Living oil-water interfaces: Buckling of droplets by cell growth at finite liquid interfaces

Cell growth on solid substrates confined by rigid boundaries has been shown to influence cell morphology and lead to the emergence of collective behavior. Here, through microfluidic experiments and time-lapse microscopy, we demonstrate that the colonization of oil droplets by the growth and division of rod-shaped bacteria depends strongly on the droplet diameter, or the interfacial curvature. For droplets larger than a critical diameter, bacteria grow and divide at the oil-water interface, leading to the self-assembly of a densely packed monolayer of bacteria that eventually encapsulates the droplet. The stress from cell growth exerted at the oil-water interface causes the droplet to buckle and the fluid interface to undergo large deformations, including wrinkling and tubulation. For droplets smaller than a critical diameter, the formation of a self-assembled monolayer is not observed and no interfacial deformations occur. A simple energetic argument, comparing the attachment energy and bending energy due to elastocapillarity of a rod-shaped bacterium at curved oil-water interfaces, is able to determine the critical diameter, or interfacial curvature, for colonization and may provide insight on the hydrophobicity of the cell wall.