Active-particle suspensions exhibit distinct polarization-density patterns in activity landscapes, even without anisotropic particle interactions. Such polarization without alignment forces is at work in motility-induced phase separation and betrays intrinsic microscopic activity to mesoscale observers. Using stable long-term confinement of a single thermophoretic microswimmer in a dedicated force-free particle trap, we examine the polarized interfacial layer at a motility step and confirm that it does not exert pressure onto the bulk. Our observations are quantitatively explained by an analytical theory that can also guide the analysis of more complex geometries and many-body effects.

Charles University Faculty of Mathematics and Physics 5 Holešovičkách 2 CZ 180 00 Prague Czech Republic

Institute for Theoretical Physics Leipzig University 04103 Leipzig Germany

Peter Debye Institute for Soft Matter Physics Leipzig University 04103 Leipzig Germany

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Spontaneous vortex formation by microswimmers with retarded attractions