A new droplet generator is described. A loud speaker driven extractor needle was immersed in a pendant drop. Pulsing the speaker extracted the needle forming a fluid ligament which will decay into a droplet. The droplets were sized by stroboscopic photographs. The droplet's size was changed by varying the amplitude of the speaker pulses and the extractor needle diameter. The mechanism of droplet formation is discussed and photographs of ligament decay are presented. The droplet generator worked well on both oil and water based pesticide formulations. Current applications and results are discussed

Slack, W. E.

English

NASA Technical Reports Server

A New Drop le t  Generator  
W.E. Slack  
Nat iona l  Aeronau t ica l  Es tab l i shment  
Na t iona l  Research Counci l  of  Canada 
Ottawa, Canada, K I A  OR6 
A new d r o p l e t  g e n e r a t o r  developed f o r  l a b o r a t o r y  u s e  a t  NAE is d e s c r i b e d .  A loud speaker  
d r i v e n  e x t r a c t o r  need le  is  immersed i n  a pendant  drop. Puls i r ig  t h e  speaker  e x t r a c t s  t h e  
need le  forming a f l u i d  l igament  which w i l l  decay i n t o  s d r o p l e t .  The d r o p l e t s  a r e  s i z e d  by 
a t r o b o s c o p i c  photographs.  The d r o p l e t ' s  s i z e  can  be changed by varying t h e  ampli tude o f  t h e  
speaker  p u l s e s  and t h e  e x t r a c t o r  n e e d l e  d iamete r .  The mechanism o f  d r o p l e t  fo rmat ion  is 
d i s c u s s e d  and photographs oE l igament  decay a r e  p resen ted .  The d r o p l e t  g e n e r a t o r  works w e l l  
on bo th  o i l  and wate r  based p e s t i c i d e  fo rmula t ions .  Cur ren t  a p p l i c a t i o n s  and r e s u l t s  a r e  
d i s c u s s e d .  
I n t r o d u c t i o n  
The NAE Droplet  Generator  is  a d e v i c e  des igned  t o  produce and p h o t o g r a p h i c a l l y  r e c o r d  f o r  
subsequent  s i z i n g  d i s c r e t e  d r o p l e t s  i n  a l a b o r a t o r y  environment.  I t  is a l s o  a u s e f u l  
ins t rument  f o r  obse rv ing  t h e  breakup o r  c o l l a p s e  o f  f l u i d  l igaments .  
D e s c r i 2 t i o n  of  t h e  Drop le t  Generator  
-- 
The d r o p l e t  g e n e r a t o r  c o n s i s t s  o f  a pendant  drop g e n e r a t o r ,  a d r o p l e t  e x t r a c t o r ,  a photo- 
g r a p h i c  r e c o r d i n g  system and a n  e l e c t r o n i c  c o n t r o l  sys tem (F ig .  1). 
Pendant Drop Generator  
The source  o f  f l u i d  f o r  t h e  d r o p l e t s  is a pendant drop. A s y r i n g e  pump d r i v e s  a hypo- 
dermic s y r i n g e  t o  supply f l u i d  t o  a c e n t r a l  p o r t  i n  a 6 . 3 5  millimeter (mm) diamete r  hor izon-  
t a l  d i s c .  I t  c o l l e c t s  on t h i s  d i s c  t o  form a pendant  drop.  The pendant  d rop  i s  i n  t h e  
o r d e r  o f  l o 6  t imes t h e  s i z e  of  t h e  d r o p l e t s  produced and t h e r e f o r e  t h e  s : ~ r i n g e  pump does  n o t  
need t o  b e  o p e r a t e d  whi le  g e n e r a t i n g  d r o p l e t s .  
