Capillary flow porometry

The technique is similar to the liquid extrusion technique in that the nanofiber mat is saturated with a wetting liquid and gas pressure is applied to one smface of the mat. The surface free energy of the liquid with the fiber mat needs to be less than that of the mat with the gas. As with liquid extrusion, the liquid coltmm occupying through-channels will be displaced by the gas. In flow porometry, the gas displaces the liquid (and continues to flow through the emptied channel as well) and the flow rate of gas as a function of the differential pressure is recorded. 

The distribution of interstitial pore sizes obtained by this technique, however, refers exclusively to that of the throat of the pores (rather than to the average size of lumen). This might be the pertinent value in characterizing liquid filter media or studying the amenability of scaffolding to cell movements within it. By contrast, the full range of pore diameters encountered within a pore contributes to the mercury intrusion data. 

The general features of wet and dry curves obtained from capillary flow porometry are shown in Figs. 5.4 and 5.5 for a porous poly(e-caprolactone) film sample. 

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Pore Size Distribution—Capillary Flow Porometry... 

Another method to measure pore size distribution is capillary flow porometry [202,203], in which a sample material is soaked with a low surface tension liquid that fills all its pores. Then, gas pressure is applied on one side of the sample in order to force the liquid out of the pores. At low pressures, the flow rate is close to zero however, as the pressure increases, the flow rate also increases and the amount of liquid inside fhe pores decreases. Thus, the flow rate is determined as a function of pressure and is then used to calculate the desired pore characteristics, such as pore size distribution, largest pore diameter, and mean flow pore diameter. 

Another technique, capillary flow porometry has been developed by Porous Materials Inc. ° to characterize battery separators.The instrument can measure a number of characteristics of battery separators such as size of the pore at its most constricted part, the largest pore size, pore size distribution, permeability, and envelope surface... 

Keywords tissue scaffolds, mercury porosimetry, capillary flow porometry, scanning electron microscopy, image analysis... 

Scaffold porosity and information on the pore size distribution can be obtained from intrusion techniques. The most commonly used methods are mercury porosimetry and capillary flow porometry. In mercury porosimetry the pressure required to fill a tissue scaffold with non-wetting mercury is monitored over a set period of time. Higher pressures are required to fill small pores than large pores a fact that can be exploited using the Washburn equation13 to extract structural information where D is the diameter of the pore at a particular differential... 

Capillary flow porometry is essentially mercury porosimetry in reverse where the increasing gas pressure required to displace a fluid (not mercury) from a fluid filled scaffold is monitored as a function of time. Higher pressures equate to smaller pore sizes again following Washburn s equation i.e. 

Table 5. Pore diameters obtained for PCL scaffolds from capillary flow porometry.

Figure 5 compares the pore size distributions of the scaffold computed from the intrusive techniques of capillary flow porosimetry and mercury porometry. From this figure it is apparent that the range of pore sizes derived from capillary flow porometry occurs over a smaller length scale than those based on mercury porometry data. This difference is expected since underlying physics of the... 

Figure 5. Comparison between scaffolds pore distributions obtained via different measurement techniques (cap = capillary flow porometry, Hg = mercury porosimetry, = median value, = median value). The error bars represent the span of the distribution.

The distribution of pore sizes can be obtained from both mercury porosimetry and capillary flow porometry. These distributions are only representations of the actual scaffold structure reflecting the limitations of the underlying physics behind each technique. For this reason it is very difficult to compare pore size distributions for complex structures, such as particulate-leached tissue scaffolds. 

Another method that uses a completely wetting liquid is capillary flow porometry." If the material of interest is brought into contact with the wetting liquid, all pores will be flooded. In order to remove the liquid a pressure across the sample has to be applied. While slowly increasing the pressure difference, at a particular pressure the gas flow steps in, which means that the pore with the largest diameter was emptied according to... 

Porometry Capillary flow porometry Median pore diameter, through-pore size distribution, permeabibty... 

Fig. 16 Before and after comparison of capillary flow porometry (CFP) pore size distributions PSDs) for cell M2 - GDLs aged in 80°C deionized water with air sparging gas for l,006h, then durabihty-tested (with fresh 3M CCM) for 664 h at 1.00 A cm". See the Appendix for durability testing conditions...

Figure 4.6 Typical curves obtained by capillary flow porometry on a nanofiber non-woven, under dry (crosses) and wet (diamonds) conditions. A curve corresponding to half the curve measured in dry conditions is also shown (plus signs). Reproduced with permission from Ref. 73, J. Membr. Sci., 2006, 286, 104 114. Doi 10.1016/j.memsci.2006.09.020. Copyright 2006, Elsevier B.V.

Mercury Porosimetry and Capillary Flow Porometry - Pore Size Determination In a mercury porosimetry measurement, pressure is used to force mercury into filling the pores and voids of the material. The method is based on the capillary rise phenomenon which exists when a non-wetting liquid climbs up a narrow capillary. As the pressure is increased, mercury infiltrates the pores to occupy a subset of the total pore space, the extent of which depends on the applied external pressure. The injected volume of mercury as a function of pressure is recorded. The pore size and distribution can be resolved using the Young and Laplace model [43]. The pore sizes that can be determined by mercury porosimetry range from a few nanometers to a few hundreds of microns. The method is invasive in that not all the mercury will be expelled from the pores and pores may collapse as a result of the high pressures. Due to this and environmental concerns about mercury pollution mercury porosimetry method is becoming less popular. 

Porosity may be measured by mercury porosimetry or by capillary flow porometry [31],... 

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