# Surface Tension

views updated May 11 2018

# Surface Tension

Surface tension, in physics, is an effect within a liquids surface layer, which causes the layer to possess characteristics similar to elastic. For instance, surface tension allows insects such as a water strider to walk on water. Surface tension, thus, is the result of the cohesive forces that attract water molecules to one another. This surface force keeps objects that are more dense than water (meaning they should not float) from sinking into it. The surface tension of water makes it puddle on the ground and keeps it in a droplet shape when it falls.

If one uses a table fork to carefully place a paper clip on the surface of some clean water, a person will find that the paper clip, although more dense than water, will remain on the waters surface. If one looks closely, the surface is bent by the weight of the paper clip much as the human skin bends when one pushes on it with a finger.

A molecule inside a volume of water is pulled equally in all directions by the other molecules of water that surround it. The molecules below it and to its sides, on the other hand, pull a molecule on the waters surface. The net force on this surface molecule is inward. The result is a surface that behaves as if it were under tension. If a glob of water with an irregular shape is created, the inward forces acting on the molecules at its surface quickly pull it into the smallest possible volume it can have, which is a sphere.

A simple apparatus can be used to measure the forces on a liquids surface. A force is applied to a wire of known length, which forms a circle parallel to the surface of the water. The force balances the surface forces acting on each side of the wire. The surface tension of the liquid, g, is defined as the ratio of the surface force to the length of the wire (the length along which the force acts). For this kind of measurement, g = F/2L. The force, F, applied to the wire is that required to balance the surface forces; L is the length of the wire. The 2 appears in the denominator because there is a surface film on each side of the wire. Thus, surface tension has units of force per lengthdynes/cm, newtons/m, ounces/inch, etc.

Water has a relatively high surface tension that decreases with increasing temperature. The increased kinetic energy of the molecules at higher temperature would oppose the forces of cohesion. In fact, at the boiling temperature, the kinetic energy of molecules is sufficient to overcome their cohesive forces of attraction and the molecules separate to form a gas.

# Surface Tension

views updated May 21 2018

# Surface tension

Surface tension is the result of the cohesive forces that attract water molecules to one another. This surface force keeps objects which are more dense than water (meaning they should not float) from sinking into it. The surface tension of water makes it puddle on the ground and keeps it in a droplet shape when it falls.

If you use a table fork to carefully place a paper clip on the surface of some clean water, you will find that the paper clip, although more dense than water, will remain on the water's surface. If you look closely, you will see that the surface is bent by the weight of the paper clip much as your skin bends when you push on it with your finger.

A molecule inside a volume of water is pulled equally in all directions by the other molecules of water that surround it. A molecule on the water's surface, on the other hand, is pulled by the molecules below it and to its sides. The net force on this surface molecule is inward. The result is a surface that behaves as if it were under tension. If a glob of water with an irregular shape is created, the inward forces acting on the molecules at its surface quickly pull it into the smallest possible volume it can have, which is a sphere .

A simple apparatus can be used to measure the forces on a liquid's surface. A force is applied to a wire of known length which forms a circle parallel to the surface of the water. The force balances the surface forces acting on each side of the wire. The surface tension of the liquid, g, is defined as the ratio of the surface force to the length of the wire (the length along which the force acts). For this kind of measurement, g = F/2L. The force, F, applied to the wire is that required to balance the surface forces; L is the length of the wire. The 2 appears in the denominator because there is a surface film on each side of the wire. Thus, surface tension has units of force per length—dynes/cm, newtons/m, ounces/inch, etc.

Water has a relatively high surface tension that, not surprisingly, decreases with increasing temperature . The increased kinetic energy of the molecules at higher temperature would oppose the forces of cohesion. In fact, at the boiling temperature, the kinetic energy of molecules is sufficient to overcome their cohesive forces of attraction and the molecules separate to form a gas.

# surface tension

views updated May 23 2018

surface tension Symbol γ. The property of a liquid that makes it behave as if its surface is enclosed in an elastic skin. The property results from intermolecular forces: a molecule in the interior of a liquid experiences a force of attraction from other molecules equally from all sides, whereas a molecule at the surface is only attracted by molecules below it in the liquid. The surface tension is defined as the force acting over the surface per unit length of surface perpendicular to the force. It is measured in newtons per metre. It can equally be defined as the energy required to increase the surface area by one square metre, i.e. it can be measured in joules per metre squared (which is equivalent to N m–1).

The surface tension of water is very strong, due to the intermolecular hydrogen bonding, and is responsible for the formation of drops, bubbles, and meniscuses (the curved surfaces of columns of liquid), as well as the rise of water in a capillary tube (capillarity), the absorption of liquids by porous substances, and the ability of liquids to wet a surface. Capillarity is very important in plants as it contributes to the transport of water, against gravity, within the plant.

# surface tension

views updated May 08 2018

surface tension (γ) Fluid surfaces may take on the behaviour of a stretched elastic membrane as a result of the tendency of a liquid surface to contract. The surface tension of a liquid is given as the tension across a unit length of the fluid surface. Surface tension is temperature-dependent and is closely associated with capillarity.

# surface tension

views updated May 23 2018

sur·face ten·sion • n. the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.