Abstract
We study a model, motivated by a bioremediation process, describing a cross-diffusion movement of a bacteria population b attracted by a chemoattractant signal c, in a nonhomogeneous stratified medium with n layers. We assume that this reaction–diffusion process is characterized by a low rate of degradation and a low diffusion coefficient of the chemoattractant, expressed in the model by a small parameter ϵ. The model consists of n systems of nonlinear parabolic equations with transmission conditions between layers. We prove a global-in-time solution for the asymptotic model setup with respect to the small parameter of the problem, for arbitrarily large initial data. Next, we deal with the control problem focusing mainly on the reduction of the chemoattractant concentration, by acting upon the initial distribution of the bacteria population b0. To this end, we prove the existence of a solution to the control problem and determine the optimality conditions.
Introduction
Chemotaxis is the biological process of directed movement of cells in response to a chemical signal emitted by a substance or by another population, called chemoattractant, and play an important role in the interaction of cells with their environment. Chemotaxis is a complex process which involves many aspects such as various species of cell populations b can interact; the movement can be either towards the higher concentration of the signal (positive chemotaxis) or away from it (negative chemotaxis); the chemical signal can be secreted by a population b itself, and not necessarily by an external source; the chemotactic process can lead to an aggregation of the attracted individuals and to a production of the chemoattractant, or contrary, it can determine the degradation of the chemoattractant. Consequently, the theoretical understanding of these processes determined a growing interest in their mathematical modelling, which at its turn raised challenging problems.Footnote†
The origin of the fundamental model is given in the work of Patlak [Citation24]. Later, Keller and Segel [Citation18–20] introduced a similar model based on another assumptions. Since then, a rich mathematical literature on various versions of the model has been emerged, mainly focusing on the well-posedness of it and we refer the reader to a very comprehensive survey in [Citation15].
In this paper, we shall denote the density of the cell population by b and the density of the population spreading the signal by c, both of them being functions of time t, and space variable ξ.
A chemotactic system with only one population of cells consists of two equations for b and c with initial and boundary conditions:
In the 1-D case, it has been shown that blow-up does not occur [Citation16] for D=1, K constant and small. When the space dimension d is greater or equal to two, the solutions generally exhibit blow-up, this being influenced by the model parameters and the characteristics of the initial data [Citation13,Citation14,Citation25]. For example, in [Citation17], a chemotaxis motion with constant diffusion coefficients is studied by using a nonlocal gradient sensing term to model the effective sampling radius of the species. In [Citation9], Dyson et al. use a nonlocal term to model the species-induced production of the chemoattractant,
in order to prevent blow-up in the d-D space, considering that the diffusion coefficients are constant. They prove the existence of solutions, which exist globally, and are L∞-bounded on finite time intervals. In [Citation28], the system (1)–(4) is considered with
f(b, c)=0 and
and it is shown that for arbitrarily large initial data, this problem admits at least one global weak solution for which there exists T>0, such that (b, c) is bounded and smooth in
The paper [Citation8] deals with two types of reaction–diffusion systems, one arising in chemotaxis and the other in angiogenesis. The first refers to Equations (1)–(4) with D(b, c), δ (b, c) and K(b, c) constant, f(b, c)=0,
and the equation for c stationary, that is,
This equation is obtained as a case limit, for
of Equation (3) in which
and the diffusion coefficient δ is of the order of
with ϵ small. For this system (as well as for that of angiogenesis type), it is shown in [Citation8] that when the Ld/2 norm of initial data is small enough (for d≥2), then there is a global (in time) weak solution that stays in all Lp spaces with
. In [Citation4], the same system but with α=0 is studied in ℝ2 and a detailed proof of the existence of weak solutions below the critical mass, above which any solution blows up in finite time in the whole Euclidean space, is given [Citation26]. The stability of the stationary solutions to a chemotaxis system was proved in [Citation11] for D=1,
and a general function
In [Citation23], existence and uniqueness are studied for slow and singular fast diffusion of the cells in the case with a stationary equation for the chemoattractant.
Besides applications in biology, a chemotaxis model can apply in environment bioremediation in the cases when specific bacteria b are injected into a polluted medium (soil or water) with the purpose of cleaning it from an inside spread pollutant c [Citation5,Citation10,Citation27].
Our study is motivated by an application to environment bioremediation and focuses on the case in which the kinetic term and the diffusion coefficient of the chemoattractant (pollutant) c have a weak influence on the flow, meaning that the rate of degradation of the chemoattractant is slow and it diffuses very little (or not at all, as in the case of oil polluting an environment).
