Exp Fluids (2014) 55:1691 DOI 10.1007/s00348-014-1691-y

RESEARCH ARTICLE

Wettability and impact dynamics of water droplets on rice (Oryza sativa L.) leaves Dae Hee Kwon • Hyung Kyu Huh • Sang Joon Lee

Received: 17 October 2013 / Revised: 8 February 2014 / Accepted: 11 February 2014 / Published online: 21 February 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract We investigated the wettability and impact dynamics of water droplets on rice leaves at various leaf inclination angles and orientations. Contact angle, contact angle hysteresis (CAH), and roll-off angle (aroll) of water droplets were measured quantitatively. Results showed that droplet motion exhibited less resistance along the longitudinal direction. Impact dynamic parameters, such as impact behaviors, maximum spreading factor, contact distance, and contact time were also investigated. Three different impact behaviors were categorized based on the normal component of Weber number irrespective of the inclination angle of the rice leaf. The asymmetric impact behavior induced by the tangential Weber number was also identified. Variation in the maximum spreading factor according to the normal Weber number was measured and compared with theoretical value obtained according to scaling law to show the wettability of the rice leaves. The contact distance of the impacting droplets depended on the inclination angle of the leaves. Along the longitudinal direction of rice leaves, contact distance was farther than that along the transverse direction. This result is consistent with the smaller values of CAH and aroll along the longitudinal direction.

1 Introduction The leaves of certain plants, such as lotus and rice, contain superhydrophobic surfaces. These plants have been widely

D. H. Kwon  H. K. Huh  S. J. Lee (&) Department of Mechanical Engineering, Center for Biofluid and Biomimic Research, POSTECH, San 31, Hyoja-dong, Pohang 790-784, Republic of Korea e-mail: [email protected]

studied because of their unique water repellent and selfcleaning characteristics (Feng et al. 2002; Koch et al. 2008). Such unique features, known as the lotus effect, are mainly attributed to micro- and nanoscale surface structures, as well as the chemical composition of wax on the surface (Barthlott and Neinhuis 1997; Neinhuis and Barthlott 1997). Experimental studies have also revealed that the wax on a lotus leaf is hydrophilic; a superhydrophobic nature is mainly caused by microscale protrusions and nanoscale roughness (Cheng and Rodak 2005; Cheng et al. 2006). The anisotropic arrangement of microscale structures, such as micropapillae, directly induces directional wettability on a leaf surface. The spatial distribution of the micropapillae on lotus leaves is homogeneous, whereas that on rice leaves is anisotropic, as shown in Fig. 1. The anisotropic distribution of the micropapillae on rice leaves causes anisotropic wettability; as a result, water droplets easily roll along the longitudinal direction parallel to the main orientation of the micropapillae pattern (Feng et al. 2002; Sun et al. 2005). Many successful attempts to mimic the directional and superhydrophobic wettability of rice leaves have been reported and highlighted (Bixler and Bhushan 2012; Lee et al. 2013). However, few experimental studies have been conducted to investigate the anisotropic wettability on a rice leaf. To date, the only available information includes the contact angle (CA) of several liquids on different breeds of rice at different growth stages (Zhu et al. 2013). In other words, some qualitative information is available, although much attention has been focused on the wettability of rice leaves. In addition, rice leaves are frequently exposed to the impact of rain drops. Therefore, the impacting dynamics and wettability of water droplets on rice leaves were experimentally investigated in this study.

