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Tunable Multimodal Drop Bouncing Dynamics and AntiIcing Performance of a Magnetically Responsive Hair Array Sang-Hyeon Lee, Minho Seong, Moon Kyu Kwak, Hyunwook Ko, Minsu Kang, Hyung Wook Park, Seong Min Kang, and Hoon Eui Jeong ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05109 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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Tunable Multimodal Drop Bouncing Dynamics and Anti-Icing Performance of a Magnetically Responsive Hair Array Sang-Hyeon Lee1, Minho Seong1, Moon Kyu Kwak2, Hyunwook Ko1, Minsu Kang1, Hyung Wook Park1, Seong Min Kang3, and Hoon Eui Jeong1,* 1

Department of Mechanical Engineering, Ulsan National Institute of Science and Technology

(UNIST), Ulsan, Republic of Korea 44919 2

Department of Mechanical Engineering, Kyungpook National University, Daegu, Republic

of Korea 41566 3

Department of Mechanical Engineering, Chungnam National University, Daejeon, Republic

of Korea 34134

*Corresponding author. Email address: [email protected] (H. E. Jeong)

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ABSTRACT

Anti-icing materials that can efficiently limit ice formation have a strong potential to replace existing anti-icing techniques, such as Joule heating, chemical release, or mechanical removal, which are usually inefficient, expensive, and environmentally harmful. In this study, an anti-icing material based on a magnetically responsive hierarchical hair array, that can actively modulate drop bouncing dynamics, is presented. The magnetically responsive hair array exhibits an immediate and reversible structural bending motion in response to an external magnetic field. The array also exhibits superhydrophobicity, regardless of its tilt angle, due to the tapered geometry of the hairs and the multiscale surface roughness of the array. Due to its dynamic structure and water-repellent characteristics, the array can induce distinct multiple modes of drop bouncing behavior by adjusting its structural bending state in a reversible fashion. Three different types of bouncing behavior, namely, quasi-pancake bouncing, directional bouncing, and macrotexture-induced droplet fragmentation, can be obtained with the vertical, tilted, and fully bent hair arrays, respectively. We demonstrate that the dynamically controllable drop bouncing behavior of the magnetically responsive hierarchical array enables the efficient and robust prevention of ice formation and accumulation.

KEYWORDS: actuation, anti-icing, contact time, drop bouncing, magnetically responsive

The formation of ice on the surfaces of structures, ranging from roads, buildings, and power lines to cars, trains, and aircraft, create extensive social, technical, and economic problems.1,2 Joule heating, chemical release, and mechanical removal techniques have been commonly 2 ACS Paragon Plus Environment

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utilized for the prevention of ice accumulation and its removal.3 However, these techniques are not only inefficient and expensive, but also cause surface damage and corrosion, as well as environmental side effects.1 To address these problems, the development of anti-icing materials, that can efficiently limit ice formation and easily remove the ice, has been actively explored.1-18 Among them, superhydrophobic surfaces (SHSs) have been suggested as excellent candidates for ice-free materials, based on their exceptional water repellent nature.1,4,7,9,11,15,19-24 However, it was observed that some droplets remained pinned on a supercooled SHS and nucleated into ice crystals on the surface.4,25 In particular, this situation tended to occur at a high humidity level.2,25,26 Once the ice forms on a SHS, it loses its superhydrophobic nature and can even become more ice-friendly.1,2,27 Slippery liquid-infused porous surfaces (SLIPS) have emerged as efficient ice-phobic materials.2,10,28-30 SLIPS utilize a textured solid infiltrated with a confined immiscible lubricant. Due to the over-layer of the slippery liquid, condensed or melted water droplets easily slide off the surface, before they freeze. More recently, SHSs with macroscale textures have been suggested as potential dropshedding and anti-icing materials, as the macrotextures significantly reduce the contact time of the bouncing droplets on the SHSs.12,13,17,18,31 Macrotextures have been reported to reduce the contact time by 37-80 % compared to planar SHSs.32 SHSs with tapered posts can also greatly reduce the contact time, as the droplets impinging on the posts can lift-off from the surface without experiencing retraction due to the upward capillary emptying motion induced by the tapered posts (pancake bouncing).12,33-36 The minimization of the contact time enables the impinging droplets to bounce off the surface before ice nucleation occurs.1,27 However, unless the SHSs with macrotextures or tapered posts are inclined away from the horizontal, the droplets will remain on the surface after rebounding, and the stationary drops on the SHSs eventually nucleate into ice crystals. Materials that can modulate the critical features of the drop bouncing dynamics such as drop contact time, morphology, and bouncing angle, in an 3 ACS Paragon Plus Environment

