  Accurate prediction of the natural frequency for the lateral oscillation of a
liquid drop pinned on a vertical planar surface is important to many drop
applications. The natural oscillation frequency, normalized by the capillary
frequency, is {mainly} a function of the equilibrium contact angle and the Bond
number (Bo), when the contact lines remain pinned. Parametric numerical and
experimental studies have been performed to establish a comprehensive
understanding of oscillation dynamics. {An} inviscid model has been developed
to predict the oscillation frequency for wide ranges of Bo and contact angles.
The model reveals the scaling relation between the normalized frequency and Bo,
which is validated by the numerical simulation results. For a given equilibrium
contact angle, the lateral oscillation frequency decreases with Bo, implying
that resonance frequencies will be magnified if the drop oscillations occur in
a reduced gravity environment.
