Differential Variance Analysis: A direct method to quantify and visualize dynamic heterogeneities

Many amorphous materials show spatially heterogenous dynamics, as different regions of the same system relax at different rates. Such a signature, known as Dynamic Heterogeneity, has been crucial to understand the nature of the jamming transition in simple model systems and is currently considered very promising to characterize more complex fluids of industrial and biological relevance. Unfortunately, measurements of dynamic heterogeneities typically require sophisticated experimental set-ups and are performed by few specialized groups. It is now possible to quantitatively characterize the relaxation process and the emergence of dynamic heterogeneities using a straightforward method, here validated on video microscopy data of hard-sphere colloidal glasses. We call this method Differential Variance Analysis (DVA), since it focuses on the variance of the differential frames, obtained subtracting images at different time-lags. Moreover, direct visualization of dynamic heterogeneities naturally appears in the differential frames, when the time-lag is set to the one corresponding to the maximum dynamic susceptibility. This approach opens the way to effectively characterize and tailor a wide variety of soft materials, from complex formulated products to biological tissues

Concentrated suspensions of Brownian beads in water: dynamic heterogeneities trough a simple experimental technique

Concentrated suspensions of Brownian hard-spheres in water are an epitome for understanding the glassy dynamics of both soft materials and supercooled molecular liquids. From an experimental point of view, such systems are especially suited to perform particle tracking easily, and, therefore, are a benchmark for novel optical techniques, applicable when primary particles cannot be resolved. Differential Variance Analysis (DVA) is one such novel technique that simplifies significantly the characterization of structural relaxation processes of soft glassy materials, since it is directly applicable to digital image sequences of the sample. DVA succeeds in monitoring not only the average dynamics, but also its spatio-temporal fluctuations, known as dynamic heterogeneities. In this work, we study the dynamics of dense suspensions of Brownian beads in water, imaged through digital video-microscopy, by using both DVA and single-particle tracking. We focus on two commonly used signatures of dynamic heterogeneities: the dynamic susceptibility, $\chi_4$, and the non-Gaussian parameter, $\alpha_2$. By direct comparison of these two quantities, we are able to highlight similarities and differences. We do confirm that $\chi_4$ and $\alpha_2$ provide qualitatively similar information, but we find quantitative discrepancies in the scalings of characteristic time and length scale on approaching the glass transition.Comment: The original publication is available at http://www.scichina.com and http://www.springerlink.com http://engine.scichina.com/publisher/scp/journal/SCPMA/doi/10.1007/s11433-019-9401-x?slug=abstrac

Microstructure and yielding of microfiber gels

Large aspect ratio cellulose nanofibers are able to a form poroelastic network at low volume fractions via aggregation and entanglement, forming a gel without significantly modifying viscosity[1]. The gels have a small but useful yield stress and a better ability to suspend particles than non-interacting higher volume fraction glasses[2] because the sparse fiber networks can significantly restructure at small strains. Yielding behavior can thus strongly depend on the fluid microstructure[3]. We study here deformation and yielding of aqueous cellulose fiber gels. Confocal imaging shows how gel yield stress relates to structural deformation rate because of localized network restructuring. Such response is advantageous to applications like surface coatings, nasal sprays, cosmetics, and foods. Understanding the mechanism of rate- and length-scale dependent yielding, and relating microstructure changes to bulk rheology[4], will enhance our ability to formulate, model, and design complex fluids with novel performance. References [1] - Solomon MJ, Spicer PT. Microstructural regimes of colloidal rod suspensions, gels, and glasses. Soft Matter, 6, 1391 (2010). [2] - Emady H, Caggioni M, Spicer P. Colloidal microstructure effects on particle sedimentation in yield stress fluids. J Rheol. 57, 1761 (2013). [3] - Joshi YM. Dynamics of colloidal glasses and gels. Annu Rev Chem Biomol Eng. 5, 181, (2014). [4] - Hsiao L, Newman RS, Glotzer SC, Solomon MJ. Role of isostaticity and load-bearing microstructure in the elasticity of yielded colloidal gels. Proc Natl Acad Sci, 109, 16029, (2012

Colloidal fibers as structurant for worm-like micellar solutions

We investigate the rheological properties of a simplified version of a liquid detergent composed of an aqueous solution of the linear alkylbenzene sulphonate (LAS) surfactant, in which a small amount of fibers made of hydrogenated castor oil (HCO) is dispersed. At the concentration typically used in detergents, LAS is in a worm-like micellar phase exhibiting a Maxwellian behavior. The presence of HCO fibers provides elastic properties, such that the system behaves as a simple Zener body, mechanically characterized by a parallel connection of a spring and a Maxwell element. Despite this apparent independence of the contributions of the fibers and the surfactant medium to the mechanical characteristics of the system, we find that the low frequency modulus increases with increasing LAS concentration. This indicates that LAS induces attractive interactions among the HCO fibers, resulting in the formation of a stress-bearing structure that withstands shear at HCO concentrations, where the HCO fibers in the absence of attractive interactions would not sufficiently overlap to provide stress-bearing properties to the system

Variations of the Herschel–Bulkley exponent reflecting contributions of the viscous continuous phase to the shear rate-dependent stress of soft glassy materials

We explore the flow behavior of concentrated emulsions for which the viscosity of the continuous phase can be significantly varied by changing the temperature. The exponents obtained by fitting the shear rate-dependent stress with the popular Herschel–Bulkley (HB) model display a systematic dependence on the viscosity of the continuous phase, revealing that viscous dissipation via the suspending fluid cannot be neglected in the description of the flow behavior of soft glassy systems. We thus propose a simple constitutive equation that accounts for three distinct dissipation mechanisms: elastic, plastic, and viscous dissipation. This three component model describes the flow behavior of soft glassy materials as accurately as the HB model, albeit maintaining a clear physical insight into the dissipation processes at work

Fluorocarbon Oil Reinforced Triple Emulsion Drops for Encapsulation and Retention of Small Molecules

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Fluorocarbon Oil Reinforced Triple Emulsion Drops

Fluorocarbon oil reinforced triple emulsion drops are prepared to encapsulate a broad range of polar and non-polar cargoes in a single platform. In addition, it is demonstrated that the fluorocarbon oil within the emulsion drop acts as an effective diffusion barrier, as well as a non-adhesive layer, enabling highly efficient encapsulation and retention of small molecules and active biomolecules in microcapsules.1111Nsciescopu

Encapsulation and Enhanced Retention of Fragrance in Polymer Microcapsules

Fragrances are amphiphilic and highly volatile, all of which makes them a challenging cargo to efficiently encapsulate and retain in microcapsules using traditional approaches. We address these limitations by introducing a new strategy that combines bulk and microfluidic emulsification: a stable fragrance-in-water (F/W) emulsion that is primarily prepared from bulk emulsification is incorporated within a polymer microcapsule via microfluidic emulsification. On the basis of the in-depth study of physicochemical properties of the microcapsules on fragrance leakage, we demonstrate that enhanced retention of fragrance can be achieved by using a polar polymeric shell and forming a hydrogel network within the microcapsule. We further extend the utility of these microcapsules by demonstrating the enhanced retention of encapsulated fragrance in powder state.113319Nsciescopu

Triple Emulsion Drops with an Ultra-Thin Water Layer: High Encapsulation Efficiency and Enhanced Cargo Retention in Microcapsules

1117Nsciescopu