Self-Stratifying Particulate Coating for Robust Superhydrophobic and Latex-Repellent Surface

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1107-1111

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

https://doi.org/10.47277/JEET/8(3)1111

Self-Stratifying Particulate Coating for Robust

Superhydrophobic and Latex-Repellent Surface

1,3,*

Sulaiman Hajeesaeh , Sobiroh Kariyo , Nantakan Muensit , Chalongrat Daengngam

,2,3

1,3

¹Department of Physics, Faculty of Science, Prince of Songkla University, Songkhla 90112, Thailand

Department of Research and Development, Faculty of Science and Technology, Fatoni University, Pattani 94160, Thailand

Center of Excellence in Nanotechnology for Energy, Prince of Songkla University, Songkhla 90112, Thailand

Received: 19/05/2020

Accepted: 08/07/2020

Published: 20/09/2020

Abstract

A technique for preparing superhydrophobic and natural latex-repellent surface requires at least two fabrication components: surface

roughness, and surface layer with low free energy. Here, multiscale surface roughness in micro-/nanoscales with low surface energy can be

simultaneously achieved through the deposition of fluoroalkyl-functionalized silica aggregates. However, the mechanical durability of such

film remains problematic. Therefore, third component such as polymer binder was incorporated carefully to improve adhesion between film-

substrate interface without deteriorating surface roughness and surface energy. In this work, we employed self-stratifying coating technique

to induce vertical phase separation between particles and polymer during film drying, such that the silica aggregates densely accumulated on

the top surface, while polymer binder concentrated near the film bottom. The governing transports during film stratification process involve

diffusion and convection driven by evaporation. Thus, this research focused on the effect of drying temperature and evaporation rate on the

anti-wetting performance of the coating. The results showed that the liquid-repellent properties of the surface improve with increasing drying

temperature, indicating the convection-dominated transport that induced substantial particle trap at the film surface. With polymer binder

added, the coatings still showed decent superhydrophobic and natural latex-repellent properties with maximum contact angles 166.4°±0.6°

and 157.5°±0.5°, as well as minimum sliding angles 2.7°±0.3° and 2.9°±0.2° for water and natural latex respectively. Also, AFM result

revealed that significant surface roughness of 581 ± 18 nm was still achievable even at high blending mass ratio of polymer binder up to half

of the silica weight.

Keywords: Superhydrophobic, Natural latex-repellent surface, Multiscale roughness, Self-stratifying coating

mechanical durability remains a major drawback for real uses.

Introduction

Also, it is quite

a contradiction to expect non-sticky

It is well known that the extreme liquid-repellent properties of

functionalized particles to adhere tightly on a substrate. The

strategy to improve the adhesion between superhydrophobic film

and substrate requires addition of fluoro-containing polymer

binder, which allows good dispersion of fluoroalkyl

functionalized silica nanoparticles into the polymer matrix.

Nonetheless, it remains quite complicated to maintain the surface

roughness and the topmost functional groups of final film, as

polymer binder tends to swamp the surface.

Therefore, an asymmetric particle distribution induced by self-

stratification during film drying is introduced, in order to produce

spatially controlled polymer blending, which does not destroy

superhydrophobic features (4). As particulate coating dries,

diffusion and convection transports determine the final particle

distribution inside the film. Particles tend to accumulate more on

coating surface when the convection dominates, i.e. high

evaporation rate, so higher surface roughness can be expected. On

the other hand, if the diffusion dominates, particles and polymer

surface are the synergistic effect of surface morphology and

chemical compositions (1). Surface free energy of coating films

can be lowered by mean of surface functionalization with

hydrophobic molecules, and the hydrophobicity can be further

enhanced by surface asperities to reach superhydrophobic state.

