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An experimental study was carried out to examine how uniform rimming flow is established for a very small volume fraction of aqueous Newtonian solutions in a partially filled rotating horizontal cylinder. There exists a certain critical volume fraction (Vc) for each solution, where the rotational speed required to achieve uniform rimming flow takes a minimum value. Counterintuitively, it takes greater rotation speeds for both larger and smaller volume fractions than this. Axial instabilities are observed for liquid volume fractions above or below this critical value. For V>Vc the defects are mainly of shark-teeth and turbulent types, while for V<Vc, fingers and rings are formed. The critical volume fraction increases with increasing fluid viscosity. Reducing surface tension increases the minimum rotational speed for V>Vc, but has very little effect for V<Vc. The lowest volume fraction for rimming flow found in the present study is 0.25%. The dimensionless minimum rotational speed Ω to achieve rimming flow is presented as a function of the dimensionless liquid volume fraction ϕ. The competing effects of fluid inertia and viscous force on rimming flow are demonstrated from a dimensionless plot of Ω versus ϕ.

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