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Modeling Dissolution

Medical Test Strips

Medical test strips used for rapid diagnostics and determination of relevant biomedical parameters have undergone considerable advances in recent years. Far from the conventional strips with absorbent fleece, new precisely-fabricated microchannels, with defined geometric structures, are now possible via Microsystems engineering. Proper geometrical design of these microstructures enables selective control of the capillary-driven flows. FLOW-3D can be used to optimize the designs of these strips.

Regardless of the type of solid, whether it is salt or acetaminophen, penicillin, or sugar, understanding how fast and how much of the solid gets dissolved into the flow is important for several applications areas, especially in medicine. The way in which the solid dissolves could lead to high blood pressure, liver or kidney strain. Also, many pharmaceutical pills are more dense or water-soluble than others and these properties directly affect how the solid interacts with the surrounding medium. All of these parameters are easily varied in FLOW-3D.

Below is a simple simulation of salt dissolution. A salt block is represented by the green component nested in the image at left. As flow is input from the left boundary into the domain it dissolves the salt. The quantity released is a function of k (Q = k(CSAT - C)) which is a constant mass transfer coefficient but takes into account the concentration of salt already in the fluid. A higher concentration in the fluid will result in less salt dissolved from the block. The images below show the concentration released into the fluid and the shape of the dissolving surface and the mass flux of salt, respectively.

A schematic of the computational domain for the test problem. The arrow indicates the inlet boundary. The initial solid salt block is represented by the green component.
Distribution of dissolved salt in fluid in a horizontal cross-section along the bottom boundary
The shape of the salt block colored by its concentration in fluid at time, t =  60 sec.

FLOW-3D's meshing method has solid geometry independence using the FAVOR™ method, and so the area and volume fractions that make up the geometry are simply rewritten at every time step as salt is dissolved from the surface. This makes the simulation very computationally efficient because no mesh refinement is required.