Oil Behaves In Pans: Why Does It Migrate To Edges?

Oil pooling to the sides of a pan is a common issue encountered while cooking. This phenomenon is caused by uneven heating of the pan, resulting in a process known as thermocapillary convection, which draws the oil towards the edges. The centre of the pan often heats up faster, causing the oil to heat up and become less viscous, allowing it to move around freely. In contrast, the oil on the sides of the pan remains thicker and stays in place. Additionally, surface tension plays a role, as liquids tend to have lower surface tension as temperatures rise, creating a gradient that pulls the oil outwards. This can lead to dry spots in the centre of the pan, causing food to stick. To prevent this issue, it is recommended to use a thicker layer of oil, a pan with a more massive bottom for even heat distribution, or moderate heat. Stirring the food regularly can also help.

Uneven heating of the pan

When a pan is heated, the section directly above the flame or heating element becomes hotter than the edges, which lose heat faster due to being exposed to the air. This creates a temperature gradient, with heat moving from the warmer areas to the cooler areas through conduction.

The oil's surface tension is affected by this temperature difference. As the oil above the heat source receives more energy, its molecules become more agitated, resulting in reduced surface tension in that region. Conversely, the surface tension at the cooler edges of the pan remains higher due to lower temperatures. This creates a surface tension gradient, with lower tension in the centre compared to the edges, causing the oil to be drawn towards the edges.

Additionally, the metal in the pan expands unevenly when heated. The centre of the pan, being in direct contact with the heat source, heats up faster than the rim, causing the pan to bow up in the middle. This "hill" in the centre, combined with the reduced surface tension, further facilitates the flow of oil towards the edges, resulting in a ring-shaped pool.

The use of thicker-based pans or pre-heating the pan for a longer duration can help mitigate this issue, allowing the rim to reach a similar temperature as the base before cooking.

Pans with a slight convex shape

The reason pans are designed with this slight bulge is to prevent permanent warping. Pans that are perfectly flat are prone to going concave and becoming "spinners". The little convexity is there so that in a higher heat situation, it will flatten out. This is generally not an issue with non-stick pans, because most people know not to get non-stick pans very hot. However, if you use a halogen or induction burner that is too small for the pan, you run a real risk of heating the middle and making a spinner anyway.

The convex shape of the pan is designed to account for the expansion that occurs when heating, to avoid the pan from permanently warping and turning into a "spinner". In theory, the pan is supposed to flatten when it is brought up to its operating temperature following the expansion that occurs, although this doesn't always work as well in practice.

Some pans may appear perfectly flat, but their ability to handle heat expansion lies in the hidden concavity at the bottom. These concavities are not readily apparent to the casual observer, but they serve an essential purpose in accommodating heat expansion and contraction during cooking. Manufacturers often cleverly shape the base with a thicker design to effectively conceal the concavity, making it less noticeable to the eye while maintaining its heat-handling capabilities.

Pans with a thin base

To prevent this from happening, it is recommended to avoid extreme temperature changes. For example, by letting the pan cool down before washing it and by heating it up gradually.

If you are using a thin-based pan, you can also try to prevent oil pooling by using more oil. This will create a thicker layer that is less likely to reach a critically thin level in the centre of the pan.

Additionally, you can try to manage the oil pooling by tilting the pan after preheating to ensure the oil coats the entire cooking surface.

Oil temperature and viscosity

Oil viscosity refers to how easily oil pours at a specified temperature. The viscosity of an oil is measured by its resistance to flow. The viscosity of oil is influenced by temperature, with higher temperatures leading to lower viscosity and thinner oil, and lower temperatures resulting in higher viscosity and thicker oil.

Thin oils have lower viscosity and pour more easily at low temperatures than thicker oils with higher viscosity. For example, an oil with a 5W-30 grade will thicken less in cold weather compared to an oil with a 10W-30 grade. At higher temperatures, the 5W-30 oil will thin out faster than an oil with a 5W-40 grade.

The Society of Tribologists and Lubrication Engineers (STLE) considers viscosity to be one of an oil's most important physical properties. It is often one of the first parameters measured by oil analysis labs due to its significance in oil condition and lubrication.

Viscosity can be measured in two ways: kinematic viscosity and absolute (dynamic) viscosity. Kinematic viscosity is defined as the oil's resistance to flow and shear due to gravity. On the other hand, absolute viscosity measures the force required to stir the oil at a constant rate, taking into account the oil's resistance to flow and shear due to internal friction.

Monitoring and controlling viscosity is crucial because of its impact on equipment life. Even small changes in viscosity can have significant effects on equipment performance and lubrication.

Stove surface not level

If your stove surface is not level, the oil in the pan will move towards the lower end of the pan. This is due to gravity and the fact that oil, being a liquid, will flow towards the lowest point in the pan.

To prevent this from happening, you can try to level your stove by adjusting its legs or using shims to raise one side. If your stove is unable to be levelled, you may need to replace it or use a wok stand to create a more level surface.

Additionally, the pan you are using may also be warped, which can cause oil to pool to one side. Check if your pan is warped by placing it on a level surface and seeing if it rocks or sits unevenly. If your pan is warped, you can try to fix it by placing it on a firm book and gently tapping the centre with a rubber mallet until it becomes flat again.

Another factor that can cause oil to move to the edge of the pan is the heat distribution. The centre of the pan is usually hotter than the edges, resulting in lower surface tension in the middle and higher tension at the edges. This causes the oil to flow from the centre to the sides. To mitigate this, you can preheat your pan before adding oil or use a thicker-bottomed pan to promote even heat distribution.

By addressing the stove levelling, pan warping, and heat distribution issues, you can minimise the tendency of oil to move towards the edge of the pan.

Frequently asked questions