Excellent Superhydrophobic Cone-Array Surfaces with Low Contact Time of Droplet Pancake Bouncing Under Various Conditions

Abstract

Superhydrophobic surfaces with a low liquid–solid contact time have huge application prospects in anti-icing, corrosion-resistant, self-cleaning, etc. Significant attempts have been devoted to reducing the contact time through altering the hydrodynamics of the process through which the droplet contacts the superhydrophobic surface. However, these works are rarely considered to be related to the influence of environmental conditions (e.g., the pH of the droplet, salinity of the droplet, droplet viscosity, and supercooled droplet impact). Here, we report various superhydrophobic cone arrays (SCAs) with low droplet impact contact times under various conditions (pH of the droplet, salinity of the droplet, droplet viscosity, droplet temperature, etc.). We demonstrate that the low contact time of the droplet impacting cone-arrays can be optimized via the critical Weber number, pillar-to-pillar spacing, and pillar height (e.g., 11.1, 350 μm, and 300 μm, respectively). The lowest droplet contact time of ~6 ms, which is reduced by more than 60% compared to conventional bouncing, can be achieved. In addition, directional pancake bouncing behaviors can achieve the largest horizontal displacement (85% of the droplet size, ~3 mm) on a tilted SCA with optimal tilt angles. These findings offer insights into the interface effect for controlling wetting that would extend the practical applications, e.g., liquid repellency, anti-corrosion, anti-icing, heat transfer, etc.

1. Introduction

Superhydrophobic surfaces have gained significant attention over the past two decades for their unique attributes, such as self-cleaning [1,2,3], anti-icing [4,5,6,7], anti-fouling [8], corrosion resistance [9], and drag-reducing [10,11]. Great efforts have been devoted to realizing superhydrophobic surfaces with low-surface-energy chemistry and micro–nano structural optimization [12,13,14,15]. We usually use the static contact angle (CA, usually larger than 150°) and rolling-off angle (RA, less than 10 degrees) to evaluate whether the surface has superhydrophobic properties. However, for many practical applications, repelling an impacting water droplet is also important [16]. As a droplet impacts a superhydrophobic surface, it spreads across the substrate to reach a maximum spreading diameter, then recoils, and eventually leaves the surface. During the process, the kinetic energy is partially transformed into interfacial energy, and then the interfacial energy is transformed into kinetic energy for the droplet to bounce off the surface [17,18]. The contact time controls the extent to which mass, momentum, and energy are exchanged between the drop and the surface [19], and reducing the contact time is often advantageous. Thus far, significant attempts [20,21,22,23,24,25,26,27,28,29,30,31,32,33] have been devoted to reducing the contact time through altering the hydrodynamics of the process whereby the droplet contacts the superhydrophobic surface. Liu et al. [20] experimentally demonstrated that the drop bouncing is intricately modulated by the surface morphology (center-to-center spacing, apex angle). Further, they reported that pancake bouncing on superhydrophobic posts enables a reduction in the contact time by a factor of over four [24]. A theoretical model involved the contact timescale (k) related to the Weber number (W_e), and the surface geometry was proposed to predict the bounce mode. Song et al. [23] demonstrated that pancake bouncing of a droplet can occur on superhydrophobic surfaces with millimeter diameters and <1 height–diameter ratios, which can be easily fabricated over large areas. However, much of these works were conducted under ambient conditions. Droplet mobility that departs from the ambient conditions (pH of the droplet, salinity of the droplet, droplet viscosity, and supercooled droplet impact) is yielding new insight into, and unveiling new requirements for, the rational design of superrepellent surfaces. Furthermore, directional removal of a droplet is also necessary for a surface to keep dry since the bouncing droplet will eventually fall back to the original impact spot on a horizontally placed substrate. Therefore, it is necessary to construct superhydrophobic surfaces with a low droplet impact contact time under various conditions and directional droplet bouncing mainly through the preparation of asymmetric superhydrophobic structures [34,35,36].

