Feng-Ming Chang

Superhydrophilicity to superhydrophobicity transition of CuO nanowire films

The surface of CuO is known for its hydrophilicity and exhibits superhydrophilic nature as nanowires are present. When exposed in the air at room temperature or treated by low temperature annealing, however, transition from superhydrophilicity to superhydrophobicity of the CuO nanowire films are observed. Since the chemical structure of the films after treatment remains the same as CuO according to x-ray photoelectron spectroscopy spectra, the superhydrophobicity may be attributed to partial deoxidation of the upmost layer of CuO surfaces into Cu2O-like hydrophobic surfaces. Nonetheless, superhydrophilicity is recovered if the superhydrophobic CuO film is subject to high temperature annealing.

High contact angle hysteresis of superhydrophobic surfaces: Hydrophobic defects

A typical superhydrophobic surface is essentially nonadhesive and exhibits very low water contact angle (CA) hysteresis, so-called Lotus effect. However, leaves of some plants such as scallion and garlic with an advancing angle exceeding 150° show very serious CA hysteresis. Although surface roughness and epicuticular wax can explain the very high advancing CA, our analysis indicates that the unusual hydrophobic defect, diallyl disulfide, is the key element responsible for contact line pinning on allium leaves. After smearing diallyl disulfide on an extended polytetrafluoroethylene (PTFE) film, which is originally absent of CA hysteresis, the surface remains superhydrophobic but becomes highly adhesive.

Anomalous Contact Angle Hysteresis of a Captive Bubble: Advancing Contact Line Pinning

Contact angle hysteresis of a sessile drop on a substrate consists of continuous invasion of liquid phase with the advancing angle (θ(a)) and contact line pinning of liquid phase retreat until the receding angle (θ(r)) is reached. Receding pinning is generally attributed to localized defects that are more wettable than the rest of the surface. However, the defect model cannot explain advancing pinning of liquid phase invasion driven by a deflating bubble and continuous retreat of liquid phase driven by the inflating bubble. A simple thermodynamic model based on adhesion hysteresis is proposed to explain anomalous contact angle hysteresis of a captive bubble quantitatively. The adhesion model involves two solid–liquid interfacial tensions (γ(sl) > γ(sl)′). Young’s equation with γ(sl) gives the advancing angle θ(a) while that with γ(sl)′ due to surface rearrangement yields the receding angle θ(r). Our analytical analysis indicates that contact line pinning represents frustration in surface free energy, and the equilibrium shape corresponds to a nondifferential minimum instead of a local minimum. On the basis of our thermodynamic model, Surface Evolver simulations are performed to reproduce both advancing and receding behavior associated with a captive bubble on the acrylic glass.

From superhydrophobic to superhydrophilic surfaces tuned by surfactant solutions

The wettability of hydrophobic surfaces is generally improved by surfactant solutions. The wetting behavior of superhydrophobic surfaces can be classified into two types, in terms of the variation of contact angle with surfactant concentration cs. Contact angle is controlled by surface tension for common linear surfactants and becomes independent of cs as cs>critical micelle concentration. Consequently, superhydrophobic surfaces remain in hydrophobic range, as reported. However, for branch-tailed surfactants such as sodium-bisethylhexylsulfosuccinate and didodecyldimethylammonium bromide, superhydrophobic surfaces can turn superhydrophilic by increasing cs owing to continuous reduction of solid-liquid interfacial tension. The superhydrophobicity is recoverable simply by water rinsing.

Tiny bubble removal by gas flow through porous superhydrophobic surfaces: Ostwald ripening

The fact that tiny bubbles are easily formed on the rough, hydrophobic surface results in difficulties in bubble detachment and removal. We show that bubbles captured by porous superhydrophobic surfaces merge into larger ones, which can detach by buoyancy. The responsible mechanism is convective Ostwald ripening because networklike pores in the superhydrophobic film remain nonwetted and provide passage for gas flow between adhered bubbles. A large bubble grows spontaneously by absorbing all small adhered bubbles due to capillary pressure differences. Our results demonstrate that porous hydrophobic film can be an efficient, passive way of bubble removal in microfluidic systems.

Drag reduction in electro-osmosis of polymer solutions

Electro-osmosis is the preferred transport mechanism in microfluidic systems. Drag reduction in electro-osmosis of polymer solutions is observed due to polymer depletion in the electric double layer (EDL). The well-known Helmholtz-Smoluchowski (HS) equation indicates that the electro-osmosis mobility is inversely proportional to the solution viscosity. For low molecular weight the polymer size (R) is smaller than the EDL thickness (λ) and the HS equation is valid. For high molecular weight (R>λ) the chains in the EDL are partially sheared and the effective viscosity is smaller than the solution viscosity. Salt addition reduces λ and can enhance drag reduction substantially.

Superhydrophobic floatability of a hydrophilic object driven by edge effect

It is generally believed that a water-repellent surface is necessary for small insects to stand on water. Through a combined experimental and theoretical study, we demonstrate that an object with hydrophilic surface can float with apparent contact angle greater than 90° due to edge effect. The apparent contact angle rises with increasing loading even to a value typically displayed only by superhydrophobic surfaces. On the basis of free energy minimization, two regimes are identified. When buoyancy controls, the meniscus meets the object with the intrinsic contact angle. As surface tension dominates, however, contact angle is regulated by total force balance.

Size-dependent electro-osmosis in a microchannel with low-permittivity, salt-free media

Electro-osmosis is the preferred transport mechanism in microfluidic systems. The electro-osmotic mobility in an aqueous solution is essentially independent of the microchannel size. However, in low-permittivity solvents, the screening effect is absent and the mobility is found to grow with the capillary radius according to our analytical theory and electro-osmotic flow experiments. Our results show that regardless of low surface charge density, electro-osmosis always occurs for long enough microchannels. Therefore, electro-osmosis provides a method for determining extremely low surface charge density associated with low-permittivity media. The implication in the response time of electronic paper is given as well.