Therefore, both surface topographical roughness and the outmost

functional groups play a crucial role in producing such superior

liquid-repellent properties. Furthermore, surface roughness in

multiple length scale has been proven as the key to achieve more

stable Cassie-Baxter non-wetting state (2). In our previous

research, we also demonstrated that tipple-scale surface

roughness, obtained from silica aggregates functionalized with

fluoroalkylsilane molecules, were superiorto from stableextreme

anti-wetting surface that can repel water or even highly adhesive

liquid like concentrated natural latex (3). As the film was mainly

composed of functionalized nanoparticles, however, its poor

Corresponding author: Chalongrat Daengngam, (a) Department of Physics, Faculty of Science, Prince of Songkla University, Songkhla

0112, Thailand. (b) Center of Excellence in Nanotechnology for Energy, Prince of Songkla University, Songkhla 90112, Thailand. E-mail:

chalongrat.d@psu.ac.th.

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Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1107-1111

will blend uniformly causing smoother surface, unfavorable for

superhydrophobic properties.

The coating solution was applied on a glass slide substrate by

simple drop-casting technique and allowed to dry at different

evaporation temperatures. During film drying, self-stratification

of the silica particles and fluorinated polymer binder are

investigated. The effect of evaporation temperature on stratified

particulate coating and the resulting liquid-repellent properties

were examined.

To accomplish the aforesaid idea of stratified drying, a 1D

vertical model of particle transport in film during drying

formation, developed by Routh and Russel, was considered (5). It

can be used to predict skin formation occurring for non-uniform

vertical drying combined with wet sintering (6, 7). This model

approach can be employed to determine whether the particle

transport is controlled by diffusion or convection through the

dimensionless Peclet number , which is defined by the following

expression

Experimental

.1 Formulation of latex-repellent coating

Fumed silica nanoparticles with primary particle size in a

range of 5-25 nm were chemically modified to alter their surface

from hydrophilic silanol groups (–SiOH) to hydrophobic

fluoroalkyl groups (–CF ) by stirring them with 0.2 ml of

퐸퐻₀

푃_푒=

(1)

퐷₀

perfluorooctyltriethoxysilane molecules in a mixture of xylenes

for 70-75 h at room temperature. The silica content in the mixed

solvent was 5 %w/v. Then, polymer binder poly(vinylidene

fluoride-co-hexafluoro propylene) (PVDF-HFP) was dissolved

in acetone for 0.5 h at room temperatureand added into the silica

solution for 2.5 %w/v amount. The coating solution was stirred

further for 0.5 h to attain homogeneous formulation.

where 푃 is the Peclet number. 퐸 is the film evaporation rate

푒

measured from the speed of the descendingsurface. 퐻 is the film

initial thickness, and 퐷 is the diffusion coefficient of particle,

which is related to the particle size described by the Stokes-

Einstein equation,

푘푇

.2 Measurement of solvent evaporation rates

퐷 =

(2)

휋휂푅

The evaporation rate of the solvent was measured by mean

of film surface descending rate in unit of mm/s by using an

ultrasonic sensor for thin film measurement system. The

evolution of drying film thickness can be tracked in real time

inside a temperature-controlled chamber, in order to study the

effect of drying temperature as shown in Figure 2.

where 푘푇 the thermal energy, 휂 is the viscosity of solvent, and 푅

is the particle effective radius. Equation (1) and (2) infer that

stratified drying may be achieved for evaporation-dominated

coating process (high 푃 ). The particles consolidation front forms

푒

at the air–solutioninterface and grows maximumpacking fraction

near the surface top. On the other hand, when diffusion dominates

LinkControl adapter

(

low 푃 ) particles concentration are predicted to remain dispersed

푒

uniformly in the film throughout the coating thickness as it dries

10).

As described in Figure 1, smaller particles with 푃 < 1

undergo faster Brownian motion as “kicked” by the solvent

molecules, and thus they are able to escape from the descending

surface into the bulk film solution during evaporation. On the

(

Ultrasonic

sensor

푒

Computer

other hand, the larger particles with 푃 > 1 experience much

푒

slower Brownian motion unable to escape away from the

descending surface. Thus, comparatively heavy particles tend to

accumulate and form skin layer at the film surface.

