Morphological Transitions of Water Channels Induced by Vertical Vibrations Paolo Sartori, † Luca Bonato, † Giorgio Delfitto, † Matteo Pierno, † Giampaolo Mistura,* ,† Ciro Semprebon, ‡ and Martin Brinkmann* ,§ † Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via Marzolo 8, I-35131 Padova, Italy ‡ Department of Physics, Durham University, DH1 3LE Durham, U.K. § Geometry of Fluid Interfaces Group, Experimental Physics, Saarland University, 66123 Saarbrü cken, Germany * S Supporting Information ABSTRACT: We report the results of comprehensive experiments and numerical calculations of interfacial mor- phologies 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. ■ INTRODUCTION Resonant oscillations of sessile drops have been the subject of recent experimental and numerical studies because of their relevance to microfluidic applications, in particular to mixing and actuation. In contrast to electrowetting 1 and magnetic actuation 2 which require polar liquids or ferrofluids, respectively, mechanical vibrations couple to drop inertia and, thus, provide a universal driving mechanism for all types of liquids. Taking advantage of the rectifying effect related to a contact angle hysteresis, horizontal and vertical vibrations with a controlled phase shift can be utilized for directed drop actuation and to let drops even climb against gravity. 3−10 Similarly, the spatiotemporal modulation of the acoustic levitation field allows continuous planar transport and processing of droplets. 11 Recent investigations have shown that the topographic micropatterns of linear grooves and ridges may increase the yield of passive fog collectors. 12,13 During growth, the droplets wetting the ridges coalesce and may form stretched out, linear morphologies, called filaments, or are collected into larger droplets which facilitate droplet shedding by gravity or other external forces. The transition between the flat channel or filament-like morphology and the localized, droplet-like bulge morphology is controlled by surface wettability 14 and the geometry of the ridges. 15 More generally, the static interfacial shapes of sessile liquid drops wetting the chemical or topographic surface patterns have been widely studied both at the applied 16,17 and fundamental levels. 14,15,18−23 Small drops wetting a rectangular stripe or annular ring, in particular, can be morphologically bistable in a certain range of liquid volumes. 15,24,25 In the latter cases, the interface of a drop in mechanical equilibrium may be found either in a flat, spread-out “channel” morphology or in a localized drop-like “bulge” morphology. Close to the point of instability, the energy barrier separating these two interfacial conformations is small, and one can expect that the transition from the metastable state into the global energy minimum can be easily triggered by external stimuli like mechanical vibrations of the underlying substrate. The present work is the natural prosecution of our static study on rectangular posts 15 and demonstrates that vibrations can indeed induce a transition from a flat channel into a localized bulge, but this transition is inhibited under certain conditions. In a small range of vibration frequencies, the liquid interface of the channel does not decay into a liquid bulge but instead displays patterns of standing capillary waves with pronounced beating and highly irregular local amplitudes. Received: July 13, 2018 Revised: October 4, 2018 Published: October 4, 2018 Article pubs.acs.org/Langmuir Cite This: Langmuir 2018, 34, 12882-12888 © 2018 American Chemical Society 12882 DOI: 10.1021/acs.langmuir.8b02370 Langmuir 2018, 34, 12882−12888 Downloaded via UNIV OF PADOVA on December 29, 2021 at 18:09:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.