Rheology of a nematic active suspension undergoing oscillatory shear and step strain flows
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We investigate experimentally the rheology of an active suspension of nematodes C. elegans under oscillatory shear. There are few experimental investigations and theoretical work on the oscillatory rheological properties including elastic and viscous moduli of micro-swimmer suspensions. The viscous and elastic moduli are evaluated by our experiments in the regime of linear viscoelasticity. These viscoelastic quantities are explored at low frequency given the active suspension viscosity and the shear elastic modulus. The experiments have revealed an anomalous behavior of the viscosity and the shear elastic modulus with the variation of the suspension volume fraction. The suspension relative viscosity decreased with the increase of active particles within a certain range of volume fraction. However, above a critical particle volume fraction, the relative viscosity increases. This observed increase of the viscosity for larger concentration is a direct consequence of formation of large and coherent oriented structures and active particle interactions. This collective behavior also increases the first normal stress difference N1, obtained through Cox–Merz rule. Three different regions are obtained regarding different involved mechanisms and a physical interpretation is provided based on the particles dipole stresslet. Step strain tests are carried out and the active relaxation time as a function of volume fractions are obtained. An intrinsic oscillatory behavior is observed regardless the volume fraction, showing the non-equilibrium condition of the active suspension.
KeywordsCollective behavior Active suspension viscosity Oscillatory shear Shear elastic modulus C. elegans
We wish to acknowledge the support of Professor Carlos Winter from the Nematode’s Molecular Biology Laboratory (ICB-USP) and Professor Vicente de Paulo Martins from Pathogens Molecular Analysis Laboratory (IB-UnB) in obtainment and cultivation of C. elegans. Also we would like to acknowledge Professor Takuji Ishikawa for the comments and discussions during the preparation of this manuscript.
The work was supported in part by the Brazilian funding agencies FAPESP—São Paulo State Research Support Foundation (Grant No. 2016/14337-5) and CNPq (Grant Nos. 552221/2009-0/142303/2015-1).
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