Roughly speaking we shall start from a model reading as
The model will be implemented into a nonhomogeneous medium, fact evidenced later by a particular space dependence of the coefficients. The aim is to investigate the way in which the solution depends on the small parameter ϵ and not to study the limit model when Accordingly, we shall not pass to the limit, but use a perturbation technique [Citation7], by which the solution is expanded in series with respect to the powers of the small parameter, and retain the systems of
-order of approximation, obtained by equating the coefficients of
in Equations (5).
The main goal of the paper is to study the possibility of controlling the environment cleaning by acting upon the initial distribution of the bacteria. More exactly, the intention is to design the initial distribution of the bacteria such that the chemoattractant mean concentration should decrease under a certain critical value ccrt within a time period T.
A secondary task of the control problem is motivated by the fact that bioremediation can be problematic because sometimes it might become difficult to remove the microorganisms released into the environment in order to let them at acceptable levels in the soil [Citation21]. Consequently, for enhancing a complete and efficient decontamination process and for avoiding higher costs of a further cleaning of the environment from the remained bacteria, we investigate by the same control problem whether a moderate bacteria proliferation, whose growth is limited by a prescribed density value bc, would be sufficient for achieving the main objective of reducing the pollutant concentration up to a value under the dangerous critical threshold ccrt. To this end, the state system is studied in the framework of an appropriate functional setting and a global-in-time solution for the asymptotic system derived from Equations (5) is obtained. Then, the existence of at least a solution to the control problem is proved and the optimality conditions proving information about the initial distribution of the bacteria b0 are computed.
2. Statement of the model and the perturbation technique
We consider a nonhomogeneous 3D right cylinder domain
where the base
is an open bounded subset of ℝ2, with a boundary of class C2. The medium nonhomogeneity is modelled by a stratification of Ω in n parallel layers along the Ox axis, the separation of the layers being determined by the different values that movement parameters may have in each layer. More exactly, a stratification can be put into evidence when certain parameters do not depend on x (here x is the stratification variable) or are constant in each layer (xi−1, xi) but have different values from a layer to the other.
Therefore, the domain Ω consists of n subdomains Ωi, having the boundaries i=1, …, n, where
are the lateral boundary of Ωi and
, i=0, …, n. The surfaces Γ0 and Γn are the external horizontal boundaries, while Γi with
are the boundaries between layers. We denote
In each layer i the chemotaxis process is modelled by two equations, one for bi and the other for the chemoattractant ci. The interaction between the layers is established by transmission conditions for bi, that is, the continuity of the solutions and fluxes. We assume that the system is closed for bi and ci, namely the fluxes across the exterior frontiers are zero. The chemoattractant ci may display jumps at the interface between layers, because the -approximation systems will not be anymore of diffusion type, as we shall see. Therefore, there is no need to specify these conditions.
The stratification is set by the values for Di, δi, ci, 0 assumed constant in each layer i, but different for two consecutive layers. Also, the expressions of the functions fi, Ki, ϕi, which are assumed not to depend explicitly on ξ, are different from one layer to another.
With these considerations, the dimensionless mathematical model for a chemotaxis movement in a nonhomogeneous stratified medium has n equations for the unknowns bi and ci (the density of the cell population and the chemoattractant concentration, respectively, in each layer i=1, …, n) and reads [Citation1]
Generally, ci and its normal derivative can have a jump when crossing the internal boundaries Γi. As we said, do not specify here these conditions because they will not intervene in the asymptotic model we shall study.
We stress that this model is presented in a dimensionless form and the constants are dimensionless parameters. They appear after the dimensionless procedure by which all dimensional variables and function are scaled by certain characteristic values indicated by the subscript a (e.g. La for length, Ta for time, Da for diffusion coefficient, etc.). We do not present the dimensionless computation, but we assert that by this procedure the dimensionless parameters are given by
If by such a procedure the computed values
and
follow to have the same order of magnitude, much smaller than that of the other parameters, we set in Equations (6)–(17)
2.1 Hypotheses
To approach system (6)–(17) we assume the following hypotheses for all i=1, …, n:
(i1) ci, 0 is constant, ci, 0≥0, and there exist an i such that ci, 0>0;
(i2) bi, 0≥0, and there exist an i such that bi, 0>0;
(i3)
,
;
(i4)
are of class C1 with respect to r1,
are bounded,
;
(i5)
are of class C2 with respect to r1 and
are bounded.
