Fabrication and characterization of microfluidic pedestal nozzles enabling geometric contact line pinning

Abstract

Microfluidic pedestal nozzles have been fabricated using self-aligned wafer-scale microfabrication methods. Through implementation of these novel fabrication strategies, nozzles with a concentric rim at the nozzle exit were made. The process allows to accurately vary the rim radius, and rim radii ranging between 36μm and 111μm are reported here. A unique and essential feature of the nozzle, which is made of hydrophilic silicon dioxide, is that the rim edge has an interior angle of 5∘ and a radius of curvature smaller than 20 nm. Due to this nanometrically sharp rim edge, geometric contact line pinning of polar liquids is facilitated. With hydrostatic and hydrodynamic measurements, the nozzle functionality is demonstrated for water and ethanol. Due to the unprecedentedly strong contact line pinning, macroscopic contact angles can be increased by more than 150∘. These measurements reveal a pressure point at which the droplet starts to grow uninhibitedly, which is predicted by a thermodynamic model based on the Gibbs free energy of pinned droplets. It is reckoned that the nozzles are useful in applications where liquids with free surfaces have to be confined to a specific micron-sized surface area, as is the case in, e.g., inkjet printing and electrospinning. To show the functionality of the pedestal nozzles, electrohydrodynamic jetting of a typical electrospinning polymer solution is demonstrated.