biosystems engineering 106 (2010) 58–67
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/issn/15375110
Research Paper
Evaporation rate and development of wetted area of water
droplets with and without surfactant at different locations
on waxy leaf surfaces
Linyun Xu a, Heping Zhu b,*, H. Erdal Ozkan c, Harold W. Thistle d
a FABE, The Ohio State University-OARDC, Wooster, OH, USA
b USDA/ARS Application Technology Research Unit, Wooster, OH, USA
c FABE, The Ohio State University, Columbus, OH, USA
d USDA Forest Service, Morgantown, WV, USA
article info The evaporation and formation of deposit patterns from single droplets deposited at
various locations on waxy leaves were investigated under controlled conditions. Leaf
Article history: locations included the interveinal area, midrib and secondary vein on both adaxial and
Received 31 August 2009 abaxial surfaces. Tests were conducted with 300 and 600 mm diameter droplets containing
Received in revised form water and a non-ionic surfactant. The ambient temperature was 25 C and relative
30 January 2010 humidity (RH) was 60%. Evaporation time and wetted area varied with the area where the
Accepted 12 February 2010 droplets were deposited on the leaf surfaces. The variation in evaporation time for 300 mm
Published online 20 March 2010 diameter droplets without surfactant on the interveinal area, midrib and secondary vein of
adaxial surface was 30% whilst the variation of wetted area was 39%. The wetted area was
significantly larger on the adaxial surface than on the abaxial surface but the evaporation
time between both surfaces was not significantly different. For the whole leaf, the average
evaporation time of 300 mm diameter droplets decreased by 44% and the average wetted
area increased by 202% when 0.25% non-ionic surfactant was added to the spray solution.
The total mean evaporation time increased 279% and the wetted area increased 166%
without the surfactant, 452% and 229% with the surfactant when droplet diameter was
increased from 300 to 600 mm. The largest deposit area was measured on the midrib of
adaxial surface with added surfactant. The 300 mm diameter droplets had longer evapo-
ration times per droplet volume and greater wetted area per droplet volume than the
600 mm diameter droplets, thereby supporting the hypothesis that increased pesticide
application efficiency could be achieved by finer spray. This study also demonstrated that
the ratio between spray coverage area and the amount of spray liquid required could be
increased by the use of surfactants, thereby offering possibilities of reduction in spray
application rates and of increase in application efficiency.
Published by Elsevier Ltd on behalf of IAgrE.
* Corresponding author.
E-mail address: [email protected] (H. Zhu).
1537-5110/$ – see front matter Published by Elsevier Ltd on behalf of IAgrE.
doi:10.1016/j.biosystemseng.2010.02.004
biosystems engineering 106 (2010) 58–67 59
1. Introduction the leaf surfaces (Kannan & Sivakumar, 2008). Surfactants are
usually used in spray mixtures to increase leaf surface
Pesticide application has ensured the production of floral and wettability and droplet spreading capability (Foy, 1993;
nursery crops of high quality, but the increased use of Ryckaert et al., 2007; Spanoghe, Schampheleire, Meeren, &
chemicals has brought public concerns about worker expo- Steurbaut, 2007), thereby reducing the amounts of pesticide
sure, environmental contamination, and adverse impacts on required (Costa et al., 2005; Kirkwood, 1993). However, exces-
vulnerable ecosystems. Nursery and floriculture crops in the sive use of surfactants might not produce uniform residual
USA states of California, Florida, Michigan, Oregon, Pennsyl- coverage on hydrophobic surfaces post-evaporation (Pierce,
vania, and Texas consumed a total of 2540 tonne of pesticide Chan, & Zhu, 2008). Water repellency on leaves varies
active ingredients in 2006, an increase of around 17% from the greatly with species and between the leaf abaxial or adaxial
amount applied in 2003 (Anonymous, 2007). surface (Holder, 2007).