We remark that equations with nonlinear terms fi do not generally admit global solutions in time [Citation12], but by the perturbation technique we shall deduce a global solution for this asymptotic model, even under the assumption of a polynomial form for fi. Thus, for all i=1, …, n, we assume
2.2 ϵ0-order and ϵ1-order approximations
We write the series expansions of all functions, with respect to the small parameter Denoting generically
and
we have
We replace these series in the system (6)–(17) and by equating the coefficients of the mth powers of we deduce the systems corresponding to
-order approximation.
The -order approximation for ci reads
The next approximations (for m≥2) lead to systems having similar forms as that for the -order approximation, and so they pose the same mathematical problems. That is why we do no longer write them.
definition 2.1
We call an asymptotic solution to Equations (6)–(17), up to the order of approximation a pair of functions
i=1, …, n,
We shall rigorously explain the definition of the solutions and
in the next sections, in which the well-posedness of these systems will be studied.
2.3 The control problem
As we have already explained, the aim of the control problem is to provide information about the necessary initial density of bacteria, such that two objectives would be achieved. The main one is related to the environment cleaning that is, to force the decrease of the chemoattractant mean concentration under a critical threshold ccrt. This means to minimize in the cost functional the mean positive part of the difference between the concentration
given by Equation (48) and ccrt. The second objective aims at realizing the necessary pollutant concentration decrease by a process limiting a too large proliferation of the bacteria. Consequently, in the cost functional we add a term expressing the restriction of the bacteria growth, by minimizing the mean positive part of the difference between
and a constant prescribed value bc. We introduce this control problem to give a quick response (rather than an extremely accurate one) and to indicate a first decision for a process evolving in real time. That is why we accept that the consideration of the
-order approximation
only, which is the dominant term in the asymptotic expansion of bi, is motivated. A more accurate computation, if necessary, may take into account the further approximations.
Mathematically, we have to minimize the cost functional
where the superscript ‘+’ means the positive part (i.e.
. A certain choice of the constant σ can induce a lower (or a higher) influence of the first term with respect to the second one in the cost functional. In particular, if the limiting condition of the proliferation of the bacteria is disregarded, the constant σ can be set zero.
Therefore, the control problem is
subject to Equations (28)–(34) and (35)–(36), where
We recall that in our model, the initial distribution of the density of the bacteria may depend on the space variable ξ (i.e.
and only the chemoattractant initial concentration is considered constant (
. A natural restriction for the initial densities of the bacteria bi, 0 (
representing the controllers in the control problem, is to impose their boundedness. The minimum and maximum bounds, bi, m and bi, M, are fixed numbers,
. However, as we see later, this restriction does not necessarily imply that the minimum of J must be reached at bi, m.
Finally, to be consistent with the requirement expressed by the first term in J, the maximum of the initial datum is chosen less than or equal to bs, the appropriate upper threshold envisaged for the solution In the case with σ=0, the choice of bi, M is free.
3. Well-posedness of the state system
In order to approach the control problem we need some information about the and
-state systems. We begin by a few preliminaries.
3.1 Preliminaries and functional setting
Since by our assumptions, ci, 0 remains constant in each layer, one notes that in Equation (28) fi depends only on and so one can denote
We rewrite the system (28)–(34) for the -order approximation without indicating the superscript ‘0’
We introduce now the global functions defined in the following way (indicated by a generic notation φ, φ0):
We note that assumptions (50)–(52) and (i1)–(i5) imply similar properties for the functions defined before. Namely, we have
System (53)–(59) will be treated in the functional framework of the Sobolev space endowed with the standard norm
and its dual
, with the pivot
, such that
. The value of
at ψ∈V is
We specify that for the writing simplicity we shall denote the scalar product and the norm in L2(Ω) by and
.
We define the operator by
Thus, the functional abstract setting of our problem is
3.2 Existence for the ϵ0-order approximation
The well-posedness for Equations (71) and (72) is concentrated in the following.
