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

Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations

We present a study of a water-in-oil microemulsion in which surfactant coated water nanodroplets are dispersed in the isotropic phase of the thermotropic liquid crystal 5CB. As the temperature is lowered below the isotropic to nematic phase transition of pure 5CB, the system displays a demixing transition leading to a coexistence of a droplet rich isotropic phase with a droplet poor nematic. The transition is anticipated, in the high T side, by increasing pretransitional fluctuations in 5CB molecular orientation and in the nanodroplet concentration. The observed phase behavior supports the notion that the nanosized droplets, while large enough for their statistical behavior to be probed via light scattering, are also small enough to act as impurities, disturbing the local orientational ordering of the liquid crystal and thus experiencing pretransitional attractive interaction mediated by paranematic fluctuations. The pretransitional behavior, together with the topology of the phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model which describes the nanodroplets simply as holes in the liquid crystal.Comment: 64 pages, 16 figures, J. Chem. Phys. in pres

Modified Gellan Gum hydrogels with tunable physical and mechanical properties

Gellan Gum (GG) has been recently proposed for tissue engineering applications. GG hydrogels are produced by physical crosslinking methods induced by temperature variation or by the presence of divalent cations. However, physical crosslinking methods may yield hydrogels that become weaker in physiological conditions due to the exchange of divalent cations by monovalent ones. Hence, this work presents a new class of GG hydrogels crosslinkable by both physical and chemical mechanisms. Methacrylate groups were incorporated in the GG chain, leading to the production of a methacrylated Gellan Gum (MeGG) hydrogel with highly tunable physical and mechanical properties. The chemical modification was confirmed by proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy (FTIR-ATR). The mechanical properties of the developed hydrogel networks, with Young's modulus values between 0.15 and 148 kPa, showed to be tuned by the different crosslinking mechanisms used. The in vitro swelling kinetics and hydrolytic degradation rate were dependent on the crosslinking mechanisms used to form the hydrogels. Three-dimensional (3D) encapsulation of NIH-3T3 fibroblast cells in MeGG networks demonstrated in vitro biocompatibility confirmed by high cell survival. Given the highly tunable mechanical and degradation properties of MeGG, it may be applicable for a wide range of tissue engineering approaches.This research was funded by the US Army Engineer Research and Development Center, the Institute for Soldier Nanotechnology, the NIH (HL092836, DE019024, EB007249), and the National Science Foundation CAREER award (AK). This work was partially supported by FCT, through funds from the POCTI and/or FEDER programs and from the European Union under the project NoE EXPERTISSUES (NMP3-CT-2004-500283). DFC acknowledges the Foundation for Science and Technology (FCT), Portugal and the MIT-Portugal Program for personal grant SFRH/BD/37156/2007. HS was supported by a Samsung Scholarship. SS acknowledges the postdoctoral fellowship awarded by Fonds de Recherche sur la Nature et les Technologies (FQRNT), Quebec, Canada. We would like to thank Dr. Che Hutson for scientific discussions

Microfluidic production of endoskeleton droplets with controlled size and shape

© 2018 Elsevier B.V. Oil-in-water emulsion droplets, containing an elastic endoskeleton that holds the droplets in various non-spherical shapes, are formed by crystallizing a portion of the oil phase into a network of wax crystals. Such structures have recently been found to provide enhanced active ingredient delivery and shape-changing responsiveness, but robust methods of producing such droplets are needed that enable control of droplet size and shape. A continuous microfluidic flow is used here to produce endoskeleton droplets whose size is controlled by fluid flow rate and whose shape is varied between spheres, ellipsoids, and rods by control of exit temperature. A wide range of anisotropic shapes is produced using a single flow channel geometry by allowing the endoskeleton droplet to relax its deformation by varying degrees in response to fluid interfacial tension. Flexible production of shaped endoskeleton droplets will expand their application in enhanced delivery, deposition testing, and additive manufacturing processes

Anomalous field-induced particle orientation in dilutemixtures of charged rod-like and spherical colloids

Charged colloids in aqueous suspension often reconfigure counter-intuitively in the presence of an external electric field. Despite decades of substantial theoretical effort, a clear understanding of such electrokinetic phenomena is only available for the electric polarization and migration of diluted spheres in low fields. The interpretation of the electric response of charged rod-like particles is far less advanced. Here we report that diluted rod-like particles suspended in a sea of smaller spherical particles having similar charge align perpendicular to the external electric field. This \u2018anomalous\u2019 orientation, which contradicts the notion that charged rods in dilute suspensions always align parallel to external electric fields, seems to be universal in dilute mixtures of charged rod-like and spherical colloids. The dependence of the perpendicular orientation on frequency and particle charge, size and concentration indicates the presence of field-induced asymmetric crowding of spherical particles around each rod, which gives rise to asymmetric electro-osmotic flows that induce a negative torque

Colloidal fiber networks under stress : shear versus gravitational load

Colloidal gels are widely used in applications to modify the mechanical properties of fluids; in particular, they can provide yield stress, i.e. the ability to display solid-like properties at rest, while flowing upon application of a mechanical load. In this contribution we describe the yield behavior of a colloidal fiber network when it is subjected either to a macroscopic shear stress, or to a gravitational load obtained by inserting density-mismatched particles into the network. Delayed yielding is observed in both experiments. However, the time scales for the yielding process strongly differ from each other both on an absolute scale and in their dependence on the effective stress applied. We discuss how delayed yielding depends on background viscosity, intrinsic network relaxations, and on network porosity and elasticity