| We present a scheme of self-propelling liquid droplets which closely mimics the locomotion of some protozoal organisms, so-called squirmers. In contrast to other schemes proposed earlier, locomotion paths are not self-avoiding, since the effect of the squirmer on the surrounding medium is weak. Our results suggest that not only the velocity, but also the mode of operation (i.e., the spherical harmonics of the flow field) can be controlled by appropriate variation of parameters. The model squirmers are experimentally realized using monodisperse aqueous droplets containing chemicals that produce a steady source of bromine ions, in an external oil phase rich in surfactant. The surfactant (mono-olein) reacts at the droplet interface with the bromine produced within the droplets, and a dynamic instability leads to gradients of interfacial tension at the droplet interface. These gradients set up Marangoni stresses at the droplet interface, thus propelling the droplets. The motion mechanism closely resembles that of a squirmer due to the surface tangential velocities at the droplet interface. The flow around the swimmers as well as its effect on the droplet motion are measured using particle image velocimetry (PIV). The PIV analysis reveals the far field flows generated by the swimmers in the surrounding liquid, leading to the emergence of bound states and oriented clusters. |
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