Theorem 3.1
Assume conditions (62)–(64), and let b0∈D(A). Then, problem {\rm (71)} and {\rm (72)} has a unique solution
Proof The proof is split into two steps and is given in [Citation1]. Further, we indicate the arguments. We assert by the properties (62)–(64) and by handling appropriate Sobolev inequalities that the operator turns out to be locally Lipschitz from V to L2(Ω), uniformly in x. In the first step, according to the method presented in [Citation3], we reduce the problem to a case with a globally Lipschitz operator, by approximating
by
It is obvious that in each layer the functions bi (meaning in fact the -approximation
have the regularity induced by b restricted to Qi, for i=1, …, n, that is,
3.3 Existence for the ϵ1-order approximation
The functions are computed by Equations (35) and (36),
Existence for the solution to Equations (37)–(43) for the -order approximation
is studied by the Lions’ theorem for the time-dependent case [Citation22] and the conclusions are [Citation1] the following.
Theorem 3.2
System (37)–(43) has a unique solution
Corollary 3.3
Problem (6)–(17) admits a unique asymptotic solution up to the order of approximation
In particular, the restrictions of the solution to each layer have the properties
4. The control problem
Using the notation (60) we can rewrite the control problem (P) as
subject to Equations (53)–(59) (equivalently Equations (71) and (72)) and (84), where
This means that b0 is the solution to the
-approximation, Equations (53)–(59), with the initial datum b0 and c is given by Equation (84). Here,
and
, and we note that
.
We recall that and is constant in all layers. Because in (P) only the
-order approximation for b and only the
-order approximation for c are involved, for the writing simplicity we shall indicate them by b and c, without superscripts.
4.1 Existence of the optimal control
Theorem 4.1
Problem (P) has at least a solution.
Proof Since J(b0)≥0, its infimum exists and let us denote it by
Let us take a minimizing sequence
,
such that
4.2 The system in variations and the dual system
Let be an optimal pair in (P). The function b* is the solution to Equations (71) and (72) with the initial datum
and c* is given by Equation (84) corresponding to b*,
Let λ>0 and introduce where
The corresponding solution to Equations (71) and (72) with the initial datum
is denoted by bλ, and we define
By a few computation, it can be shown that Y solves the system in variations (written for Yi in each layer)
We introduce now the system for the dual variable p, written for each layer,
It is obvious that it has a unique solution
4.3 Optimality conditions
Proposition 4.2
Let be optimal in (P). Then, the optimality conditions read
Proof We write that is optimal, that is,
We take
and have
whence, passing to the right-hand side, performing some computations, dividing by λ and letting λ to go to 0, we get
where
generically indicates
in each layer. This can still be written, after changing the order of integrations, as
We observe that the minimization problem admits at least a solution which can be realized not necessarily at b0=bm, but in a way expressed by relations (105). We conclude that the controller
may take either the value bi, m or bi, M on the subsets of Ωi, where pi(0, ξ) has positive or negative values. In some layers,
may reach also the value 0 (if e.g. pi(0, ξ)>0 in Ωi), because bi, m was allowed to be nonnegative. This means that in some situations it might be possible not to place bacteria in some layers at the initial time. Finally, we assert that it is difficult to say if the system (98)–(104) can have a null solution pi(0, ξ) on a subset of Ωi, but this analysis is beyond the scope of this paper. Therefore, we cannot exclude that in some circumstances it may happen that the minimum be reached for whatever
.
5. Numerical results
Some graphics revealing the feature of the process with respect to the change of b0 are presented. The simulations are made with Comsol Multiphysics (FLN License 1025226) for the system (6)–(17) in a 1D domain with the same values in all layers (in order to compare how results change for various b0) with the following dimensionless data:
The figures represent the values of b and c along time, drawn at x fixed (0, 2, 4, 6, 8, 10) corresponding to two values of the initial datum b0. In , these graphics are plotted for b0, the step function
In (right), we see that the values of c increase from x=0 to x=10. The medium cleaning is more efficient beginning with the first layers. In (right), the medium cleaning happens in a different way, being less efficient in the first layers (x∈[0, Citation2]) where the values of c remain high, but more efficient in the medium layers (x∈[Citation4, Citation6]).
6. Conclusion
We address a control problem related to a chemotactic motion of a bacteria towards a pollutant chemoattractant in a stratified medium. The control problem is focused on the reduction of the chemoattractant concentration, by acting upon the initial distribution of the bacteria population, b0. We prove that the control problem has at least a solution and provide the structure of the controller b0 in each layer.
Acknowledgement
The author thanks the reviewers for their comments and suggestions related to the control problem.
Funding
This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS –UEFISCDI [project number PN-II-ID-PCE-2011-3-0045].
Notes
†Dedicated to Mimmo Iannelli on the occasion of his 65th birthday.
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