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active and simultaneous fashion, would be of great benefit for the development of advanced ice-phobic and drop-shedding materials. Here, we present a robust anti-icing material that can efficiently limit the formation of ice, based on tunable multimodal bouncing dynamics of impinging droplets by utilizing a magnetic-responsive hierarchical hair array (MRA). The MRA exhibits reliable actuating capabilities with an immediate field response, precisely controllable tilt angle, and a maximum tilt angle of ±90° with a permanent magnet. The array also exhibits superhydrophobicity regardless of its tilt angle, due to the tapered geometry of the hairs and the roughness of the multiscale surface of the array. By utilizing the tapered structural features with high aspect ratio, reversible actuating ability, and robust water-repellency of the array, it is shown that a distinct bouncing behavior can be induced in the impinging drops by adjusting the tilt angle of the array. A semi-pancake bouncing behavior, with a significantly decreased contact time and contact area, is obtained on the vertical hair array, whereas a directional rebound behavior is observed on the tilted hair array. When the array is fully bent, it acts as a macrotexture, causing the droplet impacting on the surface to fragment into small drops, thus leading to a further reduction in the contact time. A set of anti-icing experiments demonstrate that the controlled rebound behavior of the magnetically responsive array enables the efficient and robust prevention of ice formation and accumulation.

RESULTS AND DISCUSSION

Structural Characteristics of the Magnetically Responsive Array


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Figure 1a shows the detailed structure of the fabricated MRA. As shown, an array of hairs with a high aspect ratio (AR) was uniformly constituted on a flexible elastomeric substrate through a moldless self-assembly process.37,38 As the fabrication process relies on the magnetic field-induced self-alignment of the carbonyl iron (CI) particles, a high AR MRA can be formed over a large area, without the need for micropatterned molds (Figure 1a-i).37,38 The individual hairs had a top diameter of ~20 µm, a base diameter of ~135 µm, and a height of ~2.3 mm on an average (Figure 1a-ii). The surfaces of the hairs exhibited nanoscale roughness due to the carbon nanoparticles (CNPs) coated over the array to provide superhydrophobic wetting properties (Figure 1a-iii). Due to the CI particles embedded in the array, the MRA exhibited dynamic structural changes and actuating motion, when the magnetic field was modulated by using a permanent magnet (Figure 1b-d). For example, when a rectangular magnet was placed directly below the array, the MRA maintained its original vertical position without bending, as the magnetic field was vertically oriented with respect to the bottom substrate of the array (Figure 1b). When the direction of the magnetic field was changed by moving the magnet, the array immediately responded to the field, exhibiting a structural bending (Figure 1c, d). It was observed that the tilt angle (θt), defined as the angle between the vertical axis and the MRA, was fully tunable from -90° to 90° due to the high AR and flexibility of the array (Figure 1d), and the structural bending and actuation were highly reversible. Furthermore, the MRA maintained its superhydrophobic nature regardless of the structural tilt angle, due to the roughness of the multiscale surface of the array.39-42 The static contact angles (CAs) on the vertical, 45°- tilted, and 90°- tilted MRAs were 165.6°, 162.6°, and 164.3°, respectively. The contact angle hysteresis (CAHs) on the vertical, 45°- tilted, and 90°- tilted MRAs were 3.4°, 3.0°, and 3.1°, respectively. Similar CAs and CAHs were observed for the MRAs with θt of 0–90° (Figure S1). For comparison, a SHS


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was prepared by coating CNPs on a planar PDMS substrate. The CA and CAH on the SHS were 165.2° and 3.3°, respectively.

Figure 1. Structure and dynamic motion of the magnetically responsive hair array. (a) (i) A photograph showing a fabricated magnetically responsive hair array (MRA). (ii) A scanning electron microscope (SEM) image of the array. (iii) SEM image showing the surface roughness of the array. (b) (i) Top-view, (ii) side-view, and (iii) wider-field of side view images of the vertical MRA placed directly above a permanent magnet. (c) (i) Top-view, (ii) side-view, and (iii) wider-field of side view images of the MRA with a controlled tilt angle of 6 ACS Paragon Plus Environment

