  A dense suspension of the cornstarch flowing on a very inclined wall finally
forms some ridge-like patterns of the free surface. The onset of pattern
formation is the primary target to elucidate the mechanism. In this work, based
on the continuity of fluids and the force balance, we show that the flat free
surface is unstable when the second normal stress difference $N_2$ is
negatively proportional to shear stress and the gravity component perpendicular
to the wall is weak enough. Such instability is inevitable to grow into a
ridge-like surface profile oriented parallel to the flow direction. We use the
instability criterion to predict the critical slope angle for the formation of
ridge patterns. The estimated critical angle was found to be in agreement with
experimental observations for a cornstarch suspension.
