In the MatLab code below, Figures 4 and 5 are not working.





%% RANDOM WALKS, BROWNIAN MOTION, AND DIFFUSION


% ————————— parameters
width = 5;                                           % width of pipe in mm
a = 0.01;                                         % lattice constant in mm
pw = 0;                                           % probability of waiting
num_steps = 1000000;                              % # steps for histograms
num_walks = 1000000;                       % # random walks for histograms

% ————————— calculate lattice size based on width
lattice_size = round(width/a);

% ————————— parameterize the step probabilities
A = 0.5;                               % electric field strength parameter
ph = 1/2*(1+A^2);
pv = 1 - ph;
pu = pv/2;
pd = pv/2;
pL = (ph/2)*(1-A);
pR = (ph/2)*(1+A);

% ————————— initialize random walker position
x = 0;
y = lattice_size/2;

% ————————— initialize mean squared displacement variables
msd_x = zeros(1,num_steps);
msd_y = zeros(1,num_steps);

% ————————— loop over the # of steps
for step = 2:num_steps+1
%           choice: does walker wait or make a step?
            if rand<=pw
%           walker waits
            continue;
            else
%           walker makes a step
%           calculate the step direction
            dx = zeros(1,2);
            dy = zeros(1,2);
            dx(rand<=pL+0.5) = -1;
            dx(rand<=pR+(pL/2)) = 1;
            dy(rand<=pu+0.5) = -1;
            dy(rand<=pd+(pu/2)) = 1;
%           update walker position
            x = x+sum(dx);
            y = y+sum(dy);
%           reflect at the y-direction boundary
            if y<1
                y = 2-y;
            elseif y>lattice_size
                y = 2*lattice_size - y - 1;
            end
%           calculate mean squared displacement in each direction
            msd_x(step) = (x*a)^2;
            msd_y(step) = ((y-(lattice_size/2))*a)^2;
      end
end

% —————————— 1) visualize the random walk trajectories
figure;
plot((0:num_steps)*a, (msd_y*a), 'b.')
title('Random Walk Trajectories');
xlabel('x (mm)');
ylabel('y (mm)');
xlim([0,num_steps*a]);

% ————————— 2) mean squared displacement in x-direction
figure;
n_steps = 1:num_steps;
msd_x = cumsum(msd_x) ./ (1:num_steps+1);
plot(n_steps(1:num_steps-1)*a, msd_x(2:num_steps)*a^2, 'b')
title('Mean Squared Displacement in the x-Direction');
xlabel('Number of Steps');
ylabel('Mean Squared Displacement');
xlim([0,num_steps*a]);

% ————————— 3) mean squared displacement in y-direction
figure;
msd_y = cumsum(msd_y) ./ (1:num_steps+1);
plot(n_steps(1:num_steps)*a, msd_y(1:num_steps)*a^2, 'b')
title('Mean Squared Displacement in the y-Direction');
xlabel('Number of Steps');
ylabel('Mean Squared Displacement');
xlim([0, num_steps*a]);

% ————————— 4) histogram of locations in x-direction
figure;
for num_steps = [100,1000,10000,100000,1000000]
    x_hist = zeros(1,lattice_size+1);
    for walk = 1:num_walks
        x=0;
        for step = 1:num_steps
            dx = zeros(1,2);
            dx(rand<=pL+0.5) = -1;
            dx(rand<=pR+(pL/2)) = 1;
            x = x+sum(dx);
            if x<0
                x=0;
            elseif x>lattice_size
                x = lattice_size;
            end
        end
        x_hist(x+1) = x_hist(x+1) + 1;
    end
    subplot(2,3,find([100,1000,10000,100000,1000000] == num_steps));
    bar(0:lattice_size, x_hist / num_walks, 'b');
    title(['Histogram of x-locations after ', num2str(num_steps), ' steps']);
    xlabel('x');
    ylabel('Probability');
end

% ————————— 5) histogram of locations in y-direction
figure;
for num_steps = [100,1000,10000,100000,1000000]
    y_hist = zeros(1,lattice_size+1);
    for walk = 1:num_walks
        x = 0;
        y = lattice_size/2;
        for step = 1:num_steps
            dx = zeros(1,2);
            dy = zeros(1,2);
            dx(rand<=pL+0.5) = -1;
            dx(rand<=pR+(pL/2)) = 1;
            dy(rand<=pu+0.5) = -1;
            dy(rand<=pd+(pu/2)) = 1;
            x = x+sum(dx);
            y = y+sum(dy);
            if y<1
                y = 2-y;
            elseif y>lattice_size
                y = 2*lattice_size - y - 1;
            end
        end
        y_hist(y+1) = y_hist(y+1) + 1;
    end
    subplot(2,3,find([100,1000,10000,100000,1000000] == num_steps));
    bar(0:lattice_size, y_hist / num_walks, 'b');
    title(['Histogram of y-locations after ', num2str(num_steps), ' steps']);
    xlabel('y');
    ylabel('Probability');
end