Fabrication of Superhydrophobic Textiles
211
However, if a very high pressure was applied the liquid could pass through
it. Another textile based Swiss Company Schoeller synthesized Nanosphere and
coated them on textile to make it water repellent. The lotus effect was provided onto
Nanosphere coated fabric that is involved impregnation of 3-D surface structure and
gel forming additives that repel water and during rolling off all the dirt and dust
particles are attached with the water droplet making the textile super hydrophobic
and self-cleaning [71].
2.4 Switchable Hydrophilicity and Hydrophobicity
The surface with water contact angle less than 90° is hydrophilic, less than 5° is
superhydrophilic, higher than 80° is hydrophilic and even higher than 150° is superhydrophilic. The wettability or repellency of surface is dependent upon the surface
roughness and surface chemistry. The switchable surface which combines the effect
of both superhydrophilic and superhydrophobic have been recently reported that are
smart, adaptable and stimuli responsive by change of surface chemistry. Such surfaces
can be used in many applications such as drug delivery, oil-water separation, protein
concentrators, and microfluidic pumps. There could be some triggering mechanism
depending upon the surface chemistry that can reverse the surface’s character under
different conditions. Switchability of surface could be induced by any stimulus like
light [72], pH [73], temperature [74, 75], electric potential [76, 77], solvent [78] and
mechanical stress [79].
There have been thermos-responsive polymers used to approach the switching
character of a material that undergo the phase transformation at certain temperature which is called lower critical solution temperature at which switching between
hydrophobicity and hydrophilicity occurs. Tao Sun and his colleagues worked on
thermos responsive PNIPAAm (poly N-isopropylacrylimide) thin film fabrication on
roughened and smooth silicon surface with LCST of about 32–33 °C. The mechanism
of switchability was adaptation from hydrogen bonding with solvent at temperature
below LCST to internal hydrogen bonding above the LCST. Compared to flat surface the roughened surface of silicon with thin film of PNIPAAm gave the ability to
surface to be switched between superhydrophilic at ~25 °C and superhydrophobic at
~40 °C [74].
Light sensitive switchable duperhydrophilic, superhydrophobic films have been
synthesize by hydrothermal treatment of an aqueous titanium trichloride solution
supersaturated with NaCl. These fabricated films on UV irradiation transformed
from superhydrophobic to superhydrophilic. Because TiO 2 is a photocatalyst, on
exposure to UV radiation the generated holes react with oxygen to form surface
oxygen vacancies, thus water molecules coordinate kinetically making the surface
superhydrophilic. As the surface is roughned by TiO 2 nanorods the water droplets
fill the nanogrooves by replacing the trapped air. The water contact angle will be
about below 5°. The surface will be in metastable state on water adsorption, which
can be replaced by atmospheric oxygen [80] after the films are located in the dark.
211
However, if a very high pressure was applied the liquid could pass through
it. Another textile based Swiss Company Schoeller synthesized Nanosphere and
coated them on textile to make it water repellent. The lotus effect was provided onto
Nanosphere coated fabric that is involved impregnation of 3-D surface structure and
gel forming additives that repel water and during rolling off all the dirt and dust
particles are attached with the water droplet making the textile super hydrophobic
and self-cleaning [71].
2.4 Switchable Hydrophilicity and Hydrophobicity
The surface with water contact angle less than 90° is hydrophilic, less than 5° is
superhydrophilic, higher than 80° is hydrophilic and even higher than 150° is superhydrophilic. The wettability or repellency of surface is dependent upon the surface
roughness and surface chemistry. The switchable surface which combines the effect
of both superhydrophilic and superhydrophobic have been recently reported that are
smart, adaptable and stimuli responsive by change of surface chemistry. Such surfaces
can be used in many applications such as drug delivery, oil-water separation, protein
concentrators, and microfluidic pumps. There could be some triggering mechanism
depending upon the surface chemistry that can reverse the surface’s character under
different conditions. Switchability of surface could be induced by any stimulus like
light [72], pH [73], temperature [74, 75], electric potential [76, 77], solvent [78] and
mechanical stress [79].
There have been thermos-responsive polymers used to approach the switching
character of a material that undergo the phase transformation at certain temperature which is called lower critical solution temperature at which switching between
hydrophobicity and hydrophilicity occurs. Tao Sun and his colleagues worked on
thermos responsive PNIPAAm (poly N-isopropylacrylimide) thin film fabrication on
roughened and smooth silicon surface with LCST of about 32–33 °C. The mechanism
of switchability was adaptation from hydrogen bonding with solvent at temperature
below LCST to internal hydrogen bonding above the LCST. Compared to flat surface the roughened surface of silicon with thin film of PNIPAAm gave the ability to
surface to be switched between superhydrophilic at ~25 °C and superhydrophobic at
~40 °C [74].
Light sensitive switchable duperhydrophilic, superhydrophobic films have been
synthesize by hydrothermal treatment of an aqueous titanium trichloride solution
supersaturated with NaCl. These fabricated films on UV irradiation transformed
from superhydrophobic to superhydrophilic. Because TiO 2 is a photocatalyst, on
exposure to UV radiation the generated holes react with oxygen to form surface
oxygen vacancies, thus water molecules coordinate kinetically making the surface
superhydrophilic. As the surface is roughned by TiO 2 nanorods the water droplets
fill the nanogrooves by replacing the trapped air. The water contact angle will be
about below 5°. The surface will be in metastable state on water adsorption, which
can be replaced by atmospheric oxygen [80] after the films are located in the dark.