’—
”'
——'-
_—-—-
-—-—'
_—-———
—'—I——
___————
——-——
_——.—
—_.——
_—_’—
’—
_’
—————

 

A STUDY OF CERTAIN PHYSICAL FACTORS
INVOLVED IN THE SPREADING
OF SPRAY MATERIALS

THESIS FOR DEGREE OF M. S.

WILBUR HERMAN THIES
1925 ’

 

 

 

 

 

 

A.STUDY OF CERTAIN PHYSIOAL FACTORS
INVOLVED IN THE SPREADIIG
OF SPREY MATERIALS

Presented as a Thesis
for the Degree of 1!. S.

Wilbur Herman 2313;
1925

 

 

 

il‘LLf‘ r‘ilulill '.|I|.| ‘f. i IIZ-

 

TH E815

A Studg 93: Certain Phxsical Factors
Inv _i_§ the Spreading 9; Sprax laterialg

Introduction

The development of the science of
horticulture depends to a large extent on
men's ability to control the insects and fungi
which prey upon plant life. It is for this
reason that the application of spray materials,
as a protective measure, has assumed such an
important place in the program.of the fruit
and vegetable growers of the present day.

Beginning with the work of Millardet
in the eighties, research work-in spray materials
has attracted the attention of a vast corps of
workers. Numerous independent efforts have been
made to determine, the best kind of material e
to apply in each particular instance and also
the most economical method of application. Wheth-
er the material be an insecticide or a fungicide,
a thorough covering of the plant with a minimum
of waste, is the goal to be attained.

”lam-r I?
. ‘ Q);l:)\)_$

 

 

It is the writer's purpose in this
paper. first. to review the studies which have
been made on the spreading of liquids in con-
tact with solid surfaces and second, to discuss
the question from.the standpoint of certain
physical factors. viz: (1) surface tension.

(2) velocity and (3) size of drop.

[Acknowledgment of many helpful
suggestions pertaining to the tests is made
to Professor V. R. Gardner of the Hichigan
Agricultural.College, Horticulture Department,
to Professor C. W. Chapman of the Physics Depart-
ment and other members of the Ehysics Staff, also
to Professor Louis Thompson of Kalamazoo College
for counsel in connection with the calculation of
air resistance.

Reziew 9;, Literatgg

l. Higtorioa; £93.29. 93; weeding

lany attempts have been made to in-
crease the filmpforming tendency of a spray so-
lution and to make 1t ”stick“ better by the addi-

 

 

tion of adhesive substances.

One of the earliest is mentioned in
Rapport au Hinistre de llAgriculture (1885).
Davis, an investigator, used 6 kilograms of glue
to 800 liters of copper sulfate solution, think-
ing that the glue increased its efficiency.

Soap or soap suds was early used in
contact sprays with tobacco and other materials
to insure spreading of the insecticide over the
body of the insect.

Iashburn (1891) used whale oil soap
6 lbs., to 50 gals. of a Paris Green suspension.
Galloway (1892) used soap with different fungi-d
sides to increase spreading. Fairchild (1894)
used soap with can celeste, obtaining excellent
spreading even on leaves with a heavy cuticle
such as those of the pear. Galloway (1894) used
resin soap with Bordeaux mixture, finding it
Just as effective in producing a continuous fine
on the leaf as ivory or whale oil soap.

Lowe (1896) encountered difficulty

in spreading lead arsenate over the surface of

 

 

willow leaves. He used glue, 2 qts. to 50 gals.,
successfully.

lausier (1908) studied spreading of
a number of liquids over different solids. He
realized that spreading depends on the nature of
the solid and of the liquid, but considered the

 

difference as due principally to the surface
tension of the liquid. .A liquid which spreads )

 

over a particular solid in a certain definite way
nmet have a certain definite surface tension value.
if it has this value, it will spread regardless of
the nature of the liquid. (A.spray containing

50 gms. of soap to 10 liters of water with either
50 gme. of oil tar or 10 gms. of formaldehyde

 

is considered as meeting the surface tension re-
quirements of the leaf).

Gastine (1912) recommended the use of
saponin to reduce the surface tension of spray
materials and thus insure spreading.

vermorel and Dantony (1912) conclude
that surface tension of a liquid is not an index

of its spreading power. Solutions with the same

 

ability to spread may have different surface
tensions. Two distinctions are made, (1) where
the liquid is able to touch the surface, as water
on a potato leaf, and (2) where the liquid rests
on the surface without touching it, as water on
a cabbage leaf, in which case there is a film.of
air between leaf and water. A.solution of sodimm
oleate was found to spread on a cabbage leaf but
not on grape leaf, while a saponin solution of
higher surface tension than the sodium cleats
spread readily on e grape leaf. They believed
therefore that surface tension influenced spread-
ing less than surface viscocity. The difference
in the behavior of the above solutions is thought
to be due to the surface concentration which
takes place rapidly in the saponin solution after
the film.has formed, thus increasing the surface
viscosity to such an extent that the solution
was unable to collect in drops.

floors (1921) states that a concentra-
tion of material in the surface layer reduces

surface tension and since different materials

 

 

vary as to the time required to concentrate in

the surface layer, measurements of surface tension
by ordinary means are not comparable. Furthermore,
if surface concentration takes place so rapidly

as to cause the film to become viscous, a decided
lowering of the surface tension should be apparent.
No consideration is taken of the possibility of

a concentration of materials at the interface of
the leaf and the liquid. This should.result in

a lowering of the interfacial tension and thus
tend to produce spreading.

