wettability and impact dynamics of water droplets on rice (oryza sativa l.) leaves
TRANSCRIPT
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: [email protected]
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
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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
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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
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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
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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