ILASS-Americas 29th Annual Conference on Liquid Atomization and Spray Systems, Atlanta, GA, May 2017

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Observation of the Coating Process in Spray Coating

Jianchi Huang, Zhihao Yuan, Siyi Gao, Jianshan Liao, Morteza Eslamian1

University of Michigan-Shanghai Jiao Tong University Joint Institute

Shanghai, 200240, China

Abstract

Spray coating is a facile coating and deposition process with numerous existing and emerging applications. Howev-

er, it is a stochastic process comprising impingement of many droplets, which upon impact on a heated substrate

may dry individually or coalesce first to make a thin liquid film and then dry to make a thin solid film. There is very

limited knowledge on how this process occurs; therefore in this work, high speed imaging is used to visualize the

spray coating process. Two model solutions including food-dye with properties similar to those of water, and PE-

DOT:PSS, a polymeric solution, are sprayed onto glossy paper and regular glass substrates. Substrates are kept at

room temperature and elevated temperature of 80 °C. In some cases, a vertical ultrasonic vibration is imposed on the

substrate to study its effect on the coating process. In conclusion, it is observed that the spray coating process is

highly random and stochastic. A higher substrate temperature results in better coating process. Imposed vibration in

the case of glossy paper substrates results in better droplet spreading and a more uniform coating, whereas in the

case of glass substrate results in droplet “walking” on the substrate. Further systematic study is required to better

understand the process.

1 Corresponding author: Morteza.Eslamian@sjtu.edu.cn & Morteza.Eslamian@gmail.com

ILASS-Americas 29th Annual Conference on Liquid Atomization and Spray Systems, Atlanta, GA, May 2017

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Introduction

A spray is a dynamic collection of droplets gener-

ated by the fluid mechanics process of liquid atomiza-

tion, using an atomizer or spray nozzle. Atomization

and sprays is an important field of multiphase flow flu-

id mechanics, with theoretical and practical implica-

tions [1]. Sprays have ubiquitous presence in our daily

life and in numerous industrial applications, such as

spray cooling, internal combustion engines for fuel at-

omization, agricultural sprays, fire sprinkler systems,

spray painting, and more sophisticated and emerging

applications, such as deposition of thin films of func-

tional materials, using spray coating and spray pyrolysis

[2]. Some of the emerging applications of spray coating

are as follows: fabrication of emerging solution-

processed thin film devices, such as polymer [2-5],

quantum-dot [6] perovskite [7], chalcopyrite [8] and

kesterite [9] solar cells, as well as thin film transistors

[10], sensors and actuators [10], and various functional

layers, such as conducting thin films using graphene

[12-13], and other thin film devices [14].

Although spray deposition is a versatile, facile,

scalable and low-cost fabrication technique, it is a com-

plex and multistep fluid dynamics process, including

atomization of the precursor solution, droplet flight

from the nozzle tip to the substrate, droplet impact dy-

namics, spreading, coalescence, solvent evaporation,

and finally the formation of a coating or thin solid film

[1]. Therefore, due to the unsteady nature of sprays,

various methods have been employed to either generate

a more uniform and controllable spray or to control the

coating process. For instance, ultrasonic atomization is

an effective way to obtain a narrow droplet size distri-

bution [5], and electrostatic sprays produce sub-

micrometer mono-dispersed spray droplets for better

deposit formation [15]. Development of new multi-hole

nozzles, such as that in inkjet printing can be also used

to generate a uniform spray or streams of droplets [16].

To control and improve the deposition process after

spray droplet impingement, various optimization strate-

gies may be followed, such as using multiple spray

passes, controlling spray flow rate, and substrate tem-

perature, e.g. [4, 17, 18-20]. Recently, a method has

been developed in which ultrasonic vibration is im-

posed on the substrate to improve spreading, and to

obtain more uniform thin solid films [7, 13, 21-23].

