrate (i.e., secondary sliding angle, SSA) and the critical releasing height (i.e., H_c) of water droplets have been introduced and measured with the help of commercial equipment (drop shape analysis, DSA). Our results indicate that SSA, H_c and the phase diagram based on the combination of them work well in describing the sliding/pinning behaviors of droplets along highly adhesive substrates. The developed strategy makes it possible to distinguish sliding/pinning behaviors of water droplets along different surfaces as well as different adhesions between them. It can act as an efficient supplement to conventional methods.

1. Introduction

The wetting phenomenon has attracted broad interest since it is of great importance in kinds of applications from microfluidics systems to material science [1,2]. For instance, the first thing during the formation of an adhesive bond is the interfacial molecular contact via wetting [3]. The hydrophobic and hydrophilic surfaces have been defined according to static contact angle (CA). In general, the former and the latter correspond to contact angle values in the ranges of (0° < θ < 90°) and (90° < θ < 180°), respectively. The former and the latter correspond to the higher and lower magnitudes of contact angles, respectively. Especially, the surface with CA above 150° is the so-called superhydrophobic surface, which is always associated with hierarchical roughness [4,5,6,7]. In other words, water contact angle exhibits direct dependence on both surface composition and surface morphology. On one hand, it is facile to determine surface free energy (e.g., Owens–Wendt method [8]) and CA values with the help of commercial equipment. On the other hand, surface texture makes it possible to enlarge the wettability difference via increasing/decreasing water contact angle. In particular, Peta K performed pioneering investigation and developed the effective method of selecting the best scales for observing wetting phenomena [9]. To describe contact angle hysteresis, the supporting substrate is inclined continuously until water droplets begin to slide. This tilted angle is named as the sliding angle (SA). In the critical scenario before sliding, the advancing (at the lower part) and receding (at the higher part) angles correspond to the maximum and minimum contact angles, respectively [10,11,12]. Both CA and SA (as well as advancing and receding angles) have been employed to characterize the surface wetting. For instance, lotus leaf exhibits high CAs and low SAs. The enhanced mobility of water droplets on it can be attributed to the reduced solid–liquid contact resulting from air pockets in roughness [13]. Based on this scenario, the superhydrophobic surface has also been defined as the surface with SA below 5° [14]. On rose petals, however, water droplets cannot slide even at a 90° tilt, although their surface also exhibits the contact angle above 150°. This is the so-called highly adhesive (super-)hydrophobic surface. It has been well known that it is the hydrophilic defects that prevent the continuous movement of water droplets along this kind of surface [15,16,17,18]. In this case, sliding angle fails to distinguish different surface wetting phenomena.

To characterize the wetting behaviors on a highly adhesive hydrophobic surface, force-based techniques have been regarded as excellent solutions. For instance, Drelichb and co-workers developed an efficient way to identify the adhesion/spreading forces between water droplets and glass with the help of a force curve from a high-sensitivity microelectronic balance [19]. During their measurements, much attention has been paid to the force and droplet movement in the normal direction. To describe the sliding behaviors of droplets along surfaces, Tadmor measured the lateral adhesion forces at the solid–liquid interface for the first time using a centrifugal adhesion balance [20]. Xue et al. determined the lateral adhesion/friction force quantitatively with the help of a capillary sensor in which the deflection of the capillary has been converted to the friction force based on Hooke’s law [21,22]. Recently, this method has been further optimized by Pesika. In their investigation, the capillary sensor was replaced by a ring probe, thereby allowing for the measurement of higher friction force [23]. Gao and Berger measured the adhesion force between water droplets and substrate and found that this force can be divided into static and kinetic regimes, which is analogous to solid–solid friction [24]. After that, McHale and co-workers performed important theoretical investigations from the view of surface free energy. Their results indicated that the in-plane frictional force is proportional to the normal component of surface tension force. The obtained equation can fit the literature data well [25].