Pesticide application is complicated by the use of a variety Yu, Zhu, Frantz, et al. (2009) and Yu, Zhu, Ozkan, Derksen,
of delivery equipment and methods, the varied physical and Krause (2009) investigated droplet evaporation rates and
properties of chemical sprays, the diverse crops and their the area covered by pesticide droplets on hydrophobic and
growth habits, the numerous pests and diseases, disparities in hydrophilic slides, and waxy and hairy leaves, under the
operator skills, the uncontrollable weather conditions, the controlled environmental conditions. They found that evap-
extensive worker safety and environmental regulations, and oration dynamics and post-evaporation deposit formations of
the economics related to the benefits of pesticide applications. pesticide droplets on waxy and hairy leaf surfaces were
greatly influenced by the spray mixture additive, droplet size,
Over the past few decades, research into pesticide spray and relative humidity (RH). For the droplets ranging from 246
application technology has mostly concentrated on methods to 886 mm diameter, and RH in the range 30–90%, the addition
and equipment to improve the accuracy of spray delivery to of surfactant increased the area of spray coverage 4.5–10.1
targets. This has included the development of air-assisted times that on the hairy leaves and 3.4–4.1 times that on the
sprays and electrostatic sprays; investigations into nozzle waxy leaves. However, they only investigated the interveinal
selection, and the optimisation of application rates. However, area of adaxial leaf surfaces.
there are few studies on the behaviour of droplets deposited
onto targets following delivery. The behaviour of droplets on The leaf surfaces attacked by pests vary with the type of
plant surfaces contributes to controlling the physiological and insect or disease. Different insects have their favourite feeding
biological processes. Absorption and uptake of active ingredi- locations on leaves. For example, many weevils like to feed on
ents by leaves can be increased when the duration of sprays on interveinal area of leaves; cottonwood leaf beetles chew the
leaf surfaces is increased (Knoche & Bukovac, 1994; Knoche, interveinal area particularly on the abaxial surface of willow
Petracek, & Bukovac, 2000). The absorption of chemicals may leaves, and aphids and whiteflies extract sap from the
cease after droplets deposited on leaves evaporate (Ramsey, secondary veins and midribs of leaves (Dreistadt, Clark, &
Stephenson, & Hall, 2005). Also, if droplets evaporate too Flint, 2004). There is little information on droplet spreading
rapidly and do not spread out on leaves then chemical residues and evaporation on particular parts of the leaf surfaces to
may form crystals which have low retention on leaves. On the establish pesticide spray application strategies to control
other hand, if water droplets persist on leaves this may accel- these insects.
erate the germination of certain pathogens (Bradley, Gilbert, &
Parker, 2003; Huber & Gillespie, 1992). Fully understanding the In an effort to provide quantitative information for end
physical process of spray droplet evaporation and residual users in order to increase pesticide application efficiency for
pattern formation on target surfaces is essential not only in controlling particular insects or diseases, the objective of this
improving pesticide application efficiency but also in mini- research was to determine the evaporation time, droplet
mising off-target contamination. spreading process and spreading area of droplets with and
without the non-ionic surfactant at three different positions
The leaf morphology which varies with the plant variety (i.e. interveinal area, secondary vein, and midrib) on adaxial
and species affects the formation of droplet deposition and and abaxial surfaces of waxy leaves.
retention on leaf surfaces (Beattie & Marcell, 2002; Brewer,
Smith, & Vogelmann, 2007; Costa, Martins, Rodella, Duarte, 2. Materials and methods
& Costa, 2005; De Ruiter, Uffing, Meinen, & Prins, 1990;
Smith, Askew, Morris, Shaw, & Boyette, 2000). The structures Leaves were selected from a container-grown two-year old
on the leaf surface include the cuticular wax, veins, hairs and Japanese Tree Lilac (Syringa reticulate). At the time of experi-
other protrusions. The microstructures of the epidermal cell ments, the tree height was 2.5 m and its diameter at 0.18 m
and epicuticular wax crystals play important roles in repelling above the container surface was 29 mm. This species of tree
water from leaf surfaces (Guo & Liu, 2007; Holder, 2007; was selected for the experiments because its leaves have
Wagner, Fu¨ rstner, Barthlott, & Neinhuis, 2003). The two waxy characteristics and its surface structure at interveinal
main forms of epicuticular wax on leaf surfaces are crystalline area, secondary veins and midrib on both adaxial and abaxial
and amorphous. Crystalline wax resists droplet spreading on surfaces can be identified clearly (Fig. 1). Puckering on leaves
leaf surfaces (Wang & Liu, 2007). formed valleys along the veins on the adaxial surface and
ridges on the abaxial surface. The adaxial surface of leaves
The penetration performance of insecticides, herbicides or was glabrous with few sparse 100 mm long hairs along the
fungicides into plant tissues largely depends on the wetta-
bility of leaf surfaces which is influenced by the topography on
60 b i o s y s t e m s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 5 8 – 6 7
Fig. 1 – Positions and locations selected for droplets to control unit consisted of a humidifier, a dehumidification unit,
deposit on adaxial and abaxial leaf surfaces. an air mixing tank, two RH probes, a micro data-logger, and
associated electronics which provided the target holding
midrib. There were epicuticular crystalline waxes on the chamber with air at a constant RH ranging from 10 to 90%. The
midrib of the adaxial surface. The abaxial leaf surface had target holding chamber was insulated and was used to posi-
trichomes with a mean hair length of 300 mm and a mean tion targets and single droplets in X–Y directions along the
distance between hairs of 700 mm. The abaxial surface was plane of the leaf surface. The image acquisition assembly was
more hydrophobic than the adaxial surface as shown by the a stereoscope and an Insight Firewireª, high-definition digital
contact angle for 300 mm droplets which without surfactant camera mentioned above. A detailed description of the
was 90 in the interveinal area of the adaxial surface and 102 experimental system was reported by Yu, Zhu, Ozkan, et al.