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Fig. 1 SEM image of a rice leaf surface. a Micropapillae arranged along the longitudinal direction of the leaf. b Nanoscale structures on the leaf surface

Fig. 2 Schematic diagram of the experimental setup used to determine the impact dynamics of a water droplet on the adaxial surface of a rice leaf

2 Experimental setup Rice leaves (Oryza sativa L.) at the six- and seven-leaf stages were selected as test samples and prepared from a greenhouse in POSTECH. Fresh leaves were cut into several strips to fit in the field of view of the imaging system used in this study. The adaxial (upper) side of the rice leaf was cleaned by air to prevent any physical damages to the nano- and microscale structures of the surface. The abaxial (lower) side of the leaf was then attached to a glass slide by using a double-sided adhesive tape. Distilled water (density q = 997 kg/m3, viscosity g = 889 lPa s, and surface tension c = 72.8 mN/m) was used to measure the wettability and impact dynamics on rice leaf. The contact angle (CA), roll-off angle (aroll), and contact angle hysteresis (CAH) on the adaxial surface were measured using a goniometer (SmartDrop, Femtofab Inc.). A schematic of the experimental setup used to observe the droplet impact phenomena on a rice leaf surface is depicted in Fig. 2. The impacting droplets were generated from a capillary needle with gauge number 27 (inner diameter was approximately 0.21 mm). The initial diameter (D0) of the impacting droplet was maintained constant

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at 2.52 ± 0.01 mm, and the terminal velocity (V) was controlled by adjusting the height of the capillary needle. The images of each impacting droplet were consecutively captured at a frame rate of 1 9 104 (with a time interval of 0.1 ms between consecutive frames) by using a high-speed camera (FASTCAM SA 1.1, Photron Inc.) and a macro zoom lens (Nikon AF Micro-Nikkor 60 mm). The exposure time was 20 ls. The impact images were captured along two directions, namely longitudinal and transverse directions, of the rice leaf to investigate the anisotropic behaviors on the leaf surface. The inclination angle (a) of the rice leaf was measured from the horizontal plane (Fig. 2). In this study, the impact dynamics was studied at a = 0°, 30°, 60°, and 80°. To specify the imaging and measurement directions, we represented the captured images and measured data as La (or Ta) when the surface was inclined at a with respect to the longitudinal (or transverse) direction of the rice leaf, such that the imaging axis was perpendicular to the longitudinal (or transverse) orientation of the leaf.

3 Results and discussion 3.1 Sessile droplet on rice leaf CA, aroll, and CAH were measured along the longitudinal and transverse directions of the rice leaves to verify the anisotropic wettability of rice leaves. CA of the sessile droplets on the rice leaves exceeds 140°, and the leaf surface exhibits a superhydrophobic feature (Fig. 3). CA measured at the two perpendicular orientations of the rice leaf is clearly distinct. For the prepared test samples, CA measured perpendicular to the longitudinal orientation of the rice leaf (CA on L0°) is 143.2° ± 8.1°, whereas CA on T0° is 166.9° ± 7.5°. This kind of anisotropic or directional difference is also observed in the measured values of CAH and aroll (Fig. 4). The measured CAH and aroll are 16.3° ± 5.9° and 11.7° ± 4.9° on L0°, respectively, but these values are respectively 59.7° ± 16.7° and 37.9° ± 7.9° on T0°. The smaller values of CAH and aroll along the longitudinal direction

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Fig. 3 Sessile droplets on the adaxial surface of a rice leaf. Contact angles are measured perpendicular to the a longitudinal and b transverse orientations of a rice leaf

Table 1 Experimental parameters of impacting droplets tested in this study

Fig. 4 Contact angle hysteresis and roll-off angle of water droplets on the rice leaves

quantitatively indicate an easy roll-off along the longitudinal direction of the rice leaves. 3.2 Impact phenomenology For an impacting droplet with a diameter D0 and a terminal velocity V, several key parameters that describe the impact dynamics were determined. These parameters are Weber number (We), Reynolds number (Re), Ohnesorge number (Oh), and impact number (P), which have been defined as pffiffiffiffiffiffiffi We = qV2D0/c, Re = qVD0/g, Oh ¼ We Re; and 4/5 P = We/Re . The ranges of the experimental parameters of the impacting droplets in this study are summarized in Table 1. According to Schiaffino and Sonin (1997), the ranges of We, Re, and Oh indicate that the drop impact is highly inertial and capillary driven. In addition, viscous effect was negligible from the small value of P. Clanet et al. (2004) suggested that the drop impact is in the capillary regime at P \ 1. In this impact regime, the impacting droplets exhibit three different impact behaviors on the rice leaves at a = 0° (Fig. 5). The impacting droplets with We \ 9.27

Parameters studied

V (m/ s)

Min.

