Nanofiber nonwovens are finding increasing applications in the filtration industry particularly in processes involving biotechnology. For such applications, pore volume is very important. Through pore diameter, pore throat diameter, and permeability are also important pore structure characteristics. Nanofiber mats are normally sensitive to pressure and are often brittle. Therefore, the characterization technique should be such that the pore structure is not distorted. In this investigation, the applicability of the techniques, Mercury Intrusion Porosimetry, Liquid Extrusion Porosimetry, and Liquid Extrusion Flow Porometry for pore structure characterization of nanofiber mats have been investigated. The results obtained by the three techniques have been critically examined.

Characterization Technique Mercury Intrusion Porosimetry
Mercury is non-wetting to most materials. It does not enter the pores spontaneously. Intrusion of mercury into pores occurs due to pressure applied on mercury. Pressure is used to compute pore diameter [1].

Equation 1
D = - 4gcos u/p

D is pore diameter, g is surface tension of mercury, u is contact angle of mercury, and p is pressure on mercury for intrusion into the pore. Intrusion volume gives pore volume.
The technique measures pore volume and pore diameter of through and blind pores. As shown in Figure 1, all diameters and volumes of through and blind poresare measured.

Liquid Extrusion Porosimetry
In liquid extrusion porosimetry, the pores are spontaneously filled with a wetting liquid and the liquid is extruded from pores by a non-reacting gas.It can be shown that the differential pressure is related to pore diameter.

Equation 2
D = 4gcos u/p

D is pore diameter, g is surface tension of wetting liquid, u is the contact angle of the wetting liquid, and p is differential pressure.

In this method, the volume of extruded liquid is measured in addition to the differential pressure. In order to allow the extruded liquid to flow out and prevent the gas to escape, a membrane is placed under the sample such that the largest pore of the membrane is smaller than the smallest pore of interest in the sample (Figure 2a). The pores of the sample and the membrane are filled with a wetting liquid and pressure on gas is increased to displace the liquid from pores of the sample. The gas pressure is inadequate to empty the pores of the membrane. Therefore, the liquid-filled pores of the membrane allow the extruded liquid from the pores of the sample to flow out while preventing the gas to escape. The measured volume of the liquid flowing out of the membrane gives through pore volume. Differential pressure yields through pore diameter and variation of volume with pressure yields through pore surface area. Flow rate of excess liquid maintained on the sample yields liquid permeability (Figure2b).

This technique measures only the volume and diameters of through pores (Figure 3) [2]. Blind pores are not measured (Figure 1). As shown in Figure 4, only some of the diameters of the through pore are measured, where as mercury intrusion measures all pore diameters (Figure 1).

Liquid Extrusion Flow Porometry Capillary Flow Porometry
In this method, the pores of the sample are filled with a wetting liquid. The liquid is emptied by a pressurized gas permitting gas to flow through the empty pores. The differential pressure required to empty a pore of diameter D is given by Equation 2. It shows that the largest pore is emptied at the lowest pressure and initiates gas flow. With increasing pressure, smaller pores are emptied and gas flow increases. The differential pressures and gas flow rates through dry and wet samples are measured.

In the dry sample, the flow rate increases with increase in pressure. In case of the wet sample, initially there is no flow because all the pores are filled with the liquid. At a certain pressure, the gas empties the largest pore (Equation 2) and gas flow starts through the wet sample. With further increase in pressure, smaller pores are emptied and the flow rate increases until all the pores are empty and the flow rate through the wet sample is the same as that through the dry sample. This is schematically illustrated in Figure 4. The half-dry curve in this figure is computed from the dry curve to yield fifty-percent of flow through dry sample at the same pressure. The dry and wet curves yield the bubble point, the mean flow pore diameter, flow distribution and pore fraction distribution of through pores. The dry curve yields gas permeability and envelope (through pore) surface area. Liquid flow rate gives liquid permeability. Capillary flow porometry measures only the throat diameter of each through pore (Figure 4). One diameter per through pore is measured. Blind pores are not measured (Figure 1).