Vortex flow evolution in a growing microdroplet during co-flow in coaxial capillaries

International audienceUsing Micro-Particle Image Velocimetry (µPIV), the convective flow inside a silicone oil droplet was investigated in details during its formation in coaxial capillaries under co-flow in a water/glycerol mixture continuous phase. The analysis of µPIV measured flow field revealed that two characteristic flow areas exist in the droplet in formation: an inflow zone and a circulation zone. The intensity of vortex flow in these zones was estimated by calculating the average angular velocity of these vortices under the condition of no shear for different dispersed phase and continuous phase flow rates and for different viscosity ratios between the two phases. The evolution of the vortex flow pattern inside the droplet was investigated thoroughly all the way from the step of their formation to the step of free-moving droplet. The results of this study are important for understanding the mixing processes inside the droplet at different stages of its formation

Dripping and jetting of semi-dilute polymer solutions co-flowing in co-axial capillaries

This work is focused on the mechanisms of the dripping and jetting flow modes of viscoelastic semi-dilute polyacrylamide aqueous solutions co-flowing with silicone oil in co-axial capillaries. A phase diagram of the dripping, jetting, and intermediate flow modes is established. It was found that in the dripping mode, the elongation velocity of the filament between the terminal droplet and the inner capillary is controlled solely by the continuous phase rate. At the same time, the decrease in the filament diameter is due to both stretching and outflow of the polymer solution into the terminal droplet. In the jetting mode, the thread diameter was found to evolve in three stages. In the first stage, the average jet velocity increases, whereas in the second and third stages, it becomes constant and corresponds to the velocity of the continuous phase. The transition from the second to the third stage is defined by the appearance of capillary waves resulting in the formation of the beads-on-string structure. In the third stage, the filament diameter between the neighbor beads decreases exponentially and is governed by the relaxation time, which strongly depends on polymer concentration, but does not depend on the continuous phase flow rate. A simple physical model was proposed for describing the evolution of dimensions of filaments and beads during development of jet capillary instability. The universal character of the evolution of filaments and beads sizes, which is independent of concentration of semi-diluted polymer solutions and flow rates of the continuous phase, is revealed

Non Linear Strain Rate Dependency and Unloading Behavior of Semi-Crystalline Polymers

The classical viscoelastic constitutive equations do not simulate correctly the mechanical behavior of semi crystalline polymers, particularly in the case of the strain rate jumping or unloading to zero stress after tensile testing. The main experimental observations of mechanical response of polyethylene and polypropylene at small strains are reviewed. A physical model, which account for the deformation-induced evolution of the material structure, is developed and compared to the experimental observations. It is concluded that this model is the most adequate approach for the simulation of the mechanical behavior and the structure development of semi-crystalline polymers under loading and unloading

A novel numerical model for the prediction of patient dependent bone density loss in microgravity based on micro-CT images

The deterioration of the musculoskeletal system is a serious health concern for long-term space missions. The accumulated information over the past decades of space flights showed that microgravity impacts significantly the musculoskeletal system with muscle atrophy and bone loss. Until now, it has been difficult to make reasonable predictions of the bone loss for prolonged space missions due to the lack of in-space experimental data and weak understanding of the mechanobiological bone mechanisms. On earth, the healthy musculoskeletal degradation is mainly age related with osteoporosis and delayed fracture healing. A better understanding of the bone mechanobiological functions could help us improve our model predictions of the musculoskeletal health system during long-term space missions. We develop a numerical model able to predict the bone loss at the mesoscopic scale (bone trabecula) in microgravity. The model is able to correlate the calculated bone degradation mechanism with data available in the literature showing the effective bone density loss measured experimentally. An optimization algorithm is used for an average bone microstructure distribution and long-term prediction. Extrapolation is made to link the local bone loss at the structural scale with the corresponding effective bone strength. The first part of the paper details the extraction of the bone microstructure using micro-CT images and numerical model development. Next, the degradation and optimization schemes are detailed. Finally, some results are presented for long-term degradation