Drop le t  E x t r a c t o r  
The d r o p l e t  e x t r a c t o r  c o n s i s t s  o f  a h o r i z o n t a l  e x t r a c t o r  need le  which i s  connected by a 
d r i v e  s h a f t  t o  t h e  cone o f  an aud io  speaker .  The need le  d r i v e  s h a f t  is  suppor ted  i n  c bear-  
i n g  t o  main ta in  c o n s i s t e n t  need le  t r a c k i n g .  By c o n t r o l l i n g  t h e  p o l a r i t y  and ampli tude of  
e l e c t r i c a l  p u l s e s  t o  t h e  s p e a k e r ,  t h e  t i p  o f  t h e  e x t r a c t o r  need le  can be d r i v e n  i n t o  o r  
p u l l e d  o u t  o f  t h e  pendant drop.  Any aud io  speaker  capab le  of  t a k i n g  con t inuous  d i r e c t  
c u r r e n t  Loads and producing l a r g e  cone d e f l e c t i o n s  cou ld  be  used. The speaker  c u r r e n t l y  
being used is a 25 w a t t  8  inch  d iamete r  speaker .  With p o s i t i v e  and n e g a t i v e  p u l s e s  of  8  
v o l t s  ( 6 . 5  w a t t s ) ,  i t  produces an e x t r a c t o r  need le  movement o f  r4 .23 mm (F ig .  2 ) .  
Power f o r  t h e  speaker  is s u p p l i e d  by a d u a l  v a r i a b l e  v o l t a g e  power supply.  One h a l f  of  
t h e  power supply p r o v i a e s  t h e  p o s i t i v e  p u l s e s  t o  immerse t h e  n e e d l e  i n  t h e  pendant d rop  
w h i l e  t h e  o t h e r  h a l f  p rov ides  t h e  n e g a t i v e  p u l s e s  t o  e x t r a c t  t h e  need le .  A convnnt iona l  
mul t imete r  is used t o  s e t  t h e  ampli tude of  t h e  p u l s e s  a c c u r a t e l y .  
The e x t r a c t o r  need les  a r e  removable from t h e  need le  d r i v e  s h a f t  and need les  of  v a r i o u s  
d i a m e t e r s ,  r ang ing  i n  s i z e  from 9 8  microns (pm) t o  700pm, have been used. 
Photographic  Recording System 
The d r o p l e t  ge' :era+?r i n c l u d e s  a pho tograph ic  record ing  system t o  r e c o r d  pho tograph ic  
images o f  t h e  d r o p l e t s  f o r  subsequent  s i z i n g .  The camera be ing  used w i t h  t h e  d r o p l e t  gener-  
a t o r  i s  a conven t iona l  35 mm Pentax Spotmat ic  w i t h  a motor d r i v e  f i l m  advance and a remotely  
o p e r a t e d  s h u t t e r  a t t achment .  S i n c e  t h e  d r o p l e t s  be ing  photographed a r e  s m a l l ,  c o n s i d e r a b l e  
phc tograph ic  m a g n i f i c a t i o n  is r e q u i r e d  b e f o r e  they  can be a c c u r a t e l y  s i z e d .  T h i s  r e q u i r e -  
ment is m e t  by us ing  a 38 mm microscope o b j e c t i v e  l e n s  i n  l i e u  o f  t h e  conven t iona l  photo- 
o p t i c s  and it i s  mounted on a n  a p p r o p r i a t e  e x t e n d e r  tube  t o  a c h i e v e  a m a g n i f i c a t i o n  o f  10. 
The format ion o f  a d r o p l e t  i s  a dynamic e v e n t  and s h o r t  photo exposures  a r e  r e q u i r e d  t o  
o b t a i n  s h a r p  d r o p l e t  images. I n  a d d i t i o n  t h e r e  is a requirement  f o r  a h i g h  l i g h t  l e v e l  due 
t o  t h e  ex tender  tube .  A s t r o b o t a c  and a s t r o b o s l a t e ,  o p e r a t i n g  i n  a s i n g l e  f l a s h  mode have 
been adequate .  The s t r o b e s  a r e  a l i g n e d  t o  bounce l i g h t  o f f  a w h i t e  backdrop and p rov ide  a 
3  microsecond (psec )  exposure .  
