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.
Highlights
The interfacial tension γ between immiscible fluids is a material property with implications on a variety of fields, ranging from fundamental biophysics to applications in jetting dynamics
The interfacial tension of 5CB in the nematic phase was measured against aqueous solutions of sodium dodecyl sulfate (SDS), a surfactant used to achieve normal alignment, of varying concentrations
We have quantified the difference in interfacial tensions between the tangential and normal alignment cases of 5CB in pure water
Summary
The interfacial tension γ between immiscible fluids is a material property with implications on a variety of fields, ranging from fundamental biophysics (such as respiration and locomotion) to applications in jetting dynamics. Bubble deformation techniques [11,12] When it comes to long-range ordered liquids, liquid crystals (LCs), many of these techniques show significant shortcomings. Most of these issues are material-related, as the methods require data on other material parameters for calculating γ: the room temperature densities of common LCs can be extremely close to that of water [1,4,9,13], and any uncertainties in these values will propagate into uncertainty in the measurements [4]. Other techniques rely on other external properties, such as viscosity [10], which can show strong orientation dependence; others work only for specific LC phases [11,12]; and yet others, in particular rings and plates, require large amounts of material for baths and to ensure adequate contact with the apparatus, which is not appropriate when working with small quantities of precious liquids.
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