0.23

Max.

1.90

We

1.85 124.6

Re (9103)

Oh (910-3)

P (910-2)

0.65

2.07

1.03

5.36

2.08

1.29

show gentle bouncing while maintaining a blunt shape (impact behavior I). As We increases, the rim and lamella structures are formed during the spreading phase (impact behavior II). Splashing occurs (impact behavior III) at We [ 41.6. As seen in Fig. 5e, f, splashing starts right after contact, and liquid jets are developed along the longitudinal direction of the rice leaf. Therefore, the impact behaviors can be categorized based on We. Three impact behaviors are also observed at a = 30° (Fig. 6). The impact behaviors are categorized based on the normal component of We (WeN). The ranges of We for the three different impact behaviors are summarized in Table 2. We, which has been used to classify the impact behaviors on the normal surface (a = 0°), is comparable to WeN on the inclined surfaces (a = 30°, 60°, and 80°). On the rice leaves with a = 60°, the splashing behavior is not observed; only impact behaviors I and II are identified (Fig. 7a, b). At a = 80°, the maximum WeN is in the order of one. Therefore, only impact behavior I (gentle bouncing mode) is observed in the impacting droplets tested on rice leaves with a = 80° (Fig. 7c). This finding clearly reinforces the dependence of the deformation and impact behaviors on the ratio of the normal inertial forces to the surface forces (definition of WeN), which have been considered in the study of oblique impacts (Sˇikalo et al. 2005). As a increases, the tangential component of We (WeT) increases under the given impact conditions. The water droplet depicted in Fig. 6e shows asymmetric splashing patterns compared with the water droplet shown in Fig. 5e. This kind of asymmetric splashing was interpreted to be

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Fig. 5 Drop impact sequences on the rice leaves with an inclination angle of 0°. Images are captured on L0° for a, c, and e and on T0° for b, d, and f. The time interval between the consecutive image is 1.5 ms

for a, b, c, and d. The impact conditions of the droplets are a We = 2.18, b We = 2.12, c We = 29.6, d We = 29.8, e We = 68.4, and f We = 66.7

arisen from the existence of the tangential velocity of the impacting droplet relative to the surface (Bird et al. 2009). Therefore, the tangential component of We, WeT, has an influence on the asymmetry of the impacting droplet deformation. In terms of impact phenomenology, the anisotropic characteristics according to the leaf orientation are limited to the splashing case (impact behavior III). The development of liquid jets is stronger, and the satellite ejections are more frequent along the longitudinal direction compared with those along the transverse direction. No significant difference is observed along the two orthogonal leaf orientations for the other sets of impact experiments.

ðb ¼ Dm =D0 Þ on the rice leaves with respect to We (Fig. 8a) show large scattering, but these factors exhibit an evident dependence on a (Fig. 8b). The deformation of impacting droplets is closely related to WeN. Therefore, the maximum spreading factor b according to WeN exhibits a clear dependence on the inclination angle a and WeN. Considering that the impacting droplets tested in this study are in the capillary regime (P \ 1), b scales as We1/4 N (Clanet et al. 2004). All b values on the rice leaves with a = 0°, 30°, and 60° except b on the rice leaf with a = 80° were assumed to be scaled with We1/4 N . Clanet et al. (2004) mentioned that the scaling law can be applied only at WeN [ 1. Therefore, b on the rice leaf with a = 80° (WeN \ 3.20) does not scale well 1/2 with We1/4 N ; instead, b scales relatively well with WeN (1 \ WeN \ 3.20) as proposed by Bennett and Poulikakos (1993). At WeN \ 1, b of droplets on the rice leaf with a = 80° behaves as if they are in the viscous regime in which P [ 1 and b scales as Re1/5 N as shown in Fig. 8b (Clanet et al. 2004). This condition may be caused by a high WeT, which could affect droplet deformation. Considering the variations in b, we found that anisotropic wettability features are not plainly observable in many cases of impacting droplets.

