MCB111: Mathematics in Biology (Fall 2019)
week 13:
DiffusionReaction patterns
The Brusselator on steroids
Figure 1. The Brusselator + diffusion model. Adapted from K. Sims (http://www.karlsims.com/rd.html).
In class, we showed how the GrayScott model is a twospecies system that in two dimensions can produce striking spatial patterns. Here, we are going to look at another twospecies model that also produces interesting patters.
The Brusselator is just a theoretical model, and it got its name because it was proposed by investigators at the Universite libre de Bruxelles. The Brusselator is a purely reaction process, and it is interesting on its own because it can produce oscillations, thus the name Brusselator.
These are the reactions that define the Brusselator. There are two species and such that,
that is, transforms to , and transforms to but this last reaction can only occur in the presence of two additional molecules
But here, we are not going to study the Brusselator reactions alone, but the result of combining the Brusselator with a diffusion process. The and in addition to being subject to the Brusselatorâ€™s reactions, they can diffuse in both the and directions with diffusion coefficients and respectively.
We will workout this exercise in class, but that we need to do is

Write the continuous differential equations associated to the Brusselator + diffusion model in two spatial directions and .

Find the steady state(s) of the Brusselator (sans diffusions).

Implement the code to solve the equations numerically.

Make patterns!
Figure 2. Brusselatordiffusion patterns. The top figure depicts the initial conditions for species A. The bottom figure depicts the concentration of A after some time. Parameters values are described in the text. The concentration of species B is a mirror image of that of A.
Do not forget the four conditions necessary to convert the system from a steady state of the Brusselator alone, into patternforming when including the diffusion terms,
where is a steady state of the Brusselator alone.
Here are some possible sets of parameters and initial conditions that in my hands produced cool patterns. I used a square of length , and . I run it until , with .
Figure 3. Brusselatordiffusion patterns. The top figure depicts the initial conditions for species A. The bottom figure depicts the concentration of A after some time. Parameters values are described in the text. The concentration of species B is a mirror image of that of A.

Figure 2
The initial conditions are very important. Different initial conditions can change the resulting patterns significantly for the same underlying reactions. In Figure 2, The square was initialized at with having a random concentration between 0 and 3, and is zero everywhere.

Figure 3
The square was initialized at with and being zero everywhere, except for having a small square in the center with random concentrations between 0 and 3.
I know you will be creative, and for sure will get more interesting patterns than mine.

Please submit your progress by the end of the Monday 12/2 Lecture.

This homework will not be graded, unless you want us to for extra credit. In the later situation, you can resubmit an improved version together with the final.