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45° using a permanent magnet. (d) (i) Top-view, (ii) side-view, and (iii) wider-field of side view images of the MRA with a controlled tilt angle of 90° using a permanent magnet. Multiple Modes of Droplet Bouncing Behavior on the Magnetically Responsive Array Droplet bouncing behavior is directly related to the anti-icing performance, as it determines the contact time (tc) and contact area of the liquid-solid interface.1,27 To examine the bouncing behavior, droplet impact tests were conducted on the planar SHS and the MRA at seven different tilt angles (θt = 0°, 15°, 30°, 45°, 60°, 75°, and 90°), using a high-speed camera with a recording rate of 5000 frames per second (Figure 2 and S2). The initial drop radius (r0) was 1.45 mm, and the impact velocity (v0) was 0.74–1.13 m s-1, which corresponded to a Weber number (We) of 11.0–25.8. We is defined as We=ρv02r0/γ, where ρ is the density (1000 kg m-3) and γ is the surface tension (72 × 10-3 N m-1) of water. Figure 2a shows the time-lapse top and side view images of a droplet impinging on the SHS with an impact velocity, v0, of 1.13 m s-1 (We = 25.8). Immediately after the impact of the droplet on the SHS, the droplet was fully flattened and reached its maximum extension in 3.8 ms. Then, it recoiled back and bounced off the SHS after full retraction at 17.0 ms (tc = 17.0 ms). The overall bouncing behavior, such as the drop morphology and tc on the SHS, were in good agreement with previous reports.43,44 Significantly different bouncing behavior was observed on the vertically aligned MRA (θt = 0°), where the droplet penetrated the array on coming in contact with the MRA (Figure 2b). During the downward penetration, it could not fully extend in the lateral direction to the diameter observed on the planar SHS, due to its physical confinement by the array. Subsequently, the droplet rebounded and left the MRA surface in 10.8 ms (for v0 = 1.13 m s-1,

We = 25.8) (Figure 2b), indicating that the tc on the vertical MRA was reduced by 36.5 % compared with that on the planar SHS (17.0 ms). When a falling droplet touches the surface 7 ACS Paragon Plus Environment

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of a tapered pillar array, a part of the drop starts to penetrate the void space of the array due to its momentum. At this time, a part of its initial kinetic energy is stored as surface energy. During the penetration, it is decelerated by the capillary force induced by the superhydrophobic array. Subsequently, the decelerated droplet is accelerated upwards, and capillary emptying takes place, during which the stored capillary energy is converted back into kinetic energy, enabling the drop to bounce off. In this process, if the stored energy is sufficiently high, the drop can leave the surface before retraction occurs (pancake bouncing), resulting in a significant reduction in tc.12,33-36 Although the droplet that bounced off the MRA did not exhibit perfect pancake-like morphology, it left the array surface before full retraction in a quasi-pancake bouncing mode (Figure 2b). The enhanced bounce height (16.5 mm) of the droplet on the vertical MRA compared to that on the SHS (6.6 mm) demonstrates the higher kinetic energy of the droplet that rebounded on the MRA (Figure 2b). The droplets on the vertical MRA maintained overall axisymmetric dynamics throughout the process (Figure 2b). When a droplet hit the tilted MRA with θt of 15–60°, it exhibited a directional bouncing trajectory (Figure 2c and S3). For example, on the 45°- tilted MRA, the drop rebounded obliquely on the surface with a bounce angle of 74.7°, a maximum bounce height of 12 mm, and a horizontal travel distance of 10.7 mm (for v0 = 1.13 m s-1, We = 25.8) (Figure 2c). The bounce angle, bounce height, and horizontal travel distance of the directionally rebounded droplet could be tuned by adjusting the structural tilt angle of the MRA (Figure S3). The directional bouncing on the tilted MRA was caused by the unbalanced surface tension, structural elasticity, and upward capillary force.34,45 It can be seen from the top-view images, that the droplet morphology is slightly elongated along the direction of the slant of the MRA, and irregular due to the anisotropic surface morphology of the tilted array (Figure 1c and 2c). 8 ACS Paragon Plus Environment

