Liquid Crystal Shells: from Physics Mysteries, via Chemistry Challenges, to Biosensing Opportunities

Doctoral thesis (2021)

Liquid crystals (LCs) confined in a spherical geometry between two aqueous phases, i.e., a shell, exhibit unavoidable topological defects, real or virtual. In fact, the presence of defects, and their ... [more ▼]

Liquid crystals (LCs) confined in a spherical geometry between two aqueous phases, i.e., a shell, exhibit unavoidable topological defects, real or virtual. In fact, the presence of defects, and their configuration, on the shell are dictated by the LC alignment at the LC-water boundaries. With tangential alignment of the LC, we have a total of +2 defect strength on each interface, which can be distributed either over four +1/2 defects or a combination of +1 and +1/2 defects. In contrast, when the LC is radially aligned, the shell is defect free (but one can extrapolate the director field into a virtual point defect in the internal isotropic phase). The LC alignment can be tuned by adding a suitable solute in the aqueous phase.The vast majority of studies have used sodium dodecyl sulfate (SDS) for this purpose, with only a few studies employing other surfactants. We investigate various other surfactants with a positive and negative head group charge by varying alkyl chain length. Here, we focus on the minimum surfactant concentration required (below the critical micelle concentration (CMC)) for the stability of the shells as well as for ensuring the radial alignment. It turns out that surfactants at a low concentration do not necessarily impose radial alignment. However, they provide stability by decreasing the interfacial tension. Once the surfactant concentration is above the CMC, surfactant forms micelles in water. When these micelles are in the outer aqueous phase of the hybrid aligned shells, the LC alignment changes from hybrid to radial on cooling below the Krafft temperature (TK), below which the surfactant solubility is lower than CMC. Interestingly, on heating above TK, the LC alignment changes back to hybrid. It turns out that the adsorption and desorption of micelles as a result of changed solubility in water with temperature give rise to the change in the LC alignment. Next, we replace surfactant with lipids. We add lipids in the form of vesicles in the aqueous phase of pre-tangentially aligned LC shells. After some time these shells develop small spindle-shaped islands, and these islands merge and grow with time, and they even interact in intriguing ways across the LC, from the inside to the outside. After exploring various solutes in the aqueous phase, we change our LC phase from nematic to smectic A and C phases. In the smectic A phase, lunes on the shell occur due to the change in the shell cross section area from the inner to the outer interface. In this context, we present the first study on tangentially and hybrid aligned smectic C shells. The most studied LC mesogens in the shell geometry contain cyano and biphenyl groups. To test whether this is important, I also made shells with cyclohexane core mesogens. In contrast to the cyanobiphenyl-based LCs, this LC aligns radially regardless of solutes added into the aqueous phases. We investigate in detail and find that a cyano group at the terminal supports shell stability and while the aromatic rings in most LCs, for instance having a biphenyl core, give rise to the tangential alignment when in contact with surfactant-free water. [less ▲]