Microfluidic concepts for electrospinning

The contemporary physical models of electrospinning are thoroughly overviewed. A novel microfabrication technique has been employed for the microfabrication of silicon dioxide pedestal nozzles. They contain a concentrical rim with a sharp edge at the end of the tube and thereby facilitate contact line pinning of liquids and strong adhesion with polar liquids. Because the typical rim radii range between 35 and 110 micrometers, it enables a unique platform for studying pinned droplets. A pressure point is observed at which droplets start to grow uninhibitedly, and the phenomenon is modeled using the droplet’s Gibbs free energy. Using a dynamic approach in which the nozzle oscillates periodically, growing droplets were brought (close to) resonance. The contact line pinning effect is so strong that the observed contact angles were close to the theoretical maximum. Also, the droplets were oscillating in an unstable fashion after passing through resonance because their response time was larger than the period of oscillation. This phenomenon is understood qualitatively using a damped mass-spring model. Strong contact line pinning of the base of the Taylor cone onto the nozzle allows the near-field electrospinning of polyvinylpyrrolidone at a relatively large nozzle-to-collector distance of 25 mm. No characteristic fiber whipping instability has been observed, and it paves the way for electrospinning hierarchical structures. A bidimensional numerical approach is developed to model the filtering efficiency of (electrospun) fibrous mats. It shows the importance of electrically polarized (electret) fibers to reduce the most-penetrating particle size. A number of ideas are presented to electrospin with multiple pedestal nozzles in parallel, characterize the polarizability of electret fibers using Kelvin-probe force microscopy, and model the electric field close to pedestal nozzles during electrospinning.