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The tcs on the MRAs with θt of 15°, 30°, 45°, and 60° were 11.0, 11.2, 11.4, and 11.6 ms, respectively, for v0 = 1.13 m s-1 (We = 25.8) (Figure 3). These values of tc are comparable to those on the vertical MRA (10.8 ms) and reduced by 31.8–35.3 % as compared to those on the SHS. Moreover, as in the case of the vertical MRA, the droplet left the tilted MRAs before full retraction, showing that quasi-pancake bouncing and directional rebound are simultaneously attainable with the tilted array. On the highly bent MRA (θt = 75°, 90°), another distinct bouncing mode was observed, which was different from those in the case of the SHS, vertical MRA, or moderately-tilted MRA. The droplet first spread out upon impact with the surface, but the thickness of the flattened film was not uniform (Figure 2d). Thinner regions were observed in the film, along with globules at the edges. The drop morphology observed from the top-view was also highly irregular. The droplet left the surface in 11.0 ms and 8.6 ms before recoiling, on the 75°- and 90°- tilted MRAs, respectively (for v0 = 1.13 m s-1, We = 25.8). At this juncture, it was broken into a few smaller fragments. This behavior was very similar to the bouncing dynamics observed on SHSs with macrotextures.13,18,31,46,47 It was reported in previous studies, that when a droplet impacts on a superhydrophobic macrotexture, it is fragmented by the texture.13,18 As a result, there is a considerable decrease in the time for recoil and tc. When the MRA was highly bent close to the bottom substrate, individual neighboring hairs overlap with each other, and form macroscopic ridge patterns on the surface (Figure 1d and S4), which act as macroscale textures, causing the drop fragmentation and subsequent reduction in

tc. A simple mathematical calculation of the MRA geometry for different θt shows that the hairs begin to overlap with each other if they are tilted more than 75°, in agreement with the experimental results (see supporting information and Figure S5). The results of the drop


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impact tests indicate that distinctly different drop bouncing modes can be achieved, in an active and controllable manner, with the MRAs.

Figure 2. Drop bouncing behavior on a planar superhydrophobic surface and a magnetically responsive hair array with different structural tilt angles. High-speed (i) top-view and (ii) side-view images of drop bouncing on (a) the SHS, (b) a vertical MRA, (c) an MRA with a 10 ACS Paragon Plus Environment

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tilt angle of 45°, and (d) an MRA with a tilt angle of 90° (drop radius r0 = 1.45 mm; impact velocity v0 = 1.13 m s-1). (iii) Drop position at its maximum height and maximum distance after the first bouncing. Quantitative Analysis of the Droplet Bouncing Behavior on the Magnetically Responsive Array For a quantitative evaluation of the different drop bouncing modes on the MRAs, two time scales tc and t↑, and three quality factors Qp, Qf, and Qd were examined as functions of We for the MRAs with different θt. t↑ represents the time elapsed for a drop to completely empty the array after making an initial contact with the top surface.12,35 Qp (= rj/rmax) is the pancake quality, defined as the ratio of the radius of the drop (rj) when leaving the surface to the maximum extended radius of the drop (rmax).12,34,35 A value of Qp higher than 0.8 corresponds to pancake bouncing.12 Qf (= lj/2πr0) is the ratio of the top-viewed perimeter of the droplet when leaving the surface (lj) to the top-viewed perimeter of the initial spherical drop (2πr0).31 When the droplet is fragmented into a number of smaller daughter drops, the total perimeter of all the drops would increase. Qd (=L/r0) represents the ratio of the horizontal travel distance (L) of the rebounded drop to the initial drop radius (r0).48 In the case of the SHS, the drop left the SHS after experiencing full retraction (Figure 2a). Accordingly, the Qp was less than 0.4 over the entire range of We (11.0–25.8). The drop left the SHS without fragmentation for the We range 11.0–25.8, resulting in the relatively low Qf of ~0.7. The tc on the SHS was also nearly constant regardless of the impact velocity (Figure 3a), which is in agreement with previous observations that suggested that the tc on a planar SHS scales with the inertial-capillary timescale ( ~ ⁄ ) (fitting coefficient = 2.6) (Figure 4a).18,43 In the case of the vertical MRA, there was not much difference between tc 11 ACS Paragon Plus Environment

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and t↑, which reflects that the drop left the surface of the MRA soon after the upward capillary emptying process, which is a characteristic feature observed in pancake bouncing (Figure 3b).12,35 Meanwhile, Qp was 0.81–0.84 for the We range (11.0–25.8), thus confirming that quasi-pancake bouncing occurred on the vertical MRA. The value of Qp (~0.8) close to the lower limit of the pancake bouncing was mainly caused by the height of the MRA (~2.3 mm), which increased the timescale t↑. Unlike the value of tc on the SHS, tc on the vertical MRA

scales

as

(r0)1/2,

and

is

in

good

agreement

with

the

scaling

law,

↑ ~ /, with a fitting coefficient of 1.1 (Figure 4b and S6), which predicts the timescale for the drop impacting on the tapered pillar arrays.12 Here, p depicts the centerto-center pitch of the array, β is the structural parameter (β = base diameter/height of the array), and θ is the contact angle of the drop on the side of the array.

