When you heat a pan and add oil to it, the oil spreads out and moves towards the edge of the pan due to several factors, including surface tension, gravity, and the temperature gradient.
Surface tension: Surface tension is the property of a liquid that causes its molecules to stick together, forming a thin "skin" on the surface. This phenomenon is particularly evident in liquids like water and oil. When you pour oil into the pan, surface tension causes the oil molecules to attract and bond with each other, leading to the oil forming a cohesive layer.
Gravity: Gravity is constantly pulling the oil downward. As the oil spreads out on the pan's surface, gravity pulls it toward the lowest point available, which is usually the center of the pan. This downward motion, combined with surface tension, causes the oil to form a thin layer that coats the entire pan's bottom.
Temperature gradient: When you heat the pan, the temperature at the center is usually higher than the temperature at the edges. As a result, the oil in the center gets heated more quickly, causing its molecules to move more vigorously and become less dense. This creates a slightly lower density in the center of the pan compared to the edges. Less dense regions tend to rise, which causes the heated oil to move outward and towards the edges.
The combination of these factors—surface tension, gravity, and the temperature gradient—results in the oil spreading out across the pan's bottom and moving towards the edges. This behavior is also seen with other liquids, though the extent of spreading may vary depending on the specific properties of the liquid.