Cohesive strength is an important factor in determining the structure and function of biofilm systems, and cohesive strength plays a key role in our ability to remove or control biofilms in engineered systems. A micro-mechanical device has been developed to directly measure the tensile strength of biofilms and other microbial aggregates. An important feature of this method is the combination of a direct measurement of force with particle separations that occur at a scale comparable to that observed for biofilm systems. The force required to separate an aggregate is determined directly from the deflection of cantilevered glass micropipettes with a 20-40-μm diameter. Combined with an estimate of the cross-sectional area of the aggregate at the point of separation this measurement indicates the cohesive strength of the aggregate. Samples of return activated sludge (RAS) and a Pseudomonas aeruginosa biofilm were tested using the device. The measured cohesive strengths of the RAS flocs ranged from 419 to 206,400 N/m2, while many of the flocs exceeded the range of measurement of the device. Fragments of P. aeruginosa biofilm had cohesive strengths ranging from 395 to 15,640 N/m2, with a median value of 3020 N/m2. The median equivalent diameters of the particles detached from the aggregates were 32 μm for RAS and 30 μm for the P. aeruginosa biofilms.
- Tensile strength