Chappoz, G. (1913) recommends the use
of saponin in preparing sprays, considering it
superior to soap. He recognizes a general re-
lation between spreading and drop number as de-
termined by the Duclaux pipette.

Laforge (1913) points out that spread-
ing and adherence are entirely different phenmm-
ena. Addition of saponin gives good spreading
but not good adherence, while gelatin improves
both.

 

 

 

Lefroy (1915) finds that spreading
depends not only upon the surface tension of the
spray, but also upon the surface tension of the
leaf and of the tension at the interface of
spray and leaf. He realizes that the latter two
tensions cannot be measured and so recommends
determination of the surface tension of the spray
as an index of spreading quality. This should be
as low as possible.

Cooper and Nuttall (1915) believe that
certain factors may upset the relationship of
the three forces which determine the tendency
to spread. (l) Solvent action of liquid on solid,
particularly in those cases where the surface is
coated with wax or grease. (leore shows that
-this solvent action of a liquid on a solid
lowers the interfacial tension and therefore tends
to aid spreading). (2) Surface concentration.
(Cooper and Nuttall cite the experiments of
Vermerel and Dantony on the good spreading of a
spray with a high surface tension, owing to its

high surface viscocity which results from a rapid

[J
i
F

 

 

surface concentration of the solute). The state-
ment is made that the measurement of surface
tension alone is insufficient to determine spread-
ing qualities of a liquid. Cooper and Nuttall
used sheep dips as the liquids, and substituted

a heavy castor oil or vaseline for the solid.
Then by means of a Donnan drop pipette, the ten-
sion at the interface of oil and dip was measured.
They point out that this method is applicable
only to soap solutions and will not give results
with gelatin, saponin or similar solutions. Also
the surface mmst be greasy (This eliminates its
use in studying spreading on leaves). The con-
clusions of Casper and Huttall are as follows:

If Xs >81 0 31s the liquid will spread, and
Xe . (81 e Xls) e spreading power.

Aphrop

B-Surface

 

 

Diagramatic Representation of interfacial Tensions.

151 refers to tension at liquid-air interface

 

 

513 refers to tension at liquid-solid interface
Us refers to tension at solid-air interface

51s is of more significance in spreading
than is‘Ul, for, since interfacial tension varies
inversely with drop number (Donnans drop pipette),
and also inversely with spreading, the drop nuns
ber (which in reality is a measure of interfacial
tension) must vary directly with the tendency to
spread. For example, a high drop number means
high spreading power. because of low interfacial
tension. This conclusion evidently holds true
for liquids of low surface tension such as soap
solutions but does not hold true for all liquids
that will spread.

Smith (1916) used Cooper and Nuttall's
method of finding Uls in determining spreading
power of various combinations of fish oil soap
and nicotine sulfate. He found the relations of
spreading and interfacial tension similar to
those found by Cooper and Nuttall. But since

oil was used to represent the solid surface, the

IIIIIIIIIIIV" ‘“‘--———q.-nlllllll-n-____

10

experiment has no significance in a study of
leaves.

Lee (1918) studied spreading of water,
gelatin, casein, soap and paraffin emulsions on
leaves of gooseberry, and sea kale, and on leaves
covered with gooseberry mdldew. .All gave spread-
ing on gooseberry leaves, but only certain emul-
sions spread on the sea kale and on gooseberry
mildew. no explanation was offered.

Invett (1918) believes that the ability
of a liquid to hold arsenates in suspension is
a fair indication of its ability to spread. How-
ever, it is doubtful if this can'be considered
as an index of spreading over all surfaces. In a
later paper (1920) Lovett says that although the
suspension test is not an accurate index of spread-
ing, it does indicate a physical quality in the
solution much to be desired in a spreader.

Jacobson (1919) produced good spread-
ing by use of the extract from.5 lbs. of alfalfa
hay added to 100 gallons of spray solution. He

 

 

 

11

does not state whether spreading is the result
of increased surface viscocity, lowered surface
tension, or to a concentration of materials at
the leaf spray interface.

Ruth and Kelley (1922) report a series
of experiments on the use of casein as an aid
to the formation of a filmi Their conclusions
are that varietal and seasonal differences in
leaf and twig surfaces exist, and that the ease
of covering a surface varies with the season.

Hears (1921) states that a solution
containing 1/4 1b. of casein to 100 gallons of
water gave good spreading on citrus leaves but
failed unless applied with such force that the
spray penetrated between the wax particles reach-
ing the epidermds below. incidentally, it may
be stated that this is the only mention.made, in
any of the writings reviewed, cf the effect of
velocity on spreading.

woodman (1924) of England, has car-
ried on a piece of work on the physics of spray

liquids in which he used glass as the surface

 

 

 

because he believed that it resembled very closely
in some of its characteristics the surface of a
leaf. He defines a certain critical value for

the surface tension of a liquid at which he says
the.maximmm.amcunt of spray material will be

held. Failure to reduce the surface tension

to this value results in imperfect wetting of

the leaf. He found that reduction of surface ten-
sion beyond this point does not increase the wet-
ting power.

