Paper Number
NF23 

Session
Non-Newtonian Fluid Mechanics & Flow Instabilities

Title
Viscous fingering instabilities in carbon black gels

Presentation Date and Time
October 17, 2018 (Wednesday) 3:45

Track / Room
Track 7 / Plaza II

Authors (Click on name to view author profile)

1. Marsit, Badis (MIT, Mechanical Engineering)
2. Kaloga, Yacouba (MIT, Mechanical Engineering)
3. Bischofberger, Irmgard (MIT, Mechanical Engineering)
4. Divoux, Thibaut (CNRS Bordeaux, CNRS-MIT, MSE2)

Author and Affiliation Lines (in printed abstract book)
Badis Marsit¹, Yacouba Kaloga¹, Irmgard Bischofberger¹, and Thibaut Divoux^2
1Mechanical Engineering, MIT, Cambridge, MA 02139-4307; ²MSE2, CNRS Bordeaux, CNRS-MIT, Cambridge, MA

Speaker / Presenter 
Divoux, Thibaut

Text of Abstract
Pattern formation in fluids occurs in numerous physical processes in which mechanical mixing, chemical reactions, evaporation and/or surface effects play a key role. When the pattern develops in a non-Newtonian fluid, the non-linear rheology interferes with the patterning process, which often generates a richer dynamics than that commonly observed for a Newtonian fluid. Here we focus on the viscous fingering instability in a time-dependent yield stress fluid. We study experimentally the flow of a carbon black gel sandwiched in a parallel plate geometry, for which the upper plate is being lifted up at constant velocity. We show the existence of a critical initial gap spacing and a critical lift velocity, above which the flow becomes unstable, leading to the growth of viscous finger originating from the Saffman-Taylor instability at the fluid-air interface. The resulting pattern in the gel consists in a tree-like branched structure, and covers only a fraction of the plate surface. Varying the gel concentration, we demonstrate that the extent of the pattern is governed by the yield strain of the carbon black gel, while for a fixed gel concentration, the span of the pattern increases for increasing lift velocity and decreasing initial gap width. Moreover, we observe three different regimes in the selection of the wavelength: a yield stress dominated regime, a viscous dominated regime and a surface tension dominated regime. Finally, we show that the shear history of the gel has a strong influence on the pattern morphology and allows us to generate a broad variety of patterns, from heavily ramified structures composed of thin branches when the gel is rejuvenated by a preshear of high intensity followed by an abrupt flow cessation, to coarse patterns characterized by a large core and a few thicker branches when the gel is rejuvenated by a preshear of high intensity followed by a slow cessation of shear.