Cyuan-Jhang Wu

Superhydrophilicity and spontaneous spreading on zwitterionic surfaces: carboxybetaine and sulfobetaine

Zwitterionic surfaces are fabricated by grafting sulfobetaine silane (SBSi) and carboxybetaine silane (CBSi) on glass slides. Their wetting behaviors are investigated using water, polar organic liquids, and hexadecane as test liquids. For the CBSi surface, partial wetting is observed, and contact angles of water and hexadecane are lower than 10°, revealing super-amphiphilicity. For the SBSi surface, all test liquids spread spontaneously and contact angles are absent, corresponding to total wetting. The time evolution of the wetting area of a liquid drop can be divided into three types: spread-withdrawal for water, spread-pin for polar organic liquids, and continuous spread for hexadecane. The spontaneous spreading on SBSi surfaces is driven by the high solid–gas interfacial tension and can be characterized by the power law. Although zwitterionic surfaces like both water and hexadecane in ambient air, their preference for water over hexadecane is typically demonstrated by a hexadecane drop in a water environment. Nonetheless, the contact angle of the hexadecane drop is 120° on the CBSi surface, but becomes 180° on the SBSi surface. As the zwitterionic surfaces are immersed in all test liquids, bubbles generally adhere to the CBSi surface but freely move beneath the SBSi surface. Our experimental results clearly show the wettability difference between the CBSi and SBSi surfaces. The former is superhydrophilic, while the latter is total wetting.

Copper conductive lines on flexible substrates fabricated at room temperature

Flexible electronic devices often comprise conductive Cu patterns and polymeric substrates. In this article, copper conductive patterns are readily fabricated on various soft polymeric substrates at room temperature by reduction-assisted sintering with ascorbic acid (AA). Conductive lines on polyethylene terephthalate exhibit good flexibility, adhesion, electrical conductivity, and robust durability under substantial deformation. According to XRD and XPS characterization, pure Cu is obtained after reduction by AA. The sintering of Cu is demonstrated via the mechanical strength of Cu patterns. The optimal concentration of AA is about 0.5 M, which corresponds to the lowest resistivity of around 7.74 × 10−4 Ω cm. The sintering of Cu particles at room temperature can be explained by the removal of oxygen associated with reduction by AA, which drives the rearrangement of nearby Cu atoms. Good adhesion may be attributed to the structure of AA, which is like that of L-3,4-dihydroxyphenylalanine. This method may be applied to spray printing technology or paper electronics.

Reduction-assisted sintering of micron-sized copper powders at low temperature by ethanol vapor

The low temperature sintering of micron-sized Cu powders is achieved by ethanol vapor annealing. A Cu-pancake is formed and has enough mechanical strength to sustain the gravitational pull. The electrical resistivity of the Cu-pancake formed by flaky powders is lower than that by spheroidal ones because more contact area of the former facilitates the sintering process. The resistivity of the Cu-pancake grows with decreasing the annealing temperature but it is still about 10−3 Ω cm at 120 °C. The sintered Cu-pancake is characterized by X-ray diffraction and X-ray photoelectron spectroscopy to investigate the sintering mechanism. The low temperature sintering is attributed to the reduction of the native oxide on surfaces of Cu powders by the ethanol vapor. The reduced Cu is very active and tends to sinter with each other to lower the surface energy. This reduction-assisted sintering may be useful in the fabrication of conductive patterns on flexible substrates. The prepared Cu pattern on polyethylene naphthalate exhibits repeatable flexibility and acceptable conductivity.

Self-Propulsion and Shape Restoration of Aqueous Drops on Sulfobetaine Silane Surfaces

The motion of droplets on typical surfaces is generally halted by contact line pinning associated with contact angle hysteresis. In this study, it was shown that, on a zwitterionic sulfobetaine silane (SBSi)-coated surface, aqueous drops with appropriate solutes can demonstrate hysteresis-free behavior, whereas a pure water drop shows spontaneous spreading. By adding solutes such as polyethylene glycol, 2(2-butoxy ethoxy) ethanol, or sodium n-dodecyl sulfate, an aqueous drop with a small contact angle (disappearance of spontaneous spreading) was formed on SBSi surfaces. The initial drop shape was readily relaxed back to a circular shape (hysteresis-free behavior), even upon severe disturbances. Moreover, it was interesting to observe the self-propulsion of such a drop on horizontal SBSi surfaces in the absence of externally provided stimuli. The self-propelled drop tends to follow a random trajectory, and the continuous movement can last for at least 10 min. This self-propelled random motion can be attributed to the combined effects of the hysteresis-free surface and the Marangoni stress. The former comes from the total wetting property of the surface, while the latter originates from surface tension gradient due to fluctuating evaporation rates along the drop border.

