Fluid Mechanics Seminar: Babak Nasouri & Emad Chaparian

Babak Nasouri: Analytical investigation of high-inertial gas-liquid droplet microflows in flow focusing geometries 

Confined gas-liquid droplet microflows present a lot of new perspectives for microfluidic systems that require the presence of a gaseous phase. In addition to the benefits associated with the discretization of reactive and sensing processes, the highly inertial droplets generated in these systems can enable fast efficient mixing by pair collisions as well as high system throughput due to the short convective timescales involved in the droplet transport. However, predicting the behavior of the droplet in gas-liquid cases is not trivial and obtaining a detached droplet with a specific flowrate is challenging. Thus, in this talk, I will focus on the physics behind the geometry-specific droplet generation in a gas-liquid flow for a flow-focusing configuration. I will first describe the simplified model of this phenomenon and employ the equations of motion to find the net forces acting on the droplet. Then, I will perform the scaling analysis to identify the dominant forces and investigate the required force balance for the detachment of the droplet. By comparing the analytical results to the experimental ones, I will show that the analytical model can predict the dripping regime, precisely. Lastly, I will extend the obtained results to evaluate the effects of geometry on droplet detachment and show why reaching dripping-regime is easier in converging-diverging geometries.

Babak is currently a Ph.D. student in Mechanical Engineering at UBC working on fluid-structure interactions at low Reynolds numbers. Previously he earned his bachelor’s degree from Sharif University of Technology in 2012 and M.Sc. from University of Texas at Austin in 2014.

Emad Chaparian: Taylor-Couette instability of thixotropic fluids 

Hydrodynamic instability of circular Couette flow first investigated by G.I. Taylor. He showed that when the angular velocity of the inner cylinder is increased above a certain number, Couette flow becomes unstable and a secondary flow (axisymmetric vortices), known as Taylor vortices emerges. I will present a study of effect of thixotropy on the Taylor-Couette instability: A linear stability analysis is invoked in which infinitesimally-small perturbations, represented by normal modes, are superimposed to the base flow and their time evolution is monitored to infer the onset of instability. An eigenvalue problem is obtained which is solved numerically using a pseudo-spectral, Chebyshev-based, collocation method. The neutral instability curve is plotted as a function of the material parameters in the Moore model. Based on the results obtained in this work, it is concluded that the difference between the upper and lower viscosities appearing in the Moore model can have a stabilizing or destabilizing effect on circular Couette flow depending on its magnitude and the gap size. For vanishingly small viscosity difference, circular Couette flow of Moore fluid is predicted to be more stable than its corresponding Newtonian fluid, due mainly to its shear-thinning behavior. An increase in the breakdown-tobuildup ratio in the Moore model (i.e., its thixotropic time constant) is predicted to have a destabilizing effect on circular Couette flow. 

Emad is currently a Ph.D. student in Mechanical Engineering at the UBC. Previously, he completed a B.Sc. degree at the Isfahan University of Technology in 2011 and an M.Sc. degree at the University of Tehran in 2013.