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The effect of some adjuvants on sprays produced by agricultural flat fan nozzles
PII: SO26L2194(96)00065~
The effect of some adjuvants on sprays produced by agricultural flat fan nozzles MC.
Butler
Ellis, CR
Tuck
Silsoe Research Institute,
and P.C.H.
Miller
Wrest Park, Silsoe, Bedford MK45 4HS, UK
Six commercially-available agricultural spray adjuvants were mixed with water and sprayed through a medium quality flat fan nozzle to evaluate their effect on the spray formation process and droplet size distributions. Droplet size and velocity distributions were significantly changed. Ethokem, which reduced droplet sizes and LI-700, which increased them, were selected for more detailed study with medium and fine quality flat fan nozzles. The adjuvants significantly affected the variation of droplet size within the spray and the thickness of the spray fan. Photographs showed that the mechanism of spray formation was changed by the addition of LI-700 and that droplets containing Ethokem had air inclusions. Copyright 0 1996 Elsevier Science Ltd Keywords:
adjuvants;
droplet size; spray formation
In most pesticide applications, liquids are passed through spray generating systems to produce droplets that can control the pest problem on the target (crop, soil or weeds). Droplet size and velocity distributions are important in ensuring accurate delivery and retention of pesticide on the target (Lake and Marchant, 1983). Smaller droplets are more likely to be retained on leaves and provide more even coverage but are vulnerable to displacement from the target, leading to environmental contamination (Miller, 1988). Larger droplets are less prone to drift but might not be retained on target leaves. The efficacy of the active ingredient can also be dependent on droplet size. Reducing total pesticide use and minimising environmental contamination while maximising pesticide efficacy requires, amongst other factors, good control of droplet size and velocity distributions. A large selection of nozzles, capable of producing droplets with differing size and velocity distributions are available to the farmer. Nozzle selection is based on achieving the required volume distribution, flow rate and spray quality (Doble, Matthews, Rutherford and Southcombe, 1985) recommended by the manufacturer of the pesticide. Currently, spray quality produced by a particular nozzle is determined by measuring droplet size distribution when spraying water containing 0.1% Agral 90. It is not certain that the same spray quality will be achieved with the actual spray liquid, whose physical properties may differ significantly. Pesticide formulations contain the active ingredient plus other chemicals, including surfactants, which serve several purposes such as wetting and emulsification (Knowles, 1995). Adjuvants may also be added to pesticides at the time of application for a variety of reasons, all ultimately to increase the efficiency of action and application. The behaviour of the resulting spray liquid can therefore be modified by changes in
the spray liquid. Spray formation, droplet transport to the target and droplet interaction with the target can all be affected (Holloway, 1995; Tadros, 1994). This paper investigates the first part of the process; the effect of liquid properties on spray formation and transport. It is known that adjuvants can have a significant effect upon droplet sizes in agricultural sprays. A review of the effect of adjuvants on spray formation and transport has been presented in Miller and Butler Ellis (1996). Most recent studies have focussed on the effect of liquid properties on droplet size without considering the intermediate step of spray formation. However, it is not only droplet sizes but also droplet velocities that influence the susceptibility to drift and interaction with the target and effects on velocity have been reported less often. If the physical properties of the spray liquid are entirely responsible for determining the spray formation mechanisms and hence droplet size and velocity distributions, liquids containing different chemical compounds but with similar physical properties will produce sprays with similar characteristics. The work presented in this paper begins to investigate the effect of different liquids on spray formation mechanisms by using adjuvants to vary the chemical content. Dombrowski and Fraser (1954) demonstrated three mechanisms of spray formation by photographing the break-up of the liquid sheet emerging from flat fan nozzles: sheet disintegration by oscillation, sheet disintegration by perforation and rim disintegration. For flat fan nozzles operating with water, the sheet disintegrates because external disturbances, such as interactions with the air, cause oscillations in the sheet that grow and eventually result in break-up. In addition, the rims of the sheet shed droplets by forming ligaments that disintegrate. The effect of different liquids on the breakup of the liquid sheet from flat-fan nozzles was
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1997 Volume 16 Number 1
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The effect of adjuvants on sprays: M.C. Butler Ellis et al. also studied by Dombrowski and Fraser (1954) who varied density, viscosity and equilibrium surface tension by using liquids such as mercury, glycerine and ethyl alcohol and compared the structure of the liquid sheet with that obtained when spraying water. There was no attempt to relate changes in the liquid sheet geometry to droplet sizes. Viscosity and surface tension were found by Dombrowski and Fraser (1954) to influence sheet break-up. Increasing viscosity moved the position of disintegration away from the nozzle by damping out disturbances, allowing the sheet to continue for longer before break-up occurred. Increasing the surface tension reduced the spray angle and moved the position where sheet oscillations began, and consequently the point of break-up, away from the nozzle. Dombrowski and Fraser stated that adding a wetting agent to distilled water had no effect on the liquid sheet because the liquid sheet was too young for any reduction in surface tension to have occurred. In more detailed studies of the exact mechanisms of liquid sheet break-up, Dombrowski, Hasson and Ward (1960) hypothesised that the ‘vanished’ part of the liquid sheet (i.e. the part that disappears because surface tension draws the edges of the sheet towards the centre) was entirely contained in the edges. The consequence of increasing surface tension will therefore be that the flow pattern will change, the volume of liquid contained in droplets created by the disintegration of the edges will increase (and possibly the size of these droplets) and the volume of liquid in droplets created by the disintegration of the liquid sheet will fall. An equation describing the trajectory of the rims was derived by Clarke and Dombrowski (1971), which includes the dependence on surface tension. Modelling studies using Computational Fluid Dynamics software have also shown that surface tension influences the shape of the liquid sheet (Zhou, Miller, Walklate and Thomas, 1994). However, agricultural spray liquids are generally mixtures of several compounds and hence are much more complex in structure than the liquids studied by Dombrowski and Fraser (1954). Spray liquids used in practice frequently contain surfactants and so have time-dependent surface tension and also have shear rate-dependent viscosity (Hermansky and Krause, 1995). In addition, since the droplet size distribution is, not constant over the whole spray, the method of sampling droplet size and velocity distributions will influence the results (Chapple and Hall, 1993). Changing liquid properties could change droplet production from the rims, for example, while leaving the droplets from the sheet unaffected. Equilibrium surface tension of solutions can be significantly altered by the addition of a small quantity of surface active chemicals. On the time-scales involved in droplet production (which may be as short as 2 ms), the change in dynamic surface tension from that for water alone is likely to be small. However, the difficulties involved in measuring surface tension on these time scales mean that little information is available on the spray liquids used in practice. Measurements on adjuvants using the maximum bubble pressure method (Hall, Chapple, Downer, Kirchner and Thacker, 1993, Murphy, Policello, Goddard and
42
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Stevens, 1993) and oscillating jet method (Brazee, Bukovac, Cooper, Zhu, Reichard and Fox, 1994) have shown that many adjuvants reduce the surface tension at a rate dependent on concentration. Most investigations into the physical properties of spray liquids have concentrated upon surface tension (both dynamic and equilibrium) and bulk liquid viscosity. For example, Hermanski and Krause (1995) investigated the relationship between droplet size distribution and a variety of physical properties including shear viscosity, extensional viscosity and surface tension at a surface age of 20 ms. However, a surfactantcovered interface behaves as a two-dimensional body with its own elasticity and viscosity, which are related to the non-equilibrium values of the interfacial tension (Lucassendreynders and Wasan, 1993). The forces involved in atomisation are likely to be influenced as much by the physical properties of the liquid surface as the bulk liquid. Interfacial rheology is needed to understand the processes involved in both spray formation and its interaction with the target. The surface of the liquid sheet is both moving rapidly and expanding, so that interfacial shear viscosity, inter-facial dilational elasticity and interfacial dilational viscosity all play a role (Clint, 1990). The physics of sheet break-up and spray formation is therefore complicated by the addition of surface-active compounds and existing theories of spray formation are inadequate to describe all of the processes involved. An initial study of the effect of adjuvants on droplet sizes and velocities was carried out by Miller, Butler Ellis and Tuck (1995). The work described in this paper investigates this in more depth and attempts to relate some of the measured changes in the spray from agricultural flat-fan nozzles, which result from adding adjuvants to water, to changes in the mechanism of spray formation.
