title: Gravitaxis of asymmetric self-propelled colloidal particles.
abstract: Many motile microorganisms adjust their swimming motion relative to the gravitational field and thus counteract sedimentation to the ground. This gravitactic behaviour is often the result of an inhomogeneous mass distribution, which aligns the microorganism similar to a buoy. However, it has been suggested that gravitaxis can also result from a geometric fore-rear asymmetry, typical for many self-propelling organisms. Despite several attempts, no conclusive evidence for such an asymmetry-induced gravitactic motion exists. Here, we study the motion of asymmetric self-propelled colloidal particles which have a homogeneous mass density and a well-defined shape. In experiments and by theoretical modelling, we demonstrate that a shape anisotropy alone is sufficient to induce gravitactic motion with either preferential upward or downward swimming. In addition, also trochoid-like trajectories transversal to the direction of gravity are observed.
authors.affiliation: Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany.
authors.forename: Borge
authors.initials: B
authors.lastname: ten Hagen
authors.affiliation: 2. Physikalisches Institut, Universität Stuttgart, D-70569 Stuttgart, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Kümmel
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Raphael
authors.initials: R
authors.lastname: Wittkowski
authors.affiliation: Department of Mathematics, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Takagi
authors.affiliation: Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany.
authors.forename: Hartmut
authors.initials: H
authors.lastname: Löwen
authors.affiliation: 1] 2. Physikalisches Institut, Universität Stuttgart, D-70569 Stuttgart, Germany [2] Max-Planck-Institut für Intelligente Systeme, D-70569 Stuttgart, Germany.
authors.forename: Clemens
authors.initials: C
authors.lastname: Bechinger