Morphological Transitions of Water Channels Induced by Vertical Vibrations

We report the results of comprehensive experiments and numerical calculations of interfacial morphologies of water confined to the hydrophilic top face of rectangular posts subjected to vertical vibrations. In response to mechanical driving, an initially flat liquid channel is collected into a liquid bulge that forms in the center of the rectangular post if the acceleration exceeds a certain threshold. The bulge morphology persists after the driving is switched off, in agreement with the morphological bistability of static interfacial shapes on posts with large length-to-width ratios. In a narrow frequency band, the channel does not decay into a bulge at any acceleration amplitude, but displays irregular capillary waves and sloshing instead. On short posts, however, a liquid bulge can be dynamically sustained through vertical vibrations but quickly decays into a homogeneous channel after the external driving is stopped. To explain the dynamic bulging of the liquid interface, we propose an effective lifting force pulling on the drop's slowly moving center of mass in the presence of fast oscillation modes.