title: A microchip fabricated with a vapor-diffusion self-assembled-monolayer method to transport droplets across superhydrophobic to hydrophilic surfaces.
abstract: A wettability gradient to transport a droplet across superhydrophobic to hydrophilic surfaces is fabricated on combining a structure gradient and a self-assembled-monolayer (SAM) gradient. The combination of these two gradients is realized with a simple but versatile SAM technique, in which the textured silicon wafer strip is placed vertically in a bottle that contains a decyltrichlorosilane solution to form concurrently a saturated SAM below the liquid surface and a wettability gradient above. The platform fabricated in this way has a water-contact angle from 151.2 degrees to 39.7 degrees; the self-transport distance is hence increased significantly to about 9 mm. A theoretical model that approximates the shape of a moving drop to a spheroidal cap is developed to predict the self-transport behavior. Satisfactory agreement is shown for most regions except where the hysteresis effect is unmeasurable and an unsymmetrical deformation occurs. A double-directional gradient surface to alter the direction of movement of a droplet is also realized. The platforms we developed serve not only to transport a fluid over a long distance but also for a broad spectrum of biomedical applications such as protein adsorption, cell adhesion and DNA-based biosensors.
authors.affiliation: Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Yu-Hsuan
authors.initials: YH
authors.lastname: Lai
authors.forename: Jing-Tang
authors.initials: JT
authors.lastname: Yang
authors.forename: Dar-Bin
authors.initials: DB
authors.lastname: Shieh
mesh_terms: D015374:Biosensing Techniques; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D004058:Diffusion; D057927:Hydrophobic and Hydrophilic Interactions; D046208:Microchip Analytical Procedures / Q000379:methods*; D055616:Microtechnology / Q000379:methods*; D009038:Motion; D012825:Silicon / Q000737:chemistry; D013499:Surface Properties; D014835:Volatilization; D017155:Wettability
chemical_list: D004247:DNA; D012825:Silicon