Figure 3. Quantitative analyses of the drop bouncing dynamics on different surfaces. Contact time (tc), Qp, Qf, and Qd of the droplet impinging on the (a) SHS, and (b-h) MRAs with different tilt angles of (b) 0°, (c) 15°, (d) 30°, (e) 45°, (f) 60°, (g) 75°, and (h) 90° as a function of We.


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The values of tc, t↑, Qp, and Qf for the tilted MRA at θt of 15–60° were similar to those for the vertical MRA (Figure 3c-f, 4b, and S6). The values of tc and t↑ were close to each other, and Qp was 0.76–0.83 for all the MRAs tilted at θt of 15–60°, without noticeable differences among the arrays with different θt. This indicates that even when the array is tilted at θt up to ~60°, the liquid drop penetrating the array can save sufficient capillary energy to bounce off the surface before full retraction. Such quasi-pancake bouncing behavior on the vertical and tilted MRAs was enabled by the both large height (hp = ~2.3 mm) and pitch (p = ~500 µm) of the MRAs. Liu et al. revealed that pillar array with large height and spacing is necessary to induce the pancake bouncing. During their drop impact tests, they could induce the pancake bouncing on pillar arrays with pitches of 300 µm and 400 µm of pillar heights 1.2 mm, whereas they observed typical conventional rebound on an array of pitch 200 µm.12 The MRAs tilted at 15–60° had effective pitches of ~250–480 µm (peff = pcosθt) (Figure 4d), which was appropriate to induce the pancake bouncing in the present experimental conditions. Liu et al. also reported that drops impacting on a short array of height 0.3 mm showed the conventional bouncing behavior, rather than pancake bouncing. This is because if the array height is not enough, the impinging drop can touch the bottom substrate, leading to energy dissipation.12,33,35 The low height is also disadvantageous for the drop to save enough capillary energy, during the penetration, required for the pancake bouncing. The surface energy during the downward penetration (Es,y) is proportional to the square of the penetration depth (h) of the drop as shown in the equation33 , ~ 4γnℎ

(1)

where n is the number of pillars wetted by the penetrated water. Considering the dissipation to be negligible during the penetration and emptying process, the stored surface energy can be assumed to be equal to the kinetic energy (Ek,y) of the droplet in the transverse motion at 13 ACS Paragon Plus Environment

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the end of the emptying process.33,35 Therefore, Ek,y is also approximately proportional to h2, which gives the minimum array height required for the pancake bouncing. The millimeter scale height of the MRA provides enough travel distance for the droplet to save the capillary energy even when it is tilted (Figure S5). Meanwhile, as the capillary force induced by the hydrophobic array was directed along the length of the tilted array, directional bouncing was simultaneously induced on these surfaces along with the pancake rebound. Therefore, the value of Qd increased with the increase in θt (Figure 3c-f). As described above, when the MRA was tilted at larger angles of 75° and 90°, a droplet hitting the surface was splashed into multiple smaller drops for We above 17.8 (for θt = 90°) and 21.8 (for θt = 75°). Accordingly, Qf rapidly increased to 1.9–2.7 for We above 17.8–21.8, while Qp could not be defined. Due to the drop fragmentation, tc was greatly reduced to 8.6– 11.3 ms on these surfaces for We 17.8–21.8, which is a reduction in the value of tc by 33.5– 49.4 % compared with the tc (17.0 ms) on the SHS. Interestingly, this observation is in good agreement with the report by Gauthier et al., in which they observed a 44% reduction in the value of tc (~7.3 ms) on the SHS with macroscale lines at high impact velocity, compared to the value of tc ~ 13.0 ms, for a drop radius of 1.3 mm, on the same surface without the textures. Figure 4c shows the tc on the tilted MRA at θt of 75° and 90° as a function of r0 for two different impact velocities. As shown, when the droplet shows the conventional bouncing behavior at low impact velocity, the trend of tc on the highly-laid MRA is highly similar to that on the SHS, and is in good agreement with the scaling law of ~ ⁄, with a fitting coefficient of 2.24-2.28. The tc of the fragmented drops on the MRA at high impact velocity also scales as (r0)3/2. However, the fitting coefficient is reduced to 1.56-1.67 due to the reduced volume of each split drop, which agrees well with the observations that suggested that the tc of the fragmented drops with n subunits is divided by √ , compared with that of 14 ACS Paragon Plus Environment