The foregoing review of the work that
has been done leaves considerable doubt as to the
significance of the part played by surface ten-
sion in influencing the spreading of spray materi-

 

als. It has long been assumed by many people

that the surface tension of a liquid has a direct
bearing on its tendency to spread when in con-
tact with a solid surface. But several investi-
gators. have reported that certain'oth'er factors
appear to have greater influence on spreading than
does surface tension. it was, therefore, with

the idea of throwing further light on this partic-

 

ular question that a series of tests was outlined.
.And since lime sulfur is used so commonly in
spraying, that was the material selected.
1. Relation 93 SURFACE TENSION £9. Spreadipg
1133 2; Procedure '
A.stock solution was made from Niagara
Dry Lime Sulfur, 4 lbs. to 50 gallons and the
five samples treated as indicated below.
Sample No. 1 Lime sulfur and fish oil soap.
Sample No. 2 line sulfur and saponin
Sample No. 3 line sulfur and calcium.caseinate
Sample No. 4 lime sulfur and Sun 011
Sample No. 5 Lime sulfur
The surface tension of each sample was

carefully measured with a du Nucy instrument.

 

This is a device made up of a steel wire and cross
arm attachment to which is suspended a loop of
platinum wire. One end of the steel wire is
rigid, and as the dial is turned, the resulting
stress raises the platinum loop, thus lifting the
surface film. Ihen the film breaks, the reading

on the dial is observed. This is a measure of

 

14

the force exerted by the film, or its surface
tension. In this piece of work. these values

were not converted into dynes per on" because

only relative values were desired. The dial read-
ings are proportional to the surface tension in
dynes. Hence the surface tension values given in
Table l are merely degrees, read on the dial. It
will be observed from this table that the treat-
ment of the samples reduced the surface tension

by varying amounts, number 4 being only half that

 

 

 

of the stock solution.

Spreading qualities were measured by
dropping the liquids on glass, mica and paraffin.
Although no one of these surfaces is exactly
analogous to any kind of leaf or bark. the drop-
lets behave on these surfaces in a way very simi-
lar to that on surfaces that are sprayed. This
method therefore, puts to the test the question
of influence of surface tension 0‘ spreading
under different conditions.

The glass surface was cleaned very

thoroughly with chromic acid and rinsed with dis-

 

15

tilled water. The mica surface was prepared by
splitting along a cleavage plane. And the paraf-
fin surface was cleaned and levelled by scraping
with the edge of a plate of glass.

To transfer equal amounts of each solu-
tion to the above surfaces, a capillary tube was
drawn on one end of a short piece of glass tub-
ing and to the other end was attached a small
rubber bulb. The end of the capillary tube was
dipped in paraffin to prevent adherence of the so-
lution. By means of the rubber bulb the solution
was allowed to enter the capillary tube up to a
given point. The tip of the tube was then held
near the surface, all of the solution excluded at
a single point on the surface, and allowed to
spread naturally without disturbance. .About twenty
sueh areas were produced on.each surface. The
tube was then cleaned thoroughly, and the process
repeated with the next sample.

‘When the areas had completely dried,
measurements were made with a planimeter, first

discarding any area which was not circular. The

 

 

 

 

16 V

 

figures in Table 1 show averages of such measure-
ments. 7
Similar tests were made of these five

solutions on leaves of cherry and pear. But on

 

account of indistinctness of.margin, and ten-

dencies to spread along the leaf veins, the only

conclusion drawn was that spreading is slightly

greater on the upper surface than on the lower

surface of the leaf. This was true for each so-

lution and for both kinds of leaves tested.

my; 1 Showing Relation of Surface Tension

To Area Covered by Drops of Uniform
Size on Three Surfaces,--Glass, lice,

and Paraffin.

 

Hyper 95g, Surfs“, Tensign p; m

Surface IV( 82853) 1(81‘860) 111(81'882 ) 11182.87 ) $138109 )

 

i

Area of spread in square inches

61833 : e05 : e03 3 9028 3 e032 : e032
lice : .082 : .076 z .08 : .096 : .086
Paraffin : .024 : .015 : .014 : .012 : .008

 

 

 

.l a: llil)
_

 

I
Y
I

4‘- r.-- \-I

AK. \.
.

.II . . .T . T . m T I3. ... I4)!.Y.-|-)v
. _

horseman: magmas

 

m
_

.
..... , , T 43:..-

. .T . .. ._“T. _ _i, . T. _

c _ H i ..

   

  

a
4

 

1V1

17

A.study of the above data fails to re-
veal any correlation between surface tension and
fapreading, except in the case of paraffin. On a
paraffin surface the solution with the highest sur-
face tension spread over the least area and vice
versa. it would seem.the reason for this is that'
none of the solutions were strongly adsorbed by
the paraffin, or in other words the force of ad-
hesion was weak. A drop of liquid. therefore on
such a surface, will remain spherical if the sur-
face tension is high or flatten out if the surface
tension is low.

But in the case of either glass or mica,
surface tension seems to be a negligible factor
in spreading, or to be more explicit, no direct
relation is apparent. This fact seems to offer
experimental evidence that surface tension as
such, need not be considered as a factor of fun-
damental importance in determining the spreading
ability of spray drops on all types of surfaces.
And although it is undoubtedly operative when a

liquid is applied to certain surfaces, such as l

18

line sulfur on paraffin, the virtue of a low sur-
face tension value in spray materials is probably
due to the greater facility with which such a

- material may be broken into small droplets. The
importance of the size factor will be pointed out
in a later portion of this paper.

It is.a well known fact that if a liquid
is adsorbed at a solid surface, it forms a liquid
film.and we say that it “sets“ the solid. 1f the
liquid is not adsorbed by the solid no wetting
takes place. The phenomenon of adsorption depends
upon the nature of both liquid.and solid because
it has to do with the molecular forces between
the two. so measurements, therefore, which we can
make of the forces between the molecules of the
liquid, such as the force of surface tension, will
throw any light on the forces of adsorption. The
phenomena of adsorption, then, must be of funda-
mental importance in a study of the spreading of
spray materials. The tests already mentioned bear

out-this contention.

