Hierarchical Porous Materials Made by Drying Complex Suspensions
Wiki Entry by Daniel Rubin, AP225, 11/12/2012
Authors: Andr�e R. Studar, Julia Studer, Lei Xu, Kisun Yoon, Ho Cheung Shum, and David A. Weitz
Publication: A. R. Studar, et al. Hierarchical Porous Materials Made by Drying Complex Suspensions. Langmuir, 27, (3) 955-964 February 2011
Key Words: Porous materials, hierarchical, complex suspensions
Electrospinning (electrostatic fiber spinning) is a material fabrication technique used to generate sub-micrometer fibers from polymers and proteins. By extruding a viscous polymer solution from a needle, into an electric field, one is able to form large amounts of very fine, solid fibers at a collection plate. Due to their extremely high surface area, fine porosity, and small diameter, electrospun fibrous mats have been constructed for many different applications including bioengineered tissue scaffolds and water filtration membranes. Figure 1 depicts an electrospun fibrous mat composed of poly-ethylene oxide (PEO).
Despite the broad interest in both single nanofibers and nanofibrous mats, there is litte known about the fundamental physics and process engineering that determines fiber diameter and homogeneity. Specifically, what physical parameters are resopnsible for converting a millimeter scale column of polymer solution to a sub micron fiber, and how might we control it better? To begin, consider Figure 2, a series of images taken of the polymer solution undergoing the transition to a nanofiber. Moving from the needle tip of the syringe to the collecting plate, you first encounter the Taylor cone, followed by the straight jet (2.A), the bending region (2.B(, and finally the whipping jet (2.C).
As the polymer solution approaches the bending region, it rapidly becomes unstable, resulting in the aforementioned whipping instability. The model presented herein is based on the observation that the wavelength of the whipping fiber is significantly longer than that of the straight jet and, therefore, the straight jet can be considered a long, slender object. By utilizing a balance of charge, mass, and a differential momentum balance allows the authors to focus on the length of the field, diameter of the jet, and Surface charge distribution. OF particular interest in the interplay between the whipping instability which leads to fibers and the axisymmetric breakup instability which results in polymer droplets being sprayed to the collector.
The main operating parameters used to adjust fiber diameter and consistency are the electric field and flow rate. When considering the solution itself, its viscosity and conductivity are of particular importance. In Figure 3, one sees the regimes that result in fibers and droplets, as well as their corresponding voltage and flow rate for a single polymer at a fixed percentage.
To this day the fine points of the electrospinning process are not well understood. However, this was the first publication to document a few very relevant factors, not the leas of which is that the jet instability is one fiber that whips rapidly rather than a family of fibers splitting at point. When all variables are considered, electrospinning becomes exceedingly complex (conductivity of solution, surface charge, flow rate, electric field, etc. THerefore honing in on what is experimentally tractable and fitting it to a model adds substantial value to the discussion of the topic.
Y. M. Shin, et al. Electrospinning: A whipping fluid jet generates submicron polymer fibers. Appl. Phys. Lett., Vol. 78, No. 8, 19 February 2001