Femtosecond laser interaction with polystyrene and its wettability characteristics
Date of Issue2017
School of Mechanical and Aerospace Engineering
NTU-SIMTech Joint Laboratory
Ultrashort pulsed laser polymer processing has attracted much interest recently. Ultrashort pulsed laser has an advantage in the processing of transparent and dielectric materials due to its high intensity and short pulse duration. Many investigations have been conducted in this area. However, there are still important issues yet to be clarified and resolved. These include a rigorous and quantitative investigation on the nonlinear absorption of laser power by the irradiated substrate, the associated thermal effects and heat conduction on the transparent materials (e.g. a transparent polymer). In addition, it is known that a short pulsed laser could modify the surface wettability of a polymer, but the linkage of the modification to nonlinear absorption has never been correlated. Polystyrene (PS) is a versatile polymer with many applications; many of these applications can take advantage of the different wettability if available. As PS is transparent to the wavelength of femtosecond laser employed for this investigation, PS is a natural choice for the current investigation. In the current study, the nonlinear absorption behavior of thick transparent PS samples were theoretically analyzed and experimentally investigated by applying the z scan technique. The thick PS sample was treated as a stack of thin samples and the nonlinear absorption behavior at different laser powers were investigated. The nonlinear absorption coefficient of PS was quantified to be 0.000695 m/W. The absorbed laser energy by the PS substrate will subsequently be converted to heat energy, and be conducted away. This femtosecond laser induced thermal effects during the laser-polymer interaction process have been investigated experimentally and by simulation. Experimentally, the temperature changes at different sample z positions associated with different laser intensities were directly measured using an infrared camera calibrated for temperature measurements. For the same incident laser power, it was observed that the temperature increased more when the sample was placed near to the focal position. This was due to the higher laser intensity at the focal position, resulted in higher laser power absorption and thus higher sample temperature. A 3D FE model based on the observed nonlinear absorption was constructed to simulate the heat conduction within the substrate at different z positions. The simulated results were compared with the experimental results. Good agreements were obtained between the simulated and experimental results at z = 2 mm (with ablation) and z = 4 mm (without ablation) with less than 2% difference between them. At z = 0 mm (with ablation), the simulated temperature was higher than the experimentally measured results, with a difference as high as 21.5%. This was due to laser-induced ablation with plasma formation at the surface at this high laser intensity. A simple estimate shows that only a small amount of energy was used for material vaporization during ablation. Instead, plasma generation during ablation was deduced to consume a significant amount of energy, but still much less than the energy absorbed by the substrate. Indeed, heat conduction as a result of the absorbed laser power was significant and dominant, with or without ablation. In addition to the investigation of laser-material interaction mechanism, the femtosecond laser-induced surface wettability modification of PS has also been studied. The surface water contact angle at different z positions (thus laser intensities) and absorptions were studied. The results show that at high laser intensity, the laser power absorption was high; the surface became very rough and hydrophobic. At low laser intensity, the surface became relatively much smoother and hydrophilic. For a rigorous investigation, different surface structures were created on the surface to modify the surface to either highly hydrophilic or superhydrophobic. The results show that with micro-scale grooves created on the sample surface, the surface became very rough resulting in hydrophobic and even superhydrophobic (WCA =156.2o) surfaces. Under these conditions, surface structure was the main factor that determines the surface wettability, and was consistent with the Cassie-Baxter’s model. The surface wettability of hydrophobic surfaces was examined to be stable over time after laser treatment as the surface structure remained the same over time. With properly spaced micro-pits created on the surface, the PS sample became more hydrophilic. The surface wettability increased with the depth, size and density of the micro-pits. This was consistent with Wenzel’s model. Since the pit depth was shallow, the substrate maintains high transparency after the laser treatment. After laser treatment, WCA recovery was found on hydrophilic surfaces, especially on highly hydrophilic surfaces. This was due to the decreased polar groups over time.