Spray coating is essentially the process of impact,

splashing, spreading and coalescence of many droplets

on the substrates, and subsequent drying of individual

impinged droplets or a formed thin liquid film, to obtain

a resulting thin solid film or coating. The impact dy-

namics of individual droplets has been studied for more

than a century. In general, droplet spreading is a com-

petition between capillarity, surface energy, and inertia

forces, ultimately reaching equilibrium with minimum

free energy of the system. When liquid droplets impact

the substrate with moderate velocity, the initial wetting

is dominated by inertia. The spreading, however, is

determined by surface properties and other present

forces. After this rapid and dynamic initial spreading

period on the order of milliseconds for a millimeter-

sized droplet, the droplet may vibrate, recede or pinned

to the surface and reach an equilibrium state in a longer

time [24-33].

To better understand the spray coating and inkjet

printing process, the coalescence and interaction be-

tween multiple droplets should be understood. There

are several studies on the interaction of two or few im-

pinged droplets. For instance, the interaction of two

impacting drops at the surface of a solid substrate and

the theoretical modeling of this phenomenon were per-

formed by Roisman et al. [34]. They varied the drop

diameter, the impact velocity, the time interval between

the impact of two drops, and the distance between the

impacting drops. In their theoretical model, the surface

tension, wettability, viscosity, gravity, and inertia were

taken into account. Dalili et al. [35] performed experi-

ments to observe the coalescence of highly viscous liq-

uid droplets deposited onto a flat, solid steel plate.

Droplets were deposited sequentially in straight lines or

square droplet arrays to form lines or films. They found

the lowest droplet overlap ratio (defined as droplet

overlap distance divided by droplet spread diameter) at

which a continuous liquid film could be formed. Wu et

al. [36] studied lattice Boltzmann simulation of impact

of one and two droplets onto a substrate. In the case of

impact of two droplets, coalescence and possible de-

tachment of droplets was studied, where it was ob-

served that several segments accompanying with their

detachment from the surface form. In a similar study,

Raman et al. [37] studied successive droplet impinge-

ment on a solid surface, where the effect of various

parameters on droplet interaction was explored, numer-

ically. Sarojini et al. [38] studied the dynamics of

spreading and coalescence of conducting polymer drop-

lets of poly (3,4-ethylenedioxythiophene) : poly (sty-

renesulfonate) (PEDOT:PSS) on a solid substrate, im-

pacting at an offset to understand the process of inkjet

printing. The effect of governing parameters on droplet

coalescence was studied and different spreading re-

gimes were identified. Cossali et al. [39] reported re-

sults on secondary atomization produced by the impact

of three water drops impacting simultaneously on a

heated wall, below and above the Leidenfrost tempera-

ture.

There are also numerous studies concerning the in-

teraction between sprays and surfaces, e.g. [40-43].

Kuhlman and Taylor [40] performed a series of experi-

mental measurements on a dense spray impacting onto

an unheated, smooth, and flat surface, where the film

3

thickness was measured by optical methods. However,

the details of the droplet interaction and film formation

were not captured. Labergue et al. [41] studied heat

transfer aspects of impinged spray droplets onto a heat-

ed surface. Spray impact onto flat and rigid walls was

studied by Kalantari and Tropea [42], where droplet

size and velocity and film thickness were measured. In

another experimental and numerical study, Yoon et al.

[43] investigated the impact of water spray droplets

onto a heated cylinder, where droplet size and velocity

downstream of the cylinder was studied.

The forgoing literature review shows that the hy-

drodynamics and heat transfer of impinging multiple

droplets and spray droplets onto surfaces has been stud-

ied by various workers. However, considerably limited

studies have focused on the process of the thin liquid

and solid film formation. Therefore, given that the ulti-

mate goal of spray coating is to form a defect-free and

uniform thin solid film, the present preliminary work

focuses on observation of spray droplet interaction on

the substrate. Glass and glossy paper substrates and

food-dye aqueous solution, as well as PEDOT:PSS are

used as the model substrates and fluids, respectively.