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the undivided drop.18 Qd increased with We for the 75°- tilted MRA, indicating that the fragmented drops have a directed momentum on this surface with structural directionality after the impact. In contrast, directionality in the splashed drops was not observed on the 90°tilted array as the Qd values were close to zero. Figure 4e is a phase diagram that shows the summary of the distinct multiple drop bouncing dynamics observed on the MRAs depending on the values of θt and We. For θt = 0°, the quasi-pancake bouncing can be expected for a wide range of We (9.3–31.0), while the directional bouncing and quasi-pancake rebound are simultaneously observed for 15° ≤ θt ≤ 60°. For θt ≥ 75°, the drop fragmentation feature is dominant for We above ~15, while conventional bouncing is expected for We below ~15. The energy consumption of the droplet during the impact and rebound on the MRAs is now estimated. For the impinging droplet, the following equation can be established, based on the simple principle of conservation of energy:33 !, + , = !,$ + ,$ + %&

(2)

where Ek,0 and Es,0 are the initial kinetic and surface energies of the drop at impact, respectively. Ek,j and Es,j represent the final kinetic and surface energies of the drop at takeoff, respectively. Edis is the energy consumption during the impact and rebound. By assuming the drop morphology at impact and take-off to be a sphere and plate (perimeter lj, thickness hj), respectively, Equation 2 is expressed as

%& = '$ ($ ( + )*$ ∙ ℎ$ 4' ,

(3)

where rj and vj are the radius and velocity of the drop leaving the surface, respectively. Figure 4f shows the estimated energy dissipation of the droplet depending on the values of θt and We. It can be seen that, the total energy loss is noticeably lower in the cases of the vertical and 15 ACS Paragon Plus Environment

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tilted MRAs at θt of 15° ~ 60°, compared with that in the SHS, mainly due to the stored capillary energy during the water penetration. Droplets lost less energy in the MRAs with smaller θt for identical We. In particular, drops impinging on the vertical MRA showed significantly lower energy loss compared with that in the SHS (Figure 4f) while the energy loss increased with θt. This is because drops that collide with the more-tilted arrays undergo higher head loss.49 Moreover, the MRAs with higher θt have higher contact area with the drop during the impact and rebound as shown in Figure S7, which also contributes to the higher frictional dissipation. When the drop rebounds, without fragmentation (We < 15), from the MRAs tilted at 75° and 90°, the energy loss is significant and even higher as compared to that in the SHS. It is assumed that this is because the droplet cannot save the capillary energy on these highly-laid arrays while the increased surface area of the MRAs (as compared to the SHS) increases the frictional dissipation. When the drop was fragmented at high We above 15, the total energy loss became slightly lower than that in the SHS (Figure 4f). It was observed that the kinetic energy loss in the case of the MRAs tilted at 75° and 90° was much higher than that in the SHS (Figure S8), as evident from the low rebound height of the splashed drop in the case of the highly tilted MRAs (Figure 2d). Instead, the fragmented drops have relatively high surface energy, which contributes to the decrease in the total energy loss in the case of the highly tilted MRAs (Figure 4f). With the increase of the We, the energy loss increased while the coefficient of restitution (e) of the drop decreased (Figure 4f and S8c), which agrees well with the observations in a prior study.50


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Figure 4. Analyses of drop bouncing dynamics on different surfaces. (a) Measured drop contact time (tc) (circles and triangles) on the SHS with drop radius for different impact velocities compared with theoretical prediction (dotted line). tc scale as (r0)1.5. (b) Measured tc on the MRA with tilt angle of 0°–60° as a function of drop radius for two different impact velocities fitted with theoretical prediction (dotted line). tc scale as (r0)0.5. (c) Measured tc on the MRA with tilt angle of 75°, 90° as a function of drop radius for two different impact velocities fitted with theoretical prediction (dotted line). tc scale as (r0)1.5. (d) Schematic illustration showing the drop penetration into the slanted MRA. (e) Phase diagram showing different regimes of the four different bouncing modes depending on the We and array tilt angle. (f) Energy loss (Edis) of the drop impinging on the SHS and MRA with different tilt angles with We. E0 represents the initial energy of the drop at impact for the corresponding We.