Herein, we report on excellent superhydrophobic cone-array surfaces with low contact times of droplet pancake bouncing in various conditions. The vertical and tilted superhydrophobic cone arrays (v-SCA and t-SCA) are fabricated by a replication technique followed by an electroplating process [37] and chemical etching or the spray coating technique. We first optimize the influencing factors (Weber number (W_e), pillar-to-pillar space, and pillar height) on the droplet impact behaviors under ambient conditions. With the optimized structures (the critical We, pillar-to-pillar space, and pillar height are 11.1, 350 μm, and 300 μm, respectively), we reveal the droplet impact behaviors under various conditions (pH of the droplet, salinity of the droplet, droplet viscosity, and droplet temperature). We further demonstrate that pancake bouncing with a low droplet contact time (~6 ms, reduced by more than 60% compared to conventional bouncing) can be realized on the v-SCAs. In addition, directional pancake bouncing behaviors were regulated on t-SCAs with different tilt angles, where the largest horizontal displacement is up to 85% of the droplet size (~3 mm). This study indicates that the SCAs can effectively control the contact time of droplet pancake bouncing under various conditions. This investigation of the bouncing behaviors under different conditions would extend the practical applications to include liquid repellency, anti-corrosion, anti-icing, heat transfer, etc.

2. Materials and Methods

2.1. Fabrication of Vertical Superhydrophobic Cone Arrays (v-SCAs)

A commercial sewing needle fixed onto a programmable 3D platform was firstly exploited to punch regular holes on a commercial high-density polyethylene to prepare the mold (Figure S1a). Parameters such as the depth and center-to-center space can be controlled by the digital controller. Then, polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning, Freeland, MI, USA) was used to replicate the structures of the mold. After peeling off the mold, PDMS cone arrays with different center-to-center spaces and pillar heights can be obtained. The dimension parameters such as the center-to-center space (s) and pillar height (H) can be controlled by the digital controller. Thus, the cone arrays with different center-to-center spaces (s = 250 μm, 300 μm, 350 μm, 400 μm) and pillar heights (H = 200 μm, 300 μm, 400 μm, 500 μm, 600 μm) are obtained by using PDMS. For the vertical SCA (i.e., v-SCA), an electroplating followed by etching technique [37] was used to fabricate the nanostructure on the PDMS cone arrays. The electroplating solution for copper-plating the PDMS cone arrays was prepared by mixing 25 g of sulfuric acid (Sigma-Aldrich, Darmstadt, Germany), 2.5 g of hydrochloric acid (Sigma-Aldrich, Darmstadt, Germany), 80 g of copper sulfate (Sigma-Aldrich, Darmstadt, Germany), 50 g of formaldehyde (Sigma-Aldrich, Darmstadt, Germany), and 500 mL of deionized (DI) water and stirring for 1 h at room temperature. Before Cu electroplating, a 5 nm thick Au film was deposited on the PDMS cone arrays as the conductive layer. The PDMS cone arrays with the 5 nm thick Au film (the cathode) were copper-plated using a pure copper frame (the anode) at an applied voltage of 3 V for 5 s. A layer of pure copper was deposited onto the PDMS cone arrays. The as-fabricated surface was chemically etched in a freshly mixed aqueous solution of 2.5 mol/L sodium hydroxide and 0.1 mol/L ammonium persulphate at room temperature for ~60 min, followed by thorough rinsing with deionized water and drying in an oven at room temperature under vacuum. Finally, all v-SCAs were treated by using 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane under vacuum in an oven for 5 h at 90 °C (Figure S2). X-ray Photoelectron Spectroscopy (XPS) was used to confirm the success of the coating process (Figure S3a).

