## STEM

Topics cover understanding physics at a high school level

#### What is the surface tension of a fluid?

• KR

Hello,

What is the surface tension of a fluid?

Thank you.

Ansys Employee

Hello,

Surface tension is the force acting on the interface when two or more phases are in contact with each other. Assume a gas-liquid interface: say water - air. In the bulk region of stationary water, each water molecule is surrounded by other water molecules in a uniform manner. Therefore, the liquid molecule experiences zero net force. A similar observation can be made for the bulk region in the gas phase. However, the liquid molecules at the interface are surrounded by other liquid molecules at the bottom and gas molecules at the top. The cohesive forces of attraction between the liquid molecules are larger than cohesive forces between the gas molecules. This results in a force imbalance between the liquid and gas molecules at the interface. This force imbalance tends to produce an effect similar to a stretched membrane at the interface and is responsible for minimizing the overall surface area of fluid surfaces. This property of fluid has the dimension of force per unit length.

Thanks.

• KR
Thank you for this response! How does this differ from the contact angle?
• prajput
Ansys Employee

Hi @Kremella

Contact angle at a meniscus refers to the angle made by the outline of the meniscus. This angle is always measured through the liquid surface. It quantifies the wettability of the liquid. If the contact angle is < 90 degrees, the fluid tends to wet the surface (in case of water, this is called hydrophilic surface). The meniscus is more curved if the contact angle is > 90 degrees (again, for water, this is gives rise to hydrophobic surfaces).

Thank you.

• KR
Thank you for this answer. You used this word 'meniscus' in your answer. I do not quite understand this. How do you define a meniscus?
• prajput
Ansys Employee

Hi,

A meniscus is generally the interface between two immiscible fluids such as air and water or water and oil, which can either be either concave or convex. The word comes from Latin and indicates a curved object. Different liquids can have different menisci. Water for example has a concave meniscus while mercury has a convex meniscus.

I hope this helps.

Thank you.

Ansys Employee

HelloÂ @kremella

Surface tension is the force acting on the interface when two or more phases are in contact with each other. Assume a gas-liquid interface: say water - air. In the bulk region of stationary water, each water molecule is surrounded by other water molecules in a uniform manner. Therefore, the liquid molecule experiences zero net force. A similar observation can be made for the bulk region in the gas phase. However, the liquid molecules at the interface are surrounded by other liquid molecules at the bottom and gas molecules at the top. The cohesive forces of attraction between the liquid molecules are larger than cohesive forces between the gas molecules. This results in a force imbalance between the liquid and gas molecules at the interface. This force imbalance tends to produce an effect similar to a stretched membrane at the interface and is responsible for minimizing the overall surface area of fluid surfaces. This property of fluid has the dimension of force per unit length.

Thanks.

Ansys Employee

Hello @Kremella

Surface tension is the force acting on the interface when two or more phases are in contact with each other. Assume a gas-liquid interface: say water - air. In the bulk region of stationary water, each water molecule is surrounded by other water molecules in a uniform manner. Therefore, the liquid molecule experiences zero net force. A similar observation can be made for the bulk region in the gas phase. However, the liquid molecules at the interface are surrounded by other liquid molecules at the bottom and gas molecules at the top. The cohesive forces of attraction between the liquid molecules are larger than cohesive forces between the gas molecules. This results in a force imbalance between the liquid and gas molecules at the interface. This force imbalance tends to produce an effect similar to a stretched membrane at the interface and is responsible for minimizing the overall surface area of fluid surfaces. This property of fluid has the dimension of force per unit length.

Thanks.

Ansys Employee

HelloÂ @kremella

Surface tension is the force acting on the interface when two or more phases are in contact with each other. Assume a gas-liquid interface: say water - air. In the bulk region of stationary water, each water molecule is surrounded by other water molecules in a uniform manner. Therefore, the liquid molecule experiences zero net force. A similar observation can be made for the bulk region in the gas phase. However, the liquid molecules at the interface are surrounded by other liquid molecules at the bottom and gas molecules at the top. The cohesive forces of attraction between the liquid molecules are larger than cohesive forces between the gas molecules. This results in a force imbalance between the liquid and gas molecules at the interface. This force imbalance tends to produce an effect similar to a stretched membrane at the interface and is responsible for minimizing the overall surface area of fluid surfaces. This property of fluid has the dimension of force per unit length.

Thanks.