Understanding the diffusional tortuosity of porous materials: an effective medium theory perspective

Gao, Xuechao, Diniz da Costa, Joao C. and Bhatia, Suresh K. (2014) Understanding the diffusional tortuosity of porous materials: an effective medium theory perspective. Chemical Engineering Science, 110 55-71. doi:10.1016/j.ces.2013.09.050


Author Gao, Xuechao
Diniz da Costa, Joao C.
Bhatia, Suresh K.
Title Understanding the diffusional tortuosity of porous materials: an effective medium theory perspective
Journal name Chemical Engineering Science   Check publisher's open access policy
ISSN 0009-2509
1873-4405
Publication date 2014-05-01
Year available 2013
Sub-type Article (original research)
DOI 10.1016/j.ces.2013.09.050
Volume 110
Start page 55
End page 71
Total pages 17
Place of publication Kidlington, Oxford, United Kingdom
Publisher Pergamon
Language eng
Formatted abstract
The interpretation of experimental data on transport in porous materials is often based on the use of a single representative pore size, overlooking effects of the pore size distribution (PSD) and pore network connectivity, and fitting a tortuosity into which all such uncertainties are consigned. Using literature data on the diffusion of N2, Xe and i-C4H10 in mesoporous Shell silica spheres, we demonstrate that the tortuosity depends on the choice of the representative pore radius as well as gas species. Both the Knudsen model and the Oscillator model considering dispersive fluid-solid interactions, developed in this laboratory, are found to adequately interpret the data in conjunction with effective medium theory (EMT) by fitting a network coordination number instead of tortuosity. This insensitivity to model is due to the large mean mesopore radius of 7.4nm for this silica; however, the Oscillator model is found to yield a value of the coordination number closer to the range of values expected for this material. Using the EMT we demonstrate that the tortuosity is dependent on temperature and diffusing species, because of differences in temperature dependence between the conductance at the representative pore radius and the true conductance which depends on the network connectivity and PSD. In the slip flow regime, which is obtained at large pore size, we show that the superposition of Knudsen and viscous mechanisms leads to temperature and species dependence of tortuosity, because of the different pore size dependence of the two contributions. This leads to different limiting tortuosities and PSD dependence in the Knudsen and viscous flow regimes. These critical aspects are largely unappreciated in the literature, and even systematic variations of tortuosity with temperature or diffusing species usually overlooked, often leading to misrepresentation of the underlying mechanism.
Keyword Adsorption
Diffusion
Effective medium theory
Nanomaterials
Porous media
Tortuosity
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Chemical Engineering Publications
Official 2014 Collection
 
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Citation counts: TR Web of Science Citation Count  Cited 13 times in Thomson Reuters Web of Science Article | Citations
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