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  
1
,2,3  
2
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  
2
3
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  
1
mechanical durability remains a major drawback for real uses.  
1
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.  
9
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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  
2
Experimental  
2
.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  
3
퐸퐻0  
=  
,
(1)  
0  
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  is the Peclet number.  is the film evaporation rate  
measured from the speed of the descendingsurface.  is the film  
0
initial thickness, and  is the diffusion coefficient of particle,  
0
which is related to the particle size described by the Stokes-  
Einstein equation,  
푘푇  
2
.2 Measurement of solvent evaporation rates  
퐷 =  
0
(2)  
6
휋휂푅  
The evaporation rate of the solvent was measured by mean  
E
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,  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 ). The particles consolidation front forms  
at the airsolutioninterface and grows maximumpacking fraction  
near the surface top. On the other hand, when diffusion dominates  
LinkControl adapter  
(
low ) particles concentration are predicted to remain dispersed  
uniformly in the film throughout the coating thickness as it dries  
10).  
As described in Figure 1, smaller particles with  < 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  
Computer  
other hand, the larger particles with  > 1 experience much  
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  
2
.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  
(silica) > 1 was obtained by particle functionalization with  
fluoroalkylsilane compound, and they was blended with smaller  
2
.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  (polymer) < 1.  
1108