A marine biofilm flow-cell for screening antifouling marine coatings using optical coherence tomography

Fabbri, Stefania, Dennington, Simon, Stoodley, Paul and Longyear, Jennifer, Elise (2017) A marine biofilm flow-cell for screening antifouling marine coatings using optical coherence tomography. The 5th International Conference on Advanced Model Measurement Technology for The Maritime Industry, , Glasgow. 11 - 13 Oct 2017.

Abstract

A novel fouling marine flow-cell was designed and fitted with a top clear 5 mm thick plastic lid to allow real time imaging of the biofilm using optical coherence tomography (OCT). The OCT was used to analyse biofilm removal and mechanical properties during shear-stress experiments. The OCT measures intensity depth profiles from translucent samples such as biofilms. Consecutive scans provide a cross-sectional view of the biofilm structure which are then combined to give volumetric representations. The scanning speed of the OCT reached up to 30,000 scans/s and covers a field of view of 9x9 mm2. The bottom plate of the flow-cell was machined to allow the insertion of fouled microscope slides (25 x 55 x 1 mm). Marine biofilms were grown on spray coated (inert coating) slides in seawater for up to 2 years to test mechanical properties (triplicates). Marine biofilms were grown dynamically on 6 different antifouling coatings (A, B, C, D, E, F) for 8 weeks to test biofilm removal (duplicates). Marine biofilms were also grown statically and dynamically on an antifouling coating G to assess biofilm removal. Biofilm mechanical behaviour and removal were assessed by increasing (load cycle) or decreasing (unload cycle) the flow velocity (and therefore shear stress) in a stepwise manner over the entire pump range. Each step interval lasted 30 s except at the highest flow which was held for 5 min before starting the unloading cycle. The OCT was set to measure 10 xz-cross sections along the flow for each velocity step. 3D C-scans were also acquired before the loading cycle and at the end of the unloading cycle. The OCT images were analysed using ImageJ and Matlab. The angle of deformation of individual biofilm clusters were measured for each shear stress to obtain a stress/strain curve. Stress/strain curves showed classic viscoelastic biofilm behaviour. From the initial linear region of the load cycle the shear modulus
(G) was estimated to be G = 46.2 ±5.43 Pa (n = 3). The biofilm also showed a residual strain εR = 0.28± 0.01 (n = 2). The % cross-sectional area removed (%A) as a function of the shear stress was measured from the OCT images for each antifouled slide. The %A value increases exponentially for all the antifouling coatings until a shear stress of ~25 Pa, when it reached a plateau. Considering a shear stress of 15 Pa, %A of coating C (A% = 75%) was significantly higher than the value of the other coatings showing best performance. The %A of the biofilm grown on coating G statically (A% = 68%) was lower than the value of the biofilm grown dynamically (A% = 82%). These results show that the marine biofilm flow-cell combined with OCT can be used to assess mechanical properties of marine biofilms and detect differences (in terms of removal) in biofilms grown on different coatings. Future testing will focus on assessing how mechanical properties of biofilms interact with their physical properties (roughness, thickness, extent) to produce drag.

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Contributors

Author: Paul Stoodley

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