New ways to explore colloid anisotropy in self-assembly, active matter, Liquid crystals, Supraparticles and self-propulsion

Abstract

Establishing novel synthesis protocols to design a new type of colloids has become an important aspect of soft matter science. For instance, several processes (self-assembly/self-propulsion) in the colloid soft matter are governed by the size, shape, and composition of colloids. Therefore, to design such colloids in a controlled way is highly desirable. The primary objective of this thesis is to cover several synthesis methodologies of anisotropic colloids (ranging from few nanometers to few micrometers) and present their self-assembly/self-propulsion. In chapter 1, we present the review on several essential topics in colloid soft matter science and the main topics in the current dissertation. Considering the advantages of a seeded-growth procedure, we have developed a facile synthesis method to fabricate active matchstick type particles using hematite seeds and presented in Chapter 2. We explored the modified seeded growth method and produced monodisperse hematite-silica matchstick particles. In addition to the improved monodispersity, an important result of this synthesis is; all the hematite seed particles were oriented with an orientation of 90 degrees with respect to the long axis of the silica rods. Such orientation of the hematite seeds is useful to study the self-assembly under the influence of external magnetic fields. The details of the synthesis and effect of partially hydrolyzed silica precursor (TEOS) on the orientation of seed at the tip of the silica rod are presented in Chapter 3. Inspired by the interesting role that partially hydrolyzed TEOS can play in the synthesis of the rods as was shown in our group before, we developed a versatile synthesis route to make the normal silica rods into more interesting silica rods in different ways; such as matchstick silica rods with nanoparticles filled heads. The synthesis details were explained in chapter 4. Anisotropic rod-like molecules are well known for forming LC phases, and these phases are widely studied by using rod-like colloids as a model system. The best known and widely exploited are uniaxial nematic phases, which have applications in displays. Similarly, bent-core mesogens or banana-shaped molecules are a popular topic of study in the LC community as they are predicted to form interesting biaxial and other new nematic phases upon assembly. However, due to experimental limitations, these phases cannot be investigated experimentally on the single-particle level. In Chapter 5, We introduce a robust synthesis route to make bent silica rods (BSR) and present the essential parameters to tailor the bending angle and length of each segment. In Chapter 6, we studied the phase behavior of the improved bent silica rods mentioned above (synthesized in chapter 5) and analyzed the interesting LC (splay-bend nematic) phases that were not yet observed on the molecular level. Since the particles were fluorescently labeled, we could examine the self-assembly of the sedimented BSRs using fluorescent confocal microscopy on the particle level. In chapter 7, we study the SA of sharp and rounded cube-shaped iron oxide NPs in spherical confinement and studied the dependence of SA on the shape of the individual building blocks.