How can I change the color between the two circles?

% Parameters for the inner and outer circles
radius_inner = 1;    % Radius of the inner (solid sphere)
radius_outer = 1.5;  % Radius of the outer (porous shell)

% Create a figure
figure;
hold on;
axis equal;

% Generate points for the circles
theta = linspace(0, 2*pi, 100);

% Coordinates for the inner circle
x_inner = radius_inner * cos(theta);
y_inner = radius_inner * sin(theta);

% Coordinates for the outer circle
x_outer = radius_outer * cos(theta);
y_outer = radius_outer * sin(theta);

% Plot the outer circle and fill the area between circles with a color
fill([x_outer, fliplr(x_inner)], [y_outer, fliplr(y_inner)], [0.9, 0.9, 0.9], 'EdgeColor', 'k', 'FaceAlpha', 0.5); % Light gray color for the porous shell

% Plot the inner circle with a different color
fill(x_inner, y_inner, [0.8, 0.8, 0.8], 'EdgeColor', 'k'); % Darker gray color for solid sphere

% Set axis limits and hide axes
axis([-2 2 -2 2]);
axis off;
hold off;

Answer 1

[Voss]

Change the third argument to the first fill() call. Example:

% Parameters for the inner and outer circles
radius_inner = 1;    % Radius of the inner (solid sphere)
radius_outer = 1.5;  % Radius of the outer (porous shell)

% Create a figure
figure;
hold on;
axis equal;

% Generate points for the circles
theta = linspace(0, 2*pi, 100);

% Coordinates for the inner circle
x_inner = radius_inner * cos(theta);
y_inner = radius_inner * sin(theta);

% Coordinates for the outer circle
x_outer = radius_outer * cos(theta);
y_outer = radius_outer * sin(theta);

% Plot the outer circle and fill the area between circles with a color
fill([x_outer, fliplr(x_inner)], [y_outer, fliplr(y_inner)], [0, 0.9, 0], 'EdgeColor', 'k', 'FaceAlpha', 0.5); % Green color for the porous shell

% Plot the inner circle with a different color
fill(x_inner, y_inner, [0.8, 0.8, 0.8], 'EdgeColor', 'k'); % Dark gray color for solid sphere

% Set axis limits and hide axes
axis([-2 2 -2 2]);
axis off;
hold off;

[shreen elsapa]

@Voss thanks

I need the following:

% Parameters for the solid sphere and porous shell
a = 1;   % Radius of the solid sphere
b = 1.5; % Outer radius of the porous shell
% Create the figure
figure;
hold on;
axis equal;
% Plot the solid sphere
theta = linspace(0, 2*pi, 100);
x_solid = a * cos(theta);
y_solid = a * sin(theta);
fill(x_solid, y_solid, [0.8 0.8 0.8], 'EdgeColor', 'k'); % Gray color for solid sphere
% Plot the porous shell
x_shell = b * cos(theta);
y_shell = b * sin(theta);
fill(x_shell, y_shell, [0.9 0.9 0.9], 'EdgeColor', 'k', 'FaceAlpha', 0.5); % Lighter shade for porous shell
% Define plot boundaries and resolution for porous structure
x_limits = [-2, 2];
y_limits = [-2, 2];
n_pores = 1000; % Number of pores (random points)
% Generate random pore positions
rng(1); % For reproducibility
x_pores = x_limits(1) + (x_limits(2) - x_limits(1)) * rand(n_pores, 1);
y_pores = y_limits(1) + (y_limits(2) - y_limits(1)) * rand(n_pores, 1);
% Filter out pores that are inside the solid sphere and porous shell
dist_to_solid = sqrt(x_pores.^2 + y_pores.^2); % Distance from each pore to the center
pores_outside = (dist_to_solid > b); % Only include points outside the porous shell
% Plot the pores outside the porous shell
plot(x_pores(pores_outside), y_pores(pores_outside), 'b.', 'MarkerSize', 10);
% Labeling solid sphere and porous shell
text(0.3, -0.3, 'solid sphere', 'HorizontalAlignment', 'center', 'VerticalAlignment', 'middle', 'HorizontalAlignment', 'right', 'Interpreter','latex','FontSize',14,'FontName','TiemsNewRoman');
text(b,-2,-2, 'porous micropolar', 'HorizontalAlignment', 'right', 'Interpreter','latex','FontSize',14,'FontName','TiemsNewRoman');
% Set plot limits and hide axes
axis([-2 2 -2 2]);
axis off;
hold off;

[Voss]

Rather than having the outer circle semi-transparent and on top the inner circle, make the outer circle underneath the inner circle by creating the outer circle first and remove its transparency (i.e., make it fully opaque).

