Auto-phoretic nanorods driven up the wall by gravity

Gravitaxis is the directed upward motion of micro-organisms against gravity, and is observed for a few ciliated organisms like~\textit{Chlamydomonas},~\textit{Euglenas}~or~\textit{Paramecium}. Lacking a dedicated sensor, their gravitactic response relies on bottom-heaviness or shape anisotropy to induce a bias in their swimming direction. Here we study the gravitaxis of heavy self-electrophoretic Janus nanorods that move upwards on a steeply inclined substrate. Comparisons in experiments and simulations between homogeneous and bottom-heavy nanorods reveal two mechanisms contributing to the gravitactic response of the latter: a buoyancy torque and hydrodynamic interactions with the wall. We show that lubrication forces induce an effective fore-aft asymmetry on nanorods that reinforces the orientation bias to move up the steep wall against gravity.