Materials
and methods
Adjuvant selection The six adjuvantiwater mixtures (Table Z) were those used previously (Miller et al., 1995). They were chosen to cover a range of chemical types and used at concentrations within manufacturers’ label recommendations. Nozzles
Two 110” flat-fan nozzles designated F110/1.6/3 and F110/0.6/3 (Lurmark, kemetal) were used over a range of pressures between 1 and 5 bar. The nozzles were mounted on an x-y transporter so that the spray fan could be moved above the measuring instrument allowing the full width and thickness to be sampled (Lake and Dix, 1985). Droplet sizing and velocity
measurements
Two instruments were used to measure droplet size and velocities. The first was a 1-D Particle Dynamics Analyser (Dantec Ltd, Bristol, UK) which uses the technique of Phase Doppler Analysis (PDA) configured to measure droplets up to 875 urn with a resolution of
The effect of adjuvants on sprays: M.C. Butler Ellis et al. Table 1. Adjuvants used in
initial experiments
Trade
supplier
name
UK
Description
Axiom
Joseph Batson & Co
97%
Agral
Zeneca
Non-ionic
Codacide
Oil
Spraymate
Microcide Midkem
Ethokem LI-700
Silwet L-77
Crop Protection Ltd Ltd
Concentration
highly refined wetting
95% emulsifiable Cationic
mineral
1%
oil
agent, 900 g I-’ alkyl phenol ethylene vegetable
surfactant,
Newman
Agrochemicals
Penetrating
Newman
Agrochemicals
Organosilicone wetting agent, polyalkyleneoxide heptomethyltrisiloxane 800 g I-’
5 pm and velocities in the range 7 to second was an optical shadowing device, comprising a 2D-GA1 optical array imaging probe, made by Particle Measuring Systems (PMS), Boulder, Colorado, USA, which measured droplets between 14 and 1250 urn with a resolution of approximately 20 urn. Both instruments were used in order to compare results and to assess whether the addition of adjuvants affected the optical properties of the droplets sufficiently to prevent the PDA from measuring droplet size correctly. Previous work has shown that only a high density of internal bubbles or emulsion droplets prevents the PDA from measuring droplet size correctly (Tuck, Butler Ellis and Miller, in preparation). In addition, since both instruments have different characteristics when sampling sprays with high airborne droplet concentrations, they were used in parallel to improve confidence. The PMS does not adequately sample the small droplets in a dense polydisperse spray cloud, such as those produced by agricultural nozzles, because the small droplets can be shadowed by the larger droplets. This means that the estimate of the percentage of liquid volume contained in droplets less than 100 urn diameter from PMS data is less reliable than that from PDA data. The PDA can miscalculate the size of some droplets that have a particular trajectory through the sample volume, known as the ‘trajectory effect’ and ‘slit-effect’ (Dantec Newsletter, 1995), resulting in fictional droplets with large diameters being measured. The PMS was used to determine the true maximum size of droplets, and the PDA data were cut off at this maximum value to give the best estimate of the droplet spectrum. approximately 21 ms-‘. The
Sampling
All measurements were made at a vertical distance of 350 mm below the nozzle. A full scan, where the whole spray was sampled by scanning across the thickness at different horizontal positions, was carried out to confirm results obtained previously with scans through a single position, vertically below the nozzle, along the short axis (Miller et al., 1995). Following this, two adjuvants, Ethokem and LI-700 were chosen for more detailed work. Ethokem had the greatest effect in reducing droplet size and LI-700 was one of the adjuvants that significantly increased droplet size. Individual scans across the thickness of the spray were
surfactant
0.1%
1%
oil
870 g I-’ polyoxyethylene
acidifying
oxide
tallow amine
750 g I-’ soyal phospholipids modified
0.5% 0.5% 0.15%
made at various horizontal positions to determine the pattern of droplet sizes. This was carried out with two nozzle sizes, an F110/0.6/3 nozzle and an F110/1.6/3 nozzle, as representative of typical nozzles creating medium and fine quality sprays respectively (Doble et al. ) 1985). For full scans, each sample contained at least 16,000 or 10,000 droplets for the PDA and the PMS, respectively. For scans at different horizontal positions, the number of droplets in each sample declined towards the edge of the spray and was as low as 500 droplets at the most extreme positions. However, the range of droplet sizes at the edges of the spray is lower so, although the accuracy is reduced because of the low droplet numbers, the value of VMD is still reliable. The thickness of the spray fan at each horizontal position could be estimated by defining the edge of the spray as the position where the number of droplets exceeded 0.5 droplet s-‘. This enables the horizontal cross-sectional ‘footprint’ to be determined at 350 mm below the nozzle.
Photographs
of spray formation
and droplets
Droplets produced by the two flat fan nozzles using two adjuvants were collected on silicone oil (Dow Corning DC 200/1000 cs) to allow their structure to be seen under a microscope and photographed. The break-up of the liquid sheet from the flat fan nozzles at pressures between 1 and 5 bar was also photographed to observe any differences in spray formation. The flash light source was provided by a PalFlash 500 (Pulse Photonics Ltd) with the slit removed and a diffuser added. The light was focussed into the camera by an enlarger condenser. The flash and condenser lens were situated behind the spray. A Hasslebad camera with bellows attachment and 150 mm lens was used with magnification set on two and settings of l/1000 s at F16-18. The film was an FP4 rated at 100 ISO.
Dynamic surface tension
The surface tension of the test spray liquid made with the selected adjuvants, 0.5% Ethokem and 0.5% LI700, were measured using the method of maximum bubble pressure, (BP2, Kruss, Hamburg, Germany) with bubble frquencies between 2 and 10 Hz.
Crop Protection 1997 Volume 16 Number 1 43
The effect of adjuvants on sprays: M.C. Butler Ellis et al. Table 2. Measurement of droplet size distribution made with PDA; full scan,
VMD,
Spray liquid
pm
PDA %vol. < 100 pm
256 234 215 247 260 268 216
Water Ethokem LI-700 Agral Axiom Codacide Silwet L77
Fl 10/l.6/3 nozzle at 3 bar VMD,
Mean liquid velocity, ms-’
2.9 4.8 1.6 3.6 2.6 2.0 1.5
PMS %vol. < 100 pm
pm
291 284 318 288 301 318 322
1.8 6.5 8.8 7.3 8.2 8.7 9.1
Mean liquid velocity, ms-’
1.1 1.2 1.5 1.2 1.4 1.3 1.4
5.2 5.1 1.3 5.5 5.9 1.0 8.2
20
Results and discussion Droplet size and velocity
distributions
The volume median diameters (VMD), the percentages of spray volume contained in droplets
16
&a”Vi Ziai
where a is the droplet diameter, V is the droplet velocity and i indicates summing over all droplets. Droplet size distributions showed little difference between the two instruments for all the adjuvants, apart from those that relate to the limitations of the technique, i.e. the tendency of the PMS to underestimate the small droplet fraction and of the PDA to create large droplets. Because the distributions measured with the PMS and PDA were comparable for all adjuvants, it was assumed that all the spray liquids were measurable with the PDA, even though some of them formed emulsions (Codacide, Axiom and LI-700). The maximum droplet size for analysis was determined from the PMS distributions as 700 urn for the F110/1.6/ 3 nozzle. PDA distributions for all adjuvants were therefore truncated at 700 urn to calculate VMDs shown in Table 2. The size of droplets produced by flat-fan nozzles has clearly been altered by the addition of these adjuvants. VMDs were both increased and decreased relative to water and the percentage of spray volume contained in the small droplets, which are most prone to drift, was changed. The adjuvants LI-700 and Ethokem, which gave the largest changes in VMD, were sprayed through an F110/0.6/3 nozzle and measured using the PDA and PMS with a full scan. The PDA data was truncated at 500 urn for analysis, shown in Table 3. Mean velocities of droplets in each size category Table 3. Measurement of droplet size distribution made with a full scan, Fl 1O/0.6/3 nozzle at 3 bar VMD, Water Ethokem LI-700
44
198 189 216
km %vol. < 100 pm Mean liquid velocity, ms-’ 6.4 9.0 3.5
4.7 4.0 5.9
Crop Protection 1997 Volume 16 Number 1
04 0
.