on the abaxial surface. All the information about the leaf (2009).
surface characteristics was obtained in the laboratory by
measuring the leaf surfaces under a stereoscope (Model To measure droplet-spread area and evaporation time on
SZX12, Olympus, Japan) using an Insight Firewireª, high- the adaxial and abaxial surfaces of fresh leaves, two 20 by
definition digital camera (Model SZX-TB1, Olympus, Japan). 20 mm sections that included the interveinal areas, midrib
For the experiments, leaves were picked from branches in the and secondary veins were cut from the basal and distal
middle of the canopy after the foliage was fully developed. portions of a leaf (Fig. 1). Each section was mounted on a glass
The mean surface area of a leaf was 3650 mm2, the mean leaf plate with the double-sided tape and then placed on
length was 79 mm, and the mean leaf width was 57 mm. a manually operated X–Y mechanical stage inside an
environmentally-controlled chamber. The position on the leaf
A custom-built system used for experiments mainly con- section where the droplet would be deposited was positioned
sisted of a single-droplet generator, RH control unit, a target under the camera lens within a focus range and at a desired
(i.e. leaf) holding chamber, and an image acquisition assembly magnification. When the adjustments were completed,
(Fig. 2). The droplet generator was a microprocessor-based a droplet was discharged. Three positions, each located on the
timed mode, air-powered fluid dispenser (Model 2405, EFD interveinal area, secondary vein, and midrib of each section,
Inc., East Providence, RI, USA) that could produce a single respectively, were selected for the test. Because of the risk of
droplet with diameters ranging from 200 to 2000 mm. The RH damage to the fine structure of unused surface when
mounting a leaf with double-sided adhesive tape, each leaf
was tested for only one surface (either adaxial or abaxial) but
with two sections. For each measurement, only one droplet
was deposited at one position. Following deposition, sequen-
tial images of the droplet spreading and evaporation process
were obtained. After the images were taken at the first posi-
tion, the same process was repeated for the second and third
positions with droplets of the same size and formulation. The
leaf section was discarded after measurement at the third
position. In our previous study, Yu, Zhu, Frantz, et al. (2009)
and Yu, Zhu, Ozkan, et al. (2009) established relationships
between wetted area, evaporation time and droplet size on the
flat waxy leaf area for the droplet diameters ranging from 246
to 886 mm. In this study droplets of 300 and 600 mm diameter
were chosen because the objective was to determine the
influence of deposition location on droplet spreading and
evaporation.
