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Title: Tuning Water Adhesion on Biomimicking Superhydrophobic MnO2 Films
Keywords: superhydrophobic, adhesion, MnO2, robustness, nanotube
Issue Date: 29-Jul-2011
Citation: ZHAO XIAODAN (2011-07-29). Tuning Water Adhesion on Biomimicking Superhydrophobic MnO2 Films. ScholarBank@NUS Repository.
Abstract: Wettability of solid is important for both fundamental researches and technical applications, ranging from industrial coating to microfluidic engineering. Specifically, non-wettable surfaces with high contact angles and small sliding angles, called superhydrophobic or ultrahydrophobic, have received a large amount of attention in recent years. Water drops that come into contact with superhydrophobic surfaces remaining a nearly spherical shape, with contact angle close to 180? have been reported. These surfaces are of practical interest due to it water-repellent, antisticking, and self-cleaning properties. Recently, special attention has been focused on the strong adhesive superhydrophobic or more properly superhydrophobic-like surfaces that enable a nearly spherical water droplet to be firmly pinned on the surfaces. Such novel superhydrophobic surfaces are expected to have particular applications in open microdroplet devices with respect to increasing the need for controlled transport of small volumes of liquids in localized chemical reactions, bio/chem analysis assay, single-molecule spectroscopy, and tissue engineering. Currently, one of the challenges in this area is to design and fabricate smart surface systems that are able to adjust the wetting properties on demand. Therefore, the aim of my doctoral dissertation is to reveal a general route to prepare superhydrophobic surfaces with tunable adhesion and to explore new techniques to modulate adhesion on superhydrophobic surfaces in a fast and in-situ manner. The stability is crucial for the functional superhydrophobic surfaces and lab-on-chip devices in real applications, and therefore it is necessary to design and fabricate robust superhydrophobic surfaces in the first step. Inspired by the structure of lotus leaves, the hierarchical MnO2 nanopropeller array (NPA) was designed and fabricated by a two-step hydrothermal method. The robustness of superhydrophobicity was confirmed by the water droplet squeezing test. In order to uncover a general route to prepare superhydrophobic surfaces with controllable adhesion, we investigated the intrinsic correlation with structural features and the adhesive force. It was found that the tunable adhesion on a superhydrophobic MnO2 nanostructured film can be achieved by fabricating different patterns including meshlike, ball cactus-like and tilted nanorod structures. The marvelous modulation range of the adhesive forces from 132.4 to nearly 0 ?N endows these superhydrophobic surfaces with extraordinarily different dynamic properties of water droplet. This pattern-dependent adhesive property is attributed to the kinetic barrier difference, resulting in the different continuity of the three-interface contact line. In-situ manipulating water adhesion on superhydrophobic surfaces was realized by application of a small Direct Current (DC) bias, maintaining large contact angles of water droplets. Upon this technique, the measured adhesive force of a 3 ?L water droplet increased monotonically with increasing negative voltage, reaching a maximum of 130 ?N at 22 V, 25 times higher than the original value. It follows that the nearly spherical water droplet can be controllably pinned on the substrate, even if the substrate is turned upside down. This remarkable electrically controlled adhesive property is ascribed to the change of contact geometries between the water droplet and MnO2 nanotube arrays, on which water droplets exhibit different continuities of TCL. As the modulation in this manner is in situ, fast, efficient and environment-friendly, this kind of smart material with electrically adjustable adhesive property is expected to find various applications in biotechnology and in lab-on-chip devices.
Appears in Collections:Ph.D Theses (Open)

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