 

 

______-__

 

——'”_

19

it seems only reasonable to infer that
this molecular affinity termed by Langmuir as
”secondary valence,‘I which is undoubtedly electri-
cal in its nature, is largely responsible for
the behavior of spray drops as they strike a solid
surface. And to the extent that spreading is
determined by adsorption, the composition of the
spray materia1.must be of prime importance. This
is a problem for the manufacturer of spray materi—

als rather than for the grower who uses theme

Wat rica mot W
|'I‘or a liquid to set a solid in the pre-

sence of air, the liquid.mnst be adsorbed more
strongly than the air and must displace it. Since
the adsorption of liquids is selective, it follows
that one liquid will be adsorbed more by a solid
than will a second liquid and that consequently
the first will displace the second from contact
with the solid. lo systematic study of this
phenomenon seems to have been made.("Bancroft's

“Colloid Chemistry“)

 

20

Even though the principles which under-
lie the phenomenon of adsorption are not fully
understood, its application to even day life and
also its bearing on the behavior of spray materials
are very evident from the illustrations here given.

Inrd.Rayleigh (1902) states that in his
opinion the contact angle is zero, if the liquid
is actually adsorbed by the solid surface. The
same idea is expressed by Harkins and Brown (1919).
If this is true it is obvious that no adsorption
takes place between lime sulfur and paraffin.

Kerosene will displace water in contact
with copper and water will displace kerosene in
contact with quartz (Pockels,‘Wied. Ann. 67, 669,
1899). This explains the reason for using a rag
dipped in alcohol.with which to wipe off a lamp.

Alcohol will displace oil in contact
with metals. (Bofman, Zeit, Phys. Chem. 83,

386, 1913).

“Linseed oil will displace water in

contact with white ma (Cruikshank and Smith in

 

 

21

in The Manufacture of Paint, 92, 1915). This
shows the function of vaseline in preventing the
rusting of metals.

In the kitchen, housewives wet moulds
- before putting in corn starch or gelatin, they
butter pans before making fudge, and grease
griddles before making flapjacks. These are all
cases of selective adsorption. it is claimed that
aluminum griddles do not need to be greased be-
cause the air in the oxide film keeps the cake
from sticking.

Some of the methods of removing grease
spots from.olothing depend upon selective ad- I

sorption (Lake, Journ. Phys. Chem» 20, 701,

 

 

1916).

 

Gelatin is adsorbed so strongly by
glass that on drying it may tear off the surface
of the glass. 0n the other hand, it will not
adhere to plates of mica or calcite. (Bancroft)

Certain plastic clays, high in silica,
adhere to iron in a remarkable way (Trans. Am.

Ceramic So. 14, 610, 19121

 

22

Graham.(1867) reports that fifteen times
as much oil of sweet almonds was retained by
palladium foil as of water.

The foregoing observations of the phe-
nomena of adsorption will suffice to emphasize
the close relation between the behavior of sprays
and the composition of liquid and solid.

The following tests were outlined with
the idea of determining what influence, if any, the
velocity of a drop has upon its tendency to spread.
For within certain limits velocity is under the
grower's control and it is important for him to
understand how it contributes to the effectiveness

of the spraying operation.
- 2. Relatiog 9}: mm to Spreading

On account of the physical impossibility
of studying the effect of velocity of drops on
spreading where a large number are released at
once (as true a spray nozzle), the device shown
in Figure 2 was used.

A.capillary tube similar to the one pre-

viously described was drawn from.the lower end of

 

 

A! Capillary Tube for
inlet of Air

3- Tinohcock

C- Telescope

D- Scale

3- Burette containing
Spray Material

F- Standard

6- Capillary Tube for
Release of Drops

   

 

   

Fig. 2 Apparatus used in Studying Effect
of Velocity on Spreading

 

 

24

a burette. The upper end was fitted with a rubber
tube and a second capillary tube with pinchcock,
to regulate the rate at which drops were released.

Tests were made with Bordeaux mixture
(4-4-50) and with liquid lime sulfur (1-30).

No direct comparison is possible be-
tween the three tables given below (ii, iii, and
IV) because different sized capillary tubes were
used. But in each case a definite increase in
area of spread is apparent as the striking velo-

city is increased.

Taple';; Shogipg Relation Between Velogity
and _Spr eading

(Bordeaux Mixture on Glass)

Height of Fall (cm) Area (sq. one)
Practically 0 .148
1 .451
4 - .819
9 1.103

16 1.29

 

25

(Note- Drops at top
of photograph have
fallen from a height !
of 120mm: those at ‘
bottom have fallen
fifi’é’cm. , Inter-
mediate drops have
fallen fro heights
of 48 can, 111.. and
wqcm. ,respectively .) 1

 

Photograph showing areas covered by drops which
have fallen from heights indicated in Table II.

 

Tag“ 111 (Lime sulfur, fairly large drops)

Height of 111 (cm) Area (sq. cm.)
1 .297
4 .593
9 .819
16 .98

 

26

Table 5!; (Idle sulfur, drops smaller, delivered

from smaller tube).

