Laser-Induced Pyrolytic Jetting of Porosity Variable Silica Thin Film for Fog Harvesting

Laser direct writing (LDW) is a renowned maskless manufacturing technology that employs laser beams for the direct creation of micro and nanostructured devices. It overcomes the limitations of conventional multi-step manufacturing processes such as lithography and molding. LDW has a significant impact in a multitude of sectors, spanning from microelectronics and optics to medical devices and beyond, thanks to its merits, including rapid prototyping, customization flexibility, precise control, and adaptability to various materials.<br/>For PDMS, its exceptional optical transparency across a broad spectrum of wavelengths fundamentally constrains the potential applications of laser patterning technology. However, recently, successive laser pyrolysis (SLP) has been proposed as an on-demand laser-based transparent polymer patterning process, relying on the controlled sequence of photothermal pyrolysis phenomena guided by a continuous-wave (CW) laser. After effectively removing the pyrolysis by-product, SLP offers the rapid digital patterning of high quality 2D and even 3D PDMS microstructures, which can be directly utilized without the need for supplementary post-processing steps.<br/>Moreover, the by-products generated in the SLP process, including SiC and the silica layer on the surface, present versatile opportunity for utilization as microstructure in various applications including surface treatment such as wettability modification. In our study, to harness the extensive utility of the surface silica layer, we employed the SLP process on a PDMS-glass hybrid substrate, facilitating the creation of a tunable silica film with controlled porosity through the pyrolytic jetting phenomenon.<br/>The PDMS-glass hybrid substrate is prepared through the deposition of a thin PDMS layer onto a soda-lime glass substrate via a spin-coating method. During the SLP process, the gas generated as a result of PDMS pyrolysis is effectively contained by the glass substrate, directing gas concentration towards the PDMS surface, thereby facilitating the requisite pressure for the ejection of the silica film. Furthermore, the glass substrate plays a dual role by not only acting as a gas barrier but also composing the PDMS layer, thereby optimizing the efficiency of the silica film pyrolytic jetting process by providing a stabilizing counterforce.<br/>In a more detailed approach, we have developed a microporous silica film with dimensions of approximately 10 μm through the optimization of laser power, laser scanning speed, and scanning interval, in the pyrolytic jetting process. Furthermore, by adjusting these variables, we achieved the capability to tune porosity under 50%. Through water contact angle measurements, it was determined that silica films with approximately 45% porosity exhibited hydrophilic characteristics, while those with around 10% porosity demonstrated hydrophobic properties.<br/>The microporous silica film was employed in a fog harvesting application, wherein it was affixed to a 2 cm x 2 cm surface area of a PTFE rod. Real-time measurements of the harvesting rate resulted in an average yield of approximately 55 mg/min. This level of performance aligns with that of several mesh-style fog harvesting applications sharing analogous structures, underscoring the potential viability of this approach in the field.