After the droplet was deposited at a desired position on
a leaf surface, sequential images of the droplet evaporation
process were taken and stored in a computer at regular, timed
intervals using an imaging program (Spot 4.1, Diagnostic
Instruments, Inc., Sterling Heights, MI, USA). The interval
between sequential images was 2 s for 300 mm diameter drop-
lets and 4 s for 600 mm diameter droplets. Evaporation times
were computed using the timed interval and the total number
of sequential images from the start of droplet deposition to the
completion of evaporation. The wetted area of droplets, which
was defined as the maximum spreading area of a droplet on the
leaf surface following deposition, was measured using the
Polygonal Hand-trace feature of the ImageProPlus program
(version 4.1, Media Cybernetics, Bethesda, MD, USA). The
standard area of each pixel grid for the area measurement
ranged from 1.3 Â 10À6 to 8.6 Â 10À6 mm2 depending on the
amplification used to record images. The imaging program was
biosystems engineering 106 (2010) 58–67 61
Air Foot Pedal
Pressure Digital
Source Camera
Droplet Stereo Microscope Image
Generator Acquisition
Lights Environmental Computer
Humidity & Microsyringe Controlled
Temperature Chamber Humidity &
Temperature
Sensor
Sensor
Droplet
Leaf Fan
Air Out
Input Chamber
X-Y Platform Adjustment
Handle
Air Mixing 60%RH
Chamber 250C
Air Pump
Humidifier Dehumidifier Computer
Data Logger
and Controller
Fig. 2 – Schematic diagram of the experimental system used to determine the droplet spreading and evaporation on leaf
surfaces.
calibrated using a Zeiss 0.01 mm micrometre slide. During and air temperature inside the chamber were controlled at 60%
image analysis, each image was enlarged to fill a 400 mm sized and 25 C, respectively.
computer screen so that the droplet boundaries could be easily
traced and visualisation of the droplet evaporation process Because the difference in either wetted area or evaporation
could be clearly observed. time between two droplet diameters was clear, data analysis
was separated for the two diameters. Significant differences
Two types of spray solution were selected for the experi- among the treatments for each droplet diameter were ana-
ments. One was distilled water, and the other was distilled lysed using ProStat version 3.8 (Poly Software International,
water mixed with a surfactant Triton X-100 (Acros Organics, Inc., Pearl River, NY, USA). A one-way analysis of variance
Fair Lawn, NJ, USA). The concentration of the surfactant in (ANOVA) was used to test the null hypothesis that all
distilled water was 0.25% (v/v). Triton X-100 (C14H22O(C2H4O)n)
is a water soluble non-ionic surfactant with a molecular Table 1 – Variables and test conditions to investigate
weight of 646.86. It is commonly used as detergent in evaporation time and wetted area of a single droplet on
biochemistry laboratories. Pesticides were not added to each waxy leaf surfaces
solution because our earlier studies with pesticides formu-
lated by their manufacturers using surfactants (Yu, Zhu, Droplet diameter (mm) 300, 600
Frantz, et al., 2009) had demonstrated that the evaporation Leaf surface Adaxial, abaxial
time and wetted area of droplets on waxy surfaces were not Location on surface Basal, distal
significantly affected by the addition of pesticide. Position on surface Interveinal area,
midrib, secondary vein
The variables and test conditions used are listed in Table 1. Spray solution Distilled water,
There were 48 treatments including two droplet diameters, distilled water with surfactant
two leaf surfaces, two sections on each surface, three positions RH (%) 60
on each section, and two spray solutions. Each treatment used Ambient temperature (C) 25
five leaves, representing five replications. For every test, RH
62 b i o s y s t e m s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 5 8 – 6 7
Fig. 3 – Deposition pattern of 300 mm diameter droplets without the surfactant on: (a) interveinal area of adaxial surface,
(b) interveinal area of abaxial surface, (c) midrib of adaxial surface, (d) midrib of abaxial surface, (e) secondary vein of adaxial
surface, (f) and secondary vein of abaxial surface.
treatments had equal means using Duncan’s method. If the wetted area or evaporation time between the two positions
null hypothesis was rejected, the multiple comparison (one was on the basal section and the other was on the distal
procedure was used to determine differences among means of section) for each interveinal area, midrib and secondary vein
the treatments. All differences were determined at the 0.05 on the same surface (adaxial or abaxial). The final data anal-
level of significance. The first round of comparisons demon- ysis combined the two positions on the same surface for each
strated that there was no significant difference in either treatment as 10 replications.