Height of fall (cm) Area (sq. cs)

1 .168

4 .226

9 , .303

is .595

25 .452
as .477

49 - _ .529
64 .555

81 (some spattering) .574

100 (considerable spattering; areas too
irregular to measure accurately)

The reason for selecting the heights
of fall shown above was to give striking veloc-
ities in an arithmetic progression. For example,
if a large drop falls from a height of one inch,
nits striking velocity will be approximately half
that of a drop which has fallen a height of four
inches, and one-third that of a drop which has
fallen nine inches, and so on. (This follows
from the well known law of falling bodies,--
Velocity (V) squared is equal to twice the pro-
duct of the acceleration of gravity (g) and the
height (h) .'. V 3,5—35 or V varies as h. Cor-

 

 

 

|
I.
I4

 

27

rection of this formula will be taken up under
'lhthematical Aspect of.Air Resistance").

The data shown in Tables III and IV
were obtained by using lime sulfur (surface
tension-110) on a perfectly clean glass plate.
No spreader was used. Areas were calculated
from.diameters determined by use of a Filer li-
ercmeter.

Tables V and VI indicate a different
set of conditions. In both of these cases,
drops were released from heights ranging from
one to one hundred centimeters as before but the
areas were found to be identical, regardless of
the height of fall.-

1933!. MSulfur (l - 30) mmgi;
(Surface tension-53)

Drops of the above solution were
permitted to fall on a clean gigs. plate from
heights corresponding to those in Table IV. Re-

sult -- Areas were identical)

 

 

 

 

28

m 1; L132 331311; (1 - SONngace Tensions-m)
(Drops of the above solution were per-

mitted to fall on.a pgggzzin plate from heights

corresponding to those in Tables iv and v. Result--

Areas were identical).

In the last two cases cited, the drop
of spray material could be observed to flatten
out at the instant of contact with the surface
but on account of a lack of attraction between
liquid and solid, the margin of the drop imme-
diately receded toward the center of the area in-
volved.

Iran the tests Just described, one would
infer that the application of spray drops with
considerable velocity is desirable upon surfaces
which do not permit of intimate contact between
spray and surface. in this class, are included
surfaces covered with hair-like projections, or
other irregularities, and also those holding pre-
viously adsorbed materials which might be dis-

,placed sufficiently by a driving spray to permit

 

 

29

the solution to reach the surface itself. Un-
doubtedly a driving spray is desirable on the
rough bark of trees to penetrate the crevices

but on the leaves and fruit, added velocity is
very apt to cause inaury of more consequence than
any benefit to be derived frmm‘better covering.
Such inaury is probably due to abrasion or mechan-
ical breaking of the tissue by the drop itself
and by the particles of solids carried in suspen-
sion.

If we refer again to Tables V and VI,
and consider them.as illustrating two possible
orchard situations with regard to spray material
and surfaces to be covered, it becomes apparent
that velocity cannot be depended upon to increase
spreading of all materials on every type of sur-

face.

msuieal £522.91 2: m 13.21212!
91 £1; Resistapge

The following table shows that large
drops of a liquid.are retarded less in their mo-
tion through the air than are small drops.

 

 

30

The first column gives the character-

istic diameter of drops, and the second column

gives the rate of fall under the pull of gravity,

after falling from a sufficient height to attain

a constant velocity. (Both values are in centi-

meters.)

Taplg fl; (Humphrey, ”Physics of the Air,“ 1925’.

page 268)

2221.112; Em
Fog .001 on
list .01

_ Drizzle ' .02

l‘ Light Rain .045
Itoderate Rain .1
Heavy Rain . .15
.hcessive Rain .21
Cloudburst .5

25

75
200
400
600
600
700

Inseam mm

.5 cm/sec

This table was utilized to determine the

true striking velocity of the drops involved in

Table IV .

 

 

31

Instead of the formula VI fig

we have V: Y2—gh C. SQTEE- . 93.0:ng 540 flaw“)

which is the true striking velocity, having elimi-
nated the retarding effect of the air. The con-
stant c varies for each size of drop, and is found
by dividing the acceleration of gravity (g) by

the velocity given in the table above. Thus, for
excessive rain, c8 %%%.m 1,53

Applying the formula to this size drop,
falling from heights of 4, 25, and 81 cm.respective-
1y we found that the striking velocities as com-
puted by the formula “Ts—g3 are off by 77.. in

and 212, respectively. These corrections were

1

then applied to the data given in Table iv to see

 

if there were a constant increase in area as the

 

 

velocity is increased. In other words, does
doubling the velocity double the area? It was
found in this case (lime sulfur on glass), that
the increase is far short of that amount. And
since this represented only a special case, both

as regards liquid and surface, the proportionality

 

32

factor would have no significance, and is therefore

omitted from this report.

it is quite evident from the foregoing
discussion that, in the case of drops within the
size range ordinarilyapplied, the actual strik-
ing velocity would not be such as to have any
appreciable influence on the tendency to spread,
for it should be borne in mind that the smaller
drops suffer greater retardation than the larger
ones.

3. Relation 9; SE 93; D39}; _t_9_ Spreadigg

lore time was devoted to this portion
of the investigation than to either of the pre-
ceding studies, first, because of the extreme
difficulty in the technique involved, second,
because of the large number of'measurements ne-
cessary to make the data conclusive, and third,
because the “size factor' gave greater promise
of throwing light upon the problems of spraying,
since it is subJect to the greatest variation of

any of the physical factors and at the same time

 

 

 

 

33

is quite largely under the control of the man
who Operates the sprayer.

