\n Surface tension refers to the tensile force acting on the surface of a liquid in the tangential direction. Surface tension is caused due to attraction of the surface molecules by the molecules in the bulk (interior) of the liquid. Surface tension tends to minimize the surface area of the liquid. Surface tension is responsible for a variety of phenomena such as shape of liquid drops, formation of soap bubbles, and the rise/fall of liquid column in a narrow channel (i.e. capillarity).\r\n
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\r\n The molecules in a fluid interact with other molecules of the fluid via intermolecular attractive forces (also known as cohesive forces). A molecule in the bulk of the fluid is evenly surrounded by neighboring molecules (as illustrated schematically in figure below). Thus, a molecule in the bulk of the fluid is pulled equally in every direction by its neighboring fluid molecules. As a result the net force acting on a molecule in the bulk of the fluid is zero. However, the molecules on the fluid surface are surrounded by neighboring molecules on only one side of the surface. Therefore, a surface molecule is not pulled equally in all the direction due to lack of neighboring molecules on the other side of fluid surface. Thus, the surface molecules are subjected to a net inward force, which makes the liquid surface to behave as if its surface is in a state of tension. This tensile force acting on the fluid surface is called surface tension.
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\r\n Because of the surface tension, the surface of a liquid acts as if it is a stretched elastic membrane. Surface tension is that it forces the liquid surfaces to contract to the minimum area.\r\n
\n The notion of surface tension can also be explained through an energy-based argument. The molecules in the bulk are evenly surrounded by neighboring molecules, but the surface molecules are surrounded by neighboring molecules/atoms only on one side (compared to interior molecules). Thus, in a way, a surface can be thought as created by taking a bulk liquid and breaking it in two halves through action of external work.\r\n The external work done in creating the surface is stored as the surface energy ― in other words, the surface molecules/atoms tend to have higher energy than the interior molecules. For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized number of boundary molecules results in a minimal surface area. As a result of surface area minimization, a surface will assume the smoothest shape it can (mathematical proof that \"smooth\" shapes minimize surface area relies on use of the Euler–Lagrange equation). Since any curvature in the surface shape results in greater area, a higher energy will also result. Consequently, the surface will push back against any curvature in much the same way as a ball pushed uphill will push back to minimize its gravitational potential energy.\r\n
\r\n\n The surface tension of a liquid is measured using an instrument called sliding wire device. The device comprises of a rectangular frame with a movable (sliding) side, as shown schematically in the figure below. The device is coated with a liquid to form a film. The surface tension of the liquid causes the film to contract, and as a result, the sliding wire moves, if not prevented. The surface tension of the liquid can be calculated by measuring the force required to keep the wire from moving as follows:\r\n $$F = \\gamma l$$\r\n assuming \\(\\gamma\\) as the surface tension and \\(l\\) is the edge length.\n
\r\n\n Capillarity refers to the movement of (rise or fall) of a liquid column within narrow spaces (such as inside of a narrow tube, pores of a porous material etc.) in the absence of external forces such as gravity or an applied pressure drop. Capillarity occurs due to the action of cohesion, adhesion and surface tension. Cohesion refers to the attractive forces between the liquid molecules, and adhesion refers to the attractive forces between the liquid molecules and solid molecules.\r\n Capillary rise occurs when the adhesive forces between the liquid and tube surface are stronger than the cohesive forces between the liquid molecules.\r\n On the other hand, fall of the fluid column occurs when the cohesive forces between liquid molecules are stronger than the adhesive forces between the liquid and the solid surface.
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\n Capillarity refers to rise or fall of the liquid column within a narrow tube\r\n partially dipped in the body of a liquid without the assistance of a\r\n external force such as gravity or a pressure difference. Due to surface\r\n tension, the meniscus or the surface of the liquid column is curved. The\r\n curvature of the meniscus depends on the contact angle between the liquid\r\n and the wall surface. For example:\r\n
\n The applet below can be used to calculate the height of the liquid column in a capillary tube for any liquid.\r\n The parameters required for the calculation such as the liquid density, its surface tension, and radius of the capillary tube can be prescribed using the input windows.\r\n The default values represent a capillary tube of diameter 1 mm immersed in water.\n
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