Local spots of viscous electron flow in graphene at room temperature and moderate mobility

Dominating electron-electron scattering enables viscous electron flow exhibiting hydrodynamic current density patterns such as Poiseuille profiles or vortices. The viscous regime has recently been observed in graphene by non-local transport experiments and mapping of Poiseuille profiles up to room temperature. Here, we probe the current-induced surface potential maps of graphene field effect transistors using scanning probe microscopy at room temperature. We discover the appearance of{\mu}m large areas close to charge neutrality, where the current induced electric field opposes the externally applied field. By estimating the scattering lengths from the gate dependence of local electric fields, we find that these areas exhibit a dominating electron-electron scattering as expected for viscous flow. We map the respective meandering electric fields and carefully rule out artifacts such as by source-drain voltage induced local doping. Our results imply that viscous electron flow is omnipresent in graphene devices, even at moderate mobility.