Active Suspensions have Nonmonotonic Flow Curves and Multiple Mechanical Equilibria

Area of Science:

• Physics of complex fluids
• Soft matter physics
• Biophysics

Background:

• Active fluids, like bacterial suspensions, display unique mechanical properties distinct from passive fluids.
• Understanding the behavior of confined active matter under external forces is crucial for various applications.

Purpose of the Study:

• To investigate the unconventional mechanical properties of confined active fluids, specifically bacterial suspensions, under shear.
• To theoretically model and predict the conditions under which these fluids exhibit negative viscosity.

Main Methods:

• Development of a minimal theoretical model for an active liquid crystal with no free parameters.
• Analysis of the model to predict fluid behavior at steady-state under shear conditions.
• Comparison of theoretical predictions with existing experimental measurements.

Main Results:

• Prediction of a specific concentration window for bacterial suspensions where negative viscosity emerges.
• Quantitative agreement between the theoretical model and experimental data for E. coli suspensions.
• Identification of non-monotonic local velocity profiles and stress-strain rate curves as causes for negative apparent viscosity.

Conclusions:

• Confined active fluids, such as E. coli suspensions, can exhibit negative viscosity under shear.
• The observed negative viscosity is linked to non-monotonic flow profiles and stress-strain relationships.
• This behavior implies that fixed stress and fixed strain rate conditions are not interchangeable for active fluids.