André Luciani

Influence of shear and elongation on drop deformation in convergent–divergent flows

Abstract The effects of shear and elongation on drop deformation are examined through numerical simulation and experiment. A two-dimensional formulation within the scope of the boundary element method (BEM) is proposed for a drop moving under the influence of an ambient flow inside a channel of a general shape, with emphasis on a convergent–divergent channel. Both the drop and the suspending fluid can be either Newtonian or viscoelastic of the Maxwell type. The predicted planar deformation is found to provide accurate description of the physical reality. For example, small drops, flowing on the axis, elongate in the convergent part of the channel, then regain their circular form in the divergent part, confirming the experimental observations. Drops placed off-axis are found to rotate during the flow. These drops thus have longer residence time as well as larger and irreversible deformation than those moving on the axis. Both theory and experiment show a difference in deformability for Newtonian and viscoelastic drops in a slit flow. Initially, a Newtonian drop is reluctant to deform, but then deformation is rapid. A viscoelastic drop initially deforms readily, but then the deformation slows down. The slit flow does not flatten drops whose diameter is at least 10 times smaller than the slit gap. The effects of shear and elongation stress, the viscosity ratio, the drop diameter-to-channel-gap ratio, the initial drop position, the interfacial tension, and elasticity of the dispersed and ambient phases were examined using the BEM.

Fiber–fiber interaction in concentrated suspensions: Dispersed fiber bundles

A model is proposed to describe the rheology of planar randomly oriented concentrated fiber bundle suspensions in a shear-thinning matrix. The approach is that, at high fiber volume fractions, the dominant interaction mechanisms are friction and lubrication at the fiber–fiber contact points. A fiber pull-out technique is used to measure the force per unit length exerted on a single fiber tow of elliptical cross section embedded in a bulk suspension. By varying the tow velocity, fiber volume fraction, resin viscosity, and suspension structure, the factors affecting the lubrication and frictional components of the interaction forces were analyzed. The lubrication force is related to the flow behavior of the neat resin. The theoretical equations derived in this work allow for the computation of a shear viscosity of the suspension, which is in good agreement with experimental evidence. It is shown that dispersed fiber and dispersed fiber bundle suspensions are yield stress fluids.

A rheological characterisation technique for fast UV-curable systems

Using a rheometer coupled with a UV-light generator, a photo-rheometry set-up has been developed to study the viscoelastic properties of UV-coating systems during fast curing. Due to a high reaction rate, the viscoelastic properties have to be evaluated using a special procedure. This technique was found suitable to obtain reliable rheological data during the fast photo-reaction, allowing the determination of gel points occurring within less than 1 s.

Surface pattern formation in UV-curable coatings

Some UV-curable coatings display matte surfaces after cure if they have undergone a certain period of leveling at a temperature above their glass transition temperature and the melting point of any crystalline co-reagents present in the formation. The matte finish of these coatings is due to the presence of coherent surface wrinkles after cure, which are similar in appearance to those induced by differential thermal contraction, when a metal layer is sputtered onto a rubbery or viscous substrate. However, the wrinkles in the UV-cured coatings appear under isothermal conditions, and it is, therefore, inferred that they are due to the dynamics of internal stresses induced by through-thickness variations in the extent of curing.

Mixing homologous high viscosity ratio polymer blends in converging flow

Abstract Several studies have shown that deformation and breakup of dispersed droplets is easier in extensional than in shear flow field and this is particularly true for systems with high viscosity ratios. The simplest way to generate a significant extensional flow field in compounding equipment is to use converging flow conditions. In this work, the mixing efficiency of converging flow has been investigated as a function of the entry profile and the flow rate by using a capillary rheometer equipped with dies of different geometries. The material used for this study was a bimodal polyethylene presenting inhomo-geneities due to the high viscosity ratio between the low and high molecular weight fractions. The results indicated that the mixing performance depended strongly on the die geometry and flow rate. A critical flow rate was observed for each particular entry profile at which the highest efficiency was observed. This critical flow rate was found to correspond to the onset of melt flow instabilities.