Difference between revisions of "Hierarchical Porous Materials Made by Drying Complex Suspensions"

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[[Image:Fig11.jpg|500px|thumb| Figure: 1]]
 
[[Image:Fig11.jpg|500px|thumb| Figure: 1]]
  
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).  
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Porous structures containing pores at different length scales are often encountered in nature and are important in many applications. While several processing routes have been demonstrated to create such hierarchical porous materials, most methods either require chemical gelation reactions or do not allow for the desired control of pore sizes over multiple length scales.Wedescribe a versatile and simple approach to produce tailor-made hierarchical porous materials that relies solely on the process of drying. Our results show that simple drying of a complex suspension can lead to the self-assembly of droplets, colloidal particles and molecular species into unique 3D hierarchical porous structures. Using a microfluidic device to produce monodisperse templating droplets of tunable size, we prepared materials with up to three levels of hierarchy exhibiting monodisperse pores ranging from 10 nm to 800 μm. While the size of macropores obtained after drying is determined by the size of initial droplets, the interconnectivity between macropores is strongly affected by the type of droplet stabilizer (surfactants or particles). This simple route can be used to prepare porous materials of many chemical compositions and has great potential for creating artificial porous structures that capture some of the exquisite hierarchical features of porous biological materials.
  
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).
 
  
 
[[Image:Fig2a.jpg|left|500px|thumb| Figure: 2]]
 
[[Image:Fig2a.jpg|left|500px|thumb| Figure: 2]]
  
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.
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[[Image:FIg3a.jpg|600px|thumb| Figure: 3]]
 
[[Image:FIg3a.jpg|600px|thumb| Figure: 3]]
  
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.
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==Discussion==
 
==Discussion==

Revision as of 06:29, 12 November 2012

Wiki Entry by Daniel Rubin, AP225, 11/12/2012

General Information

Authors: Andre 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

Summary

Figure: 1

Porous structures containing pores at different length scales are often encountered in nature and are important in many applications. While several processing routes have been demonstrated to create such hierarchical porous materials, most methods either require chemical gelation reactions or do not allow for the desired control of pore sizes over multiple length scales.Wedescribe a versatile and simple approach to produce tailor-made hierarchical porous materials that relies solely on the process of drying. Our results show that simple drying of a complex suspension can lead to the self-assembly of droplets, colloidal particles and molecular species into unique 3D hierarchical porous structures. Using a microfluidic device to produce monodisperse templating droplets of tunable size, we prepared materials with up to three levels of hierarchy exhibiting monodisperse pores ranging from 10 nm to 800 μm. While the size of macropores obtained after drying is determined by the size of initial droplets, the interconnectivity between macropores is strongly affected by the type of droplet stabilizer (surfactants or particles). This simple route can be used to prepare porous materials of many chemical compositions and has great potential for creating artificial porous structures that capture some of the exquisite hierarchical features of porous biological materials.


Figure: 2


Figure: 3


Discussion

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.

Reference

A. R. Studar, et al. Hierarchical Porous Materials Made by Drying Complex Suspensions. Langmuir, 27, (3) 955-964 February 2011