Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the first normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. Application of the latter to the normal stresses requires that the droplet diameter be known. At high shear rates the droplet diameter changes nearly inversely proportional to the shear rate. This results in a first normal stress difference proportional to shear rate and hence a ‘normal viscosity’ can be defined. This is used to compare the data with the available theoretical predictions. At low shear rates deviations from a constant normal viscosity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoelastic components have been used as well to investigate whether this would result in deviations from the behaviour observed for mixtures of Newtonian fluids.     

The rheological and microstructural characterisation of the non-linear flow behaviour of concentrated oil-in-water emulsions

This paper reports experimental observations on the rheology and microstructure of concentrated oil-in-water emulsions stabilised by macromolecular and low-molecular-weight emulsifiers. From the four different samples that were tested, certain general trends were identified and the measured linear viscoelastic and non-linear viscoelastic properties were compared using a phenomenological factored integral constitutive equation with a continuous relaxation spectrum. The self-consistency of the model was quite good in two cases (vegetable protein and polyoxyethylene glycol non-ionic surfactant emulsifiers) with the general exception of low steady shear rate data where the model overpredicted experimental stress measurements. It is shown that the fitting of the experimental data is sensitive to a wide range of inter-relating factors. In addition, optical observations of the sheared materials consistently showed low shear rate surface slip and this observation was correlated with the rheology miss match. For some systems the optical microstructure studies reveal a range of behaviour for the emulsions dependent on emulsifier composition. Wall slip, microdomain movement, chaining and changes in droplet size distribution were all observed under different conditions and in some cases it has been possible to correlate these microstructure observations with the sample rheology. Received: 10 November 1998 Accepted: 24 November 1998     

RHEOLOGICAL MODIFICATION OF EPOXY RESINS WITH NANO SILICA PREPARED BY THE SOL-GEL PROCESS*

Nano silica-modified epoxy resins were synthesized by the sol-gel process. The materials have themorphological structure of nano particales dispersed in the epoxy matrix. The dispersed phase formed aphysical network in the resin and thus influenced the rheological behavior greatly. However, the nano silicadid not show a significant influence on the mechanical properties of the cured resins.     

电解质和阳离子表面活性剂对MMH/粘土体系胶体性能的影响

通过流变参数、电泳淌度和pH值的测定,研究了NaCl、MgCl₂、Na₂SO₄等电解质和阳离子表面活性剂(十六烷基三甲基溴化铵)对铝镁混合金属氢氧化物(MMH)/钠土(Mt)悬浮体胶体性能的影响.发现所研究的电解质和表面活性剂都能使MMH/Mt悬浮体的Bingham屈服值降低,但不同处理剂影响pH值和滤失量的变化趋势却不同.认为电解质中的阳离子以影响粘土颗粒间的缔合为主,而阴离子以影响MMH的荷电性能为主,前者使滤失量上升,pH值下降;后者使滤失量下降,pH值上升.电解质对MMH/Mt悬浮体滤失量和pH值的影响取决于二者相对能力的大小.阳离子表面活性剂由于在粘土上吸附后不仅影响颗粒之问的缔合,也能使其润湿性反转,导致MMH/Mt悬浮体的滤失量和Bingham屈服值的变化幅度明显高于无机电解质.     

Unimer Exchange Is not Necessary for Morphological Transitions in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that—unexpectedly—unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.     

Transient and Dissipative Host–Guest Hydrogels Regulated by Consumption of a Reactive Chemical Fuel

The transient self-assembly of molecules under the direction of a consumable fuel source is fundamental to biological processes such as cellular organization and motility. Such biomolecular assemblies exist in an out-of-equilibrium state, requiring continuous consumption of high energy molecules. At the same time, the creation of bioinspired supramolecular hydrogels has traditionally focused on associations occurring at the thermodynamic equilibrium state. Here, hydrogels are prepared from cucurbit[7]uril host–guest supramolecular interactions through transient physical crosslinking driven by the consumption of a reactive chemical fuel. Upon action from this fuel, the affinity and dynamics of CB[7]–guest recognition are altered. In this way, the lifetime of transient hydrogel formation and the dynamic modulus obtained are governed by fuel consumption, rather than being directed by equilibrium complex formation.