2.2. Fabrication of Superhydrophobic Cone Arrays with Tilt Angles (t-SCAs)

The vertical PDMS cone arrays were first silanized using 1H,1H,2H,2H-perfluorodecyltriethoxysilane. The negative mold of the PDMS cone arrays (Figure S1b) was obtained after a replica process. Then, the vertical cone arrays were fabricated using the shape memory polymer (SMP) [38]. Bisphenol A diglycidyl ether (BADGE, Sigma-Aldrich, Darmstadt, Germany), polyether amine (jeffamine D230, HUNTSMAN, Shanghai, China), and 1-decamine (DA, Aladdin, Shanghai, China) were mixed at a molar ratio of 4:1:2. Bubbles were removed in the vacuum desiccator. The mixture was poured onto the negative PDMS mold and then covered by a glass sheet as the substrate. Next, it was placed into the oven to cure at 100 °C for 1.5 h and then cured at 130 °C for 1 h. After cooling to room temperature, the mold was removed to obtain a vertical cone array of epoxy resin. The vertical cone arrays were heated to T_g (61 °C) on a hot plate for 5 min, followed by covering with clean PDMS (thickness: ~3 mm) and shearing along one direction using a cylindrical iron bar until it cooled down completely. The samples were heated again at different temperatures (below T_g) for different times to obtain cone arrays with different tilt angles. The spray coating solution was prepared as follows: 0.12 g of hydrophobic fumed silica nanoparticles (20 nm, Aerosil R202 (EVONIK, Essen, Germany)) and 2.5 g of dispersant (Nanosil 9009 (Nanosil, Sunnyvale, CA, USA) is a blend of nanosilica with PDMS and organosilane dispersed in butylacetate) were added to make a translucent suspension. Then, ultrasonic dispersion was continued for another half an hour. The PDMS cone arrays were sprayed using an airbrush (H set, Paasche Airbrush Co., Kenosha, WI, USA) at a distance of 10 cm with a N₂ pressure of 58 psi. The samples were air dried for 30 min. XPS was used to confirm the success of the coating process (Figure S3b). The t-SCAs were effectively achieved.

2.3. Characterization of Superhydrophobic Cone Arrays

SEM images of the superhydrophobic cone arrays were obtained by an environmental scanning electron microscope (Quata FEG 250, FEI, Boston, MA, USA) with a high-vacuum mode and an accelerating voltage of 10 kV using samples without magnetization. The water contact angles and drop rolling-off angles were measured by an optical contact angle meter system (OCA20, DataPhysics, Filderstadt, Germany) at ambient temperature. Drops of 10 µL were deposited on the samples, and the average CA and RA values at three different positions were taken as the final data. The dynamic contact angles on a flat PDMS surface were measured by an optical contact angle meter system (OCA20, DataPhysics, Filderstadt, Germany) at ambient temperature. The droplet volume was 15 µL.

2.4. Observation of Droplet Bouncing Behaviors

The bouncing experiments on the superhydrophobic cone arrays were recorded and observed by using a high-speed camera (Phantom V9.1, Vision Research, Wayne, NJ, USA) with a spatial resolution of 960 × 240 pixels at a frame rate of 8000 fps. The radius and volume of the droplet were controlled by a syringe pump. Water droplets (density = 0.99820 g/mL, surface tension = 72.8 mN/m) of 17.9 µL were generated from a fine needle equipped with a syringe pump (RWD Life Science, Shenzhen, China) from pre-determined heights. The bouncing experiments at various temperatures were conducted by placing the SCAs on a cold stage (STIK Co., LTD, Shanghai, China) with a temperature controller. The contact time was defined as the time interval between the first contact of the droplet with the substrate surface and the second departure from the surface (Figure S4). Pancake bouncing was defined as a ratio (Q) [24] of the lateral extension diameter of the water droplet when it detached from the surface (d_jump) and the maximum lateral extension diameter of the water droplet in the jumping processes (d_max) of larger than 0.8, that is, Q = (d_jump/d_max) > 0.8. The Weber number (W_e) is defined as $We=\rho v^2{r}_0/\gamma$, where $\rho$, $v$, $r_0$, and $\gamma$ relate to the density, impact velocity, radius, and surface tension of water droplet, respectively.

3. Results

3.1. Fabrication of Vertical SCAs (v-SCAs) and Tilted SCAs (t-SCAs)

Figure 1a illustrates the typical process of the SCA fabrication (see Section 2). A commercial sewing needle was used to punch holes in the high-density polyethylene so as to obtain a cone negative mold (step 1). By using polydimethylsiloxane (PDMS), the positive cone arrays are prepared via the replica method (step 2), i.e., the vertical cone arrays can be made effectively (step 3). With the method of Cu electroplating and chemical etching followed by silanization (step 4), the v-SCAs can be achieved successfully. Based on the v-SCAs, the t-SCAs can be further achieved via the process of molding followed by pressing and then the spray coating technique with SiO₂ nanoparticles (steps 5–8). Thus, two styles of v-SCAs and t-SCAs are achieved successfully. Figure 1b shows an SEM image of the v-SCA with a center-to-center spacing of ~300 μm and pillar height of ~600 μm, where a needle-like nanostructure covers the-SCA (see the inset). Figure 1c shows an SEM image of the t-SCA (tilt angle: 60°) with a center-to-center spacing of ~300 μm and pillar height of ~600 μm, where there is a nanoparticle coating over the t-SCA. The surfaces of the v-SCA and t-SCA exhibit a superwettability property with an apparent contact angle of 154° ± 1.2° and sliding angle of ~0° (Figure S5).

