[Paper Review] Bacterial floc mediated rapid streamer formation in creeping flows

Mahtab Hassanpourfard, Zahra Nikakhtari+5 more|arXiv (Cornell University)|Apr 1, 2015

Microfluidic and Bio-sensing Technologies34 references39 citations

TL;DR

This study reveals a rapid, floc-mediated mechanism for bacterial streamer formation in creeping flows (Re ≈ 10⁻³), where pre-formed Pseudomonas fluorescens flocs adhere to micropillars and deform into filamentous streamers in under one second due to hydrodynamic shear. Unlike slow biofilm-mediated streamers (hours), this process is driven by large, recoverable elastic strains in flocs, leading to sudden clogging in microfluidic devices, with streamers exhibiting viscous relaxation over longer timescales.

ABSTRACT

One of the central puzzles concerning the interaction of low Reynolds number (Re<<1) fluid transport with bacterial biomass is the formation of filamentous structures called streamers. In this manuscript, we report our discovery of a new kind of low Re bacterial streamers, which appear from pre-formed bacterial flocs. In sharp contrast to the biofilm-mediated streamers, these streamers form over extremely small timescales (less than a second). Our experiments, carried out in a microchannel with micropillars rely on fluorescence microscopy techniques to illustrate that floc-mediated streamers form when a freely-moving floc adheres to the micropillar wall and gets rapidly sheared by the background flow. We also show that at their inception the deformation of the flocs is dominated by recoverable large strains indicating significant elasticity. These strains subsequently increase tremendously to produce filamentous streamers. Interestingly, we find that these fully formed streamers are not static structures and show viscous response at time scales larger than their formation time scales. Finally we show that such novel streamer formation can lead to rapid clogging of microfluidic devices.

Motivation & Objective

• To investigate the mechanism of bacterial streamer formation in low Reynolds number flows.
• To determine whether pre-formed flocs, rather than biofilms, can initiate rapid streamer formation.
• To quantify the timescale, deformation dynamics, and viscoelastic response of streamers formed from flocs.
• To assess the implications of this mechanism for microfluidic device clogging.

Proposed method

• Used a microfluidic device with a staggered array of 50 µm PDMS micropillars to create creeping flow (Re ≈ 10⁻³).
• Tracked floc dynamics using dual-color fluorescence: GFP-labeled P. fluorescens and 200 nm red fluorescent polystyrene beads embedded in EPS.
• Measured strain evolution via particle tracking to compute stretch ratio and principal velocity gradient tensor.
• Applied computational fluid dynamics (CFD) with Comsol Multiphysics® to simulate flow, using incompressible Navier-Stokes and continuity equations.
• Calculated non-dimensional velocity gradient tensor and extracted the principal eigenvalue (1L) to quantify local deformation.
• Monitored real-time surface coverage and clogging progression via time-lapse imaging.

Experimental results

Research questions

• RQ1 What triggers rapid streamer formation in creeping flows, and how does it differ from biofilm-mediated streamer formation?
• RQ2 Can pre-formed bacterial flocs initiate streamers independently of biofilm development?
• RQ3 What are the deformation characteristics (elastic vs. viscous) of flocs during streamer formation?
• RQ4 How fast do streamers form, and what is their viscoelastic response over time?
• RQ5 To what extent does floc-mediated streamer formation lead to microchannel clogging?

Key findings

• Streamer formation from flocs occurs in less than one second, a dramatic contrast to biofilm-mediated streamers that form over hours.
• Floc deformation is dominated by recoverable large elastic strains at inception, indicating significant viscoelasticity.
• The principal velocity gradient (1L) remains approximately constant in fully formed streamers, indicating steady-state deformation.
• Streamer structures exhibit viscous relaxation over timescales longer than their formation time, indicating non-elastic behavior.
• Rapid streamer formation leads to catastrophic clogging, with surface coverage surging from 5% to 37% in under 30 minutes after initial accumulation.
• Flocs preferentially attach to micropillars at upstream stagnation points, with 50% attachment probability on the upstream half of each pillar.