The spray conditions are fixed and the effect of ultra-

sonic vibration imposed on substrate during the spray

coating process is investigated, using high speed imag-

ing.

Experimental

Bare glass and white glossy papers (20 mm × 20

mm) were used as the substrates. The bare glass sub-

strate was transparent, impermeable and smooth,

whereas the glossy paper was also smooth, but permea-

ble and opaque. Different surface conditions resulted in

different wettability, droplet dynamic phenomena and

overall different coating process. To obtain a color so-

lution for visualization purposes, 0.5 g of food dye

powder (carmine pigment, purchased from Dyestuffs

Research Institute Co, Ltd, Shanghai, China) was dis-

solved in 200 mL of deionizer water to prepare red-

color food dye solution with a concentration 0.25 wt%.

The solution was stirred by magnetic stirrer for a few

minutes before spraying. Pristine PEDOT:PSS aqueous

solution (Sigma-Aldrich, USA) consisting of 0.5 wt.%

PEDOT and 0.8 wt.% PSS was diluted in water with

volume ratio of 2:1, respectively. A pressurized spray

gun was used to atomize the solutions. The sprayer was

mounted on an aluminum frame at a fixed distance

from the substrate. The hand sprayer and a color high

speed camera (Photron, model FASTCAM SA5, Japan)

were turned on simultaneously to observe the coating

process. The high speed camera was set at 3000 frames

per second (fps) in impact dynamics tests. The tempera-

ture of the substrate was adjusted by placing the sub-

strate on a hot plate. In some experiments, the hot plate

was placed and secured atop an ultrasonic transducer

box with a frequency of 40 kHz and maximum power

of 50 W. The ultrasonic transducer was mounted on the

upper wall of a steel box transmitting the vibrations to

the substrates [23].

In this work, the atomizing air pressure, solution

concentration, and spray flow rate are kept constant,

and the effect of varying choice of substrate and solu-

tion, substrate temperature, and the magnitude of im-

posed vibration are studied. Various combinations of

operating conditions were tested in order to find the

optimal air pressure and flow rate to obtain a stable

spray condition, leading to suitable air pressure of 1 bar

and spray flow rate of 1.29 ± 0.09 mL/min. The nozzle

tip to substrate distance was kept constant at 163 mm.

Table 1 summarizes the variable operating conditions

investigated in the experiments. The average velocity of

the spray droplets upon impact is estimated to be

around 0.3 m/s. To visualize the impact and spreading

of single drops of food-dye and PEDOT:PSS solution

on glass and paper substrate, droplets with the size of

4.2 mm were dropped from the distance of 40 cm from

the substrates, and the drop impact behavior was rec-

orded at 3000 fps. Figure 1 shows the schematic of the

experimental setup used in this work.

Figure 1. Schematic of the experimental setup: (a)

high speed camera; (b) metal frame; (c) spray gun; (d)

container of the spray gun; (e) light source; (f) ultrason-

ic transducer box; (g) signal generator; (h) hot plate; (i)

control box of the hot plate; (j) substrate; (k) laptop.

Results and Discussion

Figure 2 shows the impact dynamics of food-dye

and PEDOT:PSS droplets on glass and glossy paper

substrates, dropped from a distance of 40 cm. The fig-

ures help understand the spray coating process which is

comprised of impact and interaction of many small

droplets. Several observations are made as follows.

Within few milliseconds, the droplets reach their equi-

librium state. The impact dynamics of food-dye and

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PEDOT:PSS solution droplets on both glass and glossy

paper is similar. In all four cases, the droplets spread to

their maximum spreading within 10 ms by their initial

momentum and kinetic energy. The rim and finger for-

mation is observed also. After achieving maximum

spreading, given that the substrates were not chemically

and thermally treated to increase their surface energy,

the droplets are not pinned onto the substrate, and there-

fore the receding phenomenon is observed. After about

50 ms, the droplets nearly reach their steady state with

somewhat uniform distribution of the liquid within the

lamella, even though the final shape is not circular. The

spreading on glass substrates, however, is more uniform

and circular in all directions, due to the smooth and

homogenous surface of glass compared to glossy paper.

