Measuring the Anisotropy in Interfacial Tension of Nematic Liquid Crystals

Abstract

Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic interfacial tensions. Measuring the interfacial tension γ of an LC with conventional methods, such as pendant drops, can be challenging, however, especially when we need to know γ for different LC aligning conditions, as is the case when we seek Δγ, the interfacial tension anisotropy. Here, we present measurements of Δγ of the common synthetic nematic LC compound 5CB against water using a microfluidic droplet aspiration technique. To ensure tangential and normal alignment, respectively, we add poly(vinyl alcohol) (PVA) and sodium dodecylsulfate (SDS), respectively, as a stabilizer and measure γ for different concentrations of stabilizer. By fitting the Szyszkowski equation to the data, we can extrapolate to zero-stabilizer concentration, obtaining the γ of 5CB to pure water for each alignment. For normal alignment, we find γ⊥=31.9±0.8 mN·m−1, on the order of 1 mN·m−1 greater than γ||=30.8±5 mN·m−1 for tangential alignment. This resonates with the empirical knowledge that 5CB aligns tangentially to an interface with pure water. The main uncertainty arises from the use of polymeric PVA as tangential-promoting stabilizer. Future improvements in accuracy may be expected if PVA can be replaced by a low molar mass stabilizer that ensures tangential alignment.

Designing Biological Microsensors with Chiral Nematic Liquid Crystal Droplets

“…A limiting condition is that NLCs function normally as a binary switch, transitioning between a bright "on" state and a dark "off" state. Only in some extreme cases can transient states of tilted alignment be observed, making intermediate concentrations distinguishable, but a complete change of LC alignment is typically observed at concentrations well below the critical micelle concentration (CMC): 17,23 once full switching has occurred, it is difficult to extract further information, if any, from an NLC interface. 22,24 A less explored but equally interesting direction is the use of chiral nematic liquid crystals (CLCs), also known as cholesteric liquid crystals, for sensing applications.…”

Designing Biological Micro-Sensors with Chiral Nematic Liquid Crystal Droplets

“…Generally, for thermotropic liquid crystals (such as 5CB), k 33 > k 11 > k 22 35 . This means that the normal/homeotropic anchoring case is favoured in a droplet; however, the surface energy associated with normal/homeotropic anchoring (γ ⊥ ) is higher than for tangential/planar anchoring (γ ) 16 . The total energy is thus described by a combination of both the surface and the volume terms.…”

“…A full analytical treatment is beyond the scope of this paper; however, the effects of this are that, since γ < γ ⊥ , but k 33 > k 11 , there is a threshold of surfactant or amphiphile adsorption necessary to reduce γ so that the energy of the splay configuration is more favourable. Often this interfacial tension decrease is sufficient at very low concentrations of surfactant (on the order of μM 16 ) to induce switching, as shown in Figure 3(e). We notice the effects of this in terms of switching time, with smaller droplets switching somewhat more quickly.…”

“…The size of our droplets was in the range 14.3-87.4 μm (34.5±12.6 μm; mean ± standard deviation). In order to stabilize the droplets and prevent their coalescence (owing to the considerable LC-water interfacial tension 16,29 ), we used 87-89% hydrolyzed poly(vinyl alcohol) (PVA), a polymer typically used to stabilize LC shells and droplets while still preserving the anchoring conditions provided by pure water 30 . We therefore use PVA solution to suspend our droplets in future stages because it gives the same alignment as pure water.…”

“…A limiting condition is that NLCs function normally as a binary switch, transitioning between a bright "on" state and a dark "off" state. Only in some extreme cases, transient states of tilted alignment can be observed, making intermediate concentrations distinguishable, but a complete change of LC alignment is typically observed at concentrations well below the critical micelle concentration 11,16 : once full switching has occurred, it is difficult to extract further information from an NLC interface.…”