An experimental investigation on inertia motion and deformation of Boger drops falling through Newtonian media

In the present study, deformations and drag coefficients of viscoelastic drops (Boger drops) with different elasticity falling into the air and oil are investigated experimentally. The main object of the present study is an evaluation of the fluid elasticity effects on drop dynamics by comparing the results with Newtonian cases (water, ethanol and aqueous solution of sodium dodecyl benzene sulfonate). A CMOS high-speed camera is used to capture images of falling drops and the cinematic calculations are performed using image processing. Here, a theoretical correlation is presented to describe the drag coefficient as a function of Reynolds number based on Newton’s second-law. In addition, effects of viscosity ratio ( k ), elasticity number ( En ) and Bond number ( Bo ) on the drag coefficient are studied. The results express that the drag coefficient increases by increasing viscosity ratio. For drops with the same viscosities, by increasing elasticity number and reducing Bond number, the drag coefficient is enhanced. It is also observed that the deformation of Newtonian drops falling in the air is periodic. These oscillations are caused by the interaction between surface tension forces and hydrodynamic pressure and the presence of internal circulation flows leading to instability on the surface of droplets. The amplitude oscillation for viscoelastic drops is remarkably lower than Newtonian drops. By increasing the viscosity of exterior fluid (when drops fall in the oil), periodic deformation of Newtonian drops is gradually damped and spherical drops are changed to elliptical ones. These changes are also observed for viscoelastic droplets.