- 1Department of Chemical Engineering, Northeastern University, Boston, MA, United States
- 2Department of Chemical and Biological Engineering, Tufts University, Medford, MA, United States
- 3Department of Bioengineering, Northeastern University, Boston, MA, United States
- 4Department of Electrical and Computer Engineering, Northeastern University, Boston, MA, United States
- 5Laboratory of Systems Pharmacology, Program in Therapeutic Science, Harvard Medical School, Boston, MA, United States
Exact analytical and closed-form solutions to the transient diffusion in bi-layer composites with external mass transfer resistance are reported. Expressions for the concentrations and the mass permeated are derived in both the Laplace and time domains through the use of the Laplace transform Inversion Theorem. The lead and lag times, which are often of importance in the characterization of membranes and arise from the analysis of the asymptotic behavior of the mass permeated through the bi-layer composite, were also derived. The presented solutions are also compared to previously derived limiting cases of the diffusion in a bi-layer with an impermeable wall and constant concentrations at the upstream and downstream boundaries. Analysis of the time lag shows that this membrane property is independent of the direction of flow. Finally, an outline is provided of how these transient solutions in response to a step function increase in concentration can be used to derive more complex input conditions. The importance of adequately handling boundary layer effects has a wide array of applications such as the study of bi-layers undergoing phenomena of heat convection, gas film resistance, and absorption/desorption.
1. Introduction
The mathematical description of the transient behavior of a composite membrane and, in particular, bi-layer composite membranes, has important implications in drug release devices from planar matrix devices (Cabrera et al., 2006; Cabrera and Grau, 2007), in drug delivery through the skin (Ghanem et al., 1987; Couto et al., 2014), in the study of the cornea (Cooper and Kasting, 1987; Couto et al., 2014), studying the role of membranes in keeping compounds from contaminating the surroundings (Kalbe et al., 2002; Edil, 2003), in vacuum insulation panels that creates high performance thermal insulation (Garnier et al., 2011), mass diffusion of neutral species in the fabrication of multi-layer thin-film (Goldner et al., 1992), metallic thermal protection system (Gu et al., 2016), semiconductor composite in bi-layer organic solar cells (Hatton et al., 2007), biodegradable bi-layer film barrier using gelatin and chitosan (Rivero et al., 2009), gas transport in inorganic/organic hybrid structures (Jang and Han, 2009), and gas barrier of organic-inorganic hybrid coatings (Minelli et al., 2010), When modeling the phenomena of heat conduction or gas permeation, the exchange of heat or chemical at the surfaces contacting with the medium are often assumed to occur “infinitely” fast. Mathematically, this translates into the assumption that the temperature (for heat conduction) or concentration (for mass transfer) of the bi-layer at the surface equates that of the contacting medium, simplifying greatly the calculations necessary to find an analytical solution. The contacting medium is generally considered to be maintained at a constant temperature or concentration. To account for boundary effects as shown in Figure 1A, whether this means heat convection, diffusion-limited reaction at the surface or mass transfer resistance, the constant boundary condition can be altered to:
with
FIGURE 1. (A) Schematic of a bi-layer composite with mass transfer resistance and (B) representative examples of the mass permeated or, equivalently, the integrals of the upstream and downstream fluxes J (shown in red and blue, respectively).
There have been considerable efforts in developing solutions to bi-layer systems involving the mass-transfer limited boundary conditions (Equation 1). Ramkrishna and Amundson (Ramkrishna and Amundson, 1974), as well as (Mikhailov et al., 1983), have proposed solution schemes based on solving the eigenvalues and eigenfunctions of a Sturm-Louiville type system to study the transient heat conduction behavior of composites undergoing convection type boundary conditions. Antonopoulos and Tzivanidis (Antonopoulos and Tzivanidis, 1996) outlined a procedure to evaluate transient heat conduction in n-layer composites with a convection type of boundary condition combining the separation of variables technique with an orthogonal expansion of the functions. Yet, the analytical, as opposed to numerical, evaluation of the eigenvalues, eigenfunctions, and the integral relationships arising from these approaches generally remains challenging for a bi-layer composite undergoing convective effects. This technique was further developed to investigate the approximate behavior of heat conduction with combined radiative and convective boundary conditions (Miller and Weaver, 2003) through a linearization of the radiative effects. An ‘analytical’ method of this problem was also developed by de Monte (de Monte, 2000; de Monte, 2002) based on Vodicka’s approach and applied to one-dimensional media to study unsteady heat conduction and extending to n-layers. We also note related works by Goldner et al. (1992) who obtained the analytical equations to a bi-layer diffusion system subjected to an impenetrable wall boundary using the Inversion theorem of Laplace transforms. Subramanian and White (Subramanian and White, 2001) derived an exact solution to a bi-layer composite undergoing galvanostatic boundary conditions using a modified separation of variables method.
