dc.description.abstract | Owing to numerous scientific and industrial applications involving fluid/substrate interfaces, the material surfaces with various types of wettability are needed. After suitable surface modifications, substrates can show specific wetting behavior and possess high-value applications. For instance, superhydrophilic surfaces can be used for anti-fog, anti-reflection, and anti-biofouling fields; in contrast, superhydrophobic surfaces possess the self-cleaning ability and have the potential to be utilized for anti-smudge materials and reducing energy consumption during mass transport. Synthesis and investigation of the surfaces with extreme wettability (superhydrophobicity or superhydrophilicity) can shed light on the development of surface modification.
Gas bubbles in aerated drinks are often present in our daily life and nature. Their motion along the material surfaces involve the interactions among three phases (gas/liquid/solid) and play an important role in fluid transport such as the dynamic microfluidic. The difficulties of industrial processes and the products encountered in the transport phenomena, such as the bubble plunger in micro-fluidic systems and syringe devices, can be resolved by studying the motion of bubbles. In this work, the wetting behavior of superhydrophilic surfaces and bubble motion on superhydrophobic surfaces have been investigated both experimentally and theoretically.
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 10o, 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 120o on the CBSi surface, but becomes 180o 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.
Solute separation of aqueous mixtures via distillation 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 interfacial area per unit volume but is halted by the self-pinning phenomenon with the addition of nonvolatile solutes including small molecules and polymers. 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 as large as that of a drop of 10 l on a poly(methyl methacrylate) surface. The dramatic enhancement of evaporation by the area expansion is demonstrated by the simultaneous formation of fog atop the liquid film and it is very useful for solute separation and concentration. During anisotropic spreading, the solutes are quickly precipitated out within 30 sec, showing the rapid solute-water separation. After repeating 6 times of the spreading process dropwisely 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 and much less energy consumption and thus it has great potentials for the applications of solute separation and concentration.
Tiny bubbles readily stick onto substrates due to contact angle hysteresis (CAH). Nevertheless, tiny bubbles can slide slowly on a tilted surface with ultralow CAH since capillarity is overcome by buoyancy. It is surprising to observe experimentally that bubbles of 3~15 l (diameter 1.79~3.06 mm) slide beneath a tilted superhydrophobic surface at a vertical ascent rate faster than freely rising ones of high Reynold numbers ~O(102). As the tilted 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 sliding bubbles is attributed to their substantially small frontal areas on superhydrophobic surfaces. | en_US |