Chemical Engineering - Fluid Mechanics - Discussion

Surface tension is also important at the interface between a liquid, a gas, and a solid. For example, a meniscus occurs when the surface of a liquid touches a solid wall, as most readily noticed when a capillary tube is placed in a liquid. Consider a glass capillary tube inserted into a liquid, such as water. The water will rise up the tube to a height, h, because surface tension pulls the surface of the water towards the glass, as shown. The meniscus is the curved surface at the top of the water column.

The contact angle is defined as the angle between the liquid and solid surface, as shown in the sketch. Contact angle depends on both the liquid and the solid. If theta is less than 90°, the liquid is said to "wet" the solid. However, if theta is greater than 90°, the liquid is repelled by the solid, and tries not to "wet" it. For example, water wets glass, but not wax. Mercury does not wet glass.

Water wets glass Mercury does not wet glass.

Mercury in a glass flask is a good example of the effects of the ratio between cohesive and adhesive forces.

Because of its high cohesion and low adhesion to the glass, mercury does not spread out to cover the top of the flask, and if enough is placed in the flask to cover the bottom, it exhibits a strongly convex meniscus, whereas the meniscus of water is concave.

Mercury will not wet the glass, unlike water and many other liquids, and if the glass is tipped, it will 'roll' around inside.

The reason we consider the surface area of the entire mass of mercury, including the part of the surface that is in contact with the glass, is because mercury does not adhere at all to glass. So the surface tension of the mercury acts over its entire surface area, including where it is in contact with the glass. If instead of glass, the tube were made out of copper, the situation would be very different.

The reason should be the force of cohesion. It is because adhesion is the property by which things gets wet after having a contact with the liquid while surface tension makes the surface intact. Cohesion is the force responsible for surface tension and therefore it restricts the liquid from adhering with the other surfaces. Cohesion is the force among the molecules of the same matter.

When liquid water is confined in a tube, its surface (meniscus) has a concave shape because water wets the surface and creeps up the side. Mercury does not wet glass - the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass. Then what's the correct answer?

Mercury does not wet glass - the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass. When liquid mercury is confined in a tube, its surface (meniscus) has a convex shape because the cohesive forces in liquid mercury tend to draw it into a drop.

Mercury does not wet glass - the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass. When liquid mercury is confined in a tube, its surface (meniscus) has a convex shape because the cohesive forces in liquid mercury tend to draw it into a drop.

Hello Sachin !

surface tension of any fluid particle acts radially over the tangential components so that the fluid particles try to obtain a minimal surface area. In this stake the mercury droplet gets spherical and thus have minimum contact just at only one point on the glass

When liquid water is confined in a tube, its surface (meniscus) has a concave shape because water wets the surface and creeps up the side. Mercury does not wet glass - the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass.

Surface tension is a property that arises due to the intermolecular forces of attraction among the molecules of the liquid. In the case of mercury taken in a glass tube, adhesive forces are weaker than cohesive forces. So, the mercury molecule does not wet the glass.