|
The dynamics of active colloidal suspension and swimming microorganisms are strongly affected by solid-liquid and air-liquid interfaces. In this contribution, we discuss the motion of E. coli at air-liquid boundary. We experimentally observed and characterized the motion of both single E. coli and microcolonies. Both of them follow circular trajectories. Single bacteria preferentially show a counter-clockwise motion in agreement with numerical results obtained by means of numerical solutions of the Stokes equations obtained via a boundary element method (Pimponi et al. J. Fluid Mech. 2016, vol. 789, pp. 514–533). Instead, no preferential rotation direction is observed for microcolonies suggesting that their motion is due to a different physical mechanism. We propose a simple mechanical model where the microcolonies move like rafts constrained to the air-liquid interface to partially explain the experimental data. Finally, we observed that the microcolony growth is due to the aggregation of colliding single-swimmers, suggesting that the microcolony formation resembles a condensation process where the first nucleus originates by the collision between two single-swimmers.
|