Mechanical, pH and Thermal Stability of Mesoporous Hydroxyapatite

The stability of mesoporous hydroxyapatite (HAP) powder was studied following treatments of ultrasound, pH and heating. HAP was found to be mechanically stable up to (and including) 1 h continuous ultrasonic treatment in water. The HAP structure was also stable to pH, evidenced by practically identical XRD and FTIR spectra over the pH range 2–12. The surface area increased progressively with increasing acidity, reaching a maximum of 121.9 m 2 g −1 at pH 2, while alkaline conditions decreased the surface area to a minimum of 55.4 m 2 g −1 at pH 12. Heating in air had a significant influence on the structural and morphological properties of HAP, which underwent dehydroxylation to form oxyhydroxyapatite (OHAP) at temperatures ≥ 650 °C, and β-tricalcium phosphate (β-TCP) ≥750 °C. The surface area decreased at elevated temperatures due to agglomeration of HAP crystals by sintering, which was associated with an increased particle size

Origin of the negative wake behind a bubble rising in non-Newtonian fluids

International audienceThe present work aims at understanding the behavior of individual bubbles in non-Newtonian fluids. By means of a Particle Image Velocimetry (PIV) device, the complete flow field around either a single non-spherical bubble rising in polyacrylamide (PAAm) solutions or a solid sphere settling down in the same fluids shows for the first time the similar coexistence of three distinct zones: a central downward flow behind the bubble or the sphere (negative wake), a conical upward flow surrounding the negative wake zone, and an upward flow zone in front of the bubble or the sphere. This excludes then the possible influence of the interface deformation on the negative wake. A theoretical lattice Boltzmann scheme coupled to a sixth-order Maxwell model was developed for computing the complex flow field around a solid sphere. The good agreement with the experimental measurements provides evidence that the physical mechanism responsible for the negative wake in such fluids could be related to the fluid's viscoelastic properties

Passage of a Gas Bubble through a Liquid-Liquid Interface

International audienceThe present study aims at investigating the passage of a gas bubble at a plan liquid-liquid interface both experimentally by using a high-speed video camera and numerically through the volume-of-fluid (VOF) approach. A Newtonian silicone oil was used for the light phase while two different liquids, a Newtonian Emkarox (HV45) solution and a non-Newtonian poly(acryl amide) (PAAm) solution, were employed as the heavy phase. The passage of a gas bubble, generated from a submerged orifice, was followed during its rise in each liquid phase and in particular at the liquid-liquid interface. The original curve of the bubble's position vs time gave interesting insight into the dynamic behavior of the interface. Experimental results show the effect of the bubble size as well as the rheological properties of the heavy phase on the bubble's retention time at the liquid-liquid interface. The preliminary numerical results obtained by the VOF approach are in qualitative agreement with the experimental data