In addition to the importance of modeling the transient response in a permeation process, especially for slowly diffusing processes, the “integral permeation” method has played a key role in the determination of the physical properties of a membrane, and in particular, its constitutive diffusion coefficients and permeability constants (Rutherford and Do, 1997). By dividing the permeation process into its transient and steady-state components as shown in Figure 1B, one can represent the transient component by a “time lag” parameter. This concept was introduced in the work of Daynes (1920) and Barrer and Rideal (1939). These findings outlined the now widely used relationship of the time lag (tlag) for a single layer with assumptions of constant concentrations maintained on either side of a single-layer material:
with L being the thickness and D being the diffusion coefficient of the material. Also in Figure 1B, the steady-state components of the upstream and downstream mass permeated are extended using the dashed lines and their x-axis intercepts represent the lead time t+ and the time lag tL. The forward mean first passage time is provided by
In this work, the closed-form and exact transient solutions to a single-layer material and a bi-layer composite undergoing mass transfer resistance at its outside boundaries are derived using the Laplace Inversion Theorem (Jaeger, 1950). The time lag and steady-state components of the transient response to an input step function are also derived. This work aims to support the characterization of bi-layer composites that have significant boundary layer effects. The presented solutions are also shown to be a generalization to various existing solutions in the literature. In addition to the similarities between diffusion and heat conduction, we also show how the solution forms presented in this work can be used to study the transient responses of bi-layer composites to inputs more complex than the usual step function.
2. Theoretical Analysis
A schematic of the problem is shown in Figure 1A. For both the single-layer and bi-layer derivation, the flow within the composite is assumed to follow Fick’s second law with mass transfer resistance occurring at the external boundaries. The assumed direction of the flow is from left to right.
2.1. Diffusion in a Single-Layer with Transfer Resistance
Let us note that the following derivation of the single-layer is equivalent to that of Ash (2001). The key differences with that work are the use of different constant definitions and the application of the nondimensionalization of the parameters. This subsection was written in an effort to provide results that are consistent between the previously derived single-layer and the newly derived bi-layer solutions as well as making comparisons with other results in the literature under one set of notations.
2.1.1. Single-Layer Diffusion with Upstream and Downstream Transfer Resistance
The phenomenon of diffusion is modeled using “Fick’s 2nd Law,” i.e., Crank (1979):
subject to the boundary and initial conditions:
and
The convention C(x,t) is used throughout the paper. B1 and B2 are the Biot numbers for systems with heat conduction undergoing heat convection at the outside boundaries, the mass transfer Biot numbers in mass diffusion processes or the reaction constant for a first-order reaction. The Biot number is a quantity that relates the inner diffusion to the mass transfer resistance at the surface. Let’s note that when both B1 and
Dimensionless variables for the concentration, time, and position are introduced:
The dimensionless quantity τ is also known as the mass Fourier number, which can be thought as the ratio between the diffusive transport rate and the storage rate. By using the introduced dimensionless variables and transforming Equation 3 into the Laplace domain, the relation with time is transformed into a dependence on the complex variable s:
The solution for
and is subjected to the transformed boundary and initial conditions:
and
Solving for
The mass permeated can be calculated as follow:
and the corresponding dimensionless mass permeated is provided by:
The dimensionless mass permeated expressions for the upstream and downstream fluxes,
and
Applying the Inversion Theorem for Laplace transforms (Jaeger and Carslaw, 1959), we find that the eigenvalues of the time-domain solution are given by the roots of:
and the time-domain solutions provided by Equations 19–21.
The time lag as described by Siegel (1991) for the single-layer problem was given by:
Using the work in Siegel (1991), the lead time was also derived:
2.1.2. Single-Layer Diffusion With Constant Upstream and Downstream Concentrations
By letting
concentration:
and downstream mass permeated:
Noting that letting
2.1.3. Single-Layer Diffusion With Downstream Transfer Resistance
Finally, letting
and
These expressions are equivalent to the solutions presented in Gough and Leypoldt (1980).
2.2. Diffusion in a Bi-layer With Transfer Resistance
2.2.1. Bi-layer Diffusion With Upstream and Downstream Transfer Resistance
The governing equations of the bi-layer system shown in Figure 1A are provided by the Fick’s 2nd law:
subject to the external boundary conditions:
and the interface boundary conditions:
The upstream interface is set at x1 = L1 and the downstream interface is located at x2 = L2. Equation 32 does not contain a second term on the right-hand side (assumes that
Noting that
with the transformed boundary conditions:
The constant
One can further calculate the dimensionless upstream and downstream mass permeation expressions (as illustrated in Figure 1B):
and
with the definitions:
and
respectively.
