The iridescent metallic green beetle, Chrysina gloriosa, which selectively reflects left circularly polarized light, possesses an exoskeleton decorated by hexagonal cells (approximately 10 microm) that coexist with pentagons and heptagons. The fraction of hexagons decreases with an increase in curvature. In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the core disappears in dark field. With use of confocal microscopy, we observe that these cells consist of nearly concentric nested arcs that lie on the surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. These textures provide the basis for the morphogenesis as well as key insights for emulating the intricate optical response of the exoskeleton of scarab beetles.
We show that visualization and analysis of capillary-driven thinning and pinch-off dynamics of the columnar neck in an asymmetric liquid bridge created by dripping-onto-substrate can be used for characterizing the extensional rheology of complex fluids. Using a particular example of dilute, aqueous PEO solutions, we show the measurement of both the extensional relaxation time and extensional viscosity of weakly elastic, polymeric complex fluids with low shear viscosity η < 20 mPa·s and relatively short relaxation time, λ < 1 ms. Characterization of elastic effects and extensional relaxation times in these dilute solutions is beyond the range measurable in the standard geometries used in commercially available shear and extensional rheometers (including CaBER, capillary breakup extensional rheometer). As the radius of the neck that connects a sessile drop to a nozzle is detected optically, and the extensional response for viscoelastic fluids is characterized by analyzing their elastocapillary self-thinning, we refer to this technique as optically-detected elastocapillary selfthinning dripping-onto-substrate (ODES-DOS) extensional rheometry.A ddition of a dilute amount, even 1−400 ppm (parts per million), of a high molecular weight polymer like poly(ethylene oxide) (PEO, M w > 10 6 g/mol) to a solvent like water is observed to significantly change the fluid response to extensional or stretching flows. 1 Examples include enhanced pressure drop in porous media flows, 1a suppression of rebound in drop impact studies, 2 a discernible birefringence around a stagnation point in cross-slot flows, 3 delayed breakup in dripping, spraying or jetting, 1b,4 and possibly turbulent drag reduction. 5 The influence of polymers is even more remarkable for dilute, aqueous solutions as the measured shear viscosity η(γ) appears to be Newtonian, and elastic modulus, relaxation time, and the first normal stress difference are not measured, or manifested, in steady shear or oscillatory shear tests carried out on the state-of-the-art torsional rheometers. 6 Macromolecular solutions typically exhibit a large and measurable resistance called extensional viscosity, η E , to streamwise velocity gradients characteristic of extensional flows 1b,7 and undergo stress relaxation with a characteristic extensional relaxation time λ E . However, for dilute, aqueous solutions, quantitative measurements of both η E and λ E remain beyond the capability of commercially available devices like CaBER (capillary breakup extensional rheometer). A countable few measurements of extensional relaxation time in dilute aqueous solutions presented in the recent literature 6,7d require bespoke instrumentation not available or easily replicable in most laboratories. The aim of the present study is 3-fold: to describe an extensional rheometry protocol that can be recreated virtually in any laboratory (quite inexpensively for high viscosity fluids), to characterize the extensional viscosity and extensional relaxation time for dilute, aqueous polymer solutions, and to pr...
Globular proteins influence the flow, microstructure, phase behavior and transport of biofluids and biomolecules in the mammalian body. These proteins are essential constituents of food, drugs and cosmetics, and their dynamics determine the physical properties and application of these multicomponent materials. In conventional rheological studies conducted using typical geometries on torsional rheometers, solutions of globular proteins are commonly reported to have a solid-like response at concentrations as low as 0.03% by weight. Typical explanations invoke the presence of long-range repulsions that are stronger than electrostatic interactions. In this study, we probe the bulk and the interfacial viscoelasticity of surfactant-free bovine serum albumin (BSA) solutions using a stress-controlled torsional rheometer, augmented by microfluidic rheometry and interfacial rheometric measurements. We demonstrate that the origin of this yield-like behavior, which is manifested as a highly shear-thinning bulk rheological response, lies in the formation of a film of adsorbed protein, formed spontaneously at the solution/gas interface. We provide direct interfacial rheometric measurements to study the concentration-dependent viscoelasticity of the adsorbed protein and we describe a simple, but quantitative, additive model useful for extracting the interfacial viscosity contribution from bulk viscosity measurements over a wide range of shear rates.
Stream‐wise velocity‐gradients associated with extensional flows arise in thinning liquid necks spontaneously formed during jetting, printing, coating, spraying, atomization, and microfluidic‐based drop formation. In this contribution, we employ Dripping‐onto‐Substrate (DoS) rheometry protocols to measure the extensional rheology response of intrinsically semi‐dilute polymer solutions by visualizing and analyzing capillary‐driven thinning of a columnar neck formed between a nozzle and a sessile drop. We show that extensional viscosity that quantifies the resistance to stream‐wise velocity gradients is orders of magnitude higher than the shear viscosity. Although shear flows only weakly perturb the chain dimensions, extensional flows can strongly stretch and orient the chains, thus influencing both intra‐ and inter‐chain interactions. We find that the extensional relaxation times for intrinsically semi‐dilute PEO solutions in a good solvent for five different molecular weights increase linearly with concentration, exhibiting a stronger concentration dependence than observed for dilute solutions, or anticipated by blob models, developed for relaxation of weakly perturbed chains in a good solvent. The observed distinction between the concentration‐dependent relaxation dynamics of intrinsically dilute and semi‐dilute solutions arises due the complex influence of stretching, conformational anisotropy, and polymer concentration on excluded volume and hydrodynamic interactions of flexible, highly extensible polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1692–1704
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