3.3 Maximum spreading factor The maximum spreading factors of the bouncing droplets except the splashing droplets (impact behavior III) were investigated to evaluate the anisotropic or directional wettability of the rice leaves (Fig. 8). The maximum spreading factor is defined as the ratio of the maximum spreading diameter (Dm) to the initial diameter of a droplet before impact (D0). The maximum spreading factors

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Fig. 6 Drop impact sequences on the rice leaves with an inclination angle of 30°. Images are captured on L0° for a, c, and e and on T0° for b, d, and f. The impact conditions of the droplets are a WeN = 1.94 (We = 2.59 and WeT = 0.65), b WeN = 1.64 (We = 2.19 and WeT = 0.55), c WeN = 30.6 (We = 40.7 and WeT = 10.1), d WeN = 31.7 (We = 42.2 and WeT = 10.5), e WeN = 70.2 (We = 93.6 and WeT = 23.4), and f WeN = 71.6 (We = 95.4 and WeT = 23.8)

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Table 2 Impact behaviors (I: gentle bouncing, II: bouncing with rim and lamella, and III: splashing) of water droplets and the corresponding We ranges on the normal and inclined leaf surfaces Impact behavior

Normal (0°)

30°

We (=WeN)

We

60° WeN

WeT

We

80° WeN

WeT

We

WeN

WeT

I Min.

2.12

0.53

1.85

0.46

1.39

2.59

0.08

2.51

Max. II

9.27

15.0

2.11

11.2

1.58

3.76

53.5

13.4

40.1

103

3.10

99.9

Min.

16.0

19.9

14.9

5.00

63.0

15.8

47.2

–

–

–

Max.

37.5

50.1

37.5

12.6

125

31.1

93.4

Min.

41.6

56.1

42.1

14.0

–

–

–

–

–

–

Max.

120

99.3

74.5

24.8

III

Fig. 7 Drop impact sequences on the rice leaves with inclination angles of a, b 60°, and c 80°. Time intervals between consecutive images are b 6 ms and c 3 ms. The impact conditions of droplets are a WeN = 2.07 (We = 8.28 and WeT = 6.21), b WeN = 29.9 (We = 119 and WeT = 89.8), and c WeN = 2.08 (We = 68.9 and WeT = 66.8)

3.4 Contact distance and time Another important factor that quantitatively shows the wettability of the leaf surface, particularly an inclined surface, is the contact distance of bouncing droplets. However, limited experimental studies are available on this

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Fig. 8 Maximum spreading factor (b) of impacting water droplets on the rice leaves according to a We and b WeN

aspect. The contact distance is defined as the distance between the contact point of a droplet and the point where its detachment occurs (Fig. 7c). The contact distance is closely related to the performance of the self-cleaning effect. For a given degree of rolling motion, the impacting droplet that travels longer distance on the surface is obviously more advantageous for self-cleaning.

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Fig. 10 Contact times of impacting droplets on the rice leaves. Contact times with respect to a V and b VN are shown

leaves (Fig. 10). The contact time of bouncing droplets does not depend on the impact velocity V over a wide range (Richard et al. 2002). The contact time of a bouncing qffiffiffiffiffiffiffiffiffiffiffiffiffi  droplet corresponds to capillary time defined as qD30 c.

