Morphology Phase Diagram of a Droplet on a Vertical Fiber
Date Issued
2011
Date
2011
Author(s)
Chu, Yen-Hsu
Abstract
Droplet-on-fiber is commonly observed in our daily life and is closely related to dyeing fabrics and fiber-based microfluidic devices. The wetting behavior of droplet-on-fiber is different from that of drop-on-plane due to the global cylindrical shape, however, the systems of droplet-on-fiber are more complicated and therefore are less studied. A combined numerical simulation and experimental observation were conducted to investigate the equilibrium shape of a droplet on a fiber. In this work, the energy calculation was accurately performed by using a powerful modeling tool, the Surface Evolver, which is based on the finite-element simulation. The phase diagram of droplet-on-fiber is also established by using SE. The droplet-on-fiber systems are studied in three parts. First, we simulated the droplets on a horizontal fiber in the presence of gravity. Our results show that the fiber diameter affects the size of the coexistent regime greatly. Then, we conducted a simulation of droplets on a vertical fiber with a superhydrophobic plane. From the simulation results and the experimental observations, we found that the clam-shell configuration appears for small volume and large contact angle, while the barrel shape is formed for large volume and small contact angle. In the third part, we divided the vertical fiber into two sections: the upper section is hydrophilic while the lower section is hydrophobic. The agreement of the droplet shape between the SE simulations and experimental observations are quite well. We also found that both barrel and clam-shell configurations can coexist in certain conditions and thus indentified the existence of the muiltiple stable states. There are four regimes in the phase diagram, including (I) clam-shell only, (II) coexistence of barrel and clam-shell, (III) barrel only, and (IV) falling-off in the shapes of clam-shell. Moreover, due to the gravitational effect, the upper boundary of barrel only regime shifts downward, and the domain of the coexistent regime shrinks when increasing the fiber diameter.
Subjects
droplet
vertical fiber
Surface Evolver
Type
thesis
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