No abstract

A new time-optimal rheometry technique is presented. It consists in applying to the material a continuous exponential frequency sweep and analyzing its response by means of Fourier transforms. The properties of this method are that it takes the least possible time to perform the linear viscoelastic measurement in a given frequency range, and also presents an optimal signal to noise ratio in Fourier space. After validation against classical methods, it is used to characterize the gelation of a dental alginate. Properly time-resolved measurements of the frequency dependent viscoelastic modulii are presented. This allows to evaluate rigorously the gel point, using the Winter and Chambon criterion. Analysis of the data shows that a fractal networks forms inside the material after a lag time of about 350s, with a subsequent fast evolution (less than two minutes) towards the final structure.

Mixing of viscous non‐Newtonian fluids plays an important role in many industrial processes (wastewater treatment, methanization, etc.). In some cases, mixing by gas injection can be more interesting than mechanical mixing. The present study focuses on the gas injection in yield stress fluids. The influence of the air flow rate, fluid rheological properties, and geometrical configuration on an air jet impinging the bottom wall of a tank containing a yield stress fluid has been considered. Focus has been placed on the air cavity present at the injection point. The trends of two key parameters of the cavity have been characterized: its maximum diameter and frequency detachment. Correlations based on the characteristic dimensionless numbers governing the flow have been derived. These correlations show that the apparent viscosity has an effect on the cavity's frequency but a low influence on its diameter which is mainly governed by the air flow inertia.