Ihenever an attempt is made to break
a liquid into small drops by forcing it thru
a capillary tube, a limiting size is very soon
reached. This is to be expected from the very
nature of the orifice. If the tube is of large
diameter the curvature of the sides is slight,
but as tubes of smaller diameter are used, the
curvature increases with a resultant increase in
the effect of surface tension in maintaining
rigidity in the issuing column of liquid. (For
the same reason, a drop of small diameter is more
stable than one of large diameter). Very small
tubes, then, emit a continuous stream.or a series
of drops at such frequent intervals that they
are difficult to manage, since we must confine
our measurements to areas over which the indi-
vidual drops spread.

The device shown in Figure 2 was found

suitable for fairly large drops, that is, drops

 

 

 

 

 

34

down to approximately .008 of a cubic centimeter.
Such a drop has a diameter of about .25 cm.

Briefly the method of using this appa-
ratus was as followsz-oThe burette was filled.
and the drops allowed to fall into a beaker while
a count was being taken. The telescope was ad-
Justed, and at the instant that the surface of
the column reached a given graduation on the burette,
a helper gave the signal. Drops were then counted
until a second signal denoted that one cc of
liquid had flowed thru the tube. Usually three
checks were made of the number of drops and these
seldom.varied by more than one or two drops per cc.
Then, with drops falling at the same rate as when
the count was made and with the tip of the capil-
lary tube very near the glass plate, individual
drop areas were produced by moving the plate
horizontally on the table. When a sufficient
number of these had been obtained the spray ma-
terial was removed from the burette, the tube was

cleaned and the tip broken off to secure an opening

 

 

35

of different diameter. The tip of the tube was
paraffined as before and the counting and plat-
ing process repeated. in each case the same
distance between tip of capillary tube and
glass plate was.maintained to eliminate the
velocity faster.

Table VIII gives the results of
three such drop measurements. lbntion should
be made of the fact that in all of these area
determinations only circular areas were con-
sidered. If a drop showed a tendency to spread
nmre in one direction than another it was assumed
that that part of the surface was not perfectly
clean. In every case, the area shown in the
table is an average of at least twenty individual

drop areas.

1

 

 

36

Table VIII (Showipg Relation Between Size g§_

Rap and Area 9; Spaced.) Drops were formed
by use of capillary tube.

No. of Drops Average Diameter Area (in cm2)covered
per cc of Drops (cm) by lcc of solution
34 .253 77.28 ‘
111 .258 79.92
120 .251 81.12

it will be observed from.this table
that there is an increase in area covered by
a given volume, if the material is applied in
smaller drops.

Previous mention has been made of
the impossibility of duplicating the finer
spray drops with a capillary tube. Those Just
described are of the magnitude of excessive rain.
For finer particles of spray materials an en-
tirely different method of transfer was necessary.
Figure 3 shows the tools used in producing

smaller individual drops.

 

 

37

     

 

   

 

   

Fig. 3 Tips of Knitting Needles were ground
differently, thus giving each a
different liquids-holding capacity.

By means of the knitting needles
shown in Figure 3, transfer of droplets of

varying size was made possible. licroscope

 

 

 

 

38

slides of the best glass obtainable were used.
These were cleaned thoroughly as in previous
tests and the spray material plated as follows.
A.small amount of the lime sulfur solution was
poured out on a clean glass plate and the tip
of one of the needles inserted. Upon lifting
the needle, a certain amount of the material
clung to the tip in a drop. The slide was
touched lightly, thus releasing part of the ma-
terial and the operation repeated. .As many
areas were placed on one slide as possible, with-
out overlapping. Three such slides were pre-
pared with each needle in order to eliminate

error in determining the average drop area.

 

Figure 4 shows a series of six slides.

 

The one at the extreme right contains 500

drop areas and was made by using the needle in
the extreme right of Figure 3. The average
area covered by these individual drops was

.0048 square centimeters.

 

39

""-'°'-I-==s-:--.. . .. ..
b. .0. 00 .23.: «E: ' :1? ::§; .5: ,
.ieee3: .0....: 3 . ,3

.'. .. 6.:e.

1'. . 0: . v. .e.:: .. :O'.’ 0 'a- e s‘e'. A :'.'.'-

p0:

 

Figure 4. Slides showing Drops Transferred by
Needles Pictured in Figure 3.
The task of determining the number of
drops per co in the tests Just mentioned in-
volved very careful weighing. The slide was

weighed empty, and again after the drops had

 

4O

dried thoroughly. This difference divided by
the number of drops on the slide gave the dry
weight of one drop of material. (An average
weight was taken from.the three slides) Then
by carefully measuring, drying and weighing one
so of the solution, and dividing this weight by
the dry weight of one drop, the number of drops

per cc was determined.

Table g; (Showing Relation Between Size g£_Drop

agg.Argalgg,Sprgag). Drops were formed by
use of knitting needles.

No. of Drops Average Diameter Area (in cm?)

per cc. of Dro Areas covered by lcc of
(cm. solution

550 .451 ' 37.9

710 .416 96.6

1015 .394 125.si///
1220 .540 100.7

1555 .306 115. V///
2060 .269 107.6‘
2250 .250 109.5
3500 ,.231 138.3 V///
6600 .156 126.2
47100 .075 228.

In the curve, accompanying this table,

the values obtained from.three of the above

 

41

 

 

 

 

 

 

720a

' .

has

Has

(30

 

L1;

 

. , _ . .
i .

A --.-o—-«>— —- o—~e—o-————A-(
. . ,

.Iud tesl

 

 

21.-
1

6
O
_ ._ -..I+.._-._

7 0| CID]-

 

 

 

1:619.i .i

 

 

t1 wit) .lvl’

 

 

e... r.

