From topological surface states to metabolically driven flows in stratified fluids

In this talk, I will present two a priori unrelated systems, a geophysical and a biological one, and show why they have in common the physics of stratified fluids. Geophysical fluids such as the ocean or atmosphere are stratified environment and can carry internal gravity waves that can transport energy and momentum over large distances, thereby affecting large-scale circulation patterns, as well as the transport of heat, sediments, nutrients, and pollutants. When the density stratification is not uniform, internal waves can exhibit resonances, tunneling, and frequency-dependent transmissions. In the ocean, the interplay between heat diffusion and salt diffusion can lead to large regions with spatially periodic density profiles called thermohaline staircases. Drawing an analogy from the electronic band structure of one-dimensional crystals, I will demonstrate with laboratory experiments the existence of band gaps in a periodically stratified fluid. By measuring the transmissivity of internal waves, we show that a spatially periodic stratification can completely prevent the propagation of internal waves for a range of frequencies. We also observe the existence of surface states, exponentially localized near a boundary and with their frequency in the band gap. Using analytical and numerical modeling, we show that these are formally equivalent to topological surface states found in one-dimensional topological insulators and photonic crystals.

In the second part of this talk, I will focus on cellular growth on the surface of liquids. Despite the importance of fluid flow for transporting and organizing microbial populations, few laboratory systems exist to systematically investigate the impact of advection on their spatial evolutionary dynamics. I will discuss how we can address this problem by studying the morphology and genetic spatial structure of microbial colonies growing on the surface of a viscous substrate. I will show that S. cerevisiae (baker’s yeast) can behave like “active matter” and collectively generate a fluid flow many times larger than the unperturbed colony expansion speed, which in turn can produce mechanical stresses and preferential growth. Combining laboratory experiments with numerical modeling, I will demonstrate that the coupling between metabolic activity and hydrodynamic flows can generate positive feedbacks and lead to the complete fragmentation of the initial colony or drive the formation of growing microbial jets.