Janus Particles at the Air-Water Interface: Single particle behavior, particle-laden interfaces, and implications for self-assembly

Recent advances in fabrication techniques have allowed for the synthesis of anisotropic Janus particles with precise control of the wetting properties on each hemisphere. Interfacial assembly of Janus particles is of particular interest as the anisotropy and amphiphilicity of these particles can be tuned in order to drive self-assembly and thus be used to template microstructures in a bottom up fashion. The ability to pattern particles at interfaces has a multitude of applications such as achieving films with tunable optical properties, making antireflective surfaces, improving the resolution of E-paper displays and creating water repellant materials. Furthermore, anisotropic particles at interfaces can be applied as emulsion stabilizers and catalysts. In order for these applications to be realized, a better understanding of particle orientation, stability and particle-particle interactions is necessary. This thesis work is focused on understanding how orientation and amphiphilicity impact the behavior of individual Janus particles at the air-water interface as well as Janus particle-laden interfaces. Individual Janus particles are studied both theoretically, via free energy modelling, as well as experimentally, via colloidal atomic force microscopy. Here, we have found that Janus particles display higher interfacial stability than isotropic particles, even at non-equilibrium orientations. Additionally, the effect of amphiphilicity is orientation dependent with equilibrium orientations showing higher adhesion forces and contact angles as amphiphilicity increases, whereas the effect of amphiphilicity is negligible for non-equilibrium orientations. Interparticle interactions are studied using Janus particle-laden interfaces with various wetting properties on a Langmuir trough. From these studies we find that for Janus particles below a certain degree of amphiphilicity, almost half of the particles are observed to be in non-equilibrium orientations at the air-water interface. Particle orientation becomes increasingly uniform as amphiphilicity increases. Additionally, the collapse mechanism changes from irreversible wrinkling to reversible wrinkling and folding as we increase the hydrophobicity of the gold hemisphere of the particle. The knowledge gained from single and multi-particle experiments showcases how interfacial orientation and amphiphilicity can be used to stimulate future applications of anisotropic particles at interfaces. One remaining barrier to realizing these applications is synthesizing high quality patchy particles and nanoscale Janus particles. To this end, a density gradient centrifugation (DGC) technique has been tested and enables the synthesis of micron-scale patchy particles and Janus particles on the length-scale of light. The DGC technique will aid future work involving larger quantities of patchy particles and for studies of the application of Janus particles to the field of optics