wetting behavior of high energy electron irradiated porous...

CHAPTER 8

Wetting Behavior of High Energy

Electron Irradiated Porous

Superhydrophobic Silica films

Chapter 8 Wetting Behavior of High Energy Electron Irradiated

Porous Superhydrophobic Silica Films

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Chapter 8

Wetting Behavior of High Energy Electron Irradiated Porous

Superhydrophobic Silica films

8.1 Introduction

The nature of adhesive forces between water and hydrophobic materials

has been a subject of great interest. Water repellency of lotus leaf has been

attracting considerable attention in fundamental and biomimetic researches. A

model surface for superhydrophobicity and self-cleaning is provided by the

leaves of the lotus plant (Nelumbo nucifera), having an combination of the

hierarchical structure and waxy cuticle with low surface energy which brings

the water contact angle of greater than 150º and a sliding angle of less than 10º

[1]. Research into the fabrication of superhydrophobic surfaces, has aroused

much attention due to their potential applications ranging from self-cleaning

surfaces to friction-reduction surfaces for microfluidic channels to sensors [2-

6]. Water on such a surface forms a perfect spherical pearl, and both the contact

area and the adhesion to the surface are dramatically reduced [7-9]. Several

studies have reported that by finely controlling the micro-/nanostructure or

chemical composition of a surface, the adhesion between the superhydrophobic

surface and the pearl-like water droplet can be changed, being either very weak

or very strong [10-12]. Guo Z.G. et al [12] reported a sticky superhydrophobic

surface conveying both large static water contact angle (>150º) and amazingly

large water sliding angle (>90º) made with a hydrophilic engineering material.

The adhesion mainly comes from the van der Waals force produced by the

liquid-solid interface between water droplet and the coated substrate [13]. The

adhesion force between the as-prepared surface and a water droplet should be

very small, causing the water droplet to slide easily, since the three-phase

contact line is discontinuous on the surface [14].

Diverse methods to achieve a wettability modification of solid materials

have been developed, including electrostatic [15], electrochemical [16], and

photochemical [17] modification, either permanent or reversible, in addition to



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the many methods based on radiation-induced damages [18-19]. Recently,

Aronov et al [20] investigated the wettability inversion (transition from

hydrophobic to hydrophilic state) of silicon dioxide ultrathin film by electron

irradiation. In the present work, the non-sticky superhydrophobic silica films

were converted into sticky superhydrophobic silica films using high energy

electron irradiation (7 MeV). Although, both of the static water contact angles

of the pristine and irradiated silica film exceeds 150º, the adhesive forces

between the coatings and water droplet are very different.

8.2 Materials and methods

8.2.1 Sample preparation

The superhydrophobic silica films have been prepared by a single step

sol-gel process using dip coating method. The chemicals used were

methyltrimethoxysilane, trimethylchlorosilane (Sigma-Aldrich chemie,

Germany), methanol, hexane (s.d.fine-chem limited, Mumbai), and ammonia

(NH3, sp.gr.0.91, Qualigens fine chemicals, Mumbai). All the reagents were

used as received.

The coating solution was prepared under basic condition from the

MTMS, CH3OH, and H2O in molar ratio of 1:14.28:4.76 respectively with 4M

NH4OH. After stirring the coating solution for 30 minutes, the glass substrate

was immersed vertically into the bath containing the coating solution and then

withdrawn with a speed of 5 mm per second, which is a necessary speed to

occur the hydrolysis and condensation of the liquid coatings, simultaneously.

The coating was taken twice on the same substrate with same molar ratio. The

silica films were dip coated on glass substrates prior to gel formation. The films

were dried at room temperature (27ºC) for 1 hour to produce chemical bonds

between the deposited sol and the substrate. All the films were thermally cured

at 100ºC for 1 hour with ramping rate of 1ºC per minute to densify the films.

The films were soaked in modifying solution consisting of 6 vol. % TMCS in

hexane for 2 h at 50ºC. The samples were then washed with hexane. The

modified silica films were thermally cured at 200ºC for 1 h with ramping rate



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of 1ºC per minute. The silica films so produced were taken out of the oven,

after it was cooled to the ambient temperature.

8.2.2 Electron irradiation experiment

The electron irradiation of the silica film was carried out by the 7 MeV

linear electron accelerators (LINAC) set up at the Bhabha Atomic Research

Centre (BARC), Mumbai, India, the details are described elsewhere [21]. The

silica film sample was kept in front of the exit of the LINAC at a distance of 12

cm where the liquid samples (cell dimensions: 10 mm × 10 mm × 50 mm) were

kept for the pulse radiolysis study. The dose absorbed by aqueous solutions per

pulse was determined using a chemical dosimeter, an aerated aqueous solution

containing 5 × 10-2

mol dm-3

potassium thiocyanate (KSCN) [22]. This dose

value was used to calculate the total dose delivered in a sample under a

repetitive irradiation condition. Electron pulses of 2 µs time duration with a

peak current, 70 mA were used for the irradiation of the sample. The electron

flux was about 3 x 1012

electrons / 2 µs / cm2. The sample was irradiated by the

electron pulses at a repetition rate of 12 pulses per second for about 6.5 s,

accounting for a cumulative dose of about 10 kGy.