Fig. 9 Contact distances of impacting droplets on the rice leaves. Contact distances with respect to a We, b WeN, and c WeT are shown

The contact distances of the impacting droplets on the rice leaves are shown in Fig. 9. It is clearly shown that the larger the inclination angle is, the farther the contact distance is for a given We of an impacting droplet. Unlike impact behaviors and maximum spreading factor, contact distance is not solely dependent on WeN. Figure 9b shows a considerable gap between the contact distances at different a. Contact distance largely depends on WeT, although a small gap is present between a (Fig. 9c). This small gap in the contact distance can be attributed to the contact time of the impacting droplet on the rice

However, contact time increases as V decreases when V is smaller than a certain threshold value. In this study, the contact time of impacting droplets increases as the normal component of impact velocity (VN) decreases at VN \ 0.2 ms-1 (Fig. 10b). The VN of the many impacting droplets on the rice leaves with a = 80° is less than the threshold value (0.2 ms-1) because of a high a. Therefore, the contact distance at a = 80° should be farther than that at other a values as long as traveling speeds (sliding velocity) are the same. Sliding velocities on the rice leaves were also measured, and the results are depicted in Fig. 11. Sliding velocity depends on WeT; sliding velocities at a = 80° are consistently higher than those at other a values. Therefore, a far contact distance on the leaf surface at a = 80° is attributed to long contact time and high sliding velocity. In addition, the contact distances along the longitudinal direction are

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Fig. 11 Sliding velocities of impacting droplets on the rice leaves with respect to a WeN and b WeT

larger than those along the transverse direction (particularly at a = 30°). This result is evident because lower CAH and aroll along the longitudinal direction, which were introduced earlier, indicate that a small resistance to droplet motion is present along the longitudinal direction. It is interesting to note that the inclination angle of the rice leaves in their natural state reaches a maximum of 80° with respect to the longitudinal direction. From these results, we can presume that the rice leaf utilizes a long contact time and high sliding velocity of water droplets to maximize self-cleaning performance.

4 Conclusion In this study, the wettability and impact dynamics of water droplets on rice leaves (Oryza sativa L.) were experimentally investigated to provide quantitative information regarding anisotropic or directional features. CA, CAH, and aroll were measured along the longitudinal and transverse directions of the rice leaf. CAH and aroll along the longitudinal direction are smaller than those along the transverse direction. Therefore, water droplets deposited on

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rice leaves easily roll off along the longitudinal direction of the leaf surfaces, indicating anisotropic wettability. The impact dynamic parameters, including impact behaviors, maximum spreading factor, and contact distance along the two orthogonal orientations of the leaf were investigated with varying a. Three different impact behaviors (gentle bouncing with blunt shape, rim and lamella formation, and splashing) are observed with respect to WeN. Irrespective of a, the impact behaviors are clearly categorized based on the range of WeN. In addition, asymmetric deformation of the impacting droplets caused by WeT is observed. However, the anisotropic wettability in terms of impact phenomenology is limited to the splashing case. The maximum spreading factor b based on WeN, except at a = 80°, scales well with We1/4 N . This condition implies that these droplet impact behaviors were in the capillary regime. This finding is consistent with droplet deformation that mainly depends on the ratio of normal inertial forces to surface forces. At a = 80°, b scales as WeN far from that for the capillary regime, which may be attributed to high WeT. The contact distance of bouncing droplets on rice leaves shows a definite dependence on a. As a increases, VN decreases. At a = 80°, VN of many bouncing droplets are smaller than the critical value, at which contact time increases as VN decreases. Therefore, the droplets sliding on the rice leaves with a = 80° have more time to travel on the surface. In addition, the contact distances along the longitudinal direction of leaves are farther than those along the transverse direction. This condition is clearly explained by low CAH and aroll values along the longitudinal direction, indicating that the droplet motion along this direction exhibits low resistance. These results explain the mechanism by which rice leaves utilize droplets to maximize the self-cleaning effect. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (No. 2008–0061991).

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