Falling sand simulation collision detection C++ and SFML - c++

Question

I could really use some help. I am trying to make a falling sand simulation and have some basic code down, but I can't figure how to do collision detection. Here is my code so far:
//SFML Include
#include <SFML/Graphics.hpp>
//Include Vectors
#include <vector>
//Main Loop
int main()
{
//Window Init
sf::RenderWindow window(sf::VideoMode(720, 480, 32), "Yip Yip Physics", sf::Style::Default);
//Global Envoirmental Variables
static float gravity = 0.25;
static float frict = 3;
//Particle Structures
struct Particle
{
//Shape
sf::RectangleShape rect;
//Cell Position
sf::Vector2f cell_pos;
//Vel and frict
float slide_vel = 3;
float grv_vel = 0;
//Init Particle (size, color and origin)
void init()
{
rect.setSize(sf::Vector2f(3, 3));
rect.setFillColor(sf::Color::Green);
rect.setOrigin(rect.getSize());
}
};
//Particle Setup
std::vector<Particle> particles;
//Window Loop
while (window.isOpen())
{
//Update
if (sf::Mouse::isButtonPressed(sf::Mouse::Button::Left))
{
//Make and Position Particle
Particle part;
part.rect.setPosition(sf::Vector2f(sf::Mouse::getPosition(window)));
//Initalize the Particle and add to vector
part.init();
particles.push_back(part);
}
//Pixel Update
for (auto& p : particles)
{
//Draw in window
window.draw(p.rect);
}
//Display
window.display();
//Event Variable
sf::Event event;
//Window Event System
while (window.pollEvent(event))
{
//Close Window
if (event.type == sf::Event::Closed)
{
window.close();
}
}
//Window Clear
window.clear(sf::Color::Black);
}
//Return 0
return 0;
}
The code works in making the particles, putting them in a vector and drawing them to the screen, but I don't know where to go for collision. I don't know where to look and would really appreciate help on it or direction on where to find it. I have heard a bit about cellular automata but have no idea how to do it or if it's the best option.