8.3 Results and discussion

8.3.1 Surface Morphological Studies

The two-dimensional morphological study of the pristine and irradiated

superhydrophobic silica films has been carried out using SEM image. Figure

8.1 (a and b) shows the SEM images of pristine and irradiated

superhydrophobic silica films at 20000x magnification, respectively. The

pristine superhydrophobic silica film shows porous morphology. Whereas, in

case of the irradiated superhydrophobic silica film, the pores are filled up due

to high energy electron irradiation, and surface shows compact morphology.

The separated grains in the pristine silica film get agglomerated in case of

irradiated silica film.



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Figure 8.1 (a): SEM image of pristine superhydrophobic silica film.

Figure 8.1 (b): SEM image of irradiated superhydrophobic silica film.



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8.3.2 Atomic Force Microscopy

The wettability of a surface is dependent on its chemical composition,

but also on the topography. Figure 8.2 (a and b) shows the typical three

dimensional atomic force microscopy (AFM) image of the pristine and

irradiated superhydrophobic silica film. The image was recorded at 5×5 µm2

planar in contact mode. The root-mean-square (RMS) roughness value of both

films was analyzed with AFM. The surface of the pristine silica film was rough

than the irradiated silica film. The pristine and irradiated silica film showed a

RMS roughness value of 73 and 46 nm, respectively. The decrease in surface

roughness value in the case of irradiated silica film contributes to lower static

water contact angle.

Figure 8.2 (a): AFM image of pristine superhydrophobic silica film.



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8.3.3 Fourier transform infrared spectroscopy

The chemical composition of the pristine and irradiated silica films was

investigated by the FT-IR spectroscopy using the KBr method in transmission

mode. Several characteristic absorption peaks were observed in the range 450

to 4000 cm-1

. Figure 8.3 shows the FT-IR spectra of pristine and irradiated

silica films. The strong absorption peak at 1074 cm-1

corresponded to the Si–

O–Si asymmetric stretching vibration [23]. The presence of this peak confirms

the formation of a network structure inside the film. The absorption bands

observed at around 2950 cm-1

and 1400 cm-1

are due to stretching and bending

of C-H bonds and the peaks observed at 847 cm-1

are due to the Si-C bonds

[24]. The peak at around 1600 cm-1

and the broad absorption band at around

3400 cm-1

are due to the –OH groups [25]. In the case of irradiated silica films,

the intensity of the absorption peak of C-H and Si-C decreased slightly and O-

H peak at 1600 cm-1

is considerably broadened than the bands for free O-H

stretching. It may be associated to the presence of adsorbed water in irradiated

Figure 8.2 (b): AFM image of irradiated superhydrophobic silica film.



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samples. The decrease in intensity of C-H bands may be associated with

irradiation induced cleavage of C-H bonds and some C-H modes disappear due

to hydrogen abstraction [26].

8.3.4 Static and dynamic water contact angle measurements

The influence of surface microstructure on superhydrophobicity of

pristine and irradiated silica films is demonstrated by the investigation of static

water contact angle, water sliding angle and adhesive force between the film

Figure 8.3: FT-IR spectra of pristine and irradiated silica film.



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surface and the water droplet. An optical microscope lens and a CCD camera

were employed herein to take images of the water droplet. First the irradiated

silica film was placed on the moving plate of goniometer, a 10 mg water

droplet was suspended on a needle tip (the droplet just in contact with the top

surface of the film) as shown in figure 8.4 (a). Then the film surface was

moved horizontally at a rate of 0.1 mm/s away from the needle tip. When the

surface slowly moved and reached its maximum value as depicted in figure 8.4

(b), implying that the adhesion force between the surface and the water droplet

increased gradually from the beginning. Again in the next set of experiments

(figure 8.4c), when the needle tip was moved vertically at a rate of 0.1 mm/s

away from the film surface, the drop sticks strongly to the film surface. After

this point, since the water droplet was separated eventually from needle tip, the

drop rests on the surface making water contact angle of 153º as shown in figure

8.4 (d). The experiment was repeated at least 4 times on the irradiated film

surface. Each experiment showed the same pinning behavior of water droplet

on different spots of the film surface. The same experiment was also applied

for pristine silica film as shown in figure 8.5 (a-c). Although, the film surface

was moved horizontally at a rate of 0.5 mm/s, the water droplet slides

effortlessly on film surface in contact with needle tip and began to run to the

side, keeping a large static contact angle of 165º. When the needle tip with

water drop was touched and moved vertically from the film surface, the drop

was not stacked to the film surface and moved with the needle tip, showing

negligible adhesion force between the film surface and the water droplet.