Modify its color as desired by modifying the third input to fill().

% Parameters for the solid sphere and porous shell
a = 1;   % Radius of the solid sphere
b = 1.5; % Outer radius of the porous shell
% Create the figure
figure;
hold on;
axis equal;
theta = linspace(0, 2*pi, 100);

% Plot the porous shell
x_shell = b * cos(theta);
y_shell = b * sin(theta);
fill(x_shell, y_shell, [0 0.9 0], 'EdgeColor', 'k'); % green color porous shell

% Plot the solid sphere
x_solid = a * cos(theta);
y_solid = a * sin(theta);
fill(x_solid, y_solid, [0.8 0.8 0.8], 'EdgeColor', 'k'); % Gray color for solid sphere

% Define plot boundaries and resolution for porous structure
x_limits = [-2, 2];
y_limits = [-2, 2];
n_pores = 1000; % Number of pores (random points)
% Generate random pore positions
rng(1); % For reproducibility
x_pores = x_limits(1) + (x_limits(2) - x_limits(1)) * rand(n_pores, 1);
y_pores = y_limits(1) + (y_limits(2) - y_limits(1)) * rand(n_pores, 1);
% Filter out pores that are inside the solid sphere and porous shell
dist_to_solid = sqrt(x_pores.^2 + y_pores.^2); % Distance from each pore to the center
pores_outside = (dist_to_solid > b); % Only include points outside the porous shell
% Plot the pores outside the porous shell
plot(x_pores(pores_outside), y_pores(pores_outside), 'b.', 'MarkerSize', 10);
% Labeling solid sphere and porous shell
text(0.3, -0.3, 'solid sphere', 'HorizontalAlignment', 'center', 'VerticalAlignment', 'middle', 'HorizontalAlignment', 'right', 'Interpreter','latex','FontSize',14,'FontName','TiemsNewRoman');
text(b,-2,-2, 'porous micropolar', 'HorizontalAlignment', 'right', 'Interpreter','latex','FontSize',14,'FontName','TiemsNewRoman');
% Set plot limits and hide axes
axis([-2 2 -2 2]);
axis off;
hold off;

[shreen elsapa]

@Voss thanks so much

[Voss]

You're welcome! Any questions, let me know. Otherwise, please "Accept" this answer. Thanks!

[shreen elsapa]

@Voss if I want to reverse the regions the annulus porous with the above color

and outside clear fluid

[Voss]

Explain.

[shreen elsapa]

% Parameters for the solid sphere and porous shell
a = 2;   % Radius of the solid sphere
b = 3; % Outer radius of the porous shell (increased space between annulus)
% Create the figure
figure;
hold on;
axis equal;
theta = linspace(0, 2*pi, 100);
% Plot the surrounding fluid (gray region) outside the porous shell
% Increase the fluid region to cover a larger area
%fill([-4 4 4 -4], [-4 -4 4 4], [0.5 0.5 0.5], 'EdgeColor', 'none'); % Increased gray fluid region
%fill([-4 4 4 -4], [-4 -4 4 4], [0 0 1], 'EdgeColor', 'none');  % Blue color
fill([-6 6 6 -6], [-6 -6 6 6], [1 1 1], 'EdgeColor', 'none');  % White color for the fluid region
% Plot the porous shell (within radius b)
x_shell = b * cos(theta);
y_shell = b * sin(theta);
fill(x_shell, y_shell, [1 1 1], 'EdgeColor', 'k'); % Green color for porous shell
% Plot the solid sphere (within radius a)
x_solid = a * cos(theta);
y_solid = a * sin(theta);
fill(x_solid, y_solid, [0.8 0.8 0.8], 'EdgeColor', 'k'); % Gray color for solid sphere
% Define plot boundaries and resolution for porous structure
x_limits = [-3, 3]; % Adjusted limits to accommodate expanded fluid region
y_limits = [-3, 3];
n_pores = 500; % Number of pores (random points)
% Generate random pore positions within the annular region (between a and b)
rng(1); % For reproducibility
radii = a + (b - a) * rand(n_pores, 1); % Random radii between a and b
angles = 2 * pi * rand(n_pores, 1); % Random angles between 0 and 2*pi
% Convert polar coordinates to Cartesian coordinates
x_pores = radii .* cos(angles);
y_pores = radii .* sin(angles);
% Plot the pores within the annulus
plot(x_pores, y_pores, 'b.', 'MarkerSize', 10);
% Labeling solid sphere and porous shell
text(a,3.5,2, 'Soft Sphere', 'HorizontalAlignment', 'center', 'VerticalAlignment', 'middle', 'FontSize', 14, 'FontName', 'Times New Roman');
text(b,-3.5,3, 'Micropolar Fluid', 'HorizontalAlignment', 'right', 'Interpreter', 'latex', 'FontSize', 14, 'FontName', 'Times New Roman');
% Set plot limits and hide axes
axis([-4 4 -4 4]);  % Adjusted plot limits to accommodate the expanded fluid region
axis off;
hold off;

