Paper Number
GR11 

Session
Gallery of Rheology Contest

Title
Stability of viscous fingering patterns in colloidal gels

Presentation Date and Time
October 17, 2018 (Wednesday) 6:30

Track / Room
Gallery of Rheology / Woodway Foyer

Authors (Click on name to view author profile)

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

Author and Affiliation Lines (in printed abstract book)
Irmgard Bischofberger1, Badis Marsit1, Yacouba Kaloga2, and Thibaut Divoux2
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 are involved in numerous physical process in which mechanical mixing, chemical reactions, evaporation and/or surface effects play a key role. In the case the pattern develops in a non-Newtonian fluid, the non-linear rheology interferes with the patterning process, which often generates a richer dynamics than the one 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 lift velocity, above which the flow becomes unstable, yielding 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. 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 figures, from heavily ramified patterns composed of thin branches when the gel is rejuvenated by a preshear of high intensity followed by an abrupt flow cessation, to a coarse pattern characterized by a large core and fewer thick branches, when the gel is rejuvenated by a preshear of high intensity followed by a slow cessation of shear.