Suppose, P o is the atmospheric pressure on the concave side and the pressure on the convex side of the meniscus is less than the atmospheric pressure which is given by 2σ/R (R is the radius of the meniscus). Let a meniscus with the top side be concave and the lower side is convex. On the other hand, liquids have convex meniscus depressed during capillary action. Liquids have concave meniscus rise in the capillary tube. The formula which is used to calculate the height of any liquid in the capillary tube is given by, This phenomenon is known as surface tension. This creates a sheet of molecules above the surface of the liquid. When liquid moves upward, these forces help in the journey of the molecules because molecules are attracted to each other.ĭue to cohesive forces, molecules of liquid come close to each other. Such kinds of forces exist between the same type of molecules. This results in the stacking of liquid molecules on each other along the solid surface at the edges of the meniscus. These forces are responsible for the adhesion of liquid molecules on the solid surface.Īt the sides edges of the meniscus, these forces are stronger than cohesive forces. For example, one type of molecule from liquid and other types of molecules from solid. These are the forces involved in capillary action:Īdhesive forces occur between the molecules of different substances. When liquid molecules come to close solid molecules attract or repel each other. There are some interactions between the molecules of liquid and solid.

When a liquid moves along the solid surface during capillary action. In this case, adhesive and cohesive forces are weak. Meniscus side edges are in upward directions. This type of meniscus is like a cave or has depression from the top side. When the contact angle of liquid is less than 90 o, this creates a concave meniscus. Impure water is also an example of a rising liquid.ĭepending on the angle of contact (θ) and forces (cohesion, adhesion, surface tension), the meniscus has three types of shapes. For example, methyl iodide has an angle of contact of 29 o. They can rise in the capillary tube by capillary action. Liquids that have a contact angle (θ) of less than 90 degrees are called rising liquids. This increases the height (h) of the liquid because its value is also positive.

If the contact angle (θ) is smaller than 90 o, the value of Cosθ will be positive in the quadrant.The contact angle (θ) is directly proportional to the height (h) of the liquid. This results in the formation of an angle at a point, where tangents are in contact called the angle of contact (θ). However, the tangent to the solid surface is drawn toward the liquid. Tangent to the liquid surface is drawn away from the solid. The angle of contact (θ) is an angle between the tangents on liquid and solid surfaces. The rising of liquid (height h) in a glass tube by capillary action also depends on the angle of contact (θ). The narrow capillary tube has water at a higher level while the broader capillary tube has water at a lower level. The height of the water is inversely proportional to the radius of the capillary tube. In the above illustration, water moves upward direction by capillary action. To calculate the height of liquid in a capillary tube, this formula is used, Increasing the diameter of the tube decreases the height of the liquid and vice versa. The height (h) of the liquid depends on the diameter (radius r) of the tube. Molecular attractions arise due to forces, like adhesive, cohesive, and surface tension.ĭuring the process, liquid moves through a narrow glass tube called a capillary. This is caused by the attraction between the molecules of liquid and solid. Capillary action is the movement of liquid in the upward direction with the solid surface.