Fig. 4 – Deposition pattern of 300 mm diameter droplets with the surfactant on: (a) interveinal area of adaxial surface,
(b) interveinal area of abaxial surface, (c) midrib of adaxial surface, (d) midrib of abaxial surface, (e) secondary vein of adaxial
surface, (f) and secondary vein of abaxial surface.
biosystems engineering 106 (2010) 58–67 63
3. Results and discussion
3.1. Droplet spreading and evaporation process
After droplets deposited on leaf surfaces, they began to
spread and evaporate. The process of the spreading and
evaporation varied with the location (interveinal area, midrib
or secondary vein) and leaf surface (adaxial or abaxial) where
the droplets landed, and the spray solution formulation (with
or without the surfactant). Fig. 5 shows the spreading area of
300 mm diameter droplets with and without the surfactant at
different times before complete evaporation on the inter-
veinal area of abaxial and adaxial surfaces (Fig. 5(a)), on the
midrib of both surfaces (Fig. 5(b)), and on the secondary vein
of both surfaces (Fig. 5(c)). After they were deposited on all
three locations on both adaxial and abaxial surfaces, the
300 mm diameter droplets without the surfactant reached
their wetted area (or maximum spreading area) within a very
short period of time when the first sequential image was
taken (Table 2). However, with the surfactant added, the time
for the droplet to reach its wetted area extended considerably
(Table 2). It took over 12.0 s for the 300 mm diameter droplet
with surfactant to spread on the midrib of the adaxial
surface. After reaching a maximum, the spreading area
remained constant, or decreased very slowly for an extended
period of time, before complete evaporation occurred (Fig. 5).
The 600 mm diameter droplets had a spreading process very
similar to that of the 300 mm diameter droplets at different
locations on both adaxial and abaxial surfaces (Fig. 6);
however, larger droplets required a much longer time to
reach the wetted area. For example, the spreading time to the
wetted area on the interveinal area of adaxial surface was
2.6 s for a 300 mm diameter droplet and 25.3 s for a 600 mm
diameter droplet when both droplets contained the
surfactant.
3.2. Evaporation time and wetted area Fig. 5 – Changes in spreading area of 300 mm diameter
droplets with and without the surfactant on (a) interveinal
3.2.1. 300 mm diameter droplets area, (b) midrib, and (c) secondary vein of adaxial and
Table 2 shows the wetted area, time to reach the wetted area abaxial surfaces during the spreading process after
and the evaporation time for 300 mm diameter water droplets deposition (B Adaxial surface without surfactant,
with and without surfactant after being deposited on the : Abaxial surface without surfactant, C Adaxial surface
interveinal area, midrib and secondary vein of adaxial and with surfactant, and ; Abaxial surface with surfactant).
abaxial waxy leaf surfaces. Without the surfactant, the evap-
oration time of 300 mm diameter droplets at the interveinal
area, midrib and secondary vein on the adaxial surface was
significantly, or partial-significantly, different while the
difference was not significant on the abaxial surface. Droplets
had the longest evaporation times on the interveinal area of
each leaf surface, followed by secondary vein while the droplets
on the midrib had the shortest evaporation time. For example,
the mean evaporation time of 300 mm diameter droplets was
86 s, 60 s and 74 s in the interveinal area, midrib and secondary
vein on the adaxial surface, respectively. The difference in the
average evaporation times among the interveinal area, midrib
and secondary vein was 30% on the adaxial surface and 18% on
the abaxial surface. Thus, the variation in evaporation time of
droplets on the adaxial surface was greater than that on the
abaxial surface. Droplets without the surfactant on the adaxial
64 b i o s y s t e m s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 5 8 – 6 7
Table 2 – The wetted area, time to reach the maximal wetted area, and evaporation time of 300 mm diameter water droplets
with and without the surfactant after deposition on the interveinal area, midrib and secondary vein of adaxial and abaxial
leaf surfaces at 60% RH and 25 8C. Standard deviations are presented in parentheses. Values in a column followed by
a different letter are significantly different ( p < 0.05)
Leaf surface Position Without surfactant With surfactant
Ad (mm2) T1e (s) T2f (s) A (mm2) T1 (s) T2 (s)
Adaxial Interveinal area 0.091c (0.011) 0 86ab (11) 0.251c (0.063) 2.6bc (1.9) 50b (7)
Midrib 0.121b (0.011) 0 60c (9) 0.661a (0.317) 12.0a (6.7) 38c (10)
Secondary vein 0.150a (0.031) 0 74bc (9) 0.455b (0.169) 3.8bc (2.7) 41c (8)
Mean 0.121 73 0.456 43
Abaxial Interveinal area 0.088c (0.015) 0 96a (23) 0.187c (0.040) 0.5a (0.4) 62a (8)
Midrib 0.087c (0.007) 0 79abc (7) 0.155c (0.028) 0.8bc (0.6) 38c (9)
Secondary vein 0.091c (0.011) 0 89ab (16) 0.199c (0.012) 6.5ab (5.0) 40c (5)
Mean 0.089 88 0.180 47
d A – Wetted area, or maximum spreading area after droplet deposition.
