Abstract
We have studied experimentally the collective behavior of self-propelling liquid droplets, which closely mimic the locomotion of some protozoal organisms, the so-called squirmers. For the sake of simplicity, we concentrate on quasi-two-dimensional (2D) settings, although our swimmers provide a fully 3D propulsion scheme. At an areal density of 0.46, we find strong polar correlation of the locomotion velocities of neighboring droplets, which decays over less than one droplet diameter. When the areal density is increased to 0.78, distinct peaks show up in the angular correlation function, which point to the formation of ordered rafts. This shows that pronounced textures, beyond what has been seen in simulations so far, may show up in crowds of simple model squirmers, despite the simplicity of their (purely physical) mutual interaction.
Highlights
Large-scale patterns emerging in a crowd of interacting self-driven elements are known for a wide range of biological systems, such as a host of sparrows, a school of fish, an army of ants or bacterial colonies
The complexity of the active elements thereby varies considerably, and so do their mutual interactions. The latter can be sorted in purely physical effects, such as hard core repulsion or hydrodynamic interaction, and biological signaling, such as olfactoric and visual signals, or chemotaxis, which is present in many microbial settings
While theoretical works have concentrated on active elements which were greatly simplified in their shape and interactions [1, 2,3,4,5,6,7,8], most experiments to date have been performed with bacterial colonies [9,10,11,12,13]
Summary
Large-scale patterns emerging in a crowd of interacting self-driven elements are known for a wide range of biological systems, such as a host of sparrows, a school of fish, an army of ants or bacterial colonies. Squirming motion is appealing for the study of the hydrodynamics of micro-scale swimming, since the velocities in the near and far field around such a swimming organism can be described well analytically [4, 16, 17], and are similar to the flow fields around moving spherical objects. Such squirming organisms may be modeled by self-propelling liquid droplets, which is what we pursue in the present paper
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