Janus particles with large wetting contrast for self-assembly applications

• Main Author: Tan, Jasmine Si Jia
• Other Authors: Chen Zhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/137099
• Institution: Nanyang Technological University

Named after the Roman two-faced god, Janus particles are a class of materials that contain more than one property, either stemming through its core or on the particle surface. By being non-homogeneous, these particles have an inherent directionality and can self-assemble based on their environment. Given the numerous possibilities of property combinations on Janus particles, they have a potential to be used in a vast number of applications. For instance, a particle containing hydrophilic and hydrophobic domains on opposite hemispheres is a more effective emulsifier than a homogeneous particle containing only one of these domains. Despite decades of established research, the lack of commercialization of Janus particles is mainly hindered by the inability of existing methods to attain sufficient yields. Often, methods place an emphasis on creating highly uniform and monodisperse Janus particles, and this usually limits the generation of high yields. Therefore, the objectives of this research are to develop novel methods for the fabrication of Janus particles, such that the methods are scalable, with yields viable for practical applications. The principles behind the development methods are addressed, and characterization studies of the fabricated Janus particles are performed. The prepared particles are also examined based on practical applications. Throughout this work, four methods were developed to produce Janus particles with contrasting wettabilities. These methods are based on the same 3-step strategy conceived. Most reported methods employ an additive strategy that involves building up the particle or applying a partial surface coating. Here, an ultraviolet light based destructive strategy, which is novel in the field, is introduced. Particles are first pre-functionalized with a surface coating, then they are assembled at various interfaces in the second step. In the final step, the assembled particles are exposed to ultraviolet light, which induces the photodegradation of the surface coating. Semiconductor type metal oxide particles were examined, due to their photocatalytic abilities, which enable them to produce reactive species for the photodegradation process. Different alkylsilane-type surface functionalizations ranging from oleophilic-hydrophobic, oleophilic-superhydrophobic, to amphiphobic could be photodegraded. Owing to the unidirectional nature of light, only particle surfaces exposed to the light are photodegraded, leading to the recovery of the particles’ intrinsically amphiphilic surface. This leads to the formation of Janus particles, which contain partial coatings and amphiphilic surfaces. The four developed methods differ at the second step, where particles are assembled across the following interfaces: air-water, wax-water, water-silicone oil, and air-solid. A range of particle types were prepared: with partially oleophilic-hydrophobic, oleophilic-superhydrophobic, or amphiphobic surfaces in combination with partially amphiphilic surfaces. The obtained Janus particles are characterized based on their differences with unfunctionalized particles and fully-functionalized particles (not subjected to ultraviolet light). A variety of techniques, including contact angle analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, electron microscopy, and self-assembly in immiscible systems was employed. For application studies, the Janus particles were used to encapsulate cargo dissolved in water within an oil bulk. The resulting emulsions were stable for at least a month with no visible phase separation. On inducing emulsion destabilization over a month, the emulsions provided a gradual, controlled release of cargo. The release profiles were also different under different ionic and pH environments. Finally, the Janus particles were also incorporated in a working sunscreen formulation, where they facilitated a more even spread of sunscreen on skin, leading to better sun protection performance. This research provides a novel strategy to Janus particle fabrication. The strategy is envisioned to extend to other particle types, even non-photocatalysts, and on virtually any organic surface coating, making it highly versatile. The methods are scalable and can produce high yields. Though resulting particle uniformity is not heavily emphasized, the produced particles have collectively displayed Janus behaviour, which is more relevant to practical applications. This finding eases the requirement of strict particle assembly or anchoring during the process, which is a major limitation to generating high yield.