Spray Droplet Size Predictions Using Microfluidic Extensional Rheometry

Seher Ozkan, Surya Kamin and Sounak Sarkar

Ashland Specialty Ingredients, Corporate Research Center, Bridgewater, NJ

INTRODUCTION

Materials

CONCLUSIONS

RESULTS

Materials: 0.1% w/w aqueous solutions polyethylene oxide (PEO), hydroxypropyl (HP) guar, hydroxypropyl cellulose (HPC), polyvinyl pyrrolidone (PVP) and hydrophobically-modified polyacetal-polyether (HM-PAPE) copolymers.

Results show that Ca numbers calculated from extensional viscosity data show good correlation with mean droplet sizes while shear viscosity was not an effective parameter. Increasing molecular weight increases droplet size within the same polymer family with a constant surface tension. For different polymeric structures, the presence of hydrophobic functional groups and polymer flexibility contribute to droplet formation due to changes in the surface tension and extensional viscosity values at high shear rates.

Spraying low viscosity polymeric solutions is a common practice for many industrial applications such as food processing, cosmetics, pharmaceutical manufacturing, agricultural and coatings. Dilute solutions are exposed to a wide range of shear and extension rates associated with short relaxation times. Even a small amount of polymer can alter the character of capillary break-up during droplet formation due to uncoiling when it is placed in an extensional flow with a strain rate exceeding the slowest molecular relaxation. The existence of capillary, inertial, elastic and viscous effects on small length and time scales experienced during the spraying process complicates the prediction of spray droplet sizes using conventional rheological measurements. It is important to measure the extensional viscosities at relevant shear rates with appropriate sensitivity, avoiding gravitational and evaporation effects.

Methods A microfluidic device, with a hyperbolically-shaped contraction/expansion flow channel geometry (e-VROC) was used to measure apparent extensional viscosity indices to overcome the inertial effects and potential evaporation. Their shear viscosity values at high shear rates were measured using straight rectangular flow channel geometry (m-VROC). The static/ dynamic surface tension values were measured and Capillary numbers of the solutions (Ca) at the narrowest point of the spray nozzle were calculated

0

1

2

3

4

5

6

7

Re

du

ced

Vis

cosi

ty

reduced extensional viscosity at 7000s-1

reduced shear viscosity at 290000s-1

Force Balance at necking point of the filament

Fluid Rheology

Surface Tension

y = 0.0071x - 0.7452R² = 0.9369

00.20.40.60.8

11.21.41.61.8

2

290 310 330 350 370 390

Re

du

ced

ext

en

sio

nal

vis

c

Droplet size (D 0.5), micron

Correlation between droplet size and extensional viscosity for Guar derivatives

y = 0.0036x + 0.2978R² = 0.9271

0.960.98

11.021.041.061.08

1.11.12

190 200 210 220 230Re

du

ced

ext

en

sio

nal

vis

c

Droplet size (D 0.5), micron

Correlation between droplet size and extensional viscosity for Cellulosics of

increasing molecular weight

y = 0.0015x - 0.3053R² = 0.9819

0.00

0.05

0.10

0.15

0.20

0.25

0.30

290 310 330 350 370 390

Ca

Nu

mb

er

Droplet size (D 0.5), micron

Effect of functional group for cellulose and guar derivatives

y = -0.0007x + 0.3254R² = 0.9247

0.000.020.040.060.080.100.120.140.160.18

190 210 230 250 270 290

Ca

Nu

mb

er

Droplet size (D 0.5), micron

Effect of hydrophobic group for PAPE chemistry

y = 0.0035x + 0.3095R² = 0.7884

0

0.5

1

1.5

2

2.5

190 290 390 490 590Re

du

ced

ext

en

sio

nal

vis

c

Droplet size (D 0.5), micron

Correlation between droplet size and extensional viscosity across different

chemistries

y = 0.0003x + 0.1112R² = 0.7031

0.145

0.15

0.155

0.16

0.165

0.17

0.175

0.18

190 210 230 250Ca

Nu

mb

er

at 2

50

0 m

s

Droplet size (D 0.5), micron

Correlation between Droplet Size and Capillary number with dynamic surface

tension