Drop le t  Generator  C o n t 7 o l l e r  
The c o n t r o l l e r  was developed s p e c i f i c a l l y  f o r  t h e  NAE Drop le t  Generator  and i ts  f u n c t i o n  
is t o  c o n t r o l  and sequence a l l  t h e  e v e n t s  a s s o c i a t e d  w i t h  g e n e r a t i n g  and photo-recording a 
d r o p l e t  (F ig .  3 ) .  I t  can be o p e r a t e d  i n  a " s i n g l e  d r o p l e t "  mode o r  i n  a "cont inuous"  mode 
https://ntrs.nasa.gov/search.jsp?R=19820015570 2020-03-21T16:57:58+00:00Z
g e n e r a t i n g  d r o p l e t s  a t  v a r i o u s  p r e s e t  r a t e s  o f  up t o  3 d r o p l e t *  p e r  mecond. The p o r t i o n  o f  
t h e  c o n t r o l l e r  which o p e r a t e s  t h e  s t r o b e  l i g h t s  c o n t a i n s  an a d j u o t a b l e  d i g i t a l  d e l a y  timer 
(1000 50 usec s t e p s )  t o  o p e r a t e  t h e  l i g h t s  a t  t h e  a p p r o p r i a t e  i n s t a n t .  
Operat ion o f  t h e  NAE Droplet  Generator  
Adjust ing t h e  E x t r a c t o r  Needle P o s i t i o n  
The c r i t e r i o n  used t o  set t h e  e x t r a c t o r  need le  p o s i t i o n  r e l a t i v e  t o  t h e  pendant drop is 
an  a r b i t r a r y  one b u t  one which has  provided c o n s i s t e n t  r e e u l t a .  The need le  is  p o s i t i o n e d  
- v e r t i c a l l y  1 .0  rrn below t h e  pendant d rop  base  s o  t h a t  it w i l l  e n t e r  t h e  r e l a t i v b l y  f l a t  
s i d e  o f  t h e  drop. I t  i s  p o s i t i o n e d  l a t e r a l l y  s o  t h a t  t h e  t i p  o f  t h e  need le  w i l l  be c e n t e r e d  
i n  t h e  pendant drop when a f u l l  ampli tude (8  v o l t )  "IN" p u l s e  is a p p l i e d  t o  t h e  speaker .  
Generat ing Droplets  
For any given e x t r a c t o r  needle  a range of  d r o p l e t  s i z e s  may be genera ted  by vary ing  t h e  
ampli tude of t h e  "IN" and "OUT" p u l s e s  t o  t h e  speaker .  The range o f  d r o p l e t  s i z e s  (Fig.  4 )  
which can be expected is 
Dn/3 Dd Dn 
where Dd - diameter  o f  t h e  d r o p l e t  
Dn - diameter  of t h e  e x t r a c t o r  need le  
The t i m e  r equ i red  t o  form a d r o p l e t  v a r i e s  cons iderab ly  with  changes i n  t h e  "IN" and 
"OUT" p u l s e  ampli tudes  and t h e  de lay  t i m e r  must be  set t o  photo record  a t  t h e  a p p r o p r i a t e  
t ime . 
Aqpl ica t ions  o f  t h e  Droplet  Generator  
The NAE Droplet  Generator was developed s p e c i f i c a l l y  f o r  b i o l o g i c a l  a p p l i c a t i o n s .  I t  is 
being used a s  s c a l i b r a t i o n  t o o l  i n  a s s e s s i n g  a e r i a l  sp ray  d e p o s i t s  and t o  t r e a t  i n s e c t  
l a r v a e  wi th  s p e c i f i c  dosages o f  p e s t i c i d e .  The d r o p l e t  g e n e r a t o r  a l s o  appears  t o  be s u i t a b l e  
f o r  s t u d i e s  on l i q u i d  l igament breakup o r  c o l l a p s e .  
