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See detailRealignment of Liquid Crystal Shells Driven by Temperature- Dependent Surfactant Solubility
Sharma, Anjali UL; Jampani, Venkata UL; Lagerwall, Jan UL

in Langmuir (2019), 35(2019), 1113211140

We investigate dynamic director field variations in shells of the nematic liquid crystal (LC) compound, 4-cyano-4′-pentylbiphenyl, suspended in and containing immiscible aqueous phases. The outer and ... [more ▼]

We investigate dynamic director field variations in shells of the nematic liquid crystal (LC) compound, 4-cyano-4′-pentylbiphenyl, suspended in and containing immiscible aqueous phases. The outer and inner shell interfaces are stabilized by the cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), and by the water soluble polymer, poly(vinyl alcohol) (PVA), respectively. PVA and surfactant solutions normally promote tangential and orthogonal alignments, respectively, of the LC director. The rather high Krafft temperature of CTAB, TK ≈25 °C, means that its solubility in water is below the critical micelle concentration at room temperature in most labs. Here,we study the effect of cooling/heating past TK on the LC shell director configuration. Within a certain concentration range,CTAB in the outer aqueous phase (and PVA in the inner) switches the LC director field from hybrid to uniformly orthogonal upon cooling below TK. We argue that the effect is related to the migration of the surfactant through the fluid LC membraneinto the initially surfactant-free aqueous PVA solution, triggered by the drastically reduced water solubility of CTAB at T < TK.The results suggest that LC shells can detect solutes in the continuous phase, provided there is sufficient probability that thesolute migrates through the LC into the inner aqueous phase. [less ▲]

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See detailInfluence of head group and chain length of surfactants used for stabilising liquid crystal shells
Sharma, Anjali UL; Lagerwall, Jan UL

in Liquid Crystals (2018), 45(13-15), 2319-2328

We investigate the stability and textural development in nematic liquid crystal (LC) shells, with aqueous interior and exterior, as a function of the type and concentration of surfactant stabiliser of the ... [more ▼]

We investigate the stability and textural development in nematic liquid crystal (LC) shells, with aqueous interior and exterior, as a function of the type and concentration of surfactant stabiliser of the shell interfaces. The LC is the common thermotropic nematic 5CB and the surfactants are commercial, of cat- as well as of anionic type, with varying alkyl chain length. In addition to stabilising the shell interfaces, surfactants are generally assumed to promote radial (homeotropic) LC alignment, based on prior studies where the surfactant concentration was well above the critical micelle concentration (CMC). Here, we focus on the low-concentration range, below CMC. We find that both cat- and anionic surfactants can stabilise shells, although the higher water solubility of cationics can render stabilisation more difficult. We also conclude that surfactants do not necessarily impose homeotropic alignment; if the surfactant concentration is very low, the director may adopt planar alignment at the 5CB–water interface. Interestingly, the threshold concentration, where the surfactant takes control of alignment, is different for the shell inside and outside. Shells stabilised by solutions of surfactant with concentration near the threshold may therefore adopt a hybrid configuration, with homeotropic inside and planar outside. [less ▲]

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