Active Viscous Fingering
Résumé
Adding swimming bacteria to a liquid lowers its viscosity [1, 2, 3, 4, 5]. We study how this phe-
nomenon can lead to the formation of viscous Saffman–Taylor fingers which occurs when a less viscous
fluid is injected into a more viscous fluid [6]. To do this, we injected bacterial suspensions into a Hele-
Shaw cell containing the suspending fluid to prove that active fluids can induce this instability. The role
of the flow velocity, and bacterial volume fraction was then systematically studied. The Saffman-Taylor
fingers (See Fig. 1(a,b,c) observed are identical to those obtained with a pair of Newtonian fluids with
different viscosities (See Fig. 1(e)). Quater five spots experiments were also performed (See Fig. 1(f,g,h)).
They demonstrate that the addition of bacteria in the suspension reduces the breakthrough time. These
results reproduce what is observed with pairs of Newtonian fluids of different viscosities.
Our study determines the flow conditions and the bacterial volume fraction to observe the Saffman-
Taylor instability from an active fluid. For For run-and-tumble E.Coli bacteria, it requires a flow
characterized by a low shear rate (< 1s −1 ) and a suspension of volume fraction (Φ > 0.8%) greater than
the volume fraction for which the zero viscosity regime is observed.
In the conclusion, we will examine the implications of this new instability for flow applications in
porous media.