Thermostat

Figure 2: Measurement of film evaporation rates using ultrasonic sensor

.3. Contact angles and sliding measurements

An optical contact angle (CA) measurement system

Figure.1: Schematic diagram shows drying steps of particulate coating,

beginning with uniform distribution of particles at the initial time (a).

Then evaporation starts and brings particles toward, where larger silica

particles are trapped at the surface, while the smaller polymer binder

molecules can diffuse away (b). This results in high concentration of silica

particles at the film surface, and the polymer binder submerges down to

bottom (c)

(Dataphysics OCA-15EC) was employed to measure static

contact angles and droplet sliding angles (SA). CA measurement

was performed using probe liquids of water, and 35% natural

latex with droplet size ∼2 µL placed at multiple locations on a

film surface.

In this study, low surface energy silica particles with

푃_푒(silica) > 1 was obtained by particle functionalization with

fluoroalkylsilane compound, and they was blended with smaller

.4. Morphology characterizations

The surface roughness of the film was studied using an atomic

force microscope (AFM) and scanning electron microscope

fluorinated polymer binder nanoparticles with 푃 (polymer) < 1.

푒

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Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1107-1111

(

SEM), focusing on the effect of the blended polymer binder on

and smaller SA are observed for both water and natural latex

droplets.

the multiscale roughness.

The detail of the CA and SA of the samples coated with

different drying temperature is presented in Table 1. The

superhydrophobic and latex-repellent properties of the coatings

improve with faster evaporation rate. The maximum CA is

166.4°±0.6° and 157.5°±0.5°, as well as the minimum sliding

angles 2.7°±0.3° and 2.9°±0.2° obtained for water and natural

latex respectively.

Result and discussion

The effect of evaporation temperature on the microstructure

development of a drying particulate coating was investigated

from 1D model of diffusion and convection transport during

solvent evaporation. The evaporation rate 퐸 was determined from

the vertical decrement of film height per unit time, and the values

of 퐸 from different drying temperatures are shown in Figure 3a.

For all cases of drying temperatures, it can be found that the

evaporation rate was fast at the initial drying time as the acetone

Table 1: The CA and RA of water and latex

Water

Natural latex

Drying

(

with low boiling point) predominantly evaporates out of the film.

temp

Upon this fast drying, the film surface moved rapidly toward the

substrate, causing high rate of convection transport dominated at

the beginning, which resulted in the trapping of large silica

particles at the surface.

CA (°)

SA (°)

CA (°)

SA (°)

(

156.1±0.1

160.3±0.4

4.2±0.2

2.9±0.4

150.9±0.3

153.7±0.3

4.7±0.2

3.9±0.3

166.4±0.6

2.7±0.3

157.5±0.5

2.9±0.2

AFM images for surface topography of the coated film were

obtained in non-contact mode and the surface profiles of 40 C

coating condition are presented in Figure. 4. It can be observed

for the substantial surface roughness, randomly distributed

throughout the studiedarea, which is a desiredpropertiesrequired

for Cassie-Baxter anti-wetting state. The quantitative analysis of

surface RMS roughness was determined as 581±18 nm, which is

much larger than the size of the primary silica particles. This

result infers that the surface roughness was essentially composed

of a larger particle aggregates that diffused slowly and trapped at

the surface during the early stage of the film drying.

Figure 3: The evaporation rate (a) and the resulting CA and SA values

(b) of film dried at temperatures

Figure 4: AFM images of film surface structure with 40 C drying rate

After ~50 s of film drying process, the evaporation rates

slowed down as the acetone depleted. At this stage, the

evaporation rate was controlled by xylene (with high boiling

point). This particularlyslow evaporation rate near the end of film

drying process is actually needed for good coalescence of the

polymer binder to prevent film cracking.

