Mechanics of Biofilms at Oil-Water Interfaces

Biofilms form at oil-water interfaces to form viscoelastic layers which evolve with time to form an elastic skin.  Previously, we have documented the transition of Pseudomonas sp. P62 biofilms at oil-water interfaces from a viscous complex fluid to an elastic film using microrheology. In this work, we focus on the mechanics of the elastic skin, which we characterize using a pendant drop method modified to quantify anisotropic tensions in thin film layers at the pendant drop surface.

The method relies on forming a pendant drop, and allowing the biofilm to assemble on the drop surface. The drop shape is subsequently distorted from this initial state, which is treated as an unstrained base state, with shape determined by the balance of gravity and isotropic tension. The drop is then expanded or contracted, with anisotropic distortion fields and hence tension. Its shape is compared to a theoretical shape determined by integration of the Young Laplace equation for anisotropic tensions related by a constitutive model.  Upon expansion, tension in the skin increases, and the drop shape becomes more spherical. Upon compression, the tension decreases, and the drop becomes extended.

By comparing theoretical and experimental drop shapes, the Young modulus and Poisson ratio of the biofilm can be extracted, and departures of the biofilm mechanics from simple linear mechanical models (e.g. Hookean membranes) can be elucidated. We compare films created by wild-type Pseudomonas aeruginosa PA01 to those produced by knock-out species lacking motility and the ability to secrete exopolysaccharides (EPS).