e T1 – Time to reach the wetted area after droplet deposition. T1 ¼ 0 presents the droplet completed its spreading to reach the wetted area when
the first sequential image was taken.
f T2 – Complete evaporation time after droplet deposition.
surface had the mean evaporation time 17% shorter than that the wetted area increased greatly (Table 2) compared to the
on the abaxial surface. solution without the surfactant. Fig. 4 showed the wetted area
of a 300 mm diameter droplet with surfactant at the interveinal
Similarly, the wetted area of 300 mm diameter droplets area, midrib and secondary vein on adaxial and abaxial
without the surfactant at the interveinal area, midrib and surfaces. Compared to the same size droplets without the
secondary vein on the adaxial surface was significantly surfactant, the total mean wetted area increased 203% while
different while the difference was not significant on the the total mean evaporation time decreased by 44%. The
abaxial surface (Table 2). Droplets on the secondary vein had greatest difference in the wetted area between the 300 mm
larger wetted areas than they were on the midrib and the diameter droplets with and without the surfactant was 446%
interveinal area. For example, the mean wetted area of 300 mm which occurred on the midrib of adaxial surface. Under the
diameter droplets was 0.091, 0.121 and 0.150 mm2 at the same conditions, the difference was 203% and 176% on the
interveinal area, midrib and secondary vein on the adaxial secondary vein and interveinal area of the adaxial surface,
surface, respectively. The difference in the average wetted respectively. The relative increase in wetted area was 153%
area among the interveinal area, midrib and secondary vein higher on the adaxial surface than on the abaxial surface with
was 39% on the adaxial surface and 4% on the abaxial surface. the surfactant, but the evaporation time between both
Thus, the variation in wetted area on the adaxial surface was surfaces was not significantly different. The droplets on the
greater than that on the abaxial surface. Droplets without the midrib and secondary vein of the adaxial surface spread
surfactant on the adaxial surface had the average wetted area extensively along midrib, or in the direction of the secondary
36% greater than that on the abaxial surface. For the entire leaf vein (Fig. 4(c) and (e)).
including both surfaces, the relative difference in the wetted
area amongst the three different locations was 42%. The surfactant dissolved and ruptured the structure of
epicuticular hydrophobic crystalline waxes on leaf surfaces,
For water without the surfactant, the shape of all droplets resulting in a greater contact area on the surfaces. Thus, the
deposited on the waxy leaves was a segment of sphere (Fig. 3), spray coverage area per volume of spray applied can be
except for the droplet deposited on the secondary vein of the increased by using surfactants offering a possible reduction in
adaxial surface (Fig. 3(e)). The droplet on the secondary vein of the amount of spray liquid applied. For example, the mean
adaxial surface spreads more rapidly along the secondary vein wetted area of a 300 mm diameter droplet with and without the
direction than in the direction perpendicular to the secondary surfactant on an adaxial surface was 0.121 mm2 and
vein. The droplet on the midrib of the adaxial surface did not 0.456 mm2, respectively. A 3650 mm2 leaf surface when the
spread along the midrib direction (Fig. 3(c)), which might be surfactant was not used, would require 30 165 300 mm diameter
caused by the midrib of adaxial surface being adequately droplets (equivalent to 426 ml) to completely cover the adaxial
covered by epicuticular hydrophobic crystalline waxes (Smith surface of the leaf. The same leaf surface would require only
et al., 2000). In contrast, the droplets did not spread along the 8004 300 mm diameter droplets (equivalent to 113 ml) when the
direction of midrib or secondary vein on the abaxial surface surfactant was used; a 3.8 times reduction in spray volume.
because of the puckered ridges. Therefore, here the use of surfactant increased the ratio
between the coverage area and the amount of sprays required
For the 300 mm diameter droplets without the surfactant on consequently offering increased application efficiency.
