Random motion, Particles, Decreasing number of particles with time

Good morning,

I am modeling the brownian movement of particles in porous media with Matlab. The basis of the program is the random displacement in x and y of particles in a 500x500 matrix.

Rules are that there can be only one particle per matrix element (represented by a "1" in the matrix). A particle can move fom (i,j) to one of the 8 neighbours. Unpenetrable zones are represented by "-1" and free elements by "0"

The following code is the active part, actually moving the particles. The idea (good or bad :) ) is to:

Copy the matrix to avoid interferences

Check if there are free zones, ie "0" elements in the neighbourhood of the considered element

If there are possible movements, randomly get one of the empty elements and move the particle there.

The idea is rather simple I think but by running it a few times, I am losing particles as time passes. I have rechecked the use of randi and find commands and I can´t find where is the error in the code or logic.

Here is the code :

    %%Random Motion Generator
for t=1:timeF
disp(t);

tempMat=spaceMatrix;
        for i=2:size-1
            for j=2:size-1
                [row,col]=find(tempMat(i-1:i+1,j-1:j+1)==0); % Finding the Zero elements row and col return indice between 1 and 3

                  if tempMat(i,j)==1 % There is one particle               
                      if ~isempty(row) % The particle can go at at least one other cell
                          a=randi(length(row)); % Generating a random indice between 1 and length(row)
                          spaceMatrix(i,j)=tempMat(i,j)-1; % Getting the particle out of the original cell
                          spaceMatrix(row(a)+i-2,col(a)+j-2)=tempMat(row(a)+i-2,col(a)+j-2)+1; % Putting the particle in the destination cell
                      else
                          spaceMatrix(i,j)=tempMat(i,j);
                      end
                  end
              end
          end
%if t==1||t==timeF/4||t==timeF/2||t==3*timeF/4||t==timeF
    figure;
    hold on
    [row2,col2]=find(spaceMatrix==1);
    plot(row2,col2,'r.');
    [row3,col3]=find(spaceMatrix==-1);
    plot(row3,col3,'.');
    xlim([0 500]);
    ylim([0 500]);
    disp(length(row2));
%end
end

Thanks in advance for the tips and hints :)

David

[Jan]

Please use a proper code formatting as described here: http://www.mathworks.com/matlabcentral/answers/help/markup . It is always a good idea to make the question as easy to read as possible.

[David]

Thank you. It looks better now :)

Answer 1

[the cyclist]

When you check (at time t) whether a particle can go to a particular location of spaceMatrix, you correctly check that the spot was empty at time t-1. But I think you fail to check whether a different particle has already moved there during time t. You need to also check whether spaceMatrix is still empty.

You'll need to devise a rule for such conflicts.