  Hysteresis is a major feature of the solid-liquid transition in granular
materials. This property, by allowing metastable states, can potentially yield
catastrophic phenomena such as earthquakes or aerial landslides. The origin of
hysteresis in granular flows is still debated. However, most mechanisms put
forward so far rely on the presence of inertia at the particle level. In this
paper, we study the avalanche dynamics of non-Brownian suspensions in slowly
rotating drums and reveal large hysteresis of the avalanche angle even in the
absence of inertia. By using micro-silica particles whose interparticle
friction coefficient can be turned off, we show that microscopic friction,
conversely to inertia, is key to triggering hysteresis in granular suspensions.
To understand this link between friction and hysteresis, we use the rotating
drum as a rheometer to extract the suspension rheology close to the flow onset
for both frictional and frictionless suspensions. This analysis shows that the
flow rule for frictionless particles is monotonous and follows a power law of
exponent $\alpha \!= \! 0.37 \pm 0.05$, in close agreement with the previous
theoretical prediction, $\alpha\!=\! 0.35$. By contrast, the flow rule for
frictional particles suggests a velocity-weakening behavior, thereby explaining
the flow instability and the emergence of hysteresis. These findings show that
hysteresis can also occur in particulate media without inertia, questioning the
intimate nature of this phenomenon. By highlighting the role of microscopic
friction, our results may be of interest in the geophysical context to
understand the failure mechanism at the origin of undersea landslides.
