Numerical study of natural oscillations of supported drops with free and pinned contact lines

Abstract

The oscillation of droplets supported by solid surfaces is important for a wide variety of applications such as dropwise condensation. In the present study, the axisymmetric natural oscillations of a liquid drop supported by a flat surface are investigated by direct numerical simulation. The liquid-gas interface is captured using a geometric volume-of-fluid method. A parametric study is carried out by varying the equilibrium contact angle and the gravitational Bond number (Bo). Both positive and negative gravities are considered, and thus the results cover both pendant and sessile drops. To incorporate the effect of contact line mobility, the two asymptotic limits, namely, the pinned contact line (PCL) and free contact line (FCL) conditions, are considered and their effects on the drop oscillation features are characterized. The predicted oscillation frequencies for PCL and FCL serve as the upper and lower bounds for general situations. The drop oscillation is initiated by increasing the gravity magnitude for a short time. The first mode due to the drop centroid translation dominates the excited oscillation. The oscillation frequency scales with the capillary frequency, and the normalized frequency monotonically decreases with the equilibrium contact angle. For zero gravity, the computed frequencies for all contact angles agree remarkably well with the inviscid theory for both the PCL and FCL conditions. The kinetic energy correction factor is introduced to account for the additional contribution of the oscillation-induced internal flow to the overall kinetic energy of the drop. Both the frequency and the kinetic energy correction factor increase with Bo, decrease with the contact angle, and increase when the contact line condition changed from FCL to PCL. The variation of oscillation frequency due to the change of Bo is particularly significant when the contact angle is large, suggesting that the gravity effect must be incorporated to accurately predict the oscillation frequency for drops supported by hydrophobic or superhydrophobic surfaces.