Spread Fac tor  C a l i b r a t i o n s  
For a e r i a l  sp ray ing ,  i t  is customary t o  p l a c e  sampling c a r d s  i n  t h e  a r e a  t o  c o l l e c t  a 
sample o f  t h e  p e s t i c i d e .  The c a r d s  r e t a i n  t h e  s t a i n s  produced by t h e  d r o p l e t s  and f o r  nub- 
sequent  p e s t i c i d e  account ing ii: is  necessary t o  know t h e  re la t ionc ih ip  between t h e  s i z e  o f  
t h e  s t a i n  and t h e  d r o p l e t  t h a t  produced it. The d r o p l e t  g e n e r a t o r  is used t o  produce and 
s i z e  d r o p l e t s  o f  t h e  a p ~ r o p r i a t e  p e s t i c i d e .  The d r o p l e t s  a r e  c o l l e c t e d  on sampling c a r d s  
and t h e  s t a i n s  a r e  s i z e d  by t h e  N A E  F ly ing  Spot Scanner Analyzer.  The computer o u t p u t s  f o r  
t h e  d r o p l e t  and s t a i n  s i z e s  a r e  combined t o  produce a mathematical r e l a t i o n s h i p  between t h e  
s i z e  o f  a d r o ~ l e t  and t h e  s i z e  o f  t h e  s t a i n  it w i l l  produce on a p a r t i c u l a r  type  of sampling 
s u r f a c e .  The mathematical r e l a t i o n s h i p s  developed a r e  t y p i c a l l y  
Dd = C + bDs  
where Dd - d r o p l e t  diameter  (urn) 
Ds - s t a i n  diameter  (urn) 
C - a cor is tant  
b - t h e  s l o p e  o f  t h e  curve 
The s l o p e  confidence a t  t h e  90% i n t e r v a l  is t y p i c a l l y  iO.38 
Dispensing Minute Q u a n t i t i e s  o f  F l u i d  
The d r o p l e t  g e n e r a t o r  can be used a s  a t o o l  t o  d i spense  minute q u a n t i t i e s  o f  f l u i d  ( i .e .  
i n  t h e  o r d e r  o f  10" m i c r o l i t r e s ) .  I t  has  been used t o  t r e a t  sp rvce  budworm l a r v a e  w i t h  
s p e c i f i c  dosages o f  p e s t i c i d e  t o  determine r e q u i r e d  spray  parameters  f o r  adequate  i n s e c t  
c o n t r o l .  
Studying Ligament Col lapse  
F igures  5,  6 and 7 a r e  a s e r i e s  o f  p h o t o g r a ~ h s  showing t h e  Tomat ion  o f  d r o p l e t s .  The 
process  o f  forminq a ligmament and its c o l l a p s e  t o  form a d r o p l e t  i e  extremely c o n s i s t e n t .  
These photographic  sct r ies  a r e  n o t  high speed movies: they a r e ,  i n  each came, 20 photographs 
o f  20 d i f f e r e n t  d r o p l e t s .  In each c a s e  t h e  photo exposure has  been delayed by t h e  time 
shown on each photograph. 
F igures  5 and 7 show t h a t  wi th  d i f f e r e n t  combinations of "IN" ~ n d  "OUT" p u l s e  ampli tudes  
t h e  l igament may break f r e e  a t  one end f i r s t .  I n  F igure  5 it breaks  f r ~ r  from t h e  need le  
f i r s t  and t h e  needle  end o f  t h e  l igament  s t a r t s  c o l l a p s i n g  t o  form t h e  d r o p l e t .  T h i s  
p o r t i o n  o f  t h e  l igament s t a r t s  moving towards t h e  pendant drop. When t h e  pendant drop end 
o f  t h e  l igament  f i n a l l y  b reaks  f r e e ,  i t  t o o  s t a r t s  c o l l a p s i n g  t o  form a d r o p l e t  and t h i s  
p o r t i o n  o f  t h e  l igament moves away from t h e  pendant drop. S i n c e  t h e  need le  end o f  t h e  
l igament  s t a r t e d  forming a d r o p l e t  f i r s t ,  its mass i s  g r e a t e r  than  t h e  o t h e r  p o r t i o n  o f  t h e  
l igament and t h e  r e s u l t i n g  dro. ! e t  has a h o r i z o n t a l  v e l o c i t y  towards t h e  pendant d r o  . 
