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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
183
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
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
184
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
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
185
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
186
Figure 8.1 (a): SEM image of pristine superhydrophobic silica film.
Figure 8.1 (b): SEM image of irradiated superhydrophobic silica film.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
187
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
188
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
189
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
190
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
191
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
192
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
193
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
194
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
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
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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
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
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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
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
197
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.
Chapter 8 Wetting Behavior of High Energy Electron Irradiated
Porous Superhydrophobic Silica Films
198
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