We report the observation of bunching of monoatomic steps on vicinal W(110) surfaces induced by step-up
or step-down currents across the steps. Measurements reveal that the size-scaling exponent γ , connecting the
maximal slope of a bunch with its height, differs depending on the current direction. We provide a numerical
perspective by using an atomistic-scale model with a conserved surface flux to mimic experimental conditions,
and also show that there is an interval of parameters in which the vicinal surface is unstable against step bunching
for both directions of the adatom drift