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Anti-Icing Performance of the Magnetically Responsive Array The MRA not only enabled a reduction in the contact time and contact area of the droplet, but also provided an active control over the direction of the drop bounce and morphology of the impinging drop. Furthermore, it showed a prolonged water-repellency and mechanical stability even after 1,200 repetitive cycles of bending motions (See Movie S1). These features are beneficial for the development of efficient anti-icing materials, as the reduced contact time and contact area minimize heat transfer from the droplet to the frozen substrate, thereby inhibiting ice formation on the surface (Figure S9a, b). The directional bouncing property of the MRA minimizes the opportunity for the droplet to remain in contact with the substrate, which also reduces the chances of drop pinning and ice nucleation on the supercooled substrate (Figure S9c). SHSs have been widely adopted as anti-icing materials, as they show excellent water repellency at room temperature and therefore minimize the drop-substrate contact area. To compare the anti-icing performance of the SHS and the MRA, a water droplet was released on an inclined supercooled SHS and on the MRA with array tilt angles of 45° and 90° (Figure 5). The overall inclination angle of the substrates was 10°, the surface temperature of the substrates was maintained at -20°C, and the value of We corresponded to 25.8 (v0 = 1.13 m s1

). Upon the release of the droplet, it rebounded on the SHS, leaving a small residue drop at

the impinging point (Figure 5a). After the initial impact on the SHS, the droplet came into contact with the substrate twice, without freezing. However, upon a third additional impact on the substrate, the droplet became pinned on the site and was frozen into solid ice (Figure 5a). This indicates that the SHS has limited utility in the prevention of ice formation.


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In contrast, when a droplet was released on the supercooled MRA with a tilt angle of 45°, it rapidly left the substrate without any ice formation on the surface (Figure 5b). This is because the droplet cleared the substrate after only two impingements, achieving a long horizontal traveling distance due to the directional bouncing and high kinetic energy. In addition, the reduced contact time and contact area of the drop on the tilted MRA further hindered ice nucleation. When a rebounded droplet fell on the aluminum support beneath the samples, it was immediately frozen into solid ice, demonstrating the outstanding ice-phobic nature of the MRA. On the fully bent supercooled MRA (θ = 90°), the droplet rapidly broke into several fragments due to the macroscopic texture of the flattened array (Figure 5c). Subsequently, the fragmented drops left the substrate before ice nucleation could occur due to the macrotextureinduced reduction of the contact time and contact area.

Figure 5. Anti-icing performance of the conventional superhydrophobic surface and the magnetically responsive hair array. High-speed images showing a drop released onto (a) a supercooled, planar SHS, (b) a supercooled MRA with an array tilt angle of 45°, and (c) a 19 ACS Paragon Plus Environment

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supercooled MRA with an array tilt angle of 90°. The inclination angle of the samples was 10° and the sample temperature was maintained at -20°C.

To reflect the real conditions in which a stream of droplets, rather than a single drop, hit a surface, water droplets were continuously dropped over the supercooled SHS and MRA samples. As shown in Figure 6a, all the droplets falling onto the SHS were pinned and nucleated into ice crystals. After 20 drop impingements, the surface of the SHS was largely covered with accumulated ice, even though it was tilted at an angle of inclination of 10°. This result confirmed that the SHS had limited utility as an ideal ice-free material. In contrast, even after continuous droplet impacts, the MRA successfully repelled the droplets from the surface before they could freeze into solid ice at both the 45° and 90° hair tilt angles. Instead, the drops were accumulated and frozen on the aluminum support (Figure 6b, c). To further evaluate the ice-phobic properties of the SHS and MRA, a droplet was released on horizontal SHS and MRA samples (Figure S10). On the conventional SHS, the droplet rebounded from the surface, leaving a small pinned drop residue. As the takeoff occurred in the vertical direction, the droplet remained on the SHS after bouncing. Consequently, the stationary drop on the SHS was nucleated into an ice crystal (Figure S10a). In contrast, the droplet released on the tilted MRA (Figure S10b) rebounded obliquely and completely left the array, preventing ice from forming on the surface. Even if the droplet remained on the MRA, it could easily be removed by actuating the dynamic motion of the MRA (Figure S10c). These results clearly demonstrated that the MRA had superior and more robust anti-icing abilities compared to the conventional SHS.


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Figure 6. Anti-icing performance of the conventional superhydrophobic surface and the magnetically responsive hair array under continuous drop impacts. High-speed images showing continuous drop release onto (a) a supercooled, planar SHS, (b) a supercooled MRA with an array tilt angle of 45°, and (c) a supercooled MRA with an array tilt angle of 90°. The inclination angle of the samples was 10°, and the sample temperature was maintained at -20 °C.