So far, the characterization of sliding/pinning behaviors of water droplets on highly adhesive surfaces concerns the requirements on specialized instruments and high experimental skills. It remains challenging to describe the sliding/pinning behaviors on this kind of surface by means of conventional surface/interface measuring devices. In this work, therefore, a novel strategy has been developed to characterize sliding/pinning behaviors of accelerated water droplets (released from a certain height, Scheme 1) along a surface based on their collision with substrates. Two parameters, including secondary sliding angle (SSA) and critical releasing height (H_c), have been introduced. Different from the general sliding angle, SSA represents the critical tilted angle below which the bounced droplets cannot slide away along the substrate (critical scenario from Scheme 1A,B). H_c corresponds to the critical height that the water droplets can be pinned on the surface without sliding at their first collision (Scheme 1C,D). SSA, H_c, and phase diagrams of them work well in describing sliding and pinning behaviors of water droplets along different highly adhesive surfaces, enabling the distinction of different adhesions between them. The developed strategy can act as an efficient supplement to conventional methods (e.g., sliding angle) in the field of surface/interface. Furthermore, it exhibits great application potential since both SSA and H_c can be measured with the help of commercial instruments (e.g., drop shape analysis, DSA).

2. Materials and Methods

The blend of a commercial water-repellent agent (FE-8) and surfactant was taken as a model system for the investigation. The structures of them are shown in Figure 1. Both of them were kindly provided by Hangzhou Transfar Fine Chemicals Co., Ltd. (Hangzhou, China).

FE-8 and surfactant were dissolved in butanone at 70 °C, followed by casting onto pre-cleaned glass. The films with various compositions of FE-8 and surfactant were prepared upon solvent evaporation at room temperature completely. The specimen with 95 wt% FE-8 and 5 wt% surfactant was labelled as F95. In the same way, F97 (97% FE-8), F99 (99% FE-8), and F100 (100% FE-8) were obtained. The surfaces of various specimens were observed by means of atomic force microscopy (AFM, E-sweep, Seiko, Japan) in tapping mode. A tip with a force constant of 2 N/m was adopted. The surface composition was assessed by means of an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher, London, England) at room temperature. The measuring angle between the X-ray and specimen surface is 90°. The detecting thickness is less than 10 nm. The static contact angles, sliding angles, and maximum droplet volume were measured by drop shape analysis (DSA100, Kruess, Hamburg, Germany). The images of water droplets were obtained with the help of a high-speed camera (FASTCAM Nova S16, Photron, Tokyo, Japan) or a camera equipped on DSA100. In this work, each measurement has been repeated at least three times. In this work, each measurement has been repeated at least three times. The average of three parallel values was calculated according to Equation (1).

$$X=\sqrt{{\displaystyle \frac{\sum _{i=1}^N{\left(x_i-\mu \right)}^2}{N}}}$$

(1)

where X can represent the water contact angle, sliding angle, secondary sliding angle, and critical height. The parameters of X_i and μ are measured values and the average of them.

3. Results and Discussion

It has been well known that surface wettability is under the control of surface composition and morphology [26,27,28]. Therefore, we performed AFM and XPS measurements. In AFM images shown in Figure 2, our attention has been paid to the following points. (1) No obvious crack or gap can be observed in the scanned areas, indicating that the substrate (glass) has been covered by FE-8 and surfactant completely. (2) There are no hierarchical structures, but only protrusions distributing on rough surfaces. This scenario is completely different with lotus leaf and rose petals, in which hierarchical roughness contributes to a superhydrophobic surface via the Cassie state [29,30]. (3) All images exhibit comparable roughness, ranging from 20.3 nm to 24.9 nm. It is noteworthy that the root-mean-square roughness (S_a) is defined and calculated according to Equation (2).

$$S_a=\sqrt{{\displaystyle \frac{1}{A}}\iint {\left\{F\left(x,y\right)-Z\right\}}^{2}dxdy}$$