Applying the Inversion theorem, the eigenvalue relationship is given by Equation 49 and the exact solutions for the dimensionless concentrations by Equations 50 and 51. The upstream and downstream mass permeated expressions are provided by Equation 52. The denominator expression, D, is provided by Equation 54.
Two representative examples of the transient concentration expressions, c1 and c2, are presented in Figures 2A,B. The first notable distinction between the two examples is that (a) settles more quickly to its steady-state value. The primary reason for this is that (b) has a stronger mass transfer resistance occurring at the downstream boundary, acting almost like an impermeable wall that limits the flux through the bi-layer. Additionally, in (b), due to κ being of value 1, there is no sharp difference in concentration behavior at the interface between the two layers. In the case of (a), the value of 0.1 for κ leads to a sharp contrast in the slope of the concentrations between the two layers. Finally, the major contrast between accounting for mass transfer resistance or not, lays in the fact that the dimensionless concentrations at the upstream boundary is not 1 nor is it 0 at the downstream boundary when the values of B1 and B2 are finite values.
FIGURE 2. Examples of transient diffusion in a bi-layer with or without external mass transfer resistance. In (E) and (F), the input responses are time-varying and the properties of the composite are B1 = 50, B2 = 5,
2.2.2. Bi-Layer Diffusion With An Impermeable Wall at One End
There are a number of exact solutions that were previously derived for both the single-layer and bi-layer materials.
The simplified solutions assuming
and
subject to the eigenvalues of:
Let’s note that the denominator of the large fraction in both concentration expressions can be further reduced to:
and
This solution not only assumes no flux at the downstream boundary but also that the transfer at the upstream boundary happens so quickly that the concentration there is essentially constant. We note here that, when compared with the previous work on this solution by Goldner et al. (1992), the eigenvalue expression (Equation 57) looks slightly different. In the attempt by the authors to simplify Equation 57 and divide the eigenvalue expression by
A representative example of this type of bi-layer is shown in Figure 2C. Most notably, the concentration is uniform across the bi-layer at steady-state. If B1 is a finite value, then this steady-state concentration would be less than 1. The κ value of 5 here makes it such that the slope of the concentration in the first layer is sharper than the second layer. This difference, however, fades over time as the solution converges to steady-state.
2.2.3. Bi-Layer Diffusion With Constant Concentrations on Either Side
This derivation was previously presented in the work of Jaeger (1950). We simply show that by assuming that
and
with the eigenvalues provided by the roots of:
An example of this well-known case is shown in Figure 2D. The main characteristics are that the upstream concentration is always 1, the downstream concentration is always 0.
2.2.4. Extension of the Step Response Solutions to Other Types of Input Responses
Using the Laplace expressions for both the single-layer and bi-layer systems, the derivation of other types of input (e.g., oscillatory, exponential decay, etc.) can be found trivially by factoring out
Alternatively, one can apply the truncated time domain convolution:
While the time-domain expressions may be convoluted in nature, the time-dependent elements of the solutions are relatively simple. Thus, the convolution can be straightforwardly calculated for many cases such as sine, cosine, or exponential inputs.
Two representative examples are shown in Figures 2E,F. In the first case (e) with
2.2.5. Time Lag Analysis
The steady-state portion of m can be obtained from the steady-state portion of its transform,
with (01) representing the first layer and (23) representing the second layer. Using previous results that were obtained in the work of Siegel (1991), the time lags of the uncombined two layers are provided by:
Let’s note that the time lags are nondimensionalized such that
as well as the backward mean first passage time of the first layer
are also introduced to derive the time lag of the bi-layer.
Applying the time lag (
the following expression for
Similarly, the relationship in Siegel (1991) for the mean passage time in a bi-layer composite is provided by:
and
The lead time
Figure 3 shows the time lags for both the single-layer and bi-layer cases. In the single-layer case (a), which only depends on dimensionless parameters B1 and B2, an axis of symmetry (shown as a dotted white line) can be drawn across the heatmap. It indicates that the time lag for the single-layer system is independent as to whether the flow from the left to right or right to left. A similar observation to the invariance of the time lag with respect to flow direction can be made for the bi-layer system and is shown in (c) and (d). However, since the quantities κ, ρ, and
FIGURE 3. Heat maps of the dimensionless time lag
In addition to the invariance to flow direction, it is also interesting to point out that the time lag is greatly affected by the values of B1 and B2. If both of these values are low, the time lag can be extremely large which coincides with the expected behavior that having large mass transfer resistance at the surfaces is not good for reaching steady-state permeation. This also shows that, depending on the values of κ and ρ in the bi-layer case, it may be easier to bring down the time lag by increasing one of the Biot number rather than the other. This can be seen in Figure 3C, for which it is easier to bring the time-lag down by increasing B2. At the other end of the spectrum, high values for both B1 and B2 yield low time lag values.