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Figure 8.4 (a): Water droplet in touch with the irradiated film surface.

Figure 8.4 (b): Water droplet moved in horizontal direction on

irradiated film surface.



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Figure 8.4 (c): Water droplet moved in vertical direction on

irradiated film surface.

Figure 8.4 (d): Shape of water droplet on irradiated film surface.



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Figure 8.5 (a): Water droplet in touch with the pristine film surface.

Figure 8.5 (b): Non wetting behavior of water droplet on pristine film surface.



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Even though, both of the static water contact angles of the pristine and

irradiated silica film exceeds 150º, the adhesive forces between the coatings

and water droplet are very different. The irradiated silica films shows strong

adhesion with the water droplet, and water droplet can not slide off the film

surface by gravity even when the substrate is placed vertically (figure 8.6 a) or

upside down (figure 8.6 b). However, for the pristine silica films, the adhesion

is very weak. As a result, the water droplet can roll off the slightly tilted

(WSA=3º) films surface effortlessly. For a 10 mg of water droplet, pristine film

shows sliding angle of about 3º. Therefore the maximum frictional force

required to slide the water droplet was calculated to be around 5.12 µN. The

resulting irradiated film showed a complete contradictory result to that of

pristine silica film. Since the water contact angle measurements were taken at

room temperature and the entire process took only 15 s during the whole

measuring process, the evaporation of the water droplet could be ignored.

Figure 8.5 (c): Shape of water droplet on pristine film surface



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Figure 8.6 (a): Water droplet on irradiated film surface at tilt angle of 90º

Figure 8.6 (b): Water droplet on irradiated film surface at tilt angle of 180º



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In case of pristine silica films, the area of the liquid-solid interface is

very small, since a water droplet roll off on the surface easily. For the pristine

film, the surface is rougher and can trap more air than the irradiated film. When

a water droplet is placed on it, a large amount of air can be trapped between the

pores and water droplet due to its porous morphology. This reduces the area of

the liquid-solid interface to a very low level as described by Cassie-Baxter’s

model. In the Cassie-Baxter’s model, the rough surface is not wetted and can

trap air in pores. As a result, the adhesion between the water droplet and the

film is weak and can even be neglected. As a result, the contact angle is as high

as 165º, and the adhesion is very weak, the water droplet can roll off very

easily at a sliding angle of less than 3º. Thus the surface can be used for self

cleaning.

For the irradiated film, even though the static water contact angle

reaches 153º, the surface is not very rough, and can ensnare only a small

amount of air. When a water droplet is placed on the film, most of the air is

removed and the area of the liquid-solid interface is large due to the compact

morphology of the film after irradiation. In Wenzel’s model, the air trapped in

the rough surface can be piled out by water, and the substrate can be wetted.

Thus, the large area of the liquid-solid interface creates strong adhesion with a

water droplet. It is believed that the van der Waals forces between the particles

possessing hydrophilicity and the water lead to the high adhesion. This force

can balance the weight of the water droplet, keeping it suspended on the film

surface even if the surface is turned vertically (WSA=90º) or upside down

(WSA=180º). It is believed that the formation of compact microstructure and

pore fill up by high energy electron irradiation was responsible for the pinning

of water droplet on superhydrophobic surface. However, such coatings may be

useful for some intelligent microfluidic devices.

8.4 Conclusions

In conclusion, we have successfully prepared superhydrophobic silica

films by a single step sol-gel process and surface derivatization method. The



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electron irradiation induces a structural change in the surface morphology. The

porous surface morphology of pristine silica film is changed to compact

morphology (pore fill up) due to high energy electron irradiation. The surface

roughness of pristine silica film is also decreased by electron irradiation. The

pristine silica film showed static water contact angle as high as 165º, and the

water droplet can roll off (WSA = 3º) the film surface effortlessly which can be

used for self-cleaning applications. This observation suggests that the adhesion

between the film surface and water droplet is very weak. However, the same

water droplet showing static water contact angle of 153º, could not slide away

from the irradiated silica film and pinned stably even if the surface is turned

vertically (WSA = 90º) or upside down (WSA = 180º) ensuring the adhesion is

very strong. Such coatings may be useful for some intelligent microfluidic

devices. These results indicate a significant difference between the dynamic

interaction of the water droplet with the pristine and irradiated silica films. This

suggests that Cassie to Wenzel state transition takes place due to high energy

electron irradiation.



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