the entire adaxial surface, the mean evaporation time was
17% shorter than that on the abaxial surface, and the mean 3.2.2. 600 mm diameter droplets
wetted area was 36% larger. This was because the abaxial leaf Similarly to results of the 300 mm diameter droplets, as can be
surface was more hydrophobic than the adaxial leaf surface. seen in Table 3, the evaporation time and wetted area of
For the spray solution with the 0.25% v/v non-ionic
surfactant, the evaporation time of droplets decreased and
biosystems engineering 106 (2010) 58–67 65
Fig. 6 – Changes in spreading area of 600 mm diameter 600 mm diameter droplets also varied with the location
droplets with and without the surfactant on (a) interveinal (interveinal, midrib or secondary vein) and surface (adaxial or
area, (b) midrib, and (c) secondary vein of adaxial and abaxial) of leaves where the droplets were deposited, and the
abaxial surfaces during the spreading process after spray solution formulation used (with or without the surfac-
deposition (B Adaxial surface without surfactant, tant). The mean wetted area on the adaxial surface increased
: Abaxial surface without surfactant, C Adaxial surface by 21% without surfactant and 44% with surfactant, compared
with surfactant, and ; Abaxial surface with surfactant). to the abaxial surface. Also, for the total leaf surface, the
addition of surfactant in the spray solution increased the
mean wetted area by 275% while the mean evaporation time
decreased by 19%. For the same solution, except for the case
on the adaxial surface without the surfactant, the longest
evaporation time occurred on the interveinal area, followed by
the secondary vein and the midrib on each surface of leaves.
For the droplets of water without surfactant, the difference
in the wetted area among the interveinal area, midrib and
secondary vein on the abaxial surface was negligible; which
was also true for the droplets with surfactant (Table 3).
However, the difference in the wetted area on the adaxial
surface was 18% without surfactant and 57% with surfactant.
The greatest difference in wetted area between the 600 mm
diameter droplets with and without the surfactant was 553%
which occurred on the midrib of adaxial surface. Under the
same conditions, the difference was 208% and 178% on the
secondary vein and interveinal area of the adaxial surface,
respectively. The use of the surfactant increased the wetted
area at different positions (interveinal area, midrib and
secondary vein) on both adaxial and abaxial surfaces. Ulti-
mately, the uniformity of spray coverage on entire leaf
surfaces might be improved by adding a surfactant to
compensate for the variation of fine structures on leaf
surfaces.
3.3. Comparison between two droplet sizes
When the droplet diameter increased from 300 to 600 mm, the
total mean evaporation time increased 279% without the
surfactant and 452% with the surfactant while the total mean
wetted area increased 166% without the surfactant and 229%
with the surfactant. The mean wetted area was greater on the
adaxial surface than on the abaxial surface whilst the mean
evaporation time was slightly shorter on the adaxial surface
than on the abaxial surface. Without surfactant the largest
wetted area occurred on the secondary vein of the adaxial
surface while with surfactant it occurred on midrib of adaxial
surface. The longest evaporation time occurred on the inter-
veinal area on each surface of leaves for both solutions with
and without the surfactant.
The wetted area per volume of a droplet gA, or the ratio of
the wetted area of a droplet to its volume, was used to eval-
uate the spreading ability of the droplet. For the same location
on leaf surfaces with the same solution, the 300 mm diameter
droplets had greater gA values than the 600 mm diameter
droplets (Table 4). For the same solution and the same sized
droplet except for 600 mm diameter droplets on the interveinal
area with surfactant, the droplets on the adaxial surface had
greater gA values than droplets on the abaxial surface.
Similarly, the ratio gT which was evaporation time per
droplet volume was used to evaluate the droplet evaporation
rate. For the same position on leaf surfaces and the same solu-
tion, the 300 mm diameter droplets had greater gT value than the
66 b i o s y s t e m s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 5 8 – 6 7
Table 3 – The wetted area, time to reach the maximal wetted area, and evaporation time of 600 mm diameter water droplets
with and without the surfactant after deposition on the interveinal area, midrib and secondary vein of adaxial and abaxial
waxy leaf surfaces at 60% RH and 25 8C. Standard deviations are presented in parentheses. Values in a column followed by
a different letter are significantly different ( p < 0.05).
