Droplet Wetting on Holes, a Fiber, and a Tilted Plate
Date Issued
2012
Date
2012
Author(s)
Chou, Tung-He
Abstract
Wetting phenomena of a liquid droplet on substrates are ubiquitous in everyday life as well as in industrial practices. The extent of a droplet’s spread over a substrate and its equilibrium shapes in the presence of gravity are important for wetting applications such as coating, self-cleaning surfaces, droplet formation, and microfluidics. In this thesis, there are three major parts: (1) Superhydrophobic surfaces involve completely nonwetting or partially wetting roughness. Since the contact angle is closely related to liquid-gas interfacial tension, the shape of the liquid-gas interfaces within the grooves plays a key role in determining droplet wetting behavior. We consider a droplet with volume V atop holes with radius r and obtain an analytical expression for the bottom liquid-gas shape based on surface free energy minimization. The bottom shape is found in terms of the interfacial angle θ1 to depend on the hole size by V/r3 in addition to the intrinsic contact angle θ*. For a given droplet volume, the smaller the hole size (r3/V → 0), the flatter the interface (θ1 → 0). Furthermore, the flatness of the interface grows with reducing intrinsic contact angle. Numerical simulations are performed to confirm our theory. Moreover, wetting experiments where the gravity effect cannot be neglected are conducted, and the results are consistent with our numerical simulations. Here, these findings indicate that such wall-free capillarity may have potential to extract liquid from microfluidic devices and fuel cells. (2) Drop-on-fiber is also commonly observed in daily life and is closely related to digital microfluidics. The wetting behavior of it differs from that of drop-on-plane due to the global cylindrical shape. The equilibrium geometric shape of a droplet on a fiber is generally believed to take either asymmetric clam-shell or axisymmetric barrel conformation in the absence of gravity. The barrel-to-clam-shell transition is determined by a stable condition. However, experimental observations showed that both barrel and clam-shell conformations can coexist in some situations and thus indicated the existence of multiple stable states. Here, the phase diagrams of drop-on-fiber, that is, the plots of droplet volume against contact angle, are established on the basis of the finite-element simulation. When the gravity effect is absent, there are three regimes including barrel, clam-shell, and the coexistence of both; in contrast, when the gravity effect is considered, there also exist three regimes, including (I) downward clam-shell, (II) coexistence of barrel and downward clam-shell, and (III) falling-off. (3) The wetting behavior of a liquid drop sitting on an inclined plane is investigated experimentally and theoretically. We performed numerical simulations that are based on the liquid-induced defect model, in which only two thermodynamic parameters (solid-liquid interfacial tensions before and after wetting) are required. A drop with a contact angle equal to θ is first placed on a horizontal plate and then the plate is tilted. Two cases are studied: (I) θ is adjusted to the
advancing contact angle (θa) before tilting and (II) θ is adjusted to the receding contact angle (θr) before tilting. In the first case, the uphill contact angle declines and the
downhill contact angle remains unchanged upon inclination. When the tilted drop stays at rest, the pinning of the receding part of the contact line (receding pinning) and the
depinning of the advancing part of the contact line (advancing depinning) are observed. The free energy analysis reveals that upon inclination, the reduction of the solid-liquid free energy dominates the increase of the liquid-gas free energy associated with shape deformation. In the second case, the downhill contact angle grows and the uphill contact angle remains the same upon inclination. Advancing pinning and receding depinning are noted for a tilted drop at rest. The free energy analysis indicates that upon inclination, a decrease of the liquid-gas free energy compensates an increase of the solid-liquid free energy. The experimental results are in good agreement with those of simulations.
Subjects
Wetting
Droplet
Contact Angle
Free Energy Minimization
Surface Evolver
Hysteresis
Type
thesis
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