(515a) Substrate Constraints Modify the Rayleigh Spectrum of Vibrating Sessile Drops

In this work, we study the resonance behavior of mechanically-oscillated, sessile water drops. By mechanically oscillating sessile drops vertically and within prescribed ranges of frequencies and amplitudes, a rich collection of resonance modes are observed and their dynamics subsequently investigated. We first present our method of identifying each mode uniquely, according to their geometric patterns and through association with spherical harmonics. Next, we compare our estimated resonance frequencies of drops to theoretical predictions using both the classical theory of Lord Rayleigh for free, oscillating drops, and new theory by Bostwick/Steen that explicitly considers the effect of the solid surface on drop dynamics. Finally, we report observations and analysis of drop mode mixing, or the simultaneous coexistence of multiple mode shapes within the resonating sessile drop driven by one sinusoidal signal of a single frequency. During mode-mixing, distinct modes within a single drop are demonstrated to coexist, while retaining their respective individual mode dynamics. The dynamic response of a deformable liquid drop constrained by the substrate it is in contact with is of interest in a number of applications, such as drop atomization and ink jet printing, switchable electronically controlled capillary adhesion, optical microlens devices, and digital microfluidic applications where control of droplet motion is induced by means of a harmonically driven substrate.