Extensional flow of nematic liquid crystal with an applied electric field

Systematic asymptotic methods are used to formulate a model for the extensional flow of a thin sheet of nematic liquid crystal. With no external body forces applied, the model is found to be equivalent to the so-called Trouton model for Newtonian sheets (and fibres), albeit with a modified ‘Trouton ratio’. However, with a symmetry-breaking electric field gradient applied, behaviour deviates from the Newtonian case, and the sheet can undergo finite-time breakup if a suitable destabilizing field is applied. Some simple exact solutions are presented to illustrate the results in certain idealized limits, as well as sample numerical results to the full model equations.

Extensional Viscosity of Coating Colors and its Relation With Jet Coating Performance

An orifice flowmeter was used to measure the extensional viscosity of several non-pigmented fluids and paper coating colors containing calcium carbonate as pigment in the context of a jet coating application. The orifice flowmeter was first calibrated in terms of a dimensionless Euler number versus Reynolds number curve with Newtonian fluids. The calibration curve was then used to determine the apparent extensional viscosity of coating colors. In the strain rate range investigated, all the fluids were found to exhibit strain-thinning and the Trouton ratio of the coating colors was in the range 5 to 20. Jet coating tests were also carried out in order to evaluate the effect of the extensional viscosity on the jet performance. The extensional viscosity was shown to be a key parameter determining the configuration of the downstream meniscus in the web contact region.

Extensional Properties of Coating Colors at High Strain Rates

Paper coating fluids also called colors are concentrated aqueous suspensions composed mainly of mineral pigments, thickeners, binders and dispersing agents. They are applied onto moving paper web for improving the optical and printing properties. Roll coating is one of the most used technologies for paper coating, however jet coating is currently a promising technology for high-speed processes. Coating colors are submitted to high strain rates in both roll or jet coaters, therefore the extensional viscosity plays a major role in the process. An orifice flowmeter was used for measuring the extensional properties of complex rheology fluids such as coating colors. The principle of this flowmeter is based on the relationship between pressure drop and the flow rate of fluid passing through a small orifice. The flowmeter was firstly calibrated in terms of a dimensionless Euler number as a function of the Reynolds number with Newtonian fluids. The calibration curve was then used to determine the apparent extensional viscosity of coating colors. Results of extensional properties of paper coating colors are presented and compared to shear viscosity. The ratio of extensional to shear viscosity (Trouton ratio) for some coating colors was shown to exceed considerably the theoretical value of 3 expected for Newtonian fluids.

Rheological properties of suspensions with spherical particles in shear and elongational flows

The rheological behaviour of model suspensions with spherical particles was experimentally investigated in shear an elongational flows. Particular attention was focussed on the main parameters affecting the flow behaviour of suspensions such as particle size distribution, particle size, particle surface, humidity, temperature and viscosity of the matrix fluids. All variables were investigated depending on the pre-shear conditions. In this regard the validity of the time-temperature-superposition and the Trouton-ratio was verified for suspensions with spherical particles.

An Investigation of the Effect of Elongational Viscosity on Entrance Flow

A new model for strain-rate dependence of elongational viscosity of a polymer is introduced. The proposed model can capture the initial strain thickening, which is followed by a descent in elongational viscosity as the elongation rate is further increased. Effect of the four rheological parameters in the new model on a 4:1 entrance flow is analyzed. It is confirmed that the entrance pressure loss and recirculating vortices in an entrance flow grow significantly as the Trouton ratio is increased. The center-line velocity near the abrupt contraction in a 4:1 entrance flow is found to overshoot its value for a fully developed flow in the downstream channel, if the Trouton ratio has a local minima beyond the Newtonian limit of the polymer.