*
100
.
200
300
400
500
600
Droplet diameter, pm Figure 1. Relationship between droplet velocity and droplet diameter for two liquids and water, measured with PDA carrying out a full scan of the spray from the Fi 1 O/0.6/3 nozzle
through
both nozzles were altered by the adjuvants 2). Larger droplets do not slow down as rapidly as smaller ones so sprays with a larger VMD would be .expected to have higher mean liquid velocities. However, from Figure 2 it can be seen that, for most droplet sizes, droplets containing Ethokem travel slower than water droplets and those containing LI-700 travel faster. Droplet size distributions measured with full scan (Table 2) showed a smaller variation in VMD with adjuvant than those measured with a centre line scan (Miller et al. 1995), suggesting that the droplets in the centre of the spray were more affected than other parts of the spray fan. Measurements of droplet size distribution were therefore made at different horizontal positions within the spray fan, scanned across the thickness, also using both instruments with the two selected adjuvants and two nozzles. Variation of VMD with horizontal position measured with the PDA is shown in Figures 2 and 3. Measurements with the PMS were very similar and therefore are not shown. The entire droplet size distribution is not simply shifted towards larger or smaller droplet sizes, but the pattern of the droplet size spectrum across the spray fan is changed. The central part of the spray was strongly affected by the adjuvant, suggesting that it is the formation of droplets by oscillation of the liquid sheet that is most altered, rather than the rim distintegration. (Figure
I and Miller et al., 1995, figure
The effect of adjuvants
VMD, pm
on sprays: MC. Butler Ellis et al.
Footprints Analysis of the full scan droplet size distribution allowed the dimensions of the spray fan in a horizontal plane to be determined and ‘footprints’ to be estimated, as shown in Figure 4. The thickness vertically beiow the nozzle was correlated with mean liquid velocity (Miller et al., 1995) showing that the thicker the fan, the slower the mean liquid velocity. The changes in footprint thickness may contribute to the differences in droplet velocities shown in Figure I. A thinner footprint would be expected to have a higher mean liquid velocity to maintain a constant volume flow rate. Droplet photographs
t
/
-600
7n I LO”!
I
-400
-200
0
/ I
/I 200
400
I
600
Horizontal position, mm l
water
l
Water plus 0.5% Ethokem
A Water plus 0.5% LI-700 Figure 2. Variation of volume median diameter with position in the spray, measured with FDA using the Fll O/l .6/3 nozzle
VMD, pm 330 T
Droplets produced by the F110/0.6/3 nozzle at 3 bar spraying the three test liquids are shown in Plates 1-3. The droplets from the F110/1.6/3 nozzle were very similar and are therefore not shown. Air inclusions appeared in droplets containing Ethokem (Plate 2) and were observed frequently in droplets of 300 pm diameter and larger, and occasionally in droplets as small as 150 pm diameter. They did not appear to disrupt the operation of the PDA, since there were no greater differences between PDA and PMS results for Ethokem than for water alone. The appearance of these air inclusions was similar to those observed in droplets produced by twin-fluid nozzles, although less densely packed, and are not thought to be caused by, for example, droplet coalescence or overlapping droplets in the silicone oil. A few air inclusions were visible inside droplets when LI-700 was added, but considerably less than with Ethokem. However, there are clear signs of internal content, e.g. at least one white ring inside each droplet, which is only visible when LI-700 is present. Small emulsion droplets are also visible. Again, this did not disrupt the PDA measurements. The same effects are visible for both nozzles. The presence of air inclusions with Ethokem may account for the lower velocity of droplets containing Ethokem above 150 pm, shown in Figure I.
Liquid sheet photographs
I
-600
I /
-400
-200
I,“,CA
I
0
L I
200
/
i
400
600
Horizontal position, mm 6
water
n
Water plus 0.5% Ethokem
A Water plus 0.5% LI-700 Figure 3. Variation of volume median diameter with position the spray, measured with PDA using the Fl 1O/0.6/3 nozzle
in
Examples of the liquid sheet at two different pressures through an F110/1.6/3 nozzle are shown in Plates 4-9. Photographs of the F110/0.6/3 nozzle were similar and are not shown. Photographs of the liquid sheet clearly show that adjuvants can alter the dominant mode of spray formation. The differences caused by the addition of adjuvants became more apparent as the pressure was reduced. The effect of reducing the equilibrium surface tension on the liquid sheet was demonstrated by Dombrowski and Fraser (1954): using pure liquids with a surface tension lower than water, oscillations became unstable earlier, leading to break up nearer the nozzle. With Ethokem, the unstable oscillations and break-up point were further from the nozzle than with water. With LI-700, the sheet perforates, leading to spray formation were nearer the nozzle. Neither of the test liquids, therefore, behaved as pure liquid with surface tension lower than water.
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The effect of adjuvants on sprays: M.C. Butler Ellis et al. 200-
100-
O-
E -100. _ :: I!! Y ‘5 -200, c 9 8 0
---0.5x u-700 ....'. 0.5% Ethokem 200-
$ s '-Z 'iz loo2
O-
-100.
-200-l -600
-500
-400
-300
-200
-100 Position
0 along
by an F110/0.6/3
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300
400
500
-I 600
Fl 1O/l .6/3 nozzle (lower) spraying two liquids and water
nozzle at 3 bar Plate 2. Droplets with air inclusions produced by an Fl 1O/0.6/3 nozzle at 3 bar spraying water and 0.5% Ethokem
Larger droplets probably occur with LI-700 because the break-up point is nearer the nozzle where the liquid sheet is thicker. This also leads to a change in the volume distribution of spray liquid. LI-700 has a thinner spray fan because the break-up occurs nearer the nozzle before sheet oscillations have begun to affect droplet trajectory. Break-up by oscillation may lead to horizontal ligaments being thrown forwards and back-
46
200
fan width, mm
Figure 4. Footprints of the spray produced with Fl 1O/0.6/3 nozzle (upper) and
Plate 1. Droplets produced spraying water only
100
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1
wards, whereas break-up by perforation results in droplets continuing more within the plane of the nozzle. This apparently results in higher velocities, possibly because greater energy losses occur when sheet oscillations take place. The effect of LI-700 on the sheet breakup is visibly similar to the effect that is observed when heat is
The effect of adjuvants on sprays: M.C. Butler Ellis et a/.
Plate 3. Droplets produced by an F110/0.6/3 spraying water and 0.5% LI-700.
nozzle
at 3 bar
Plate 5. F110/1.6/3
Plate 4. Fl 1O/l .6/3 nozzle at 3 bar spraying
nozzle at 3 bar spraying water and Ethokem
water only
applied to the liquid sheet to alter the point of break up and increase the droplet sizes (Dombrowski, Hibbit and Strachan, 1989). This similarity occurs because the gradients of surface forces are important in determining break-up. Surface tension is very sensitive to temperature and therefore a temperature gradient results in a surface tension gradient. Surface active agents also produce gradients in surface forces as the sheet ages. This leads to an earlier sheet break-up and larger droplets. Although there was no change in surface tension apparent from considering the rims of the liquid sheet, it is possible that a sudden change occurs, which causes the sheet to suddenly disintegrate and so any change in rim trajectory is lost. However, this does not explain the reduction in VMD that occurs with other adjuvants such as Ethokem. They also have time-dependent surface forces, but the result was to prolong the life of the liquid sheet rather than reduce it. This was opposite to the effect reported by Dombrowski and Fraser (1954)
Plate 6. Fl 1O/l .6/3 nozzle at 3 bar spraying water and LI-700
where a decrease in surface tension reduced the length of the liquid sheet, but is similar to the effect that occurred when viscosity was increased. A possible explanation for this would be that, although the surface tension is falling and bulk viscosity of the liquid is likely to beunchanged, the surface viscosity could be substantially increased to an extent that over-rides the effect of the reducing surface tension. An increase in bulk viscosity may result in an increase in droplet size (Snyder, Senser and Lefebvre, 1989) because as well as increasing the sheet length it also increases the sheet thickness. In the case of increasing only the surface viscosity, the sheet thickness may remain unchanged leading to smaller droplets. It has been suggested (Clark and Dombrowski, 1971;
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The effect of adjuvants on sprays: M.C. Butler Ellis et al.
