Howard Stone, the Donald R. Dixon ’69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering

Biofilms -- sticky aggregations of microorganisms -- are the main form of microbial life and are usually considered to grow on wet surfaces ranging from riverbeds to sewer pipes to human teeth. There are few studies of how motion of the surrounding fluid affects biofilms, even though much of our living world involves flows, whether in rivers, our bodies, or human-made systems such as water purification plants.

In response to turbulence in fluid flows, biofilms have been observed to form string-like filaments called streamers, which are attached to a surface at one end while the other end floats freely in the surrounding fluid. These streamers are frequently found in rivers, hot springs, and acidic metal-rich waters, and they contribute to ecosystem processes as well as the fouling of membranes used for filtration and chemical processing, which reduces efficiency.

So, it came as a surprise to our group that when we performed very slow laminar flow experiments--in which there is no turbulence -- with bacterial solutions in microfluidic channels containing corners, we systematically observed the development of streamers floating in the fluid. The streamers were found all along the length of the channel, rather than just being confined to the surfaces. Even more surprising, the streamers were attached only to the corners in the flow channel. Otherwise, they were floating freely in the fluid with the distinguishing feature that they were floating exactly in the middle of the channel.

Using a combination of approaches from fluid dynamics, engineering and physics, we conducted systematic experiments, numerical simulations and theoretical calculations of the fluid flow around corners. Our findings, based on work that began at Harvard University and continues at Princeton, hint at the manner in which the flow develops a regular three-dimensional pattern in the neighborhood of the corner, which may be the trigger for allowing the biofilm, initially on the surface, to develop as a thread in the flow.

Our findings suggest that streamers may be much more common than previously believed and that their presence can have a major impact on various flow processes, such as how biomass accumulates in filters. Our work emphasizes the need to not only examine the surface where a biofilm grows initially, but also to examine the immediate environment around the surface. We are now investigating several related problems for how fluid motion interacts with biofilm formation and function through collaborations with faculty members Bonnie Bassler, Zemer Gitai, and Ned Wingreen, so as to study problems at the interface of fluid dynamics and the molecular biology and structure of biofilms.

Rusconi, Roberto, Sigolene Lecuyer, Laura Guglielmini, and Howard A. Stone. 2010. “Laminar flow around corners triggers the formation of biofilm streamers.” Journal of the Royal Society Interface 7, no. 50: 1293-1299.