Figure  7 shows t h e  l igament b reak ing  f r e e  from t h e  pendant drop f i r a t  and t h e  reau l tTng  
d r o p l e t  is given a h o r i z o n t a l  v e l o c i t y  away from t h e  pendant d r o p l e t .  
Figure  5 a l s o  shows t h a t  i f  t h e  d r o p l e t  c o n t a c t s  t h e  pendant drop,  both t h e  d:,>plet and 
t h e  pendant drop d i s t o r t  b u t  t h e  d r o p l e t  w i l l  bounce f r e e  from t h e  e l a s t i c  pendant d rop  
s u r f a c e .  F igure  7 is an example of t h e  d r o p l e t  having s u f f i c i e n t  h o r i z o n t a l  v e l o c i t y  t o  
over take  t h e  e x t r a c t o r  needle .  Although t h e  e x t r a c t o r  needle  is covered w i t h  f l u i d  it can- 
no t  d i s t o r t  a s  t h e  pendant drop d i d  and t h e  d r o p l e t  is "captured" by t h e  need le .  
F igure  6 shows ano ther  phenomenon which is e v i d e n t ,  t o  lesser degreea,  i n  F igures  5 and 7; 
namely, t h e  i n s t a b i l i t y  o f  a c y l i n d r i c a l  l i q u i d  j e t  o r  l igament.  Xn F igures  5 and 7 t l i e re  is 
evidence t h a t  t h e  l igament was t r y i n g  t o  break up: however, each l igament  d i d  c o l l a p s e  i n t o  
a d i s c r e t e  d r o p l e t  b e f o r e  t h e  break up could occur .  I n  F igure  6 t h e  s e p a r a t i o n  of t h e  l i g a -  
ment was delayed and t h e  l igament d i d  break up before  it could c o l l a p s e  t o  form a s i n g l e  
d r o p l e t .  
These 3 f i q u r e s  show t h a t  varying t h e  amplitude of t h e  "IN" and "OUT" p u l s e s  v a r i e s  t h e  
d r o p l e t  format ion process  a s  w e l l  a s  t h e  d m p l e t  siz,'. 
R e s u l t s  w i t h  t h e  Droplet Generator 
Droplet  S i z e s  Produced 
The parameters  which hav? a s i g n i f i c a n t  e f f e c t  on t h e  s i z e  o f  t h e  d r o p l e t s  a r e :  
- The diameter  o f  t h e  e x t r a c t o r  need le  
- The ampli tude o f  t h e  "IN" p u l s e  t o  t h e  speaker  (i .e.  t h e  immersion 
dep th  of t h e  needle  i n  t h e  pendtn t  drop)  
- The ampli tude o f  t h e  "OUT" &ae t o  t h e  speaker  (i .e.  t h e  v e l o c i t y  
a t  which t h e  needle  is e x t r a c t e d  from t h e  pendant d r o p ) .  
The v i s c o s i t y  of t h e  f l u i d  h a s  very l i t t l e  e f f  -t on t h e  s i z e  o f  t h e  d r o p l e t s  produced. 
Csing e x t r a c t o r  need les  ranging from 98 urn t o  700 Dm has  produced d r o p l e t s  ranging i n  
s i z e  from 30 u m  t o  800 pm. 
Droplet  S i z e  Consis tency 
Cur ren t  a p p l i c a t i o n s  o f  t h e  NAE Droplet  Generator have r e q u i r e d  t h e  s i z i n g  of  d i s c r e t e  
d r c ~ l e t s  and t h e  cons i s tency  of t h e  d r o p l e t  s i z e s  has  been of  minor i n t e r e a t ,  Some tests 
have been c a r r i e d  o u t  t o  determine t h e  s i z e  cons i s tency  of  t h e  d r o p l e t s  and t y p i c a l  r e s u l t s  
a r c  l i s t e d  i n  Table 1. 
TABLE 1 
Mean drop diameter  
Standard d e v i a t i o n  1.20 um 
Accuracy of  Droplet  S i z i n g  
The source o f  e r r o r s  i n  s i z i n g  d r o p l e t s  and t h e i r  e s t i m a t e d  magnitude a r e  l i s t e d  i n  
Tabla 2.  