Plate 7. Fl 1O/l .6/3 nozzle at 1 bar spraying water only
Plate 9. Fl 1O/l .6/3 nozzle at 1 bar spraying water and LI-700
surface tension, mN/m 70 60
&hAbaAAAa.wd.rr+.m~A 00
00
c
n
0
00
0
*OOOO
0000
00
40 -30 -20 -10 -0 I 0
I 2
4
6
8
10
bubble frequency, Hz
Plate 8. Fl 1O/l .6/3 nozzle at 1 bar spraying water and Ethokem
Dombrowski and Fraser, 1954) that the edges of the liquid sheet are drawn together by surface tension forces. If surface tension of the liquid sheet is affected by the addition of these adjuvants, it should therefore be possible to see differences in the trajectories of the rims. By superimposing the photograph of water at 1 bar on the photograph of Ethokem at 1 bar, it was possible to observe the the trajectories have begun to diverge at approximately 30 mm from the F110/0.6/3 nozzle and 40 mm from the F110/1.6/3 nozzle. The sheet remained wider with Ethokem than with water, suggesting that the surface tension was lower. No differences in trajectory were observed between water and LI-700, implying that the surface tension with LI700 declined slower than with Ethokem. At 3 bar, there
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A water o 0.5% Ethokem 0 0.5% LI-700 Figure 5. Dynamic surface tension of water plus 0.5% LI-700 and water plus 0.5% Ethokem measured with the maximum bubble pressure tensiometer
were no observable
differences in rim trajectories between any of the liquids since the velocities were higher and the sheet had disintegrated before any change in rim trajectory could take place. Dynamic surface tension
Measurements of surface tension of the two spray liquids, made with the maximum bubble pressure tensiometer, are shown in Figure 5. The age of the liquid sheet would correspond to bubble frequencies
The effect of adjuvants
well above 10 Hz and therefore this information is not directly relevant to spray formation. However, it does support the conclusion, drawn from comparing rim trajectories, that the surface tension of water with 0.5% Ethokem declines faster than water with 0.5% LI-700. There appears to be two different effects of adjuvants upon spray formation: one which prolongs the life of the liquid sheet by suppressing oscillations and the other which reduces it by causing sheet perforation. The former results in smaller droplets and the latter in larger droplets, compared with water alone. It is important to measure properties of the liquid in order to establish what determines the break-up mechanism and consequently whether a liquid will increase or decrease droplet size. However, the properties that are important and the methods that should be used for their determination are not yet clear. The rate of change of surface properties is likely to be crucial and therefore very short timescales of measurement will be essential. Other properties, such as surface viscosity, may be of equal or greater importance than surface tension. Because it has been shown that changing the liquid can alter the spray formation mechanism, a single model relating droplet size to physical properties of the liquid is unlikely to be successful. Modelling each spray formation process may allow the spray characteristics to be predicted, providing we know which spray formation mechanism is appropriate. The effect of liquid properties on spray formation may be a function of nozzle type and will therefore also influence spray formation from other spray systems, such as twin fluid nozzles. This needs further investigation. The variation of droplet size with liquid properties is also significant when considering nozzle classification, which is carried out using water plus 0.1% Agral 90. A nozzle that is classified as ‘fine’ for example may produce a spray that is ‘medium’ when using an active formulation. It is clearly important to ensure that nozzle classification is always carried out using the same test liquid, otherwise differences due to liquid properties rather than nozzle design may result. When the effect of liquid properties on spray formation are fully understood, it may then be possible to predict the way in which pesticide spray liquids will modify spray quality and whether its classification is likely to be significantly changed.
Conclusions
This work confirms previous reports that changes in liquid properties caused by the addition of adjuvants can lead to significant changes in the quality of the spray produced by flat-fan nozzles. Droplet sizes and velocities may be either increased or decreased and the structure of the droplets may be changed by including air bubbles, for example. The liquid sheet break-up mechanism is affected, leading to either a longer liquid sheet (resulting in smaller droplets) or a shorter one (and larger droplets). This causes changes in the pattern of droplet sizes at different positions within the spray fan. A consequence of the changing break-up mechanism is that the spray cross-sectional area can be modified. with the short axis
on sprays: M.C. Butler Ellis et al.
being shorter and the long axis being longer with some adjuvants than with water alone. These effects were consistent with the six spray liquids measured such that VMD, mean liquid velocity and spray footprint thickness were correlated. The trajectory of the rims of the liquid sheet can also be affected, showing that dynamic surface tension, which is thought to be one of the major factors in the effect of surfactants on droplet size, appears to play a part with some surfactants despite the short surface ages involved in spray formation. Insights into the effects of liquid properties on spray formation provide an indication of how spray application efficiency might be improved. However, the interaction between the droplet and the target is also strongly affected by adjuvants and therefore any models of spray formation need to be linked to work on target interaction before the full effect of adjuvants can be understood.
Acknowledgements
The authors would like to thank Mr R. Cove for his excellent photography and Dr P. J. Walklate for useful discussions. This work was funded by the Ministry of Agriculture, Fisheries and Foods. References Brazee, R. D., Bukovac, M. J., Cooper, J. A., Zhu, H., Reichard, D. L. and Fox, R. D. (1994) Surfactant diffusion and dynamic surface tension in spray solutions. Trans. A.S.A.E. 37, 51-58 Chapple, A. C. and Hall, F. R. (1993) A description of the droplet spectra produced by a flat-fan nozzle. Atomiz. Sprays 3, 477-488 Clark, C. J. and Dombrowski, N. (1971) The dynamics fan spray sheet. Chem. Engineer. Sci. 26, 1949-1952 Clint, J. industrial Properties Jones) pp
of the rim of a
H. (1990) Interfacial rheology and its application to processes. In: The Structure, Dynamics and Equilibrium of Colloidal Systems, (Ed. by D. M. Bloor and E. Wyn681-6794. Kluwer Academic Publishers
Dantec Newsletter (1995) Improved spray new Dual PDA. Vol. 2, no. 2, p 1
characterization
with the
I. and Southcombe, Doble, S. J., Matthews, G. A., Rutherford, E. S. E. (1985) A system for classifying hydraulic nozzles and other atomisers into categories of spray quality. Proceedings of the British Crop Protection Council Conference - Weeds. Bracknell, pp 1125-34 Dombrowski, N. and Fraser, R. P. (1954) A photographic investigation into the disintegration of liquid sheets. Phil. Trans. Royal Sot. 247 101-142 Dombrowski, N., Hasson, D. and Ward, D. E. (1960) Some aspects of liquid flow through fan spray nozzles. Chem. Engineer. Sci. 12,25-50 Dombrowski, N., Hibbitt, I. and Strachan, J. R. (1989) Towards drift-free spraying. Proceedings of the Brighton Crop Protection Conference - Weeds, Vol. 2, pp 663-668 Hall, F. R., Chapple, A. C., Downer, R. A., Kirchner, L. M. and Thacker, J. R. M. (1993) Pesticide application as affected by spray modifiers. Pestic. Sci. 38, 123-133 Hermanski, C. G. and Krause, G. F. (lYY5) Relevant physical measurements for adjuvants. Fourth International property Symposium on Adjuvants for Agrochemicals. Melbourne. Australia, 3-6 October 1995 (FRI Bulletin No. 193). pp 20-26 Holloway, P. J. (1995) Adjuvants for foliage-applied agrochemicals: The need for more science not serendipity? Fourth International Symposium on Adjuvants for Agrochemicals, Melbourne, Australia. 3-6 October 1995 (FRI Bulletin No. lY3), pp 167-176
Crop Protection
1997 Volume 16 Number 1
49
The effect of adjuvants Knowles,
on sprays: M.C. Butler Ellis et al.
D. A. (1995) Trends in the use of surfactants for pesticide Pest. Outlook 6, 31-36
formulations.
Lake, J. R. and Dix, A. J. (1985) Measurement
of droplet size with a PMS optical array probe using an x-y nozzle transporter, Crop Protect. 4, 464-472.
