Tactoid formation in suspensions of Pf4 virus particles and the role of impurities

In this work, we first studied the synthesis of bent-core particles and if they could be used as model particles or for the formation of lyotropic liquid crystals. We used perturbation to obtain a kink, with temperature proven to be successful. We noticed a significant dependency with the base concentration and with the extension of TEOS pre-hydrolysis. However, these two parameters are hard to control, especially the second one, resulting in a synthesis hard to reproduce.
Our main goal was to understand the behaviour of tactoids (nematic droplets in an isotropic medium) formed in suspensions of sodium alginate and Pf4 virus and the role of impurities. This virus is of increasing interest due to its role in the protection of the pathogen P. aeruginosa against antibiotics. We explored the synthesis of spherocylindrical silica rods to mimic these bacteria and modified the particles to obtain positive or hydrophobic surfaces.
We then studied the phase behaviour of the tactoids with confocal microscopy to evaluate their growth, size, and to obtain a phase diagram. We found a dependence on the size and growth time with the concentration of polymer, in which a low concentration of sodium alginate led to slow kinetics and bigger tactoids. The number density of tactoids was related to the concentration of phage. We obtained an experimental phase diagram in which isotropic and biphasic regions were obtained, with the latter being further seen as tactoids or network structures. The comparison with the theoretical phase diagram showed good agreement.
Subsequently, we investigated the interaction of tactoids with different rods (bacteria B. subtilis and E. coli and synthetic silica and SU8 rods) as impurities. Four different configurations were found and classified as no attachment, partial attachment, sandwich and encapsulation. Partial attachment was the state seen most frequently and encapsulation was only seen for E.coli and SU8 rods. We found a relationship between the length of the tactoid and the length of the rod and the aspect ratio of the rod itself for each configuration. Changes in the surface of silica rods did not result in encapsulation, indicating the particle’s topography is also important. To further understand this problem, we explored the system theoretically, by looking exclusively at the elastic energy of the encapsulated configuration, and then by investigating all energy components of a free boundary tactoid. With this second model, all the states obtained empirically were found as well as a relationship with the anchoring strength. We further noticed that the rod prefers to be close to the tip of the tactoid since the proximity of the defects minimizes the energy of the system, with high aspect ratio rods having a smaller energetic cost. Furthermore, having a quasi-2D system alters the system and encapsulation was obtained for all the impurities evaluated.
Finally, we explored the rheological behaviour of the Pf4-sodium alginate system. The presence of tactoids resulted in an increase in viscosity, but the sample was still fluid-like. Differently, we saw a transition from solid- to liquid-like at concentrations where networks were formed. These results were compatible with the observations seen with the confocal microscope and this behaviour may relate to the biofilm’s ability to protect against pathogens.

Keywords: Pf4
liquid crystal
tactoids
Advisor: Dirk, A