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2.6.1 Governing Equations
The governing equations adopted in this finite element implementation of neutral solute transport in deformable porous media are based on the framework of mixture theory [76, 24]. A single solute is considered in this presentation for notational simplicity, though the extension of equations to multiple solutes is straightforward. Various forms of the governing equations have been presented in the prior literature [56, 4], though a presentation that incorporates all the desired features of this implementation has not been reported previously and is thus detailed here.
The fundamental modeling assumptions adopted in this treatment are quasi-static conditions for momentum balance (negligible effects of inertia), intrinsic incompressibility of all constituents (invariant true densities), isothermal conditions, negligible volume fraction of solute relative to the solid and solvent, and negligible effects of solute and solvent viscosities (friction within constituents) relative to frictional interactions between constituents. These assumptions are often made in studies of biological tissues and cells. External body forces and chemical reactions are not considered.
The three constituents of the mixture are the porous-permeable solid matrix ( , the solvent ( , and the solute ( . The motion of the solid matrix is described by the displacement vector , the pressure of the interstitial fluid (solvent solute) is , and the solute concentration (on a solution-volume basis) is . The total (or mixture) stress may be described by the Cauchy stress tensor , where is the identity tensor and is the stress arising from the strain in the porous solid matrix. Because it is porous, the solid matrix is compressible since the volume of pores changes as interstitial fluid enters or leaves the matrix. Under the conditions outlined above, the balance of linear momentum for the mixture reduces to Similarly, the equations of balance of linear momentum for the solvent and solute are given by where is the apparent density (mass of per volume of the mixture), is the mechano-chemical potential and is the velocity of constituent . is the diffusive drag tensor between constituents and representing momentum exchange via frictional interactions, which satisfies . An important feature of these relations is the incorporation of momentum exchange term between the solute and solid matrix, , which is often neglected in other treatments but plays an important role for describing solid-solute interactions [56, 1, 2]. These momentum equations show that the driving force for the transport of solvent or solute is the gradient in its mechano-chemical potential, which is resisted by frictional interactions with other constituents.
The mechano-chemical potential is the sum of the mechanical and chemical potentials. The chemical potential of represents the rate at which the mixture free energy changes with increasing mass of . The mechanical potential represents the rate at which the mixture free energy density changes with increasing volumetric strain of . In a mixture of intrinsically incompressible constituents, where the volumetric strain is idealized to be zero, this potential is given by , where is the true density of (mass of per volume of , which is invariant for incompressible constituents, and is some arbitrarily set reference pressure (e.g., ambient pressure).
From classical physical chemistry, the general form of a constitutive relation for the chemical potential is , where is the universal gas constant, is the absolute temperature, is the molecular weight (invariant) and is the activity of constituent (a non-dimensional quantity); is the chemical potential at some arbitrary reference state, at a given temperature. For solutes, physical chemistry treatments let , where is the solute concentration in some standard reference state (an invariant, typically , and is the non-dimensional activity coefficient, which generally depends on the current state (e.g., concentration) but reduces to unity under the assumption of ideal physico-chemical behavior . Since this representation is strictly valid for free solutions only, whereas solutes may be partially excluded from some of the interstitial space of a porous solid matrix, Mauck et al.  extended this representation of the solute activity to let , where the solubility represents the fraction of the pore space which is accessible to the solute ( . In this extended form, it becomes clear that even under ideal behavior ( , the solute activity may be affected by the solubility. Indeed, for neutral solutes, the solubility also represents the partition coefficient of the solute between the tissue and external bath [48, 59].
When accounting for the fact that the solute volume fraction is negligible compared to the solvent volume fraction [75, 9], the general expressions for and take the form where is the osmotic coefficient (a non-dimensional function of the state), which deviates from unity under non-ideal physico-chemical behavior. Therefore, a complete description of the physico-chemical state of solvent and solute requires constitutive relations for and the effective solubility , which should generally depend on the solid matrix strain and the solute concentration.
It is also necessary to satisfy the balance of mass for each of the constituents. In the absence of chemical reactions, the statement of balance of mass for constituent reduces to The apparent density may be related to the true density via , where is the volume fraction of in the mixture. Due to mixture saturation (no voids), the volume fractions add up to unity. Since the volume fraction of solute is considered negligible ( , it follows that . Since of an incompressible constituent is invariant in space and time, these relations may be combined to produce the mixture balance of mass relation, where is the volumetric flux of solvent relative to the solid. The balance of mass for the solute may also be written as where is the molar flux of solute relative to the solid. This mass balance relation is obtained by recognizing that the solute apparent density (mass per mixture volume) is related to its concentration (moles per solution volume) via . Finally, it can be shown via standard arguments that the mass balance for the solid matrix reduces to where is the solid volume fraction in the reference state, and is the deformation gradient of the solid matrix.
Inverting the momentum balance equations in (2.6.1-2), it is now possible to relate the solvent and solute fluxes to the driving forces according to where is the solute diffusivity tensor in the mixture (solid solution), is its (isotropic) diffusivity in free solution; is the hydraulic permeability tensor of the solution (solvent solute) through the porous solid matrix, which depends explicitly on concentration according to where represents the hydraulic permeability tensor of the solvent through the solid matrix. The permeability and diffusivity tensors are related to the diffusive drag tensors appearing in (2.6.1-2) according to though these explicit relationships are not needed here since , and may be directly specified in a particular analysis. Since the axiom of entropy inequality requires that the tensors be positive semi-definite (see appendix of ), it follows that must be greater than or equal to the largest eigenvalue of . Constitutive relations are needed for these transport properties, which relate them to the solid matrix strain and solute concentration. Note that the relations in (2.6.1-10) represent generalizations of Darcy's law for fluid permeation through porous media, and Fick's law for solute diffusion in porous media or free solution.