 

 

 

 

. - - 0 ~ 4
>- ~~* s-v‘ “)"_“—T‘_"—“ v“ "4 -.__,-.
. , . 1
I I

 

 

 

 

 

 

 

 

 

 

 

fluence of velocity and size of drop is of

 

41
tests were omitted (viz. 3rd, 5th, and 8th). Even
though every precaution was taken to eliminate
chances of error, it appears that there must
have been some slip in those three values, since
the others follow in progression as indicated
by the curve.

From.these data it is evident that the
total area over which a given volume of liquid
spreads varies inversely with the size of the
drops into which it is broken. This relation
of total spread to size of drop is such that
the increase in area gradually diminishes as
we continue to decrease the size of drops. In
other words, if 1 cc of a liquid is broken up
into 500 drops, another cc into 1,000 drops and
,a third cc into 2,000 drops, allowed to fall
on uniform.surfaces and total area in each case
measured, a greater increase will be noted be-
tween the first and second cases than between
the second and third.

Any attempt to correlate the in-
fluence of velocity and size of drop is of

 

 

42

little avail because of the extreme variation
in surfaces and the multiplicity of factors in-
volved. in fact, such a correlation is impossible
except for one particular set of conditions, and
for that reason impracticable in its relation
to the problems of spraying.
Some of the drops used in this piece
of work were larger than what we consider as
making up a mist, although the smallest size*
would approximate mist particles. But there
may be as.many as several hundred thousand mist
particles in a single cc of solution and the to-
tal area covered under such conditions would un-
doubtedly be in excess of the areas here determined.
However, if mist particles are to be applied, the
velocity which may be imparted to them is quite
lhnited. ‘We cannot therefore, control both the
size of a drop and its velocity. 30 we must be
content with some sort of a compromise.
Furthermore, the drops delivered by
any nozzle will vary in size. it might even

be said that no two drops are likely to be of

 

 

43

exactly the same size.~ Drops in the center of
the ”cone" are generally larger than those
around the outside. The effect of atmospheric
resistance will therefore be felt more by some
than by others. This is the reason for some of
the material falling within a few inches of the
nozzle, while other particles may travel twenty
or thirty feet, even though the velocity at the
orifice is identical. For the reasons Just men—
'tioned we can never hope to control both size
of drop and velocity because if we apply drops
of the most desirable size their velocity can
have little or no effect upon their tendency to

spread.

Discugsigg
What is the application of the fore-

going to orchard practice? Four points stand
out quite clearly in the data presented.
First, spreading of any given spray

material depends upon its composition, because

 

44

composition determines the extent to which it

is adsorbed by the surface. And without ad-
sorption neither wetting nor spreading will occur.
incidentally a so-called I'spreader" must be
specific in its action, and its use will prove
beneficial to spreading on one particular type

of surface to the extent that it is adsorbed by
that surface. The matter of composition is a
problem for the manufacturer rather than for the
grower.

Second, velocity of spray drops is
something which should be considered from.the
standpoint of distributing the material through
the tree top rather than increasing its spread
over the surface of the leaf. it is obvious that
the velocity at any appreciable distance from
the nozzle must depend more upon the size of
the drop than upon the initial velocity imparted
by the spray outfit, unless the actual carrying
of the material is accomplished by a draft of

811' .

 

45

Third, the drops must be large enough
so that their initial velocity will carry than
throughout the range to be covered in the tree,
while from.the standpoint of maximum.area to be
covered, they should be as small as possible. in
orchard practice this means a compromise. The
spray gun offers such a compromise since it can be
instantly adJusted to throw fairly uniform drops
first of one size for close range spraying, and
then of a larger size for the higher parts of the
tree. Theoretically, the ideal outfit from the
standpoint of thorough covering, is one which
delivers a mist-like spray, always releasing it
near the surface to be covered so as to secure
suitable working velocity. The long spray rod
with a fine nozzle is an attempt to attain the
ideal in that regard.

Fourth, the éincipal advantage to
be gained from.using a high pressure, power out-
fit lies in its greater capacity or its ability

to deliver into the tree in the proper form.more

 

 

46

gallons of material per minute. If such an out-
fit is utilized to deliver larger drops,at the
same velocity, coverage per unit of spray ma-
terial is decreased, and if larger drops are
delivered at higher velocity, spray inJury re-
sults. On the other hand, if the material is
broken. into extremely small drops poorer distri-
bution through the tree tops is likely to occur.
The principal advantage, then, lies in its capacity
in terms of gallons per minute. Andunless this
advantage is capitalized by properly organizing
the spraying operation, the investment is not

netting the greatest possible return.

 

 

4?

§2EEE£I.

l. in general, a spray solution
having a low surface tension will be more likely
to spread than one whose surface tension is high,

for two reasons,--first because of its greater

 

tendency to flatten out and second, because such
a solution can be more easily broken up into small
drops. ‘But the surface tension of a liquid is
not a measure of the spreading tendency of that
liquid on all surfaces. Rather, it is one of
the factors and is probably less influential
than the interfacial tension.

2. 0f greater importance to the or-
chardist, than the factors mentioned above,
is that the spray material be adsorbed by the
surface. .Adsorption is apparently strongest
between substances of like constitution. If then,

we can secure a solution, similar in its chemi-

 

cal make-up to that of the surface upon which
it is to be applied, spreading should be greatly

enchanced.

