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

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==General Information==
 
==General Information==
'''Authors:''' Andr�e R. Studar, Julia Studer, Lei Xu, Kisun Yoon, Ho Cheung Shum, and David A. Weitz
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'''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
 
'''Publication:''' A. R. Studar, et al. Hierarchical Porous Materials Made by Drying Complex Suspensions. Langmuir, 27, (3) 955-964 February 2011
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==Summary==
 
==Summary==
  
[[Image:Fig1a.jpg|500px|thumb| Figure: 1]]
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[[Image:Fig11.jpg|400px|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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Many natural structures contain pores at varying length scales. Even within our bodies, bones and lung tissue display hierarchically pored materials. Of course, these systems are useful for a variety of applications as well including high-surface area catalytic applications and filtration devices. To synthesize structures like these, people often use gelation reactions of varying chemistries, or foaming processes. However, these techniques do not leave very much freedom to precisely control the location and size of pores, especially not systems with multiple organized pores of different sizes.  
  
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).
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[[Image:Fig2a.jpg|left|400px|thumb| Figure: 2]]
  
[[Image:Fig2a.jpg|left|500px|thumb| Figure: 2]]
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In this paper, the Weitz lab describes a  versatile and simple approach to produce hierarchical porous materials. Interestingly, it relies solely on drying. Utilizing their standard microfluidic droplet-creation technique, they monodisperse droplets of tunable size. These drops are able to template the eventual pores in the material with up to three levels of heirarchy, ranging from 10nm to 800 um. Those pores are build from a combination of surfactants and colloidal particles.  
  
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|left|500px|thumb| Figure: 3]]
  
[[Image:FIg3a.jpg|600px|thumb| Figure: 3]]
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As hinted at above, the size of the macropores is determined by the size of the droplets. However, the way in which the macropores are inreconnected is a function of the droplet stabilizer (aforementioned surfactants and colloidal particles). Because this approach is largely controlled by the physical process, it allows for chemical versatility. THis means that the same approach can be used to create number of heirarchically porous materials with a wide variety of pore sizes.  
  
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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Lastly, as depicted in Figure 3, these complex mixtures can be easily deposited on patterned or unpatterned surfaces, resulting in yet another level of hierarchical organization. Examining Figure 3B-E, one sees that with increasing magnification, a new scale of order emerges- surface pattern, macro pore, micro pore, nano-channels of interconnectivity.  
  
 
==Discussion==
 
==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.
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This is an interesting general strategy for materials synthesis. Another method used to synthesize materials like these that wasn't mentioned in the introduction is the assembly, and then calcination of organic particles (i.e. Aizenberg lab inverse opals). The size of the colloidal particles can be easily controlled to give a variety of hyper-organized pores. However, this control they display here over the interstices between pores appears to be a differentiating factor. It is also interesting to consider the steps to move from a proof of principle approach to synthesis (as shown here) to a functional approach. The mechanical rigidity of these structures is likely to be quite weak, and could be enhanced through clever surface chemistry manipulation, etc.
  
 
==Reference==  
 
==Reference==  
Y. M. Shin, et al. Electrospinning: A whipping fluid jet generates submicron polymer fibers. Appl. Phys. Lett., Vol. 78, No. 8, 19 February 2001
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A. R. Studar, et al. Hierarchical Porous Materials Made by Drying Complex Suspensions. Langmuir, 27, (3) 955-964 February 2011

Latest revision as of 21:42, 13 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

Many natural structures contain pores at varying length scales. Even within our bodies, bones and lung tissue display hierarchically pored materials. Of course, these systems are useful for a variety of applications as well including high-surface area catalytic applications and filtration devices. To synthesize structures like these, people often use gelation reactions of varying chemistries, or foaming processes. However, these techniques do not leave very much freedom to precisely control the location and size of pores, especially not systems with multiple organized pores of different sizes.

Figure: 2

In this paper, the Weitz lab describes a versatile and simple approach to produce hierarchical porous materials. Interestingly, it relies solely on drying. Utilizing their standard microfluidic droplet-creation technique, they monodisperse droplets of tunable size. These drops are able to template the eventual pores in the material with up to three levels of heirarchy, ranging from 10nm to 800 um. Those pores are build from a combination of surfactants and colloidal particles.

Figure: 3

As hinted at above, the size of the macropores is determined by the size of the droplets. However, the way in which the macropores are inreconnected is a function of the droplet stabilizer (aforementioned surfactants and colloidal particles). Because this approach is largely controlled by the physical process, it allows for chemical versatility. THis means that the same approach can be used to create number of heirarchically porous materials with a wide variety of pore sizes.

Lastly, as depicted in Figure 3, these complex mixtures can be easily deposited on patterned or unpatterned surfaces, resulting in yet another level of hierarchical organization. Examining Figure 3B-E, one sees that with increasing magnification, a new scale of order emerges- surface pattern, macro pore, micro pore, nano-channels of interconnectivity.

Discussion

This is an interesting general strategy for materials synthesis. Another method used to synthesize materials like these that wasn't mentioned in the introduction is the assembly, and then calcination of organic particles (i.e. Aizenberg lab inverse opals). The size of the colloidal particles can be easily controlled to give a variety of hyper-organized pores. However, this control they display here over the interstices between pores appears to be a differentiating factor. It is also interesting to consider the steps to move from a proof of principle approach to synthesis (as shown here) to a functional approach. The mechanical rigidity of these structures is likely to be quite weak, and could be enhanced through clever surface chemistry manipulation, etc.

Reference

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