Anti-smudge behavior of facilely fabricated liquid-infused surfaces with extremely low contact angle hysteresis property

Slippery liquid-infused porous surfaces (SLIPS) possess excellent liquid-repellency and have been applied for anti-icing and anti-fouling. Following the concept of SLIPS, a liquid-infused surface is facilely fabricated by a stretched polytetrafluoroethylene (PTFE) thread seal tape infused with perfluoropolyether fluorinated lubricant. Owing to the thin thickness of the PTFE film, this surface can be transparent and flexible. The nearly hysteresis-free property of SLIPS is manifested by the absence of hysteresis loops in the plot of contact angle (or base diameter) versus drop volume for both water and hexadecane. The anti-smudge behavior of the surface is first examined by the removal of stains by sliding drops at an inclined plane. Secondly, the anti-smudge behavior of the surface is demonstrated by the wetting competition of a hexadecane drop between SLIPS and fluorinated polyvinyl alcohol surface which is lipophobic and superhydrophobic. Because of the negligible contact angle hysteresis, almost no liquid (only 3.9%) is seized by the SLIPS after rupture. A Surface Evolver simulation is performed to understand the mechanism of the wetting competition and the result is in a good agreement with that of the experiment. Our tests indicate that the liquid-infused surface exhibits excellent anti-smudge properties.

Resisting and pinning of a nanodrop by trenches on a hysteresis-free surface

The encounter of a nanodrop with a trench on a hysteresis-free surface is explored by many-body dissipative particle dynamics to show the effect of surface roughness on droplet wetting. A free nanodrop exhibits Brownian motion and the diffusivity decays exponentially with the liquid-solid contact area. In contrast, as the nanodrop sits on a trench, its random motion is constrained. Work must be done to overcome the energy barriers for the transition between free and trapped states. The potential energy landscape is thus constructed based on the force-displacement plot. It is shown that the trench acts as a hydrophobic blemish for capture but like a hydrophilic blemish for escape. A drop always breaks up after detachment from a hydrophilic trench. Therefore, the drop tends to bypass a small trench when it meets one. The macroscopic experiments are performed by fabricating liquid-infused surfaces with extremely low contact angle hysteresis. The experimental observations agree qualitatively with simulation outcomes.

Extraordinarily Rapid Rise of Tiny Bubbles Sliding beneath Superhydrophobic Surfaces

Tiny bubbles readily stick onto substrates owing to contact angle hysteresis (CAH). Nevertheless, they can slide slowly on a tilted surface with ultralow CAH because capillarity is overcome by buoyancy. It is surprising to observe experimentally that bubbles of 3-15 μL (diameter 1.79-3.06 mm) slide beneath a tilted superhydrophobic surface at a vertical ascent rate faster than that of freely rising ones of high Reynold numbers ≈O(102). As the tilting angle increases, the drag coefficient remains essentially the same as that of a freely rising bubble, but the frontal area of the flat bubble rises monotonically. Nonetheless, the frontal area of the sliding bubble always stays much smaller than that of a freely rising bubble. Consequently, the small drag force associated with the sliding bubbles is attributed to their substantially small frontal areas on superhydrophobic surfaces.

Forced Spreading of Aqueous Solutions on Zwitterionic Sulfobetaine Surfaces for Rapid Evaporation and Solute Separation

Solute separation of aqueous mixtures is mainly dominated by water vaporization. The evaporation rate of an aqueous drop grows with increasing the liquid-gas interfacial area. The spontaneous spreading behavior of a water droplet on a total wetting surface provides huge liquid-gas interfacial area per unit volume; however, it is halted by the self-pinning phenomenon upon addition of nonvolatile solutes. In this work, it is shown that the solute-induced self-pinning can be overcome by gravity, leading to anisotropic spreading much faster than isotropic spreading. The evaporation rate of anisotropic spreading on a zwitterionic sulfobetaine surface is 25 times larger as that on a poly(methyl methacrylate) surface. Dramatic enhancement of evaporation is demonstrated by simultaneous formation of fog atop liquid film. During anisotropic spreading, the solutes are quickly precipitated out within 30 s, showing the rapid solute-water separation. After repeated spreading process for the dye-containing solution, the mean concentration of the collection is doubled, revealing the concentration efficiency as high as 100%. Gravity-enhanced spreading on total wetting surfaces at room temperature is easy to scale-up with less energy consumption, and thus it has great potentials for the applications of solute separation and concentration.