Fabrication of superhydrophobic soda-lime glass using femtosecond laser pulses

Abstract

One of the most critical surface properties for materials is water repellency or hydrophobicity. Depending on the wettability, surfaces can be classified as hydrophilic, hydrophobic, and superhydrophobic surfaces. A surface is called hydrophilic, if the contact angle of a droplet on the surface is less than 900; whereas the surface is called hydrophobic, if the contact angle is greater than 900. When the contact angle lies between 1500 and 1800, the surface is called superhydrophobic [1]. In this paper, to make superhydrophobic soda-lime glass, we fabricated periodic microgratings with width of ~ 8 μm and period of 10 μm on the soda-lime glass surface using a Ti:sapphire femtosecond laser, which emits ultra-short laser pulses at the central wavelength of 786 nm with a pulse repetition rate of 1 kHz and a pulse width of 183 fs. Periodic microgratings were fabricated in an 8 8 mm2 area at average pulse energy of 21 μJ. The laser beam was shone twice on the sample surface with a scanning speed of 5 mm/s. Fig. 1 shows the SEM images of the microstructured sample surface. Self-assembled periodic ripples were formed inside the microgratings, which were almost in the perpendicular direction to the laser shining direction, as shown in Fig. 1(b). The period of the ripples was ~ 5 μm, whereas the width was ~ 1 μm, as illustrated in Fig. 1(b). We measured the contact angle of a 4 μL water droplet on the unmodified sample surface and the microstructured surface, as depicted in Fig. 2. From Fig. 2, the contact angle of the water droplet on the microstructured soda-lime glass surface is 1550, whereas the contact angle on the unmodified sample surface is 100. This result exemplifies the superhydrophobicity of the microstructued soda-lime glass surface. In order to check the transparency of the microstructured sample surface, we measured the transmittance of light of the microstructured glass using a Spectroscopic Ellipsometer, which indicates that the transparency of the glass, containing periodic microstructures, is higher than 77% in the visible spectrum (wavelength range: 400 nm to 700 nm ).