Fabrication and Applications of Multifunctional Superhydrophobic Surfaces Based on Surface Chemistry and Morphology

Superhydrophobic surfaces are gaining great interests in both fundamental researches and technological applications, because of their unique non-wetting and self-cleaning properties. By mimicking the hierarchical surface structure of the natural superhydrophobic surface, i.e. lotus leaf, numerous artificial surperhydrophobic surfaces were developed. However, the challenge is how to fabricate superhydrophobic surfaces by a scalable and economical method. To address this challenge, our group has developed methodologies that enable the fabrication of superhydrophobic surfaces in inexpensive and potentially scalable ways, such as lamination and 3-D printing. To expand on applications, we also combined other desired functionalities into the superhydrophobic surfaces. The transparent superhydrophobic surface has a great advantage of highly visible light transmittance, which make it have potential applications for solar-cell panels, optical lens, and automobile windshields, etc. Superhydrophobicity can be achieved by constructing hierarchical roughness on the surface of low surface energy material. However, the roughness may increase light scattering and lower the transparency. To minimize the affection on transparency, roughness at small scales, i.e. nanometers, is required. In Chapter 2, I discuss the fabrication of a transparent superhydrophobic surface by dip-coating and lamination method. The polymer substrate is first coated with a layer of silica nanoparticles; the following lamination process makes the nanoparticles partially embedded into polymer substrate which increases the mechanical stability. The transparency was measured by UV-Vis spectroscopy. The surface morphology was characterized by scanning electron microscope and atomic force microscope. The mechanical stability of fabricated transparent superhydrophobic surface was evaluated by using a water flushing method. Photocatalytic properties can also be integrated into superhydrophobic surfaces, which will enhance the self-cleaning property by removing the contaminations through photo-oxidation reactions. Photocatalytic superhydrophobic surfaces also have potential applications in water disinfection, treatment of organic waste solutions, and photodynamic therapy. In Chapter 3, two different methods were developed to fabricate photocatalytic superhydrophobic surfaces: 1. The nanocomposite of TiO2 and polymer was created by a lamination method. The surface roughness was controlled by templating during the lamination. Surfaces also fabricated without the templating process. All the surfaces exhibited reversible wettability and photocatalytic properties; 2. Photocatalytic particles (TiO2 or silicon-phthalocyanine) were immobilized on the surfaces of printed polydimethylsiloxane cone shape posts. The triple-level roughness (posts, particle aggregates, and individual particles) make the fabricated surface superhydrophobic and maintaining stable Cassie state during photo-reactions. In a specially designed three-phase photo-reactor, photocatalytic reactions such as photooxidation of Rhodamine B and bovine serum albumin, and singlet oxygen trapping were studied as a function of gas phase composition. The effect of bubbling through the liquid phase, which facilitates the transmission of reactive species were also discussed in Chapter 3. Base on the photocatalytic TiO2/Polymer nanocomposite film we have made, we demonstrated an application of this film in photodegrading waste organic dye solution generated in biology teaching laboratories. Furthermore, we developed a laboratory module for an undergraduate analytical chemistry lab course. In this course, students will learn about the TiO2 photocatalytic mechanism; degrade waste solutions collected from laboratories using sunlight and theTiO2/PE catalytic bags and investigate the degradation efficiency using UV-Vis absorption spectroscopy measurements. On a superhydrophobic surface, an aqueous droplet (µL) can maintain a nearly spherical shape without wetting the surface. This geometry creates a unique environment in which chemical reactions at the solid-liquid-vapor interphase can be studied. Two types of superhydrophobic surfaces were fabricated using modified 3-D printing methods. In one case, which is discussed in Chapter 4, functionalized superhydrophobic surfaces were fabricated in which reactive particles are partially embedded into the printed PDMS posts. On this surface, interactions between the solid surface and solute molecules were studied as a function of convection within the droplet. In the second case, which is discussed in Chapter 5, glass pedestals were attached to the top of each PDMS post in the array. These glass pedestals enable the precise dispensing of nanoliter (25.0 nL ± 0.5 nL) droplets. This surface can also support larger (\u3e 1µL) droplets while exhibiting contact angles \u3e150°. Evaporation of droplets promotes the concentration of dilute solute molecules into a well-defined region that facilitates the identification of biopolymers in quantities as low as 5 attomoles, by MALDI-TOF mass spectrometry. In addition, this surface can be functionalized to selectively bind specific biomolecules that can be subsequently identified by MALDI-TOF. This type of surface is especially useful for working with precious fluids such as venom from snakes and spiders. With the advantage of precise dispensing of nanoliter droplets, we further improved the dispensing system by printing only PDMS post arrays of a special morphology structure on a glass slide to form a nano-Droplet Array Plate (nDAP). By using the nDAP dispensing system, I was able to study the effect of surfactant chemistry on the distribution of hydrophobic microbeads (35 µm) in the aqueous droplet and the dispensing properties. The number of microbeads dispensed was controlled by tuning the relative concentration of microbeads and surfactant. Multiple single-bead dispensing was achieved at optimized conditions. This work is discussed in Chapter 6