TABLE 2 
E r r o r a  i n  Drop1.e. Siz:.ng 
Source or E r r o r  
Photographic  magni f ica t ion  
Lens r e s o l u t i o n  
Film resoJ * , t i o n  
M e a s u r i n r  * =curacy t 0.1 urn 
Testa  t o  Getennine t h e  e r r o r  i . l \ . ~ ~  t o  t b e  s m  of t h e  l a s t  t h r e e  i t ems  i n  Table  2 i n d i c a t e  an 
error of s l i g h t l y  over  i 1. ; , r r ~ .  
F igure  8 shows t h e  o p t i c a l  i t ~ y o u t  f o r  t h e  d r o p l e t  g e n e r a t o r .  An e r r o r  i n  t h e  d i s t a n c e  
from t h e  l e n s  t o  t h e  d r o p l e t  w ~ u l d  induce a magni f ica t ion  e r r o r ;  h w a v e r  t h e  o p t i c a l  dep th  
of f i e l d  i s  very l i m i t e d  and d r o p l e t s  which a r e  o f f  t h e  f o c a l  p lane  s u f f i c i e n t l y  t o  produce 
a 0.38% e r r o r  c r e a t e  a very ba~:l'!y o u t  o f  focus image. I f  o u t  o f  focua d r o p l e t  images a r e  
d e l e t e d  then  t h e  magni f ica t ion  e r r o r  w i l l  be lesr than  0.38 percen t .  
ORIGINAL PAGE 
BLACK APED WHITE P H O m W  
The NAE Droplet Canerator i s  a device which will produce disescte droplets w i t h  f l u i d s  
of various viscosities (i .em heavy fuel oils or w a t e r ) .  It has been used to qenerate drop- 
l ~ t s  ranqj nq ln s ~ x e  from 30 I j r n  t o  800 wm. Images af t h e  droplets produr-d arc photoqrap!.- 
lcally recorJcd and can subsequcittlg be sized to * 1.5 urn. Once the qenerator h a s  been set 
to qcnerate a specific size 0 1  dro! ict kt will continue to pr~c luce  d r o p l e t s  with a s t a r ~ d a r d  
deviation of l ess  t h a r ~  1%. 
The consistency of t h v  drop le t  formation process also - ' the qenerator a s u i t a b l ~  
instrument f o r  okscrvinq f l u i d  1 igamcnt brcak up or collapse. 
Figure ?: N A E droplet  generator 
:taQ 
0 VOLT3 POSITIVE 
PENDANT DROP 5*SE APPLIED TO SPFAKER 
0.l5mm EXTRACTOR HEFDCL 
PENDANT DROP EITRbCTMI NEEDLE MOUNTED 
T SPEAKER 
B mfs KOARVE ~ I E D ~  
TO SPEAKER 
Figure  2: Pendant d r o p  and d rop le t  e x t r ~ c t o r  
T lME (msl 
AUTO START I 
PUSH BUTTQN 1 
I I 1 I I 
CAWERA I 1- CAMERA SHUTTER OP$N 500 ma VL 
1 
I .1345 1 I 
i I SPEAKE A I NEEDLE It. I 
CONTROL I + I 
I I 
1 I 
I 
I 
! I 1 399 I 
I I I 1 
I 
I I 3 us PULSE 4 I I I STROBE I I I I I 
LIGHTS 
Figure 3: Drop le t  generator t iming sequence, 
EXTRACTOR NEEDLE Dl AMEf ER I urn 
Figure 4: D rop le t  ~izes vs ex t rac to r  needfe d iameter  
f i g u r e  5: Ligmant collopse t o  f o rm  a droplet 
zoo  

Figure 7: Needle-captured d r o p ! e t  
FOCAL FILM 
PCn NT ( IMAGE 1 
b' PLANE 
38m LENS f 
24 r 36mm 
FILM 
FORMAT 


A new droplet generator

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