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

� 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 longitu-

dinal 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 identi-

fied. 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

studied because of their unique water repellent and self-

cleaning 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 struc-

tures, 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 superhydro-

phobic 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 wetta-

bility 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 wetta-

bility 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.

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: sjlee@postech.ac.kr

123

Exp Fluids (2014) 55:1691

DOI 10.1007/s00348-014-1691-y

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 sev-

eral 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 diame-

ter (D0) of the impacting droplet was maintained constant

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 consecu-

tively 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 cap-

tured 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 lon-

gitudinal (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 sam-

ples, CA measured perpendicular to the longitudinal ori-

entation of the rice leaf (CA on L0�) is 143.2� ± 8.1�,

whereas CA on T0� is 166.9� ± 7.5�. This kind of aniso-

tropic 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

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 deter-

mine the impact dynamics of a water droplet on the adaxial surface of

a rice leaf

1691 Page 2 of 9 Exp Fluids (2014) 55:1691

123

quantitatively indicate an easy roll-off along the longitu-

dinal 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

We = qV2D0/c, Re = qVD0/g, Oh ¼ffiffiffiffiffiffiffi

Wep �

Re; and

P = We/Re4/5. 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 cap-

illary 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

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 longitudi-

nal 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 iden-

tified (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 (Sikalo 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

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

Fig. 4 Contact angle hysteresis and roll-off angle of water droplets

on the rice leaves

Table 1 Experimental parameters of impacting droplets tested in this

study

Parameters

studied

V (m/

s)

We Re

(9103)

Oh

(910-3)

P (910-2)

Min. 0.23 1.85 0.65 2.07 1.03

Max. 1.90 124.6 5.36 2.08 1.29

Exp Fluids (2014) 55:1691 Page 3 of 9 1691

123

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 lim-

ited 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.

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

b ¼ Dm=D0ð 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 baccording 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 WeN1/4 (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 WeN1/4. 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

with WeN1/4; instead, b scales relatively well with WeN

1/2

(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 ReN1/5 as shown in Fig. 8b

(Clanet et al. 2004). This condition may be caused by a

high WeT, which could affect droplet deformation. Con-

sidering the variations in b, we found that anisotropic

wettability features are not plainly observable in many

cases of impacting droplets.

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

1691 Page 4 of 9 Exp Fluids (2014) 55:1691

123

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)

Exp Fluids (2014) 55:1691 Page 5 of 9 1691

123

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

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 obvi-

ously more advantageous for self-cleaning.

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� 60� 80�

We (=WeN) We WeN WeT We WeN WeT We WeN WeT

I

Min. 2.12 2.11 1.58 0.53 1.85 0.46 1.39 2.59 0.08 2.51

Max. 9.27 15.0 11.2 3.76 53.5 13.4 40.1 103 3.10 99.9

II

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

III

Min. 41.6 56.1 42.1 14.0 – – – – – –

Max. 120 99.3 74.5 24.8

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)

Fig. 8 Maximum spreading factor (b) of impacting water droplets on

the rice leaves according to a We and b WeN

1691 Page 6 of 9 Exp Fluids (2014) 55:1691

123

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 dis-

tance 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

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

droplet corresponds to capillary time defined asffiffiffiffiffiffiffiffiffiffiffiffiffi

qD30

�

cq

.

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 consis-

tently 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

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

Fig. 10 Contact times of impacting droplets on the rice leaves.

Contact times with respect to a V and b VN are shown

Exp Fluids (2014) 55:1691 Page 7 of 9 1691

123

larger than those along the transverse direction (particu-

larly 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 experimen-

tally investigated to provide quantitative information

regarding anisotropic or directional features. CA, CAH,

and aroll were measured along the longitudinal and trans-

verse 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

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 WeN1/4. 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 direc-

tion, indicating that the droplet motion along this direction

exhibits low resistance. These results explain the mecha-

nism 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).

References

Barthlott W, Neinhuis C (1997) Purity of the sacred lotus, or escape

from contamination in biological surfaces. Planta 202(1):1–8

Bennett T, Poulikakos D (1993) Splat-quench solidification: estimat-

ing the maximum spreading of a droplet impacting a solid

surface. J Mater Sci 28(4):963–970

Bird JC, Tsai SSH, Stone HA (2009) Inclined to splash: triggering and

inhibiting a splash with tangential velocity. New J Phys

11(6):063017

Bixler GD, Bhushan B (2012) Bioinspired rice leaf and butterfly wing

surface structures combining shark skin and lotus effects. Soft

Matter 8(44):11271–11284

Cheng YT, Rodak DE (2005) Is the lotus leaf superhydrophobic?

Appl Phys Lett 86(14):144101

Fig. 11 Sliding velocities of impacting droplets on the rice leaves

with respect to a WeN and b WeT

1691 Page 8 of 9 Exp Fluids (2014) 55:1691

123

Cheng YT, Rodak DE, Wong CA, Hayden CA (2006) Effects of

micro- and nano-structures on the self-cleaning behaviour of

lotus leaves. Nanotechnology 17(5):1359–1362

Clanet C, Beguin C, Richard D, Quere D (2004) Maximal deforma-

tion of an impacting drop. J Fluid Mech 517:199–208

Feng L, Li SH, Li YS, Li HJ, Zhang LJ, Zhai J, Song YL, Liu BQ,

Jiang L, Zhu DB (2002) Super-hydrophobic surfaces: from

natural to artificial. Adv Mater 14(24):1857–1860

Koch K, Bhushan B, Barthlott W (2008) Diversity of structure,

morphology and wetting of plant surfaces. Soft Matter 4(10):

1943–1963

Lee SG, Lim HS, Lee DY, Kwak D, Cho K (2013) Tunable

anisotropic wettability of rice leaf-like wavy surfaces. Adv Funct

Mater 23(5):547–553

Neinhuis C, Barthlott W (1997) Characterization and distribution of

water-repellent, self-cleaning plant surfaces. Ann Bot 79(6):

667–677

Richard D, Clanet C, Quere D (2002) Surface phenomena—contact

time of a bouncing drop. Nature 417(6891):811–811

Schiaffino S, Sonin AA (1997) Molten droplet deposition and

solidification at low weber numbers. Phys Fluids 9(11):

3172–3187

Sikalo S, Tropea C, Ganic EN (2005) Impact of droplets onto inclined

surfaces. J Colloid Interface Sci 286(2):661–669

Sun TL, Feng L, Gao XF, Jiang L (2005) Bioinspired surfaces with

special wettability. Acc Chem Res 38(8):644–652

Zhu YQ, Yu CX, Li Y, Zhu QQ, Zhou L, Cao C, Yu TT, Du FP

(2013) Research on the changes in wettability of rice (Oryza

sativa) leaf surfaces at different development stages using the

owrk method. Pest Manag Sci. doi:10.1002/ps.3594

Exp Fluids (2014) 55:1691 Page 9 of 9 1691

123