Abstract

It is well known that flagellated bacteria swim in circles near surfaces. However, recent experiments have shown that a sulfide-oxidizing bacterium named Thiovulum majus, whose typical size ranges from 5μm to 25 μm, can transition from swimming in circles to a surface bound state where it stops swimming while remaining free to move laterally along the surface. In this bound state, the cell rotates perpendicular to the surface with its flagella pointing away from it. Using numerical simulations and theoretical analysis, we demonstrate the existence of a fluid-structure interaction instability that causes cells with relatively short flagella to become surface bound. These results have significant implications on the initial stages of formation of the white veil, an approximately 0.5-mm-thick elastic porous medium that is the natural habitat of T. majus cells. While in this work, we have focused only on the surface binding phenomena, there are other questions related to T. majus locomotion dynamics remain unanswered. T. Majus is the second fastest swimming bacterium in nature reaching speeds up to 615 μm/s. How are these cells able to swim so fast? Do they possess an elastic hook like the well-studied E. coli and if yes, how do hooks affect the propulsion of these microorganisms. (Note: I will comment on the suitability of boundary element method at resolving very small gaps between surfaces.)