It is known that the maximum droplet spreading is a

function of Re and We numbers [44]. Both Re and We

numbers of the impinged droplets at the time of impact

are estimated to be around 500 based on the physical

properties of the solutions and the impact velocity. The

maximum droplet spreading ratio of PEDOT:PSS drop-

lets is comparable to that of food-dye solution droplets,

given that the magnitude of the Re and We numbers of

both solution droplets are comparable. It should be

mentioned that the substrates in this work were not

treated and were used as-received. Chemical and ther-

mal treatment of the substrates, which results in remov-

al of all residues from the surface and increases surface

energy, usually result in droplet pinning, and therefore

larger and thinner coating area, which is beneficial to

the coating process [44].

Figure 3 shows the process of spray coating of food

dye solution on glossy paper within several seconds and

the effect of the substrate temperature at no imposed

vibration. The stochastic nature of spray coating pro-

cess is observed. A higher substrate temperature is

found beneficial to obtain a more uniform coating. This

may be attributed to an increase in droplet spreading.

Figure 3 also shows that under the experimental condi-

tions of this work, the impinged droplets do not neces-

sarily merge to form a liquid film. Instead, they indi-

vidually dry, forming an uneven solid film. An increase

in the spray flow rate or the number of spray passes

would increase the chance for the formation of a thin

liquid film, and therefore a more uniform and intact thin

solid film, although using multiple spray passes might

adversely affect the coating due to the disturbing effect

of the forthcoming spray droplets on an existing wet

film.

Figure 4 shows the sequences of spray coating pro-

cess and the effect of substrate vibration on spray coat-

ing of food dye and PEDOT:PSS solutions on glossy

paper, at substrate temperate of 80 °C. Imposing a ver-

tical ultrasonic vibration has resulted in a more uniform

film formation on the glossy paper substrate. This is

due to the imparted low-power mechanical energy to

the substrate and droplets, which results in improved

droplet spreading and mixing of the delivered liquid to

the substrate [45]. This can have important technologi-

cal applications to obtain a more uniform coating or

thin solid films. The vibration power has to be adjusted

carefully to avoid excessive agitation of the surface

which may breakup the wet and solid film [23].

The substrate type significantly affects the process

of spray coating. In Figures 5 and 6, the experiments

were performed on bare glass substrates. Figure 5

shows the process of spray coating on glass substrates

and the effect of imposing vibration on the substrate,

using food-dye solution. The spray coating process is

different from that observed in the case of paper sub-

Run Substrate Solution Substrate temp. (℃) Vibration power (VP) (W)

1 Paper Food dye 25 0

2 Paper Food dye 80 0

3 Paper Food dye 25 10

4 Paper Food dye 80 10

5 Paper PEDOT: PSS 80 0

6 Paper PEDOT: PSS 80 10

7 Paper Food dye 80 50

8 Glass Food dye 25 0

9 Glass Food dye 80 0

10 Glass Food dye 25 10

11 Glass Food dye 80 10

12 Glass PEDOT: PSS 80 0

13 Glass PEDOT: PSS 80 10

14 Glass Food dye 80 50

Table 1. Spray coating experimental runs, indicating the choice of substrate, solution, substrate temperature, and

imposed ultrasonic vibration.

5

strates. In this case, the impinged droplets coalesce

making larger droplets on the substrate. At least during

the first few seconds captured and shown in this figure,

a film is not formed; instead the droplets tend to merge,

making large drops than forming a thin liquid film. Al-

so it appears that in this case (glass substrates), the ef-

fect of the substrate vibration is to force the individual

droplets to “walk” on the substrate, whereas when a

paper substrate is used, spreading is more profound, and

vibration results in better mixing and better coating

quality. Walking and climbing of vibrated droplets has

been observed in other works as well, e.g. [46, 47].

