Abstract
The dynamics of swimming microorganisms is strongly affected by solid‐liquid and air‐liquid interfaces. In this paper, we characterize the motion of both single bacteria and microcolonies at an air‐liquid interface. Both of them follow circular trajectories. Single bacteria preferentially show a counter‐clockwise motion, in agreement with previous experimental and theoretical findings. 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. 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. Implications of microcolony splitting and aggregation on biofilm growth and dispersion at air‐liquid interface are discussed.
Highlights
Bacteria live in different environments, continually exposed to various stimuli such as chemical compounds and physical constraints
Concerning the microcolonies, at the first frame we identified the center of the microcolony and selected two bacteria belonging to the microcolony and quite far from its center
This process can potentially accelerate the microcolony dissemination as the novel independent microcolonies constitute stable nuclei that can increase in size after collision with single swimmers
Summary
Bacteria live in different environments, continually exposed to various stimuli such as chemical compounds and physical constraints. Single flagellated microswimmers, such as E. coli, are attracted by both solid–liquid and air–liquid interfaces (Lopez & Lauga, 2014; Morse, Huang, Li, Maxey, & Tang, 2013) In both cases, circular trajectories are observed, the direction of rotation is different: at solid–liquid interface the flagellated bacteria swim clockwise (CW) (Lauga, DiLuzio, Whitesides, & Stone, 2006), while counterclockwise swimming (CCW) is observed at air–liquid interface (Di Leonardo et al, 2011; Lemelle, Palierne, Chatre, & Place, 2010). Our data show that collisions between microcolonies or between single-swimmers and microcolonies often result in a merging and that, occasionally, a small colony detaches from a large colony and starts an independent rafting
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