(2)

where A and Z are the corresponding area and average height at the calculated position, respectively. In these specimens, the difference of surface roughness can be neglected since all average values of S_a are within the experimental error bar. In the blend of FE-8 and surfactant, the former is hydrophobic and has been widely used as a water-repellent agent in the modification of textiles due to the low surface free energy. The latter exhibits good interaction with water [because of the existence of trimethylammonium chloride (Figure 1B)] and oil and can act as an excellent surfactant in a water/oil system. XPS measurements were performed to detect the compositions on film surface (not in bulk). In Figure 2E, there are signals of C, O, and F elements in the XPS spectrum. The area ratio between F and C has been employed to assess the surface composition quantitatively since there is an F element only in FE-8 (Figure 1). As shown in Figure 2F, the area ratio in F95 exhibits the lowest magnitude in all specimens. When the weight fraction of FE-8 is increased from 95% to 100%, a boost in the area ratio (F/C) from 2.8 to 5.2 can be observed. This result makes it clear that there is more FE-8 enriching on the specimen surface upon increasing the weight fraction of it in the blend. Relative to F95, the lower magnitude of water contact angle and enhanced sliding behaviors of water droplets can be expected on the surface of F100.

The contact angles of our specimens were measured via drop shape analysis in sessile mode at room temperature. In this work, there are both FE-8 and surfactant on surfaces. The former is hydrophobic because of the low surface free energy, while the latter exhibits better interaction with water due to the existence of trimethylammonium chloride (Figure 1B). On a heterogeneous surface, the contact angle (θ) can be determined by Equation (3) [31].

$$\mathrm{c}\mathrm{o}\mathrm{s}\theta =f_{1}cos{\theta }_1+f_{2}cos{\theta }_2$$

(3)

where $f_1$ and $f_2$, are area fractions of hydrophobic region and hydrophilic region, respectively. ${\theta }_1$ and ${\theta }_2$ represent the contact angles in the corresponding regions. In Figure 3A, water contact angles (CAs) increase slightly (from 116° to 120°) with increasing FE-8 in the blend. This result can be interpreted by the higher content of FE-8 on the film surface (Figure 2F) and the consequent higher magnitude of $f_1$ in Equation (3). To obtain the sliding angles, water droplets were produced on the surface of the specimen, followed by inclining the substrate continuously. As shown in Figure 3A, the droplets do not slide along the substrate even at a 90° tilt. In other words, all specimens exhibit the same sliding angle of 90°. The parameter of sliding angle, therefore, fails to distinguish sliding behaviors of water droplets on these surfaces. In this case, it is impossible to obtain advancing and receding angles since the critical scenario of water droplets sliding along a substrate cannot be achieved even when the tilted angle reaches 90°. Then, we tried to characterize the surface adhesion with the help of maximum droplet volumes [32]. It is a conventional method in which water is injected into the sessile droplet slowly and continuously until the droplet falls along the substrate (at the tilted angle of 90°). The maximum droplet volumes are measured and shown in Figure 3B. It is ~42 μL in the specimen of F95. In the case of F97, F99, and F100, it decreases to 27, 18, and 15 μL, respectively. The monotonous decrease in maximum droplet volume indicates the great difference of droplet sliding behaviors among our specimens. This difference, however, cannot be detected by means of sliding angle (Figure 3A) or advancing/receding angles. Relative to other specimens, the enhanced adhesion between water droplets and F95 originates from the higher weight fraction of surfactant since it has better interaction with water due to the existence of trimethylammonium chloride [33].

