Formation of textured Al thin film on silicone rubber to obtain superhydrophobic property
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Silicone rubber, which is widely used in electrical and biomedical applications, was modified into superhydrophobic property by the formation of a textured Al thin film on the periodic microswelling structure fabricated on silicone rubber by the 193 nm ArF excimer laser-induced photodissociation of silicone. The texturing of Al thin film occurred automatically during the vacuum evaporation only on the silicone rubber, compared with the cases of a slide glass or silicone resin substrate, which were observed by the scanning electron microscope and atomic force microscope. As a result, contact angle of water on the samples with 10- and 30-nm-thick, textured Al thin films was estimated to be approximately 162° in both cases, compared to the sample without Al of approximately 155°. Moreover, a large air gap was remarkably formed between the Al-deposited sample and water; a droplet of colored water was successfully confined in the air gap and it was released into the water when and where need.
KeywordsSilicone rubber Periodic microswelling structure ArF excimer laser Al thin film Superhydrophobic property
Hydrophobic property, repelling water, is often useful as a property of functional materials. A silicone or fluorocarbon polymer is a promising material for realizing hydrophobic property because they are composed of CH3 or CF3 groups, respectively. These polymers are still required to have a higher hydrophobicity [1, 2]. On the other hand, a superhydrophobic property of a plant leaf or insect is existing by nature, which is well-known as a lotus effect [3, 4]. These are originated in a convex–concave structure of their surfaces. Thus, the formation of a convex–concave structure is effective for developing a highly hydrophobic material. For instance, micro/nanostructuring of materials, based on the laser ablation, is good processing to bring a superhydrophobic property [5, 6, 7, 8].
A hydrophobic silicone ([SiO(CH3)2]n) has good properties such as electrical insulation, optical transparency, and heat-, cold- and chemicals-resistances, together with a bio-inert property. Thus, silicone is widely used for scientific and technological applications [9, 10, 11]. The peeling resistant and self-adhesive properties are also useful for industrial and biomedical uses [12, 13]. A sealing material for light emitting diode, biochip and nanoimprint molding are a part of the industrial and practical applications. Based on those good properties, we are going to fabricate a silicone rubber-based microdevice operating in water and other solutions. To function a silicone rubber chip as a microdevice moving in water, we should apply an electric voltage to the electrodes formed on the silicone rubber chip in water. Therefore, we have considered that the surface of silicone rubber should be modified into superhydrophobic property to form an air gap layer between silicone and water .
In our laboratory, as another approach to obtain a superhydrophobic property using laser, we used our previous photochemical processing of silicone rubber to fabricate the microstructure on the surface; when a 193-nm ArF excimer laser irradiated the surface of silicone rubber, only the main chain of Si–O bonds of silicone could be photodissociated into the lower molecules, resulting in the swelling of the laser-irradiated area [15, 16, 17, 18, 19]. For the microstructuring, we used microspheres made of silica glass of 2.5 μm diameter, which covered the entire surface of sample during laser irradiation [14, 20]. Very recently, we also found that the subsequent formation of a 50-nm-thick, textured Al thin film resulted in adding a nanostructuring effect to the microstructured silicone, showing a highly superhydrophobic property . In this paper, we extend the recent experiment to the dependence of thickness of Al thin film below 50 nm on the texturing and obtained superhydrophobic property. The reason for texturing of Al thin film during vacuum evaporation is also discussed. In addition, a droplet of colored water is successfully confined in the air gap layer between the textured Al-deposited sample and water to be released into the water when and where need, for a demonstration.
2 Experimental procedure
Silica glass microspheres of approximately 2.5 μm diameter (Nippon Shokubai KE-P250) of 60 mg was dispersed in ethanol (99.5% purity) of 20 mL, and the dispersed solution of approximately 10 μL volume was dripped on the surface of 2-mm-thick silicone rubber of 15 × 10 mm2. After the heated-air drying at 50 °C for 3 min, a single layer of the microspheres was formed on the silicone rubber, though a subsequent removal of partly overlapped microspheres was often required.
The sample was placed 80 mm apart from the outlet of the ArF excimer laser (Coherent COMPexPro110). The beam path was filled with argon or nitrogen gas at the flow rate of 10 L/min to avoid strong optical absorption of oxygen molecules in air. The ArF excimer laser was conducted onto the sample surface without a lens. The single-pulse fluence of the ArF excimer laser was approximately 10 mJ/cm2. The laser irradiation time was 30 min. The pulse repetition rate was kept constant at 1 Hz. The pulse width of the ArF excimer laser was set at approximately 20 ns. All the laser irradiations were carried out at room temperature.
After the laser irradiation, silica glass microspheres were removed by a 1 wt% HF chemical etching for 90 s and subsequent ultrasonic washing in ethanol for 10 min, emerging the periodic microswelling structure on silicone rubber. The Al thin film was deposited on the periodic microswelling structure by a vacuum evaporation of Al wire (99.99% purity). Thickness of the Al thin film was approximately 10 and 30 nm, which were measured beforehand on a slide glass using a stylus profilometer (Veeco, Dektak3). For comparison, a flat slide glass, a flat silicone rubber and a flat silicone resin coated on a polycarbonate were used to see how the surface morphology of the deposited Al thin films changes. The surface morphology was observed by a scanning electron microscope (SEM, Phenomworld, Pro) and an atomic force microscope (AFM, Hitachi, AFM5100N).
3 Results and discussion
As mentioned above, the surface roughness of silicone rubber, slide glass and silicone resin taken by the AFM was 5.7, 2.6 and 2.4 nm, respectively; the roughness of silicone rubber is slightly large. Thus, one of the reasons for the texturing of Al thin film might be due to the roughened original surface of silicone rubber. However, in case of depositing an Au thin film on the flat silicone rubber, the texturing of Au thin film did not occur. Then, as another reason, we paid attention to the self-adhesive property of silicone rubber, which is originated in a low-molecular weight silicone. Generally, silicone rubber is one of the promising substrate materials for the laser induced forward transfer (LIFT) or printing method to form thin film of metals, because silicone is soft and its surface is sticky [22, 23, 24]. Thus, when the silicone rubber was used as a substrate for the vacuum evaporation, a sticking coefficient of Al atoms might be high to silicone rubber. Migration of the deposited Al atoms on silicone rubber would be hard, showing the textured structure.
The textured Al thin film was successfully formed on the periodic microswelling structure fabricated on silicone rubber by the 193-nm ArF excimer laser-induced photodissociation of silicone to realize the highly superhydrophobic property. The texturing of Al thin film occurred automatically during the vacuum evaporation only on the silicone rubber, compared with the cases of the slide glass and silicone resin substrates. As the result, the contact angle of water on the samples with the 10- and 30-nm-thick, textured Al thin films was estimated to be approximately 162° in both cases, compared to the sample without the textured Al of approximately 155°. Moreover, the large air gap layer was remarkably formed between the textured Al-deposited silicone and water; the droplet of colored water in red was successfully confined in the air gap to be released into the water when and where need.
We thank Wisnu Setyo Pambudi and Ryota Matsunaga, National Defense Academy, for their valuable assistance. This work was supported by JSPS KAKENHI Grant Number JP18K04790.
Compliance with ethical standards
Conflict of interest
The corresponding author states that there is no conflict of interest.
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