Flow fields around pinned self-thermophoretic microswimmers under confinement.

We measure the flow field around a pinned micrometer-sized self-thermophoretic Janus particle confined in a slit pore of different slit widths. The flow field is determined with the help of gold nanoparticles, which are thermophoretically inactive and thus do not respond to the generated temperature gradients directly. The flow fields are altered when the slit pore is narrowing and reveal clear features of thermo-osmotic flows generated by the temperature gradients along the confining glass cover slip surfaces. Based on these observations, we suggest that many observations of active and passive particle clustering around phoretic swimmers are influenced by such osmotic creep flows induced at the substrate boundary.