Wicking rate

Ma and Tomasko found that the structural integrity and strength of the polymer were unchanged. They also found that the contact angle and the wicking rate were indeed altered by the procedure. Their method has an advantage over previous work in that it is less disruptive of the polymer network and therefore the surfactant-polymer interactions can be more precisely investigated. However, such a method is only applicable to a limited number of materials because of the required solubility behavior. 

Spot test Wicking test 5 min 10 min 1 cm Wicking rate 2 cm 3 cm... 

Fig. 6.7. Respective wicking rates for AGM separators with large (11 pm) and small (4 pm) pore sizes [9],...

Fluid handling properties permeability, liquid absorption (liquid absorbency, penetration time, wicking rate and wicking height, rewet, bacteiia/particle collection, repellency and barrier properties, run-off, strike time), water vapour transport, and breathability. 

One-dimensional liquid wicking rate (wicking strip test)... 

The liquid wicking rate may be measured in terms of the linear rate of advance of the liquid in a strip of nonwoven fabric in strip test. In vertical strip test, the nonwoven fabric is first conditioned at 20°C and 65% RH for 24 h. A strip of the test fabric is suspended vertically with its lower end immersed in a reservoir of distilled water (or other liquid). After a fixed time has elapsed, the height reached by the water in the fabric above the water level in the reservoir is measured. Both the wicking rate and the ultimate height that the water reached are taken as direct indications of the wickability of the test fabric. Liquid wicking in both MD and CD of the nonwoven fabric are tested to obtain the anisotropic liquid wicking properties. 

There are some differences in these test procedures. The BS 3424-18 (Method 21) specifies a very long test period (24 h) and is intended for coated fabrics with very slow wicking rates. In contrast, ISO 9073-6 2000 and NWSP OlO.l.RO (15) specifies a much shorter test time (maximum 5 min) and applies to fabrics that exhibit rapid wicking. 

The horizontal wicking strip test and downward wicking vertical strip test were also reported to obtain wicking rate and capillary pressure. These methods can be connected with a computerised image analyser to obtain dynamic wicking properties, or it can be modified to integrate with an electronic balance to monitor the mass of liquid the fabric absorbed. 

Electrical capacitance techniques were also used to monitor the anisotropic liquid absorption in multidirections in a nonwoven fabric plane. The principle of the method is based on the fact that the dielectric constant of water is about 15—40 times higher than that of normal fibres and fabrics, and therefore, the capacitance of the transducers in a measuring system wdl be very sensitive to the amount of liquid absorbed by a fabric. The computer-integrated capacitance system is able to provide both dynamic (real-time) and multidirectional measurements of the wicking rate in terms of the volume of liquid absorbed. 

Because the saturated fabric has a lower resistance to flow than a dry fabric, the rate of drainage is usually greater than the wicking rate. 

Not only the time for a hquid droplet striking through a nonwoven fabric and liquid wicking rate are important, the rate of liquid absorption (or the period of time taken for nonwoven fabrics to reach saturated status) and the capacity of a nonwoven fabric to retain the hquid, the liquid absorbency, or the liquid absorption capacity are also crucial for nonwoven applications such as hygiene, wiping, and other absorbent nonwoven materials. The methods used to assess the rate of hquid absorption and the hquid absorption capacities are dehned in INDA/EDANA and ISO standards. For example ... 

A derivation of the wicking rate is as follows The incompressible liquid assumption is valid for cryogenic and storable propellants because the wicking process typically... 

Equations (3.62) and (3.63) are the wicking rate equations for the vertical and horizontal cases, respectively. Both equations can be integrated to determine the wicking distance Lw, as a function of time. Integrating Equations (3.62) and (3.63) and applying the initial condition Lw(t = 0) = 0 ... 

The most important trend to note is that De tends to decrease for finer meshes, implying that finer meshes will have slower wicking rates than the coarser meshes. This is due to the finer meshes having a more obstructed flow path and thus a higher resistance to flow. The coarser meshes have less obstruction so that the liquid is able to flow with less required capillary driving pressure. 

Clearly the optimal mesh for a particular mission requires trading all of the aforementioned influential factors against one another. Space flight requirements which include mass flow rate, acceleration level and direction, and thermal environment dictate selection of the screen for a particular mission. The primary performance parameters governing screen channel LAD design are the bubble point (and reseal pressure) and FTS pressure drop, while secondary parameters are the wicking rate, screen compliance, and material compatibility. 

Trends in room temperature reseal pressure data mirrors trends in bubble point pressure data. All reseal pressures collected here are about 90% of the corresponding bubble point values. Operationally, this implies that only a ->10% reduction in differential pressure across the screen is required to reseal the screen and prolong the point of total LAD failure to yield a higher overall expulsion efficiency. Wicking rate test results performed in IPA align nicely with historical trends as coarser meshes outperform finer meshes. 

The 450 x 2750 is able to counteract larger evaporation rates than the 510 x 3600 due to its higher wicking rate. 

---