Lake, J. R. and Marchant, J. A. (1983) The use of dimensional analysis in a study of drop retention on barley. Pestic. Sci. 14, 638644 Lucassendreynders, E. H. and Wasan, D. T. (1993) Interfacial viscoelasticity in emulsions and foams. Food Struct. 12, l-12 Miller, P. C. H. (1988) Engineering Asp. Appl. Biol. 17, 377-384
aspects of spray drift control.
Miller, P. C. H. and Butler Ellis, M. C. (submitted)
Spray generation, delivery to the target and how adjuvants influence the process. Plant Protect. Quart., submitted
P. C. H., Butler Ellis, M. C. and Tuck, C. R. (1995) The Fourth International Symposium on Adjuvants for Agrochemicals, Melbourne, Australia,
Murphy, D. S., Policello, G. A., Goddard, E. D. and Stevens, P. J. G. (1993) Physical properties of silicone surfactants for agrochemical applications. In: Pesticide Formulations and Application Systems, Vol. 12, ASTM. STP 1146 (Ed. by B. N. Devisetty, D. G.
Chasin and P. D. Berger), Materials, Philadelphia
3-6 October 1995 (FRI Bulletin No. 193), pp 95-102
50
Crop Protection
1997 Volume 16 Number 1
Society for Testing and
Snyder, H. E., Senser, D. W. and Lefebvre, A. H. (1989) Mean drop Fluids Engineer. 3,342-347
sizes from fan spray atomizers. ASMEJ.
Tadros, T. F. (1994) Surfactants in Agrochemicals. Series, Vol. 54. Marcel Dekker Tnc, New York Tuck,
C. R.,
Butler
Ellis,
M. C.
and
preparation). Techniques for measurement velocity distributions in agricultural sprays Zhou, Q., Miller, P. C. H., Walklate,
Surfactanr Science
Miller,
P. and Thomas,
(in size and
P. C. H.
of droplet
N. H. (1994)
Fluid flow in a flat fan nozzle and its effect on spray performance. IChemE Research Event, pp 865-867
Miller,
influence of adjuvants on droplet production.
American
Received I I March 1996 Revised 8 July 1996 Accepted 10 July 1996
The effect of some adjuvants on sprays produced by agricultural flat fan nozzles MC.
Butler
Ellis, CR
Tuck
Silsoe Research Institute,
and P.C.H.
Miller
Wrest Park, Silsoe, Bedford MK45 4HS, UK
Six commercially-available agricultural spray adjuvants were mixed with water and sprayed through a medium quality flat fan nozzle to evaluate their effect on the spray formation process and droplet size distributions. Droplet size and velocity distributions were significantly changed. Ethokem, which reduced droplet sizes and LI-700, which increased them, were selected for more detailed study with medium and fine quality flat fan nozzles. The adjuvants significantly affected the variation of droplet size within the spray and the thickness of the spray fan. Photographs showed that the mechanism of spray formation was changed by the addition of LI-700 and that droplets containing Ethokem had air inclusions. Copyright 0 1996 Elsevier Science Ltd Keywords:
adjuvants;
droplet size; spray formation
In most pesticide applications, liquids are passed through spray generating systems to produce droplets that can control the pest problem on the target (crop, soil or weeds). Droplet size and velocity distributions are important in ensuring accurate delivery and retention of pesticide on the target (Lake and Marchant, 1983). Smaller droplets are more likely to be retained on leaves and provide more even coverage but are vulnerable to displacement from the target, leading to environmental contamination (Miller, 1988). Larger droplets are less prone to drift but might not be retained on target leaves. The efficacy of the active ingredient can also be dependent on droplet size. Reducing total pesticide use and minimising environmental contamination while maximising pesticide efficacy requires, amongst other factors, good control of droplet size and velocity distributions. A large selection of nozzles, capable of producing droplets with differing size and velocity distributions are available to the farmer. Nozzle selection is based on achieving the required volume distribution, flow rate and spray quality (Doble, Matthews, Rutherford and Southcombe, 1985) recommended by the manufacturer of the pesticide. Currently, spray quality produced by a particular nozzle is determined by measuring droplet size distribution when spraying water containing 0.1% Agral 90. It is not certain that the same spray quality will be achieved with the actual spray liquid, whose physical properties may differ significantly. Pesticide formulations contain the active ingredient plus other chemicals, including surfactants, which serve several purposes such as wetting and emulsification (Knowles, 1995). Adjuvants may also be added to pesticides at the time of application for a variety of reasons, all ultimately to increase the efficiency of action and application. The behaviour of the resulting spray liquid can therefore be modified by changes in
the spray liquid. Spray formation, droplet transport to the target and droplet interaction with the target can all be affected (Holloway, 1995; Tadros, 1994). This paper investigates the first part of the process; the effect of liquid properties on spray formation and transport. It is known that adjuvants can have a significant effect upon droplet sizes in agricultural sprays. A review of the effect of adjuvants on spray formation and transport has been presented in Miller and Butler Ellis (1996). Most recent studies have focussed on the effect of liquid properties on droplet size without considering the intermediate step of spray formation. However, it is not only droplet sizes but also droplet velocities that influence the susceptibility to drift and interaction with the target and effects on velocity have been reported less often. If the physical properties of the spray liquid are entirely responsible for determining the spray formation mechanisms and hence droplet size and velocity distributions, liquids containing different chemical compounds but with similar physical properties will produce sprays with similar characteristics. The work presented in this paper begins to investigate the effect of different liquids on spray formation mechanisms by using adjuvants to vary the chemical content. Dombrowski and Fraser (1954) demonstrated three mechanisms of spray formation by photographing the break-up of the liquid sheet emerging from flat fan nozzles: sheet disintegration by oscillation, sheet disintegration by perforation and rim disintegration. For flat fan nozzles operating with water, the sheet disintegrates because external disturbances, such as interactions with the air, cause oscillations in the sheet that grow and eventually result in break-up. In addition, the rims of the sheet shed droplets by forming ligaments that disintegrate. The effect of different liquids on the breakup of the liquid sheet from flat-fan nozzles was
Crop Protection
1997 Volume 16 Number 1
41
The effect of adjuvants on sprays: M.C. Butler Ellis et al. also studied by Dombrowski and Fraser (1954) who varied density, viscosity and equilibrium surface tension by using liquids such as mercury, glycerine and ethyl alcohol and compared the structure of the liquid sheet with that obtained when spraying water. There was no attempt to relate changes in the liquid sheet geometry to droplet sizes. Viscosity and surface tension were found by Dombrowski and Fraser (1954) to influence sheet break-up. Increasing viscosity moved the position of disintegration away from the nozzle by damping out disturbances, allowing the sheet to continue for longer before break-up occurred. Increasing the surface tension reduced the spray angle and moved the position where sheet oscillations began, and consequently the point of break-up, away from the nozzle. Dombrowski and Fraser stated that adding a wetting agent to distilled water had no effect on the liquid sheet because the liquid sheet was too young for any reduction in surface tension to have occurred. In more detailed studies of the exact mechanisms of liquid sheet break-up, Dombrowski, Hasson and Ward (1960) hypothesised that the ‘vanished’ part of the liquid sheet (i.e. the part that disappears because surface tension draws the edges of the sheet towards the centre) was entirely contained in the edges. The consequence of increasing surface tension will therefore be that the flow pattern will change, the volume of liquid contained in droplets created by the disintegration of the edges will increase (and possibly the size of these droplets) and the volume of liquid in droplets created by the disintegration of the liquid sheet will fall. An equation describing the trajectory of the rims was derived by Clarke and Dombrowski (1971), which includes the dependence on surface tension. Modelling studies using Computational Fluid Dynamics software have also shown that surface tension influences the shape of the liquid sheet (Zhou, Miller, Walklate and Thomas, 1994). However, agricultural spray liquids are generally mixtures of several compounds and hence are much more complex in structure than the liquids studied by Dombrowski and Fraser (1954). Spray liquids used in practice frequently contain surfactants and so have time-dependent surface tension and also have shear rate-dependent viscosity (Hermansky and Krause, 1995). In addition, since the droplet size distribution is, not constant over the whole spray, the method of sampling droplet size and velocity distributions will influence the results (Chapple and Hall, 1993). Changing liquid properties could change droplet production from the rims, for example, while leaving the droplets from the sheet unaffected. Equilibrium surface tension of solutions can be significantly altered by the addition of a small quantity of surface active chemicals. On the time-scales involved in droplet production (which may be as short as 2 ms), the change in dynamic surface tension from that for water alone is likely to be small. However, the difficulties involved in measuring surface tension on these time scales mean that little information is available on the spray liquids used in practice. Measurements on adjuvants using the maximum bubble pressure method (Hall, Chapple, Downer, Kirchner and Thacker, 1993, Murphy, Policello, Goddard and
42
Crop Protection 7997 Volume 16 Number 1
Stevens, 1993) and oscillating jet method (Brazee, Bukovac, Cooper, Zhu, Reichard and Fox, 1994) have shown that many adjuvants reduce the surface tension at a rate dependent on concentration. Most investigations into the physical properties of spray liquids have concentrated upon surface tension (both dynamic and equilibrium) and bulk liquid viscosity. For example, Hermanski and Krause (1995) investigated the relationship between droplet size distribution and a variety of physical properties including shear viscosity, extensional viscosity and surface tension at a surface age of 20 ms. However, a surfactantcovered interface behaves as a two-dimensional body with its own elasticity and viscosity, which are related to the non-equilibrium values of the interfacial tension (Lucassendreynders and Wasan, 1993). The forces involved in atomisation are likely to be influenced as much by the physical properties of the liquid surface as the bulk liquid. Interfacial rheology is needed to understand the processes involved in both spray formation and its interaction with the target. The surface of the liquid sheet is both moving rapidly and expanding, so that interfacial shear viscosity, inter-facial dilational elasticity and interfacial dilational viscosity all play a role (Clint, 1990). The physics of sheet break-up and spray formation is therefore complicated by the addition of surface-active compounds and existing theories of spray formation are inadequate to describe all of the processes involved. An initial study of the effect of adjuvants on droplet sizes and velocities was carried out by Miller, Butler Ellis and Tuck (1995). The work described in this paper investigates this in more depth and attempts to relate some of the measured changes in the spray from agricultural flat-fan nozzles, which result from adding adjuvants to water, to changes in the mechanism of spray formation.
