Nonlinear deformation and localized failure of bacterial streamers in creeping flows.

Ishita Biswas,Mohtada Sadrzadeh,Ranajay Ghosh,Aloke Kumar

doi:10.1038/srep32204

Ishita Biswas, Mohtada Sadrzadeh + Show 2 more

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https://doi.org/10.1038/srep32204

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Abstract

We investigate the failure of bacterial floc mediated streamers in a microfluidic device in a creeping flow regime using both experimental observations and analytical modeling. The quantification of streamer deformation and failure behavior is possible due to the use of 200 nm fluorescent polystyrene beads which firmly embed in the extracellular polymeric substance (EPS) and act as tracers. The streamers, which form soon after the commencement of flow begin to deviate from an apparently quiescent fully formed state in spite of steady background flow and limited mass accretion indicating significant mechanical nonlinearity. This nonlinear behavior shows distinct phases of deformation with mutually different characteristic times and comes to an end with a distinct localized failure of the streamer far from the walls. We investigate this deformation and failure behavior for two separate bacterial strains and develop a simplified but nonlinear analytical model describing the experimentally observed instability phenomena assuming a necking route to instability. Our model leads to a power law relation between the critical strain at failure and the fluid velocity scale exhibiting excellent qualitative and quantitative agreeing with the experimental rupture behavior.

Highlights

In this paper, we report a quantitative study and in-situ observation of the ultimate failure and instability of bacterial streamers formed from bacterial flocs[12] for two separate bacterial strains using a microfluidic platform
We studied the deformation and failure behavior of bacterial streamers for two separate bacterial strains by passing a floc laden flow through a specially designed microfluidic device
We found that the apparent stability of the formed streamers transitioned slowly into complex creep like deformation regimes with their own characteristic time scales in spite of stable background flow conditions

Summary

Introduction

We report a quantitative study and in-situ observation of the ultimate failure and instability of bacterial streamers formed from bacterial flocs[12] for two separate bacterial strains using a microfluidic platform. We observe that the streamer material is a complex composite soft material and remains tightly influenced by the nature and dynamics of the immersed fluid, a highly localized failure via necking is common and widespread in this type of system This may be contrasted to the global nature of hydrodynamic instabilities which indicates that even up until the terminal stages, the streamers retains significant distinctions from purely viscous jets[13]. We carry out analytical instability analysis assuming a localized failure on a simplified system incorporating mechanical nonlinearity, surface tension and fluidic loading which provides a power law scaling between the strain at failure and background flow which shows excellent qualitative and quantitative agreement with our experimental observations This highly localized failure ensures that a significant portion of the streamer, attached to the wall, is still preserved unlike global instability or shear failure at the wall. To the best of our knowledge, this is the first study that reports a quantitative in-situ observation of the failure along with a continuum mechanics model for the same

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