[shreen elsapa]

@Voss done thanks

[Voss]

Don't forget to Accept the answer that helped you.

[shreen elsapa]

@Voss how?

and I need to plot it in 3d but remove the pores from the surface of inside sphere but I can not SEE

% Define the radii of the spheres (larger values)
r1 = 5;  % Radius of the first (inner) sphere
r2 = 10;  % Radius of the second (outer) sphere

% Create the sphere coordinates for the first sphere
[Xi1, Yi1, Zi1] = sphere(100);  % 100 specifies the resolution for finer detail

% Scale the coordinates for the first sphere
Xi1 = r1 * Xi1;
Yi1 = r1 * Yi1;
Zi1 = r1 * Zi1;

% Create the sphere coordinates for the second sphere
[Xi2, Yi2, Zi2] = sphere(100);  % 100 resolution

% Scale the coordinates for the second sphere
Xi2 = r2 * Xi2;
Yi2 = r2 * Yi2;
Zi2 = r2 * Zi2;

% Define plot boundaries and resolution for porous structure
x_limits = [-12, 12];
y_limits = [-12, 12];
n_pores = 2000; % Number of pores (random points), increased for more density

% Generate random points for the porous medium in the annular region
theta = 2*pi * rand(n_pores, 1);  % Random azimuthal angles
phi = acos(2 * rand(n_pores, 1) - 1);  % Random polar angles

% Generate random radius values strictly between r1 and r2 (no pores on r1 surface)
r = (r2 - r1) * rand(n_pores, 1) + r1;  % Random radii in the annulus

% Remove points that are exactly on the surface of the inner sphere (r == r1)
r(abs(r - r1) < 1e-3) = r1 + 0.01;  % Ensure no points are exactly on the surface of the inner sphere

% Regenerate random angles for remaining pores (no need to adjust since they are valid for the entire region)
theta = 2*pi * rand(length(r), 1);
phi = acos(2 * rand(length(r), 1) - 1);

% Convert spherical to Cartesian coordinates for the pores
x_pores = r .* sin(phi) .* cos(theta);
y_pores = r .* sin(phi) .* sin(theta);
z_pores = r .* cos(phi);

% Plot the two spheres and the porous medium
figure;
hold on;

% Plot the first sphere (inner sphere)
surf(Xi1, Yi1, Zi1, 'FaceAlpha', 0.5, 'EdgeColor', 'none');
% Plot the second sphere (outer sphere)
surf(Xi2, Yi2, Zi2, 'FaceAlpha', 0.3, 'EdgeColor', 'none');

% Plot the random points (pores) in the annular region with moderate pore size
scatter3(x_pores, y_pores, z_pores, 10, 'filled', 'MarkerFaceColor', [0.5, 0.5, 0.5]);

% Adjust the axes to ensure the spheres are displayed properly
axis equal;
xlabel('X');
ylabel('Y');
zlabel('Z');
title('Larger Concentric Spheres with Moderate Pores in the Annulus (No Pores on Inner Surface)');
grid on;

% Set the view to 3D
view(3);  % This ensures the plot is in 3D view

% Add a light source to make the spheres appear more realistic
light;
lighting gouraud;

hold off;

[Voss]

Those pores might all be inside the annulus. What makes you think some of them are on the surface of the inner sphere?

Checking the distance from origin to each pore will confirm that all pores are within the annulus region:

dd = sqrt(x_pores.^2 + y_pores.^2 + z_pores.^2);
all(dd > r1 & dd < r2)

[shreen elsapa]

@Voss not removed from the surface of a

[Voss]

When viewing a 3d object in 2d, some parts of it will obscure some other parts. In this case, that means some pores in the annulus will appear in front of the inner sphere. Right?