Experimental investigation of non-Newtonian droplet collisions: the role of extensional viscosity

Giulia Finotello,Shauvik De,Johan T Padding,Jeroen C R Vrouwenvelder,Alfred Jongsma,J A M Kuipers,Fredrik Innings,Kay A Buist

doi:10.1007/s00348-018-2568-2

Giulia Finotello, Shauvik De + Show 6 more

Open Access

https://doi.org/10.1007/s00348-018-2568-2

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Abstract

We investigate the collision behaviour of a shear thinning non-Newtonian fluid xanthan, by binary droplet collision experiments. Droplet collisions of non-Newtonian fluids are more complex than their Newtonian counterpart as the viscosity no longer remains constant during the collision process. Despite the complex collision dynamics, we are able to present a complete regime map based on non-dimensional Weber (We) number and impact parameter (B). We compare the collision outcomes of xanthan, glycerol and a milk concentrate at similar impact conditions. These experiments reveal very rich and complex collision morphologies for shear thinning xanthan solution, strikingly different from Newtonian droplet collisions. Unlike glycerol and milk, xanthan collisions show no reflexive separation even at very high We number. Instead of breakup, we observe disc-like shapes with an oscillating behaviour of the colliding droplets. A detailed analysis reveals that this outcome is related to increased viscous energy dissipation and extensional effects.

Highlights

The study of droplet–droplet collision behaviour has been of considerable interest for more than a century because of its complexity as a fluid dynamics problem and because of its numerous industrial applications
In the case smaller satellites were visible in the recording before collision or if the droplet diameter difference in the stream was greater than 4%, the recordings were discarded and a new optimal frequency was individuated
Binary collisions between droplets of non-Newtonian shear thinning xanthan have been experimentally investigated with jet generation and break-up equipment

Summary

Introduction

The study of droplet–droplet collision behaviour has been of considerable interest for more than a century because of its complexity as a fluid dynamics problem and because of its numerous industrial applications. The reliable prediction of the droplet collision outcome is essential to control and manipulate the droplet size distribution to achieve the desired performance, Munnannur and Reitz (2007). Powders are often manufactured from highly viscous liquid suspensions which are non-Newtonian in nature. Non-Newtonian droplet collisions occur, for example, during the production of powdered milk. The highly viscous suspension is atomized with heated air through the pressure nozzle in the drying chamber. The very small droplets are dried by the hot air fed to the drying chamber. The prediction of the collision outcomes enables the production of high quality powder with optimized characteristics suited for the intended application

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