Rupture Dynamics and Buckling Instabilities of Free-Standing Liquid Crystal Bridges and Filaments
Final Report Abstract
One-dimensional, free-standing fluid structures are not often found in nature, but may be formed by any material that can overcome capillary (Rayleigh-Plateau) instability. Once this instability is suppressed, long filaments, with length-to-diameter ratios exceeding 1000, may be formed. Quantitative knowledge of the way in which such filaments react to instabilities or compressive stresses has found necessity in the food industry for controlling large liquid sheets or ropes for mass production. Additionally, stable fluid filaments with novel optical and electrical properties hold potential as a new type of soft actuator, so further insights into the stability limits is paramount towards successful commercial development. Not only are viscous fluids, like polymer solutions, capable of forming such filaments/bridges, but also fluids that contain some sort of molecular ordering, such as liquid crystals. In fact, several liquid crystal phases were shown to form quite stable free-standing filaments, whose mechanical, acoustical and electrical stability were previously investigated. However, there are only a few studies on the rheological properties of filaments formed from complex liquid crystal phases. As these particular phases can potentially be exploited for applications, and the postrupture and buckling dynamics were already shown to be more complicated than that for viscous Newtonian fluids, it was paramount to achieve a complete rheological profile of the way in which the filament behaves. We designed a custom heating stage in order to measure the forces exerted on the filament under extensional and oscillatory perturbations. With this device, we could apply a strain rate range of 0.001–100^1/s , achieve a force resolution under 1 nN, use of a minimal sample volume of 1 nL, and maintain a temperature range from room temperature up to 200° C. We used a bent-core liquid crystal material derived from 2-nitroresourcinol that exhibits a so-called B7 (polarization splay undulating layer) liquid crystal phase between 110-170° C. This material forms stable free-standing filaments, and in combination with our device’s resolution, we could sufficiently explore the material’s rheological properties. Upon extension, we found that the filaments exhibited an initial linear viscoelastic regime, followed by a yield point and flow regime. Further investigation into this linear regime showed that the way in which the filament relaxes depends on temperature. For temperatures close to its isotropic liquid phase, the filament behaves as a viscoelastic fluid, whereas it behaves as a viscoelastic solid for mid-range temperatures and even a purely elastic beam for the lowest temperatures close to its crystalline phase. However, we did not expect that this difference would be so drastic, giving us the first indication that this temperature range might not manifest only one phase within it. Using small amplitude oscillatory shear (SAOS), we were able to elucidate the details, such as the storage/loss moduli and the loss coefficient, of the linear viscoelastic regime. To our surprise, we discovered further evidence for a phase transition between 120° C and 130° C, manifested by the loss coefficient increasing, rather than decreasing, at 120° C, as a function of applied frequency. Future work entails investigation into the non-linear viscoelastic effects, optical confirmation of the molecular selfassembly during this transition, and possible applications in the areas of soft robotics and smart textiles.