3.2. Structure Optimization for Droplet Pancake Bouncing

Firstly, the properties of droplet pancake bouncing are revealed on a v-SCA. An individual water droplet is used to impact the v-SCA from a certain height, which is recorded with a high-speed camera at a frame rate of 8000 fps. The radius of the droplet (r₀) is 1.6 mm with a volume (V) of 17.9 μL. Droplets are dropped from different heights, corresponding to different Weber numbers (W_e). Figure 2 shows the relationship between We and the bouncing behaviors (Supplementary Video S1). Three kinds of bouncing behaviors (i.e., conventional bouncing, pancake bouncing [24], rim breakup [39]) are generated. At a low W_e = 8.9, the impacting droplet penetrates into the v-SCA (s = 300 μm and H = 600 μm) at a time of 1.4 ms, spreads to a maximum lateral extension at 3.5 ms, and recoils at 6.5 ms. Herein, the property of droplet pancake bouncing is characterized by the ratio (Q). The v-SCA can generate Q = d_jump/d_max~0.40 (Q < 0.8) at W_e = 8.9 with a total contact time of 16.6 ms (Figure 2a). As W_e increases, pancake bouncing occurs. The critical W_e for pancake bouncing is 11.1. At this W_e, the droplet detaches from the surface directly without experiencing recoiling with Q ~0.91 and a total contact time of 6.6 ms (Figure 2b). When the W_e reaches 34.5, rim breakup occurs. During the spreading process, the rim of the droplet splits and forms small droplets. The center liquid lamella retracts and bounces off the surface as a whole (Figure 2c). The contact times of the rim breakup mechanism (6.5 ms) and pancake bouncing (6.6 ms) are nearly the same.

Figure 3 shows the influence of the center-to-center spacing (s) on the pancake bouncing behavior (see Supplementary Video S2). The experiments were conducted using a v-SCA with H = 600 μm at W_e = 13.3. The contact times are 6.0 ms, 6.3 ms, 7.2 ms, 7.6 ms, and 13.0 ms, corresponding to s = 200 μm, 250 μm, 300 μm, 350 μm, and 400 μm, while the Q_s are 0.98, 0.98, 0.83, 0.80, and 0.45, respectively. The s ranged from 200 to 350 μm for v-SCAs showing pancake bouncing, while s = 400 μm exhibited a conventional complete rebound. The critical s is 350 μm for pancake bouncing. The increase in the center-to-center spacing (s) between posts leads to a longer contact time for bouncing droplets due to the combination of reduced capillary energy storage and loss of timescale synchronization [24]. On the one hand, capillary forces arise from liquid–solid interfacial tension (γcosθ_Y) and depend on the total contact line length. Increasing s reduces the number of posts per unit area (∝1/s²), thereby decreasing the total contact line length. Less capillary energy is stored during liquid penetration, weakening the driving force for vertical drainage and delaying droplet detachment. On the other hand, the drainage time (t_↑) scales as t_↑~$\sqrt{s^2{r}_0\rho /(-\beta \gamma \mathrm{c}\mathrm{o}\mathrm{s}{\theta }_Y)}$. As s increases, t_↑ grows, while the spreading time t_max∝$\sqrt{\rho r_0^3/\gamma }$ remains independent of s. The ratio k = t₁/t_max~$s/\left(r_0\sqrt{-\beta cos{\theta }_Y}\right)$ increases, causing t_↑ ≫ t_max. The droplet begins retracting before complete drainage, triggering conventional bouncing with longer contact times.

Figure 4 shows the influence of the cone height (H) on the bouncing behavior (see Supplementary Video S3). The experiments were conducted by using v-SCAs with a constant s = 300 μm at W_e = 13.3. The contact times are 14.0 ms, 7.5 ms, 6.1 ms, 6.3 ms, and 6.4 ms, corresponding to H = 200 μm, 300 μm, 400 μm, 500 μm, and 600 μm, while the Q_s are 0.47, 0.9, 0.93, 0.94, and 0.92, respectively. The v-SCAs with H = 300 μm, 400 μm, 500 μm, and 600 μm show pancake bouncing, while H = 200 μm exhibits a conventional complete rebound. It indicates a critical H of 300 μm for droplet pancake bouncing. Higher posts allow droplets to penetrate deeper into the textured surface during impact. This increases the liquid–solid contact area, amplifying the capillary energy stored via the interfacial tension difference. More energy is stored in the penetrated liquid, which is later converted into vertical kinetic energy during capillary drainage, propelling the droplet upward.