Figure 5 also shows that as the vibration power increas-

es, the number of droplets observed in the field of view

decreases, i.e. more droplets tend to move towards the

edge of the field of view, where the vibration power is

presumably stronger.

Figure 6 shows the drying stage of sprayed droplets

on a bare glass substrate by showing images taken at a

prolonged period. The substrate is glass and kept at 25

and 80 °C. The drying process is observed in the high

temperature case. The outcome of the drying process is

therefore several dried island-like liquid disks. Hence,

this figure substantiates the difficulty associated with

obtaining an intact thin solid film using spray coating.

The droplet contact angles in this work were in the

range of 40 to 60º. Optimization of the spray flow rate

and using multiple spray passes, as well as using sub-

strates with high surface energy and low droplet contact

angle would improve the coating process.

Conclusions

We attempted to visualize the spray coating pro-

cess impinged on glossy paper and bare glass sub-

strates, using food-dye and PEDOT:PSS solutions. Giv-

en that the spray droplets are very small, the camera

was not able to capture the transient details of the coat-

ing process. Nevertheless, the general process of spray

coating was observed and the effects of imposed vibra-

tion, substrate temperature, and choice of substrate and

liquid solution were studied. In conclusion, it is ob-

served that the spray coating process is highly random

and stochastic. A higher substrate temperature results in

better coating process. Imposed vibration in the case of

the glossy paper results in a more uniform coating,

whereas in the case of glass substrate results in droplet

“walking” on the substrate. Optimization of spray flow

rate and generation of a uniform spray impinged on a

high surface energy substrate may improve the coating

process.

Acknowledgments

This work was conducted in the framework of the

research course proposed to undergraduate students in

the University of Michigan-Shanghai Jiao Tong Uni-

versity Joint Institute. Financial support from the

Shanghai Municipal Education Commission and Na-

tional Natural Science Foundation of China (NSFC) to

establish and run the lab is acknowledged.

Authors Contributions

JH, ZY, SG, and JL performed the experiments and

generated the results. JH and ZY wrote the experi-

mental details and contributed to the literature review.

ME directed the work and wrote the results and discus-

sion. All authors read and approved the manuscript.

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Food-dye on glass Food-dye on paper PEDOT:PPS on glass PEDOT:PPS on paper

t =

0 m

s

t =

0.3

3 m

s

t =

0.6

7 m

s

t =

1 m

s

t =

2 m

s

t =

5 m

s

t =

10

ms

t =

25

ms

t =

50

ms

Figure 2. Impact of food-dye and PEDOT:PSS drops on glass and paper substrates, dropped from 40 cm distance.

8

T = 25 ºC; VP = 0 W T = 80 ºC; VP = 0 W

t =

1 s

t

= 2

s

t =

3 s

t =

4 s

t =

5 s

Figure 3. Process of spray coating and the effect of temperature on deposition of food dye solution on glossy paper,

in the first few seconds. The substrate vibration power (VP) was set to zero.

9

Food-dye solution;

VP = 0

Food-dye solution;

VP = 50 W

PEDOT:PSS solution;

VP = 0

PEDOT:PSS solution;

VP = 10 W

t =

1 s

t =

2 s

t =

3 s

t =

4 s

t =

5 s

Figure 4. Process of spray coating of food dye (two left columns) and PEDOT:PSS (two right columns) solutions on

glossy paper and the effect of substrate vibration. Substrate temperate was set to 80 °C.

10

VP = 0 W VP =10 W VP = 50 W

t =

1 s

t =

2 s

t =

3 s

t =

4 s

t =

5 s

Figure 5. Effect of vibration power on spray coating of food dye solution sprayed on glass substrate. Substrate tem-

perature was set to 80 °C.

11

T = 25 ºC T = 80 ºC

t =

7 s

t =

14

s

t =

21

s

t =

28

s

t =

35

s

t =

42

s

Figure 6. Drying stage of sprayed food-dye droplets on glass at two substrate temperatures. VP = 0.