Materials
and methods
Adjuvant selection The six adjuvantiwater mixtures (Table Z) were those used previously (Miller et al., 1995). They were chosen to cover a range of chemical types and used at concentrations within manufacturers’ label recommendations. Nozzles
Two 110” flat-fan nozzles designated F110/1.6/3 and F110/0.6/3 (Lurmark, kemetal) were used over a range of pressures between 1 and 5 bar. The nozzles were mounted on an x-y transporter so that the spray fan could be moved above the measuring instrument allowing the full width and thickness to be sampled (Lake and Dix, 1985). Droplet sizing and velocity
measurements
Two instruments were used to measure droplet size and velocities. The first was a 1-D Particle Dynamics Analyser (Dantec Ltd, Bristol, UK) which uses the technique of Phase Doppler Analysis (PDA) configured to measure droplets up to 875 urn with a resolution of
The effect of adjuvants on sprays: M.C. Butler Ellis et al. Table 1. Adjuvants used in
initial experiments
Trade
supplier
name
UK
Description
Axiom
Joseph Batson & Co
97%
Agral
Zeneca
Non-ionic
Codacide
Oil
Spraymate
Microcide Midkem
Ethokem LI-700
Silwet L-77
Crop Protection Ltd Ltd
Concentration
highly refined wetting
95% emulsifiable Cationic
mineral
1%
oil
agent, 900 g I-’ alkyl phenol ethylene vegetable
surfactant,
Newman
Agrochemicals
Penetrating
Newman
Agrochemicals
Organosilicone wetting agent, polyalkyleneoxide heptomethyltrisiloxane 800 g I-’
5 pm and velocities in the range 7 to second was an optical shadowing device, comprising a 2D-GA1 optical array imaging probe, made by Particle Measuring Systems (PMS), Boulder, Colorado, USA, which measured droplets between 14 and 1250 urn with a resolution of approximately 20 urn. Both instruments were used in order to compare results and to assess whether the addition of adjuvants affected the optical properties of the droplets sufficiently to prevent the PDA from measuring droplet size correctly. Previous work has shown that only a high density of internal bubbles or emulsion droplets prevents the PDA from measuring droplet size correctly (Tuck, Butler Ellis and Miller, in preparation). In addition, since both instruments have different characteristics when sampling sprays with high airborne droplet concentrations, they were used in parallel to improve confidence. The PMS does not adequately sample the small droplets in a dense polydisperse spray cloud, such as those produced by agricultural nozzles, because the small droplets can be shadowed by the larger droplets. This means that the estimate of the percentage of liquid volume contained in droplets less than 100 urn diameter from PMS data is less reliable than that from PDA data. The PDA can miscalculate the size of some droplets that have a particular trajectory through the sample volume, known as the ‘trajectory effect’ and ‘slit-effect’ (Dantec Newsletter, 1995), resulting in fictional droplets with large diameters being measured. The PMS was used to determine the true maximum size of droplets, and the PDA data were cut off at this maximum value to give the best estimate of the droplet spectrum. approximately 21 ms-‘. The
Sampling
All measurements were made at a vertical distance of 350 mm below the nozzle. A full scan, where the whole spray was sampled by scanning across the thickness at different horizontal positions, was carried out to confirm results obtained previously with scans through a single position, vertically below the nozzle, along the short axis (Miller et al., 1995). Following this, two adjuvants, Ethokem and LI-700 were chosen for more detailed work. Ethokem had the greatest effect in reducing droplet size and LI-700 was one of the adjuvants that significantly increased droplet size. Individual scans across the thickness of the spray were
surfactant
0.1%
1%
oil
870 g I-’ polyoxyethylene
acidifying
oxide
tallow amine
750 g I-’ soyal phospholipids modified
0.5% 0.5% 0.15%
made at various horizontal positions to determine the pattern of droplet sizes. This was carried out with two nozzle sizes, an F110/0.6/3 nozzle and an F110/1.6/3 nozzle, as representative of typical nozzles creating medium and fine quality sprays respectively (Doble et al. ) 1985). For full scans, each sample contained at least 16,000 or 10,000 droplets for the PDA and the PMS, respectively. For scans at different horizontal positions, the number of droplets in each sample declined towards the edge of the spray and was as low as 500 droplets at the most extreme positions. However, the range of droplet sizes at the edges of the spray is lower so, although the accuracy is reduced because of the low droplet numbers, the value of VMD is still reliable. The thickness of the spray fan at each horizontal position could be estimated by defining the edge of the spray as the position where the number of droplets exceeded 0.5 droplet s-‘. This enables the horizontal cross-sectional ‘footprint’ to be determined at 350 mm below the nozzle.
Photographs
of spray formation
and droplets
Droplets produced by the two flat fan nozzles using two adjuvants were collected on silicone oil (Dow Corning DC 200/1000 cs) to allow their structure to be seen under a microscope and photographed. The break-up of the liquid sheet from the flat fan nozzles at pressures between 1 and 5 bar was also photographed to observe any differences in spray formation. The flash light source was provided by a PalFlash 500 (Pulse Photonics Ltd) with the slit removed and a diffuser added. The light was focussed into the camera by an enlarger condenser. The flash and condenser lens were situated behind the spray. A Hasslebad camera with bellows attachment and 150 mm lens was used with magnification set on two and settings of l/1000 s at F16-18. The film was an FP4 rated at 100 ISO.
Dynamic surface tension
The surface tension of the test spray liquid made with the selected adjuvants, 0.5% Ethokem and 0.5% LI700, were measured using the method of maximum bubble pressure, (BP2, Kruss, Hamburg, Germany) with bubble frquencies between 2 and 10 Hz.