It can also be found that higher drying temperature, in general,

is associated with more rapid evaporation rate. Once the solvent

completely evaporated, highly dense silicaparticledistributionon

the surface can be achieved with the polymer binder located near

the substrate interface. The results in Figure 3b show that better

liquid-repellent properties were obtained for the coated film with

higher drying temperature (faster evaporation rate), as larger CA

The film surface morphology of 40 C coating condition was

further studied using SEM imaging technique, and it clearly

revealed hierarchically structured roughness as shown in Figure

5. The overall variation of surface heights was observed at

microscale level, featuredwith numerous nanograins produced by

the aggregate of primary silica nanoparticles. This multiscale

roughness also renderedsmall hole size of less than 1 µm that can

trap countless air pockets on the surface, resulting in a reduction

of direct contact between liquid and film surface. This minimizes

liquid-film adhesion that corresponds to very low SA values of

less than 5 for both water and natural latex. The SEM cross-

section also reveals highly porous structure at the surface

produced by silica aggregates, whereas the denser film structure

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Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1107-1111

can be found below which contains higher ratio of polymer binder

that improve the adhesion of the film with substrate. The coated

surface exhibitsdecent liquid-repellentproperties even for highly

adhesive liquid like natural latex. Figure 6 shows the dynamic

wetting properties of water and natural latex droplets sliding on

coated surfaces. As revealed by the selected sequential movie

frames, it can be observed that the fluid moves easily out of the

coated surface without any residual liquid left behind. Moreover,

recent studies confirmed that the air pockets underneath the

droplets can cause liquid slippery; thus, the liquid flows virtually

over a layer of air-solid composite, which significantly reduces

surface friction (11). This could find great potential applications

in collection and transportation of natural latex liquid with

minimum waste and cleaning cost due to adhesion.

can be clearly seen high degree of surface roughness from AFM

and SEM even for the film that was incorporated with significant

amount of polymer binder. Moreover, the coated surface also

shows good superhydrophobic and latex-repellentpropertieswith

high CA and low SA. It is expected that this simple single-step

coating technique may possibly open a new way to improve the

mechanical durability of superhydrophobic and latex-repellent

surfaces for practical applications.

(a)

(

Figure 6: Selected movie frames display the flow of water (a), and latex

droplets (b) out of coated surface without leaving any residue

Aknowledgment

The authors gratefully acknowledge the Center of Excellence

in Nanotechnology for Energy (CENE), the Department of

Physic, Faculty Science, Prince of Songkla University and the

Department of Research and Development, Faculty of Science

and Technology, Fatoni University for providing the facilities of

laboratory. The financial support from PSU-Ph.D. Scholarship

and Research Grant for Thesis, by Graduate School, Prince of

Songkla University.

Figure 5: SEM images of stratifying film dried at 40 C at top view (a) and

cross section (b)

Ethical issue

Authors are aware of, and comply with, best practice in

publication ethics specifically with regard to authorship

Conclusion

In summary, we have successfully fabricated extreme anti-

wetting surface using a solution of surface functionalized silica

aggregates blended with PVDF-HFP polymer binder to improve

film mechanical durability. During film drying, self-stratification

was employed to facilitate vertical phase separation between the

silica particles and the polymer binder. The wetting-resistant

performance of the resulting films, dried with different

evaporation rates controlled by varied temperatures, was studied.

It was found that liquid-repellent properties significantly

improved with increasing coating temperature i.e. at high

evaporation rate. This suggests that silica particles were trapped

more at the surface when the convection transport dominated. It

(

avoidance of guest authorship), dual submission, manipulation

of figures, competing interests and compliance with policies on

research ethics. Authors adhere to publication requirements that

submitted work is original and has not been published elsewhere

in any language.

Competing interests

The authors declare that there is no conflict of interest that

would prejudice the impartiality of this scientific work.

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2020, Volume 8, Issue 3, Pages: 1107-1111

Authors’ contribution

S. Hajeesaeh and S. Kariy were responsible for data

collection, data analyses and manuscript writing. C. Daengnam

and N. Muensit initiated experimental design, performed data

analysis and edited manuscript.

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