3.3. Influence of the Environmental Condition on the Droplet Bouncing Behavior

As the practical application environment is usually complex and the surface properties may change under these conditions, the study of bouncing behavior under specific conditions is essential. The bouncing behavior was tested with droplets with different pH values, after different soaking times in sea water, droplets with polyelectrolytes of positive and negative charge, and under different temperatures. Due to environmental pollution, water droplets in nature are generally not neutral. The v-SCA (s = 250 μm, H = 600 μm) was used to study the influence of pH at W_e = 13.3. Figure 5a shows the variations in the contact time with droplets with different pH values (see Figure S6 and Supplementary Video S4). The contact time remains at ~6 ms in the entire range of pH (0–14), indicating that pancake bouncing occurs for the entire range of pH. The v-SCA remains stable under acid and alkali conditions. The v-SCA also remains stable to brine. Figure 5b shows the variations in the contact time with soaking time in sea water (see Figure S7). Pancake bouncing still occurs even when soaked in sea water for 50 h. The contact time remains at ~6.5 ms. Recently, Jin et al. discovered that polyelectrolytes had a strong influence on ice propagation [30]. Given this phenomenon, we employed poly (sodium 4-styrenesulfonate) (PSS, negative charge) and poly (diallyldimethylammonium chloride) (PDAD, positive charge) to study the influence of polyelectrolytes on the bouncing behavior. Figure 5c shows the variation in the contact time with the concentration of PSS in water droplets (see Figure S8 and Supplementary Video S5). The contact time remains at ~5.8 ms for a PSS concentration from 3 to 5%, and a sudden large increase occurs at the concentration of 6% with the contact time increasing to 12.3 ms. The contact time increases to 12.6 ms when the concentration increases to 7%. The critical PSS concentration for pancake bouncing is 6%. Figure 5d shows the variation in the contact time with the PDAD concentration (see Figure S9 and Supplementary Video S6). The contact time increases slightly from 5.6 ms to 6.8 ms as the PDAD concentration increases from 2% to 3% and 4%, and a sudden large change occurs at the concentration of 5% with the contact time increasing to 12.3 ms. The critical PDAD concentration for pancake bouncing is 5%. One of the major applications of pancake bouncing is in the field of anti-icing; however, most of the previous work was conducted under normal temperature. We studied the influence of temperature on the bouncing behavior. The bouncing experiments at various temperatures were conducted by placing v-SCAs on a cold stage with a temperature controller (see Figure S10 and Supplementary Video S7). As shown in Figure 5e, for the v-SCA with s = 250 μm and H = 600 μm, at W_e = 13.3, pancake bouncing occurs from 25 °C to −15 °C, with the contact time increasing from 6.5 ms to 7.2 ms (s = 250 μm). For the v-SCA with s = 300 μm and H = 600 μm, at W_e = 13.3, pancake bouncing occurs from 25 °C to −10 °C, with the contact time increasing from 7.0 ms to 8.0 ms. Conventional rebound occurs when the temperature decreases to −15 °C. For the v-SCA with s = 350 um, pancake bouncing occurs from 25 °C to 0 °C, with the contact time increasing from 7.6 ms to 8.0 ms. Conventional rebound occurs at −5 °C and −10 °C. The droplet pinned on the SCA when the temperature decreases to −15 °C. As shown in Figure 5f, for the v-SCA with H = 500 μm (s = 250 μm) at W_e = 13.3, pancake bouncing occurs from 25 °C to −15 °C, with the contact time increasing from 6.5 ms to 7.1 ms. For the v-SCA with H = 400 μm (s = 250 μm) at W_e = 13.3, pancake bouncing occurs from 25 °C to −10 °C, with the contact time increasing from 7.1 ms to 8.0 ms. Conventional rebound occurs when the temperature decreases to −15 °C. For the v-SCA with H = 300 μm, pancake bouncing occurs from 25 °C to 0 °C, with the contact time increasing from 7.3 ms to 7.7 ms. Conventional rebound occurs from −5 °C to −15 °C.