Crop Protection 1997 Volume 16 Number 1 43
The effect of adjuvants on sprays: M.C. Butler Ellis et al. Table 2. Measurement of droplet size distribution made with PDA; full scan,
VMD,
Spray liquid
pm
PDA %vol. < 100 pm
256 234 215 247 260 268 216
Water Ethokem LI-700 Agral Axiom Codacide Silwet L77
Fl 10/l.6/3 nozzle at 3 bar VMD,
Mean liquid velocity, ms-’
2.9 4.8 1.6 3.6 2.6 2.0 1.5
PMS %vol. < 100 pm
pm
291 284 318 288 301 318 322
1.8 6.5 8.8 7.3 8.2 8.7 9.1
Mean liquid velocity, ms-’
1.1 1.2 1.5 1.2 1.4 1.3 1.4
5.2 5.1 1.3 5.5 5.9 1.0 8.2
20
Results and discussion Droplet size and velocity
distributions
The volume median diameters (VMD), the percentages of spray volume contained in droplets
16
&a”Vi Ziai
where a is the droplet diameter, V is the droplet velocity and i indicates summing over all droplets. Droplet size distributions showed little difference between the two instruments for all the adjuvants, apart from those that relate to the limitations of the technique, i.e. the tendency of the PMS to underestimate the small droplet fraction and of the PDA to create large droplets. Because the distributions measured with the PMS and PDA were comparable for all adjuvants, it was assumed that all the spray liquids were measurable with the PDA, even though some of them formed emulsions (Codacide, Axiom and LI-700). The maximum droplet size for analysis was determined from the PMS distributions as 700 urn for the F110/1.6/ 3 nozzle. PDA distributions for all adjuvants were therefore truncated at 700 urn to calculate VMDs shown in Table 2. The size of droplets produced by flat-fan nozzles has clearly been altered by the addition of these adjuvants. VMDs were both increased and decreased relative to water and the percentage of spray volume contained in the small droplets, which are most prone to drift, was changed. The adjuvants LI-700 and Ethokem, which gave the largest changes in VMD, were sprayed through an F110/0.6/3 nozzle and measured using the PDA and PMS with a full scan. The PDA data was truncated at 500 urn for analysis, shown in Table 3. Mean velocities of droplets in each size category Table 3. Measurement of droplet size distribution made with a full scan, Fl 1O/0.6/3 nozzle at 3 bar VMD, Water Ethokem LI-700
44
198 189 216
km %vol. < 100 pm Mean liquid velocity, ms-’ 6.4 9.0 3.5
4.7 4.0 5.9
Crop Protection 1997 Volume 16 Number 1
04 0
.
*
100
.
200
300
400
500
600
Droplet diameter, pm Figure 1. Relationship between droplet velocity and droplet diameter for two liquids and water, measured with PDA carrying out a full scan of the spray from the Fi 1 O/0.6/3 nozzle
through
both nozzles were altered by the adjuvants 2). Larger droplets do not slow down as rapidly as smaller ones so sprays with a larger VMD would be .expected to have higher mean liquid velocities. However, from Figure 2 it can be seen that, for most droplet sizes, droplets containing Ethokem travel slower than water droplets and those containing LI-700 travel faster. Droplet size distributions measured with full scan (Table 2) showed a smaller variation in VMD with adjuvant than those measured with a centre line scan (Miller et al. 1995), suggesting that the droplets in the centre of the spray were more affected than other parts of the spray fan. Measurements of droplet size distribution were therefore made at different horizontal positions within the spray fan, scanned across the thickness, also using both instruments with the two selected adjuvants and two nozzles. Variation of VMD with horizontal position measured with the PDA is shown in Figures 2 and 3. Measurements with the PMS were very similar and therefore are not shown. The entire droplet size distribution is not simply shifted towards larger or smaller droplet sizes, but the pattern of the droplet size spectrum across the spray fan is changed. The central part of the spray was strongly affected by the adjuvant, suggesting that it is the formation of droplets by oscillation of the liquid sheet that is most altered, rather than the rim distintegration. (Figure
I and Miller et al., 1995, figure
The effect of adjuvants
VMD, pm
on sprays: MC. Butler Ellis et al.
Footprints Analysis of the full scan droplet size distribution allowed the dimensions of the spray fan in a horizontal plane to be determined and ‘footprints’ to be estimated, as shown in Figure 4. The thickness vertically beiow the nozzle was correlated with mean liquid velocity (Miller et al., 1995) showing that the thicker the fan, the slower the mean liquid velocity. The changes in footprint thickness may contribute to the differences in droplet velocities shown in Figure I. A thinner footprint would be expected to have a higher mean liquid velocity to maintain a constant volume flow rate. Droplet photographs
t
/
-600
7n I LO”!
I
-400
-200
0
/ I
/I 200
400
I
600
Horizontal position, mm l
water
l
Water plus 0.5% Ethokem
A Water plus 0.5% LI-700 Figure 2. Variation of volume median diameter with position in the spray, measured with FDA using the Fll O/l .6/3 nozzle
VMD, pm 330 T
Droplets produced by the F110/0.6/3 nozzle at 3 bar spraying the three test liquids are shown in Plates 1-3. The droplets from the F110/1.6/3 nozzle were very similar and are therefore not shown. Air inclusions appeared in droplets containing Ethokem (Plate 2) and were observed frequently in droplets of 300 pm diameter and larger, and occasionally in droplets as small as 150 pm diameter. They did not appear to disrupt the operation of the PDA, since there were no greater differences between PDA and PMS results for Ethokem than for water alone. The appearance of these air inclusions was similar to those observed in droplets produced by twin-fluid nozzles, although less densely packed, and are not thought to be caused by, for example, droplet coalescence or overlapping droplets in the silicone oil. A few air inclusions were visible inside droplets when LI-700 was added, but considerably less than with Ethokem. However, there are clear signs of internal content, e.g. at least one white ring inside each droplet, which is only visible when LI-700 is present. Small emulsion droplets are also visible. Again, this did not disrupt the PDA measurements. The same effects are visible for both nozzles. The presence of air inclusions with Ethokem may account for the lower velocity of droplets containing Ethokem above 150 pm, shown in Figure I.
Liquid sheet photographs
I
-600
I /
-400
-200
I,“,CA
I
0
L I
200
/
i
400
600
Horizontal position, mm 6
water
n
Water plus 0.5% Ethokem
A Water plus 0.5% LI-700 Figure 3. Variation of volume median diameter with position the spray, measured with PDA using the Fl 1O/0.6/3 nozzle
in
Examples of the liquid sheet at two different pressures through an F110/1.6/3 nozzle are shown in Plates 4-9. Photographs of the F110/0.6/3 nozzle were similar and are not shown. Photographs of the liquid sheet clearly show that adjuvants can alter the dominant mode of spray formation. The differences caused by the addition of adjuvants became more apparent as the pressure was reduced. The effect of reducing the equilibrium surface tension on the liquid sheet was demonstrated by Dombrowski and Fraser (1954): using pure liquids with a surface tension lower than water, oscillations became unstable earlier, leading to break up nearer the nozzle. With Ethokem, the unstable oscillations and break-up point were further from the nozzle than with water. With LI-700, the sheet perforates, leading to spray formation were nearer the nozzle. Neither of the test liquids, therefore, behaved as pure liquid with surface tension lower than water.
Crop Protection
1997 Volume 16 Number 1
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The effect of adjuvants on sprays: M.C. Butler Ellis et al. 200-
100-
O-
E -100. _ :: I!! Y ‘5 -200, c 9 8 0
---0.5x u-700 ....'. 0.5% Ethokem 200-
$ s '-Z 'iz loo2
O-
-100.
-200-l -600
-500
-400
-300
-200
-100 Position
0 along
by an F110/0.6/3
Crop
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1997 Volume
300
400
500
-I 600
Fl 1O/l .6/3 nozzle (lower) spraying two liquids and water
nozzle at 3 bar Plate 2. Droplets with air inclusions produced by an Fl 1O/0.6/3 nozzle at 3 bar spraying water and 0.5% Ethokem
Larger droplets probably occur with LI-700 because the break-up point is nearer the nozzle where the liquid sheet is thicker. This also leads to a change in the volume distribution of spray liquid. LI-700 has a thinner spray fan because the break-up occurs nearer the nozzle before sheet oscillations have begun to affect droplet trajectory. Break-up by oscillation may lead to horizontal ligaments being thrown forwards and back-
46
200
fan width, mm
Figure 4. Footprints of the spray produced with Fl 1O/0.6/3 nozzle (upper) and
Plate 1. Droplets produced spraying water only
100
16 Number
1
wards, whereas break-up by perforation results in droplets continuing more within the plane of the nozzle. This apparently results in higher velocities, possibly because greater energy losses occur when sheet oscillations take place. The effect of LI-700 on the sheet breakup is visibly similar to the effect that is observed when heat is
The effect of adjuvants on sprays: M.C. Butler Ellis et a/.
