Selective superhydrophilic/phobic coating using capillary pressure for positive-displacement nanoliter dispensing

Abstract

Owing to the fact that liquid-dispensing devices are used in a wide variety of scientific fields, including chemistry, biology, pharmacology, and mechanics, droplet dispensing has come to be regarded as a key technology with respect to micro/nanoengineering. Positive-placement dispensing technology is being used widely because it allows for a high degree of controllability without requiring a complex dispensing system; however, adhesion and slip-related issues limit the performance of this technology. In this letter, we report a technique for the fabrication of selective superhydrophilic/phobic coatings in syringe-type positive-displacement dispensing nozzle tips. The superhydrophilic capillary in the nozzle tip was coated selectively with superhydrophobic materials by exploiting the difference in the capillary pressure. The surface of the front part of the capillary was made superhydrophobic; it thus allowed the liquid to flow without adhesion. In contrast, the back part was made superhydrophilic; it retarded the flow of the liquid, holding it in place. Together, the two surfaces minimized the volume of the dispensed droplets. High-speed images of the dispensed liquid were taken to compare the two droplet-dispensation processes. It was found that the volume of the water droplets dispensed from the selectively coated nozzle tip was as low as 27 nL and much smaller than that of the droplets dispensed from the superhydrophobic nozzle tip. (C) 2015 Elsevier B.V. All rights reserved.