To enlarge the difference in sliding behaviors of water droplets on various specimens, the droplet is released at a certain height to get the velocity of V₀ when it reaches the tilted substrate (Scheme 1A). When the higher tilted angle is adopted, we can observe the typical collision and bouncing of droplets. To show the deformation and sliding behaviors of water droplets clearly, a high-speed camera has been employed. Figure 4A shows the images at the tilted angle of 40° and releasing height of 100 mm. After knocking against the substrate, the droplet deforms into film (0.0068 s), followed by bouncing off the surface (0.0150 s). Due to the adhesion between the surface and water, there is a small “tail” below the droplet (0.0236 s). Finally, the droplet recovers to round (0.0258 s) and slides away (0.0312 s). Then, the same measurement (releasing height of 100 mm) was performed on the substrate with a smaller tilted angle. In Figure 4B (10°), we can find a similar process, including deformed film (0.0062 s), bounced droplet (0.0102 s), and tail (0.0248 s). At last, the droplet is pinned on the substrate and cannot move any longer (0.0438 s and 0.0520 s). We repeat the measurement discussed above (i.e., to track the movement of the droplet) upon increasing the tilted angle from 10° until the bounced droplet can slide away along the surface. It is noteworthy that during the following measurements, the high-speed camera has been replaced by the camera equipped on DSA, which is an efficient way to harness the full application potential of this method. Our result indicates that the bounced droplet knocks at and bounces off the substrate, followed by sliding along the substrate at the tilted angle of 15° (Figure 4C). This result has good agreement with the critical scenario of Scheme 1A,B. This tilted angle (15°), obtained based on the bounced droplet, is named as the secondary sliding angle (SSA, θ₂) to distinguish it from the general sliding angle in which the substrate is tilted continuously after the droplet is produced on the substrate. SSA corresponds to the critical tilted angle below which the bounced droplet cannot slide away along the substrate (e.g., Figure 4B, the tilted angle of 10°). The value of SSA in F95, therefore, is determined to be 15° at the height of 100 mm. According to the method discussed above, we can measure SSA for various specimens. The results are shown in Figure 5. In F95, SSA exhibits a magnitude of 15°. When the weight fraction of surfactant decreases from 5% to 0% (F100), a monotonous decrease in SSA from 15° to 7° can be observed. The composition dependence of SSAs in Figure 5 agrees well with the result of the maximum volume of water droplets (Figure 3B). The enhanced water repellency (e.g., in the specimen of F100) is a direct consequence of a lower weight fraction of surfactant. With the help of SSA, we can distinguish the sliding behaviors of water droplets on four specimens while the general sliding angle (Figure 3A) does not work. It is noteworthy that the values of SSA depend crucially on the releasing height.

In order to identify the critical conditions for sliding and pinning behaviors (critical scenario from Scheme 1C,D), two methods have been adopted. For one thing, the tilted angle has been reduced further. In this case, however, there is always a bouncing motion of water droplets due to the higher magnitude of releasing height. For another thing, the water droplet has been released at the reduced height. When the height reaches a certain value, the droplet cannot slide along the tilted surface but is pinned at its first collision on the substrate. This value is then defined as critical height (H_c) at this tilted angle. The parameter of H_c, therefore, corresponds to the critical height below which the released droplet cannot slide or bounce after the collision with the substrate. In the specimen of F97, for instance, a water droplet can slide along the substrate in the case of a higher releasing height (e.g., 100 mm). When the height decreases to 6.5 mm, the droplet is pinned on the surface at its first collision with the substrate (Figure 6A,B). The critical height of F97 is determined to be 6.5 mm at the tilted angle of 13° (SSA shown in Figure 5). In this way, we can measure critical heights of all specimens at this tilted angle (13°), and the results are shown in Figure 6C. The specimen of F95 exhibits the highest magnitude of H_c. When the weight fraction of surfactant decreases, a monotonous decrease in H_c from 9.0 to 1.5 mm can be observed in Figure 6C. Clearly, the lower magnitude of H_c is a necessary consequence of the reduced adhesion between the corresponding substrate and water droplets.

Figure 5 shows SSA at the releasing height of 100 mm. The corresponding SSA of F97 has been determined to be 13°. Then, we measured the critical heights of all specimens at this tilted angle (13°, in Figure 6C). It is noteworthy that the critical height is strongly correlated with the tilted angle, and vice versa. According to the methods discussed in Figure 5 and Figure 6, we can obtain the magnitudes of critical height at various tilted angles. The results are shown in Figure 7. In all specimens, the critical height decreases with increasing tilted angle. For instance, the critical height decreases from 1.4 mm to 0.35 mm in F95 when the tilted angle increases from 5° to 30°. The plot of critical height and tilted angle provides a phase diagram to predict the sliding/pinning behaviors of water droplets on various surfaces. In Figure 7, there are two regimes. In Regime One (in green), the releasing height is above the critical height at the corresponding tilted angle. The accelerated water droplet can slide along this surface after its collision with the substrate. This area, therefore, can be called the “Sliding Regime”. On the contrary, water droplets can be pinned on the surface when they are released according to the conditions in Regime Two, producing the “Pinning Regime” (in blue). The black dashed line in Figure 7, therefore, corresponds to the boundary line of the two scenarios discussed above for F95. The location of this boundary line plays an important role in determining the sliding/pinning behavior of the water droplet and its adhesion with the substrate. For instance, the decrease in surfactant in the blend results in the down-shift of the “boundary line” (from F95 to F100), leading to the expansion of Regime One in the phase diagram. This result indicates that water droplets exhibit the enhanced sliding behavior along the surface with higher content of FE-8 (e.g., F100). It can be attributed to reduced adhesion with the substrate and has good agreement with the maximum volume of water droplets shown in Figure 3B.

