Buoyant Droplets on Functional Fibers Rië lle de Ruiter, †,∥ Jolet de Ruiter, †,∥ Hü seyin Burak Eral, † Ciro Semprebon, ‡ Martin Brinkmann, ‡,§ and Frieder Mugele* ,† † Physics of Complex Fluids, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ‡ Dynamics of Complex Fluids, Max-Planck-Institute for Dynamics and Self-Organization, Bunsenstrasse 10, D-37073 Gö ttingen, Germany § Experimental Physics, Saarland University, 66123 Saarbrü cken, Germany ABSTRACT: In the absence of gravity, the wetting of droplets on fibers is characterized by the competition between an axisymmetric barrel morphology engulfing the fiber and a symmetry-broken clamshell morphology with the droplet sitting on the side of the fiber. In the generic case of nonzero buoyancy the cylindrical symmetry of the barrel morphology is broken, yet barrels and clamshells can still be distinguished based on their different interfacial topologies being multiply and simply connected, respectively. Next to contact angle and droplet size the capillary length appears as a third parameter controlling the droplet morphology. For droplets of variable size, contact angle and buoyancy are independently varied in experiments by use of electrowetting and density mismatch. This approachtogether with the complementary numerical calculationsprovides new insights into the gradual shifts of the stability limits in the presence of an external volume force. Overall, the parameter space for stable clamshells is found to expand with increasing gravitational forces, gradually shrinking the regimes of stable barrels and bistability. In addition, a new stability limit is introduced for the clamshell morphology related to a partial detachment of the wetting liquid from the fiber, appearing toward higher droplet volumes. ■ INTRODUCTION Droplets on fibers are widely observed in nature, for instance as dew droplets on spider webs, grass, and pine needles, and in traditional engineering applications such as cleaning of textiles. 1 More recently, inspired from nature’s way of collecting and transporting dew droplets on spider webs, digital microfluidics on a fiber 2,3 and bio-inspired fibers for water collection in fog filters 4−6 have been developed. The accumulation of droplets by directional transport is critical to these systems. It has largely been achieved by tuning the fiber shape or inclination: on a horizontal conical wire, the motion of an axisymmetric droplet is driven by a gradient in Laplace pressure. 7,8 On an inclined fiber, gravity is the driving force. 9,10 As generally observed under partial wetting conditions contact angle hysteresis hinders droplet motion. To overcome this problem, bio- inspired fibers are produced mimicking the structure of spider silk: 4−6 the so-called spindle knots have different surface roughness than the connection elements, thus exerting an additional driving force toward the knot due to the surface energy gradient. The equilibrium morphology of the droplet on the fiber influences both its attachment to and its motion along the fiber 3 due to the difference in adhesion and contact area. In the absence of buoyancy, two competing droplet morphologies exist: the rotationally symmetric pearl-like “barrel” structure and the symmetry-broken “clamshell” that sits at the side of the fiber. The former was described by Plateau 11 in the context of the instabilities of liquid films covering cylindrical fibers; the latter was first noted by Adam. 1 More generally, the two morphologies can also be distinguished by their topology: the liquid interface of the barrel is multiply connected while that of the clamshell is simply connected. This aspect distinguishes the wetting of fibers from other geometries with competing liquid morphologies such as the wetting of stripes 12−14 and the Cassie and Wenzel wetting state on structured surfaces. 15,16 The morphology transition from the barrel state to the clamshell state has been studied extensively. 17−20 These studies identified the droplet volume and the wettability of the fiber as key control parameters. Only recently, we demonstrated the existence of the inverse transition from the clamshell to the barrel state and showed that reversible switching between both morphologies is possible by tuning the wettability of the fiber using electrowetting. 21 In our combined experimental and numerical study, we identified the stability limits of both morphologies for a density matched oil−water system. Notwithstanding minor quantitative uncertainties, our study clearly revealed fundamental differences between the barrel-to- clamshell and the reverse clamshell-to-barrel transition: the former is caused by a soft mode of the barrel morphology Received: July 6, 2012 Revised: August 27, 2012 Published: August 27, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 13300 dx.doi.org/10.1021/la302726z | Langmuir 2012, 28, 13300−13306