3.4. Directional Droplet Pancake Bouncing on t-SCA

In addition to the reduction in the contact time, directional bouncing off the surface is also crucial for a surface to stay dry. Recently, oblique pancake bouncing has been reported on an inclined Janus structure fabricated by 3D print technology [34]. Unfortunately, the authors did not take the effect of the tilt angle into account since they only prepared a vertical Janus structure and an inclined Janus structure with a tilt angle of 20°. Herein, inspired by the droplet directional rolling off the wings of a butterfly [35], superhydrophobic cone arrays with tilt angles were designed to realize directional pancake bouncing off. The cone arrays with tilt angles were fabricated according to the method previously reported [38]. The strategy is to translate the v-SCA into a t-SCA with different tilt angles by using shape memory polymer (see Section 2). Figure 6 shows a comparison between the v-SCA and t-SCA for the bouncing behaviors of droplets impacted (s = 250 μm, H = 600 μm, W_e = 13.3). The tilted angles can be designed as β = 0°, 15°, 45°, 60°, and 80°, respectively (β = 0° indicates the v-SCA; β is illustrated in Figure 6b). Figure 6a shows the process of droplet impacting and rebounding vertically on the v-SCA (β = 0°). As for the t-SCA with β = 15°, 45°, 60°, and 80° (Figure 6b–e), the droplet also shows pancake bouncing (Supplementary Video S8). It indicates that the t-SCAs, along with s = 250 μm, H = 600 μm, and W_e = 13.3, can induce the pancake bouncing of droplets. Specifically, the pancake droplets are at a deflection angle (α, defined in Figure 6b) to the horizontal orientation on the t-SCA. As β increases, α increases. When β reaches 80°, α is reduced distinctly (Figure 6g). When the droplet falls onto the surface again, a certain displacement (Figure S11) is produced compared with the original position. As β increases, the displacement increases (Figure 6h). When β reaches 60°, the largest displacement occurs.

We proposed a simple model to explain these results. As shown in Figure S12, the driving force ($F_d$) can be expressed as [34]

$$F_d=F_c[\cos \left(\theta -{\alpha }_R\right)+\cos \left(\theta +{\alpha }_L\right)]={2F}_c\cos \left(\theta -{\displaystyle \frac{{\alpha }_R-{\alpha }_L}{2}}\right)\cos {\displaystyle \frac{{\alpha }_R+{\alpha }_L}{2}}$$

where ${\alpha }_L=\arctan \left(1/(tan\beta +R_b/h)\right);$ ${\alpha }_R=\arctan \left(1/(tan\beta -R_b/h)\right)$ (0 < ${\alpha }_L$ < ${\alpha }_R$ < 90); F_c, $\theta ,$ $R_b,$ $h$ represent the capillary force, the intrinsic contact angle, the base radius, the height of the pillars, and the center-to-center spacing, respectively. When $\beta =0,$ $F_d=0$. The droplet shows pancake bouncing up and down vertically. When $\beta >0,$ $F_d>0$, and the droplets bounce off away directionally. As $\beta$ increases, ${\alpha }_R+{\alpha }_L$ decreases, ${\alpha }_R-{\alpha }_L$ increases, $F_d$ increases, and the displacement increases.

4. Conclusions

In conclusion, a low contact time of droplet pancake bouncing on superhydrophobic cone arrays under various conditions was studied. The various types of superhydrophobic cone arrays are fabricated by a replication process followed by electroplating and etching process or the spray coating technique. The critical Weber number (W_e), pillar-to-pillar spacing (s), and pillar height (H) are 11.1, 350 μm, and 300 μm, respectively. Pancake bouncing occurs on the vertical SCA (i.e., v-SCA) surface for the entire range of pH values (0–14). Pancake bouncing still occurs even when soaked in sea water for 50 h. The critical PSS concentration for pancake bouncing is 6%. The critical PDAD concentration for pancake bouncing is 5%. Pancake bouncing can still occur under low temperature. The lowest droplet contact time is ~6 ms, which is reduced by more than 60% compared to conventional bouncing. In addition, directional pancake bouncing behaviors (the largest horizontal displacement is ~3 mm) are generated on the tilted angle SCA (i.e., t-SCA). We believe that the investigation of pancake bouncing under different conditions would extend the practical applications including self-cleaning, anti-corrosion, anti-icing, heat transfer.