Plate 3. Droplets produced by an F110/0.6/3 spraying water and 0.5% LI-700.
nozzle
at 3 bar
Plate 5. F110/1.6/3
Plate 4. Fl 1O/l .6/3 nozzle at 3 bar spraying
nozzle at 3 bar spraying water and Ethokem
water only
applied to the liquid sheet to alter the point of break up and increase the droplet sizes (Dombrowski, Hibbit and Strachan, 1989). This similarity occurs because the gradients of surface forces are important in determining break-up. Surface tension is very sensitive to temperature and therefore a temperature gradient results in a surface tension gradient. Surface active agents also produce gradients in surface forces as the sheet ages. This leads to an earlier sheet break-up and larger droplets. Although there was no change in surface tension apparent from considering the rims of the liquid sheet, it is possible that a sudden change occurs, which causes the sheet to suddenly disintegrate and so any change in rim trajectory is lost. However, this does not explain the reduction in VMD that occurs with other adjuvants such as Ethokem. They also have time-dependent surface forces, but the result was to prolong the life of the liquid sheet rather than reduce it. This was opposite to the effect reported by Dombrowski and Fraser (1954)
Plate 6. Fl 1O/l .6/3 nozzle at 3 bar spraying water and LI-700
where a decrease in surface tension reduced the length of the liquid sheet, but is similar to the effect that occurred when viscosity was increased. A possible explanation for this would be that, although the surface tension is falling and bulk viscosity of the liquid is likely to beunchanged, the surface viscosity could be substantially increased to an extent that over-rides the effect of the reducing surface tension. An increase in bulk viscosity may result in an increase in droplet size (Snyder, Senser and Lefebvre, 1989) because as well as increasing the sheet length it also increases the sheet thickness. In the case of increasing only the surface viscosity, the sheet thickness may remain unchanged leading to smaller droplets. It has been suggested (Clark and Dombrowski, 1971;
Crop
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47
The effect of adjuvants on sprays: M.C. Butler Ellis et al.
Plate 7. Fl 1O/l .6/3 nozzle at 1 bar spraying water only
Plate 9. Fl 1O/l .6/3 nozzle at 1 bar spraying water and LI-700
surface tension, mN/m 70 60
&hAbaAAAa.wd.rr+.m~A 00
00
c
n
0
00
0
*OOOO
0000
00
40 -30 -20 -10 -0 I 0
I 2
4
6
8
10
bubble frequency, Hz
Plate 8. Fl 1O/l .6/3 nozzle at 1 bar spraying water and Ethokem
Dombrowski and Fraser, 1954) that the edges of the liquid sheet are drawn together by surface tension forces. If surface tension of the liquid sheet is affected by the addition of these adjuvants, it should therefore be possible to see differences in the trajectories of the rims. By superimposing the photograph of water at 1 bar on the photograph of Ethokem at 1 bar, it was possible to observe the the trajectories have begun to diverge at approximately 30 mm from the F110/0.6/3 nozzle and 40 mm from the F110/1.6/3 nozzle. The sheet remained wider with Ethokem than with water, suggesting that the surface tension was lower. No differences in trajectory were observed between water and LI-700, implying that the surface tension with LI700 declined slower than with Ethokem. At 3 bar, there
48
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A water o 0.5% Ethokem 0 0.5% LI-700 Figure 5. Dynamic surface tension of water plus 0.5% LI-700 and water plus 0.5% Ethokem measured with the maximum bubble pressure tensiometer
were no observable
differences in rim trajectories between any of the liquids since the velocities were higher and the sheet had disintegrated before any change in rim trajectory could take place. Dynamic surface tension
Measurements of surface tension of the two spray liquids, made with the maximum bubble pressure tensiometer, are shown in Figure 5. The age of the liquid sheet would correspond to bubble frequencies
The effect of adjuvants
well above 10 Hz and therefore this information is not directly relevant to spray formation. However, it does support the conclusion, drawn from comparing rim trajectories, that the surface tension of water with 0.5% Ethokem declines faster than water with 0.5% LI-700. There appears to be two different effects of adjuvants upon spray formation: one which prolongs the life of the liquid sheet by suppressing oscillations and the other which reduces it by causing sheet perforation. The former results in smaller droplets and the latter in larger droplets, compared with water alone. It is important to measure properties of the liquid in order to establish what determines the break-up mechanism and consequently whether a liquid will increase or decrease droplet size. However, the properties that are important and the methods that should be used for their determination are not yet clear. The rate of change of surface properties is likely to be crucial and therefore very short timescales of measurement will be essential. Other properties, such as surface viscosity, may be of equal or greater importance than surface tension. Because it has been shown that changing the liquid can alter the spray formation mechanism, a single model relating droplet size to physical properties of the liquid is unlikely to be successful. Modelling each spray formation process may allow the spray characteristics to be predicted, providing we know which spray formation mechanism is appropriate. The effect of liquid properties on spray formation may be a function of nozzle type and will therefore also influence spray formation from other spray systems, such as twin fluid nozzles. This needs further investigation. The variation of droplet size with liquid properties is also significant when considering nozzle classification, which is carried out using water plus 0.1% Agral 90. A nozzle that is classified as ‘fine’ for example may produce a spray that is ‘medium’ when using an active formulation. It is clearly important to ensure that nozzle classification is always carried out using the same test liquid, otherwise differences due to liquid properties rather than nozzle design may result. When the effect of liquid properties on spray formation are fully understood, it may then be possible to predict the way in which pesticide spray liquids will modify spray quality and whether its classification is likely to be significantly changed.
Conclusions
This work confirms previous reports that changes in liquid properties caused by the addition of adjuvants can lead to significant changes in the quality of the spray produced by flat-fan nozzles. Droplet sizes and velocities may be either increased or decreased and the structure of the droplets may be changed by including air bubbles, for example. The liquid sheet break-up mechanism is affected, leading to either a longer liquid sheet (resulting in smaller droplets) or a shorter one (and larger droplets). This causes changes in the pattern of droplet sizes at different positions within the spray fan. A consequence of the changing break-up mechanism is that the spray cross-sectional area can be modified. with the short axis
on sprays: M.C. Butler Ellis et al.
being shorter and the long axis being longer with some adjuvants than with water alone. These effects were consistent with the six spray liquids measured such that VMD, mean liquid velocity and spray footprint thickness were correlated. The trajectory of the rims of the liquid sheet can also be affected, showing that dynamic surface tension, which is thought to be one of the major factors in the effect of surfactants on droplet size, appears to play a part with some surfactants despite the short surface ages involved in spray formation. Insights into the effects of liquid properties on spray formation provide an indication of how spray application efficiency might be improved. However, the interaction between the droplet and the target is also strongly affected by adjuvants and therefore any models of spray formation need to be linked to work on target interaction before the full effect of adjuvants can be understood.
Acknowledgements
The authors would like to thank Mr R. Cove for his excellent photography and Dr P. J. Walklate for useful discussions. This work was funded by the Ministry of Agriculture, Fisheries and Foods. References Brazee, R. D., Bukovac, M. J., Cooper, J. A., Zhu, H., Reichard, D. L. and Fox, R. D. (1994) Surfactant diffusion and dynamic surface tension in spray solutions. Trans. A.S.A.E. 37, 51-58 Chapple, A. C. and Hall, F. R. (1993) A description of the droplet spectra produced by a flat-fan nozzle. Atomiz. Sprays 3, 477-488 Clark, C. J. and Dombrowski, N. (1971) The dynamics fan spray sheet. Chem. Engineer. Sci. 26, 1949-1952 Clint, J. industrial Properties Jones) pp
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Received I I March 1996 Revised 8 July 1996 Accepted 10 July 1996