To validate the universality of new parameters, we tried to measure and compare the values of SSA and H_c on highly adhesive and lowly adhesive hydrophobic surfaces. For this purpose, both the lotus-leaf-like specimen (specimen 1) and the rose-petal-like specimen (specimen 2) were prepared according to the method reported in the literature [34,35]. Briefly, the lotus-leaf-like specimen was prepared as follows. The blend solution (chloroform as solvent) of silica nanoparticles and poly(methyl methacrylate) (PMMA) was spin-coated on pre-cleaned glass, followed by coating 1H, 1H, 2H, and 2H-perfluorooctyltriethoxysilane via chemical vapor deposition (CVD) to reduce surface free energy [from 70.1 mJ/m² (before) to 21 mJ/m² (after)]. The blend solution (chloroform as solvent) of silica nanoparticles and poly(methyl methacrylate) (PMMA) was spin-coated on pre-cleaned glass, followed by coating 1H, 1H, 2H, and 2H-perfluorooctyltriethoxysilane via chemical vapor deposition to reduce surface free energy. The surface morphology of specimen 1 is shown in Figure 8A. As a result of low surface free energy and a rough surface, specimen 1 exhibits the water contact angle of 111.5° and a sliding angle of 6.8° (Figure 8B), revealing the lotus-leaf-like (i.e., lowly-adhesive) surface. Then, a small amount of dopamine (DA) solution was spray-coated onto the surface of specimen 1, followed by in situ polymerization sufficiently. This process does not produce significant influence on surface morphology. The existence of a bit of poly-dopamine (PDA) on specimen 1 leads to the hydrophilic defects on a hydrophobic surface and the occurrence of a rose-petal-like surface (specimen 2). The water contact angle (112.3°) and sliding angle (90°) in the later (Figure 8B) indicate the highly adhesive, hydrophobic (i.e., rose-petal-like) surface. We measured the values of SSA (at the releasing height of 100 mm) and critical height (H_c) based on the methods discussed above. The results are shown in Figure 8C. SSA and H_c in specimen 2 are located at 15.2° and 7.5 mm, respectively. Both of them are much higher than those in specimen 1 (4.9° and 1.2 mm, respectively). This result indicates that two new parameters work well in the identification of sliding/pinning behaviors of water droplets along lotus-leaf-like and rose-petal-like surfaces.

4. Conclusions

The sliding/pinning behaviors of water droplets along surfaces have usually been characterized with the help of the sliding angle. This parameter, however, does not work in the case of highly adhesive surfaces. In this work, the measurement of sliding angle has been performed by replacing the static water droplet with the droplet released from a certain height. The resultant initial velocity of the droplet can enlarge the difference of adhesion on various surfaces based on the collision between the tilted substrate and the water droplet. Two parameters, including secondary sliding angle (SSA) and critical height (H_c), have been introduced. The former represents the critical tilted angle below which the bounced droplets cannot slide along the surface, while the latter corresponds to the critical height for the onset of water droplets sliding. SSA, H_c, and the phase diagram of them work well in describing the sliding/pinning behaviors of water droplets on a highly adhesive hydrophobic surface. They can act as a supplement to conventional parameters in the field of dynamic wetting, especially on highly adhesive surfaces. The strategy developed in this work exhibits great application potential since all measurements can be performed on commercial instruments.