Leaf surface Position Without surfactant With surfactant
Ae (mm2) T1f (s) T2g (s) A (mm2) T1 (s) T2 (s)
Adaxial Interveinal area 0.288b (0.025) 0 309bc (22) 0.801c (0.098) 25.3a (12.7) 263b (18)
Midrib 0.283bc (0.031) 0 292bc (20) 1.847a (0.354) 29.3a (18.0) 184d (33)
Secondary vein 0.343a (0.084) 12.0 (12.3) 279c (18) 1.058b (0.216) 20.6a (7.1) 234c (30)
Mean
0.305 293 1.235 227
Abaxial Interveinal area 0.264bc (0.015) 0 338a (29) 0.819c (0.045) 29.5a (21.1) 298a (13)
Midrib 0.237c (0.005) 0 287c (33) 0.884bc (0.261) 20.7a (10.2) 251bc (13)
Secondary vein 0.258bc (0.006) 0 325ab (16) 0.876bc (0.039) 37.7a (16.8) 262b (17)
Mean
0.253 317 0.860 270
e A – Wetted area, or maximum spreading area after droplet deposition.
f T1 – Time to reach the wetted area after droplet deposition. T1 ¼ 0 presents the droplet completed its spreading to reach the wetted area when
the first sequential image was taken.
g T2 – Complete evaporation time after droplet deposition.
Table 4 – Ratio of the wetted area of 300 mm and 600 mm In general, the evaporation time decreased as the wetted
diameter droplets to their volume (mm2/ml) area increased; however, the wetted area was not the only
factor that influenced the evaporation time. Other factors
Surface Position Without surfactant With surfactant might include the contact area of liquid–vapour interface,
uptake rate of cuticular, uptake rate of stomatal, leaf mois-
300 mm 600 mm 300 mm 600 mm ture, the hydrophobic property of wax crystals on leaf surface,
diameter diameter diameter diameter RH and ambient temperature. These factors should be studied
in the future.
Adaxial Interveinal 6.5 2.5 17.8 7.1
4. Conclusions
area
The wetted area and evaporation time of droplets varied with
Midrib 8.6 2.5 46.9 16.3 location (interveinal area, secondary vein and midrib) and the
surface (adaxial and abaxial) of leaves where the droplet
Secondary 10.6 3.1 32.3 9.4 deposited. They also varied with droplet diameter and spray
formulation (with or without surfactant).
vein
The difference in mean wetted area of 300 mm diameter
Abaxial Interveinal 6.2 2.3 13.3 7.2 droplets on the interveinal area, midrib and secondary vein of
area adaxial surface was 39% without surfactant and 62% with
Midrib 6.2 2.1 11.0 7.8 surfactant, and was 5% without surfactant and 22% with
Secondary 6.5 surfactant on the abaxial surface.
vein 2.3 14.1 7.8
For all the treatments, the mean wetted area on the adaxial
600 mm diameter droplets (Table 5). Therefore, smaller droplets surface was significantly higher than that on the abaxial, and
had relatively longer lifetimes and relatively greater wetted the mean evaporation time on the adaxial surface was slightly
areas on the leaf surface, which increased the relative efficiency shorter than that on the abaxial surface.
of active ingredients to penetrate into the plant tissues.
Addition of the surfactant into spray solutions significantly
Table 5 – Ratio of evaporation time of 300 mm and 600 mm increased wetted area and reduced the variation of deposition
diameter droplets to their volume (min/ml) formation on leaves due to the uneven leaf surface structure.
The average wetted area on the entire leaf increased 203% for
Surface Position Without surfactant With surfactant the 300 mm diameter droplets and 275% for the 600 mm diam-
eter droplets when the surfactant was added into the spray
300 mm 600 mm 300 mm 600 mm solution. The 300 mm diameter droplets had larger relative
diameter diameter diameter diameter wetted area per droplet volume than the 600 mm diameter
droplets.
Adaxial Interveinal 102 46 59 39
For all treatments, droplets on the interveinal area had
area longer evaporation times than on the secondary vein and
midrib. The 300 mm diameter droplets had longer evaporation
Midrib 71 43 45 27 time per droplet volume than 600 mm diameter droplets.
Secondary 87 41 48 35
vein
Abaxial Interveinal 113 50 73 44
area
Midrib 93 42 45 37
Secondary 105 48 47 39
vein
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