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Liquid transport in diapers in the presence of SAP
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Modern hygiene materials, such as diapers, sanitary products for women,
materials for incontinence, etc., usually include a sublayer with
distributed superabsorbent polymer (SAP). Its function is to uptake
liquid and to take it immobile away from the skin. Current SAP materials
can absorb a large amount of liquid.Some of them in this case
swell 2-3 orders of magnitude.
Computer simulation is recognized recently to be a powerfull technique,
assisting the design of hygiene materials with a better performance.
Mathematical model of porous media flow in this case is usually
based on Fokker-Plank equation, describing saturated and unsaturated flow.
The most important aspects that have to be accounted for
in the presence of SAP, are the rate of uptaking the water from SAP
particles, and the changes in the porous media characteristics
due to the swelling. The absorption of the liquid by SAP is modeled
by introducing a sink term in the mass balance equation for the liquid phase.
Such a treatment of the SAP interaction with the porous media flow is
based on the fact, that Fokker-Plank equation accounts for the mass balance
of the "free", or "mobile" liquid. At the same time the liquid trapped
by SAP becomes immobile. This sink term depends on the number of SAP particles
per unit volume, multiplied by the rate of trapping liquid by a single
SAP particle. A SAP particle can absorb
a final amount of liquid, and the rate of absorption depends on the
amount of the already absorbed liquid. Thus, a first order model
(ordinary differential equation) is suitable for modeling the rate of absorption.
Further, the model has to account for swelling.
The swelling of SAP particles can be completely
compensated by the change of the porosity, or in some cases it can also
cause expanding of the multilayered porous media. In our models we account
for these effects.
Propagation of the wetting front in the across a porous material, containing sublayer
with SAP, is presented on the animation below.
It can be seen that a SAP layer can cause dewetting in the neighbor layers,
or it can even cause flow blocking in the case of large swelling and significant
reduction of the porosity.
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| Simulation of dynamics of saturation of porous media in the presence of a SAP sublayer (click pic to see a movie) |
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Type of the project: Independent research |
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2009 |
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