 

 

 

3. Velocity seems to exert an in-
fluence on spreading if the surface is one for
'which the spray material has an attraction. In
other words, if the'material is adsorbed by the
surface, an added velocity will enhance spread-
ing by aiding it in penetrating the surface layer
which consists of a film of air or certain hm-
purities.v This penetration probably brings
about a more intimate contact between liquid and
solid, thus partially replacing the previously
adsorbed material. Adsorption, then, is apparently
a factor of greater fundamental importance than
is velocity. it is doubtful if greater velocities
are desirable in the orchard except as pointed
out in the previous discussion.

4. The area covered by a given volume
of spray material varies inversely with the size
of the drops applied. The data pertaining to
size of drops offers a strong argument in favor

of a mist spray.

 

 

 

 

49

LITERATURE CITED

 

 

Bancroft, W. D. “Applied Colloid Chemistry,"
Chap. 2, New York, 1921. (Data on Adsorption)

Chappaz, 6. Ice bouillies mouillantes. in Prog.
Agr. Algeria et Tunise: 19:187-l9l.l913.

Cooper, W. F. and Nuttall, W. H. The Theory of
Wetting and the Determination of the Wet-
ting Power of Dipping and Spraying Fluids
Containing a Soap Basis. In Jour. Agr.

Sci. 7:2, 219-239. 1915.

Fairchild, D. G. Experiments with Fungicides to
Prevent Leaf Blight of Nursery Stock. dour
Wool. 7:4, 338-353. 1894.

Galloway, B. T. Report on Experiments lads in
1891 in the Treatment of Plant Diseases. U. 8.
Dept. of Agr., Div. Veg. Path. Bul. 3, 1891.

Gastine, G. Sur l' emploi des Saponines pour la
preparation des emulsions insecticides et
des liqueurs do traitements insecticides et

anticryptogamiques. Compt. Rend. Acad. Sci.,
152: 532-534. l9l1.

 

30

Harkins, I. D., Brown, F. E., and Davies, E.

C. H. The Structure of the Surfaces of
Liquids, and Solubility as Related to the
Iork Done by the Attraction of Two Liquid
Surfaces as they Approach Each Other. Jour
Amer. Chem. Soc., 39: 3, 354-364. 1917.

Harkins, W. D., Davies, E. C. 11., and Clark, C. L.
The Orientation of Molecules in the Surface
of Liquids, the Energy Relations at Surfaces,
Solubility, Adsorption, h‘mulsification, Hole-
cular Association, and the lffect of Acids
and Bases on interfacial Tension. In Jour.
Amer. Chem. Soc., 39: 4, 541-596. 1917.

Humphreys, C. F. 'Physics of the Air“. 268,
Philadelphia, 1921. (Precipitation Values
for Rain).

Jacobson, C. A. Alfalfa Saponin. Alfalfa in-
vestigation VII. In J‘our. Amer. Chem.

Soc., 41: 4, 540-648. 1919.

Iangmuir, I. The Constitution and Fundamental
Properties of Solids and Liquids. 11 Li-
quids. 1n J'our. Amer. Chem. Soc., 39: 9,
1848-1906. 1917.

 

51

Lees, A. 11. Winter Cover Washes. in Ann. App.
13101.. 1: 3-4, 351-364. 1915.

Lovett,. A. L. Spreaders for Arsenate Sprays. 1n
Jour. Econ. Ent., ll: 1, 66-69. 1918.

Lowe, V. R. Combating the Cottonwood Leaf Beetle.
In. l. I. Agr. Exp. Sta. 15th Ann. Rept.
543-544. 1896.

llausier, -- Principles d'apres lesquels doivent
etre composes lee insecticides. In Bul. Soc.
Dee Agr., France, 63: 86-93. 1908.

lillardet and Davis. Resultats do divers procedes
de traitement sur le developpement du mildiou.
In Jour. Agr. Pratique, 2: 50,761-770. 1886.

Iocre, I. Spreading and Adherence of Arsenical
Sprays. in Tech. Bul. Ho. 2 Iinn. Agr. Exp.
Ste. 1921.

Quincke, G. Ueber den Randwinkel und die Ausbreitung
von Flussigkeiten auf festen Kdrpern'. In ;
Wisdemann's Annalen der Physik und Chemie,
neue Folge, Band 11, Do. 10, 145-194. 1877.

Ruth, I. A. and Kelley, V. W. Recent Advances
in Spraying. Paper Presented Before illinois
State Horticultural Soc. 1922.

 

52 ~ J

Smith, Loren B. Relationship Between théiettling
Power and Efficiency of Nicotine-Sulphate ‘
and Fish-oil Soap Sprays. in dour. Agr.
Res., 7: 9, 389-399. 1916.

Vermorel, v., and Dantony, E. Tension superfi-
cielle et pouvoir mouillant des insecti-
cides et fongicides. lloyen do rendre mouill-
antes toutes les bouillies cupriques cu
insecticides. 1n Comp. Rend. Acad. Sci.,
154: 20, 1300-1302. 1912.

Washburn, F. L. Practical Work with the Codling
lloth and with a Combined insecticide and
Fungicide. Ore. Agr. Exp. Sta. Bul. 10 1891.

Willows, R. S. and Hatschek, E. Surface Tension
and Surface Energy and Their influence on
Chemical Phenomena. J. and A. Churchill,
London. 1915.

Woodman, R. It. The Physics of Spray Liquids.

In Journ. of Pom. and Hort. Sci. 4: 1,
38-58. July 1924.

 

 

 

 

 

 

 

 

MICHIGAN SFATE UNIVERSHY lilJHARIES

 

ILIHHHH Hm

1293 02604 7427