Hierarchical Porous Materials Made by Drying Complex Suspensions
Wiki Entry by Daniel Rubin, AP225, 11/12/2012
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
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.
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 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.
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.
A. R. Studar, et al. Hierarchical Porous Materials Made by Drying Complex Suspensions. Langmuir, 27, (3) 955-964 February 2011