In Figure 4, the steady-state and transient solutions for the dimensionless mass permeation expressions are combined for three different set of parameter values. The slopes of the asymptotes are provided by the time-domain concentration expressions in Equations 52 and 53. The lag time is provided by Equation 70 and the lead time is calculated using Equation 73 applied to Equation 71 and Equation 72. Evidently, the slope of the upstream and downstream steady-state solutions are the same since the flux coming into the membrane needs to exit at an equal rate or there would be accumulation within the bi-layer. This type of graph is important because, oftentimes, it is easier in practice to measure the mass permeated rather than the concentration value at a fixed location. Given time series data of the mass permeated, one can use the steady-state and transient solutions provided in this work to calculate missing membrane property values of the materials. In presented examples, it can be seen that the solution that reaches steady-state has high values of both B1 and B2, an observation that can also be seen in Figure 3. One can also use the slopes of the steady-state solutions to find missing values by noting, for example, that the steady-state slopes of the mass permeated expressions do not depend on κ and proceed through process of elimination.
FIGURE 4. Examples of the time-dependent dimensionless mass permeated at the upstream (mup, in red) and downstream (mdown, in blue) boundaries for three different bi-layer membranes.
3. Conclusion
There has been a large body of literature treating the problem of diffusion in n-layer composites. Yet, the derivation of closed-form solutions remain seldom and often limited to single-layer or bi-layer systems due to the difficult and tedious nature of finding such solutions. In this work, we presented analytical solutions to the phenomena of diffusion in a bi-layer with mass transfer resistance at the external boundaries for each of the layers as well as the corresponding expressions for the asymptotes of the upstream and downstream mass permeated. We showed how these relates to previously derived solutions of diffusion in a bi-layer with an impermeable wall as well as diffusion in a bi-layer with constant concentrations on either side. The step response solutions are also extended to time-varying inputs such as exponentially decaying and oscillating inputs. Notably, both the time-domain and Laplace domain solutions to the bi-layer can be used through numerical inversion of the Laplace solution or by applying a convolution on the provided time-domain solutions.
Also, the mean first passage time that is the difference between the lead and lag times does not depend on B1. Since the lead time is dependent on the time lag, however, it is also dependent on all the four dimensionless quantities B1, B2, κ, and ρ. It was also shown that flipping the membrane materials around of the system has no bearing on the time lag for the single-layer and the bi-layer.
When comparing the generalized solutions with the simplified equations with constant concentrations on either side of the bi-layer membrane, there are notably two additional parameters in B1 and B2. It is likely that, in the absence of prior knowledge about the system, one cannot easily fit all the parameters at once from experimental data, especially when accounting for measurement errors. Knowledge about B1 and B2 can be derived most easily from studying the transient mass permeation in single-layer experiments of each of the components of the bi-layer. This is possible because the nondimensionalized lag time and the slope of the mass permeation in a single-layer membrane only depend on B1 and B2. Using experimental data from the bi-layer, one can, for example, use the fact that the slope of the mass permeation expressions are both independent of ρ. In this scenario, one can then use the lag time
Data Availability Statement
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
Author Contributions
JM conceptualized the initial idea of the paper. AT expanded these notes into a full manuscript and derived the equations presented here. ES provided help in the writing and support toward the completion of this work. All authors contributed to manuscript revision, read, and approved the submitted version.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
This manuscript could not have come to life without the contributions of JM. We hope that this work honors his memory, his passion for the field of heat and mass transport, and his dedication to nurturing the next generation of students and researchers. We also acknowledge the contributions of Kenneth A. Smith and Matthew J. Panzer for providing feedback on this manuscript.
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Keywords: analytical solution, transient heat conduction, transient diffusion, composite membrane, mass transfer resistance
Citation: Tran AP, Meldon JH and Sontag ED (2021) Transient Diffusion in Bi-Layer Composites With Mass Transfer Resistance: Exact Solution and Time Lag Analysis. Front. Chem. Eng. 2:605197. doi: 10.3389/fceng.2020.605197
Received: 11 September 2020; Accepted: 17 November 2020;
Published: 13 January 2021.
Edited by:
Adele Brunetti, National Research Council (CNR), ItalyReviewed by:
Giuseppe Genduso, King Abdullah University of Science and Technology, Saudi ArabiaDavid Alique Amor, Rey Juan Carlos University, Spain
Copyright © 2021 Tran, Meldon and Sontag. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Eduardo D. Sontag, sontag@sontaglab.org
†Deceased: Jerry H. Meldon July 18, 2017