Nanoseminar dr. Rama Kotni

Speaker: dr. Rama Kotni (Soft Condensed Matter)

Title: New ways to explore colloid anisotropy in self-assembly 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 physical processes (self-assembly/self-propulsion) in the colloidal systems are governed by the size, shape, and composition. Therefore, designing such colloids in a controlled way is highly desirable.  

The primary objective of this talk is to present several synthesis methodologies of anisotropic colloids with a main focus on rod-like colloids, to use them in self-assembly/self-propulsion. Anisotropic rod-like molecules are well known for forming liquid crystal (LC) phases and these phases are extensively studied by using rod-like colloids as a model system. Here, we established a synthesis protocol to make monodisperse hematite-silica match-stick particles. We explore the self-assembly and self-propulsion of hematite silica match-stick particles in hydrogen peroxide by trigging the photocatalytic activity of the hematite present at one end of the rod, using blue/UV light illumination. Furthermore, we establish a synthesis protocol for a new type of silica matchstick-type rods with nanoparticles filled inside. Compared with pure silica rods, these rods are interesting since they have a functional head. We study the functionality of these rods by studying their self-propulsion in dilute hydrogen peroxide (H2O2) solution.  

In addition to the conventional LC phases, bent-core mesogens or banana- or boomerang-shaped molecules have also become popular in the LC community as they can form or have been predicted to form interesting biaxial nematic phases upon assembly. Compared with conventional nematic phases, both the biaxial and the splay- and twist-bend nematic phases have intriguing new kinds of applications because of the more complex, often optical, behavior. However, these phases have not yet been studied experimentally on the single-particle level which could lead to new information and insights. Motivated to realize model systems that could form these complex LC phases and can be studied on the single-particle level, we presented a new synthesis approach to fabricate fluorescently labeled bent silica rod-like particles (BSRs). If the rod-like particles composing the LC become bent, bend deformations arise in the particle orientations, yielding a geometrical frustration which may in turn lead to the formation of intriguing nematic phases that also incorporate twist or splay deformations (the twist-bend (NTB) and splay-bend (NSB) nematic phases). Here, we show for the first time in experiments that NSB phases can be stabilized by systems of polydisperse micron-sized bent silica rods. Our results open avenues for the realization of NTB and NSB phases of colloidal and molecular LCs.