3.1 General Introduction
All the experiments were preceded in National Chiao Tung University (NCTU).
All the equipments were also conducted in our laboratories in NCTU. The reagents were purchased commercially and used by following with the directions unless specially mentioned.
All the reagents were listed alphabetically in the form of “Name {abbreviation;
chemical formula; purity; manufacturer}”. Some information will be omitted if not available or not necessary. The following text will use the abbreviation of the reagent.
Deionized and distilled water {DI water, ddH2O}
The water we used was purified with filters, reverse osmosis, and deionized system until the resistance was more than 18 MΩ cm‧ -1. DI water was used to clean and wash.
Hydrogen peroxide {H2O2; ≥30%; Sigma}
Hydrogen peroxide was mixed with sulfuric acid to form piranha solution which cleaned the wafer surface.
Sulfuric acid {H2SO4; 98%; Sigma}
Sulfuric acid was mixed with hydrogen peroxide in a 3:1 ratio to remove impurities on the wafer surface. On the other hand, we mixed it with nitric acid for corrosion. This solution was very corrosive and dangerous. We must handle it with carefulness and patience.
Ethanol {C2H5OH; 99%; Sigma}
In this study, it was used for washing and dehydration of a fresh petal.
Nitric acid {HNO3; 60-61%; Showa}
It was mixed with sulfuric acid for corrosion.
Polydimethylsiloxane {PDMS (Sylgard 184); Dow Corning}
We used PDMS to manufacture the superhydrophobic structure which was similar to petal’s surface.
Poly (vinyl alcohol) {PVA; 87-89% hydrolyzed, high molecular weight (88,000-97,000; Alfa}
In this study it was prepared as a negative mold, micro-scale structure.
Polystyrene latex microsphere {PS; 35 μm in diameter; Alfa}
We dropped the latex solution on a silicon wafer to form a two dimension close-packed PS beads. The prepared film was to mimic the micro-scale part of a rose’s petal.
Phosphoric acid {H3PO4; 5%; J. T. Baker}
Phosphoric acid was used to prepare a buffer in the fresh rose’s petal dehydration.
Osmium tetroxide {OsO4, 99.8%; Alfa}
Osmic acid was used to maintain the shape of a plant sample.
Glutaraldehyde {CH2(CH2CHO)2; 25%; MP Biomedicals Inc.}
It was mixture with other solution to maintain the shape of a plant sample.
Paraformaldehyde {OH(CH2O)nH (n = 8 - 100); powder,95%; Sigma}
It was mixture with other solution to maintain the shape of a plant sample.
Sodium phosphate dodecahydrate {Na3PO4‧12H2O, 97%; Alfa}
The solution was prepared as a buffer solution during the dehydration process.
3.2 Flat PDMS with periodic wrinkles
The flat PDMS was prepared by mixing silicone base (reagent A) and curing agent (reagent B) in a weight ratio of 10:1. After degassing, the mixture was heated at 80°C for one hour. The final PDMS, which had the size of 1 cm2 in area and 0.1 cm in thickness, was fixed on a wafer chip. Then flat PDMS on the wafer chip was immersed in a sulfuric acid solution, made of sulfuric acid/nitric acid in different volume ratio), for several seconds. After acid corrosion, it was immediately putted into a cold bath around 0°C for thirty seconds. The final step was drying the sample by nitrogen gas then vacuumed over night. Figure 3.1 illustrated the process for preparing the surface.
3.3 Petal-like PDMS surface manufacture
Experimental protocols to mimic the petal-like PDMS surface were schematized in Figure 3.2. First of all, the 2.5 wt% PS beads, which had 35 μm in diameter, were spread on a clean wafer chip treated with O2 plasma. It was dried in barometric pressure until the total water was vaporization. Then the two-dimensional close-packed PS beads were formed owing to the water vaporization. Thereupon, we spread a 15 wt% PVA solution on the sample. A PVA-PS film was peeled from the wafer when the PVA solution was dried at atmosphere pressure. The film we received was slightly polished by using sandpaper. Afterwards, it immersed in an acetone solution accompanied with ultrasonic’s bath until the PS beads were removed. Then the treated PVA film, replicated from the cross-packed PS beads, was used as a negative mold. PDMS, which had the same conditions as we mentioned in section 3.2, was spread on the PVA film. The next step was to degas for the purpose that avoid an air bobble in PDMS. When the PDMS was dried, we peeled it off and fixed it onto a clean wafer. Finally, the PDMS undergone an acid corrosion process as same as we
mentioned in section 3.2.
3.4 Fresh rose petal and chrysanthemum petal dehydration
Firstly, the fresh rose’s petal was sliced up into many part, each chip had a size of 0.5 cm2. The chip of petal was immersed in the 0.1 M solution, composed of a phosphate buffer solution had a pH value equal to 7.0, a 2 wt% glutaraldehyde solution and a 4 wt% paraformaldehyde solution, for 2 hours. The prepared petal was then immersed in the 0.1 M solution consisted of a phosphate buffer solution had a pH value equal to 7.0 and a 2 wt% osmium tetroxide solution for 2 hours. Subsequently, the treated petal was washed by the phosphate buffer solution for many times.
Afterward, the cleaned petal was processed through a dehydration process by using a series of alcohol solution, it had a concentration of 20%, 30%, 50%, 70%, 80%, 90%
and 100% respectively. The treated sample was submerged by each alcohol solution for 20 minutes and then immersed in an acetone solution for 20 minutes. It shall be repeated three times before migrating to next high concentration alcohol solution.
Finished the dehydration process, the almost dried sample had to undergo a critical point dry by a carbon dioxide liquid ensured total water was removed from the petal.
Figure 3.1The flow chart of preparing the superhydrophobic PDMS surface.
Figure 3.2 The scheme showed the protocol of manufacturing a petal-like PDMS.
3.5 Specimen characterization
(A) SEMSEM is a very useful tool for observing surface morphology of specimen. SEM has secondary electrons or backscattered electrons detectors passing the signal to computer and forming image. In this study, the micro- or nanostructure of the prepared PDMS, close-packed PS beads on wafer and duplicated PVA film were all characterized by a field-emission scanning electron microscopy (FE-SEM, JEOL-6700) operating at 10 kV accelerating voltage.
(B) FT-IR
The position of Fourier transform infrared spectrometer (FT-IR) as a useful technique for characterization of polymeric materials has been firmly established last decades. FT-IR has brought additional merits such as high sensitivity, high precision, quickness of measurement, and extensive data processing capability besides the intrinsic advantages of infrared spectroscopy such as wide applicability, nondestructiveness, measurement under ambient atmosphere, capability of providing detailed structural information, and a huge date base. We prepared the specimen by scrapping the treated surface then mixture the bits and small pieces with potassium bromide powder. After that, we grinded the mixtures to a small size as possible as we could. Finally, pressed the mixtures to a chip. The chip was then characterization by FT-IR (Nicolet Avatar 320).
(C) Inverted light microscopy
Light microscopy is widely used to observe a biological specimen. Upright light microscopy and inverted light microscopy are the most popular equipment for characterization transparent samples. In this study, we used an inverted light microscopy (Olympus CKX41) to characterize the periodic wrinkles on a PDMS surface.
(D) Contact angle meter
A measurement of a static contact angle is a general method to understand the wetting phenomena of a surface. A surface with a static contact angle larger than 150 degree is regard as a superhydrophobic surface. Herein, we investigated the anti-wetting properties of the resulting surfaces by using a contact angle meter (FTA175). The average contact angle was from five different position measurements on the same surface.