CONCLUSIONS

In summary, actively controllable and switchable multimodal drop bouncing behavior on an array of magnetically actuating hierarchical hairs has been demonstrated, which can be utilized for the efficient suppression of ice formation on its surface. Through a combination of dynamic structures and water-repellent properties of the MRA, distinct drop bouncing modes of quasi-pancake bouncing, directional bouncing, and macrotexture-induced droplet fragmentation could be obtained depending on the tilt angles of the array. A set of anti-icing 21 ACS Paragon Plus Environment

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experiments showed that reduced contact time and contact area, directional bouncing, and droplet fragmentation of the magnetically responsive hair array led to the effective prevention of ice nucleation on its surface. These robust anti-icing properties based on controllable multimodal bouncing dynamics, together with the facile moldless large-area fabrication process of the array, should contribute to the development of advanced drop-shedding and ice-free materials.

METHODS

Fabrication of the Magnetically Responsive Hierarchical Hair Array The base polymer Sylgard 184 (Dow Corning Korea, Korea), hexane (Sigma Aldrich Korea, Korea), and carbonyl iron particles (Sigma Aldrich Korea, Korea) were mixed together in a 1:1:1 weight ratio using a vortex mixer and sonicated for 30 min in a water bath. Then, a curing agent for Sylgard 184 was added to the mixture at 10 wt% of the base polymer, and the solution was sonicated again. The composite solution was spray-coated over a cured PDMS substrate placed on a neodymium magnet (magnetic flux density ~4.5 T). Subsequent thermal curing of the sprayed sample on the magnet for 2 h at 70°C in a convection oven produced a magnetically actuating self-assembled hair array. The hair array was spray-coated with 0.5 wt% carbon nanoparticles (CNPs, Sigma Aldrich Korea, Korea) dispersed in 2 mL of acetone and then dried for 1 h at 70°C, resulting in a magnetically responsive hierarchical hair array with superhydrophobicity.

Scanning Electron Microscope (SEM) Imaging 22 ACS Paragon Plus Environment

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Scanning electron microscope (SEM) images of the fabricated samples were obtained using a HITACHI S-4800 microscope (Hitachi, Japan). To avoid charging effects, the samples were coated with a Pt layer (~5 nm thick) using a metal sputter coater (K575X sputter coater, Quorum Emitech, UK).

Contact Angle Measurements The contact angles (CAs) of water droplets were measured using a contact angle analyzer (SDLAB 200TEZD, Femtofab, Korea). The static CAs were measured by gently placing 10 µL deionized (DI) water droplets on the magnetic hair array. The advancing and receding CAs were measured by increasing and decreasing the volume of the water droplet on the array.

Analysis of Drop Impact Behavior on the Magnetically Responsive Hair Array DI water droplets with four different diameters of 2.0, 2.45, 2.9, and 3.5 mm were released from a controlled height onto the planar SHS and MRA samples with different tilt angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°). The tilt angles of the array samples were controlled by adjusting the positions of a neodymium magnet (magnetic flux density ~4.5 T). The experiments were performed under atmospheric conditions (25°C, relative humidity 50 %). The drop bouncing processes on the samples were recorded at 5000 fps using a high-speed camera (Phantom Miro M310, USA) from the top and side of the samples.


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Analysis of the Anti-Icing Properties of the Magnetically Responsive Hair Array DI water droplets with diameters of 2.9 mm were released from a controlled height onto the supercooled planar and MRA samples with different tilt angles (0°, 45°, and 90°). The temperature of the samples was maintained at -20 °C using a thermoelectric cooler. The relative humidity was maintained at 50 % and the ambient temperature was 25 °C. The MRA samples were purged with nitrogen to avoid frost formation.26 The drop bouncing processes on the supercooled samples were recorded at 5000 fps using the high-speed camera.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions S.-H.L. and M.S. contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the Mid-career Researchers Supporting Program funded by the National Research Foundation of Korea (NRF) (2016R1A2B2014044).

ASSOCIATED CONTENT Supporting Information. 24 ACS Paragon Plus Environment

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The Supporting Information is available free of charge on the ACS Publications website at https://pubs.acs.org. Calculation of the MRA geometry for different tilt angles. Supplementary Figures: CAs and CAHs on the SHS and MRA, high-speed images and trajectories of the drop bouncing on the MRA with different tilt angles, conceptual illustration showing the MRA at different tilt angles, geometry of the MRA with different tilt angles, contact time of the impinging drop on different surfaces as a function of drop radius, contact area of the impinging drop on the SHS and MRA, kinetic and surface energies of the drop at take-off for the SHS and MRA, conceptual illustration showing the drop bouncing dynamics on the SHS and MRA, high-speed images showing the drop release onto the supercooled SHS and MRA in a horizontal position. Supplementary Video: A video showing the 1,200 repetitive cycles of structural actuating motions of the MRA.


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