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Mechanical Measurements

par douarche - publié le





A specific set up has been recently built to measure the mechanical property of the biofilms floating on top of liquid (see the device designed by Sandrine Mariot in the figure). Actually the setup can be used for other materials of weak elasticity.
In the experiments, a portion of a floating biofilm adheres to two vertical plates. The right-sided plate is connected to a micro-translation stage (PHYSIK INSTRUMENTE GmbH \& Co) able to move distances ranging from few micrometers to centimeters at different speeds. The other facing plate is connected to a force sensor composed by a double-cantilever spring which stiffness is precalibrated by tensile measurements on various elastic well-defined materials (gels of Polydimethylsiloxane) and by a capacitive gauge set behind the spring (FOCALE nanotech). In our device, the cantilever stiffness is equal to 19 N/m. Then when the biofilm is stressed by a deformation, it pulls out the facing plate and the spring to a distance that is measured by a capacitive gauge. In our typical experiments, the spring deflects less than 0.05 mm and the corresponding force reaches one milli-Newton. Without any stress, we still detect a fluctuation of the spring position. This noise fixes our minimal limit above which we can detect a force signal. We find a minimal deflection threshold equal to 7 μm corresponding to a force 0.15 mN.

In the picture of the force device, a yellow biofilm portion grown in a rectangular dish and located between two vertical plates is strained mechanically. The strained biofilm pulls out a double-cantilever which deflection is measured by a capacitive gauge.