Surface tension - Simple English Wikipedia, the free encyclopedia. Surface tension is an effect where the surface of a liquid is strong. The surface can hold up a weight, and the surface of a water droplet holds the droplet together, in a ball shape.

Some small things can float on a surface because of surface tension, even though they normally could not float. This property is caused by the molecules in the liquid being attracted to each other (cohesion), and is responsible for many of the behaviors of liquids. Surface tension has the dimension of force per unit length, or of energy per unit area.

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The two are equivalent—but when referring to energy per unit of area, people use the term surface energy—which is a more general term in the sense that it applies also to solids and not just liquids. In materials science, surface tension is used for either surface stress or surface free energy. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area.

Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer.

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In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical. The spherical shape minimizes the necessary . A molecule in contact with a neighbor is in a lower state of energy than if it were alone (not in contact with a neighbor). The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors (compared to interior molecules). So, the boundary molecules have a higher energy.

De la superficie la composición superficial de la corteza terrestre es distinta a la de su centro. Que es poco profundo tan sólo es una.

For the liquid to minimize its energy state, the number of higher energy boundary molecules must be minimized. The minimized quantity of boundary molecules results in a minimized surface area. So, 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. Studying water shows several effects of surface tension: A. Rain water forms beads on the surface of a waxy surface, such as a leaf. Water adheres weakly to wax and strongly to itself, so water clusters into drops.

Superficial Appearance

Surface tension gives them their near- spherical shape, because a sphere has the smallest possible surface area to volume ratio. B. Formation of drops occurs when a mass of liquid is stretched. The animation shows water adhering to the faucet gaining mass until it is stretched to a point where the surface tension can no longer bind it to the faucet. It then separates and surface tension forms the drop into a sphere. If a stream of water were running from the faucet, the stream would break up into drops during its fall.

Gravity stretches the stream, then surface tension pinches it into spheres. Objects denser than water still float when the object is nonwettable and its weight is small enough to be borne by the forces arising from surface tension. The surface of the water behaves like an elastic film: the insect's feet cause indentations in the water's surface, increasing its surface area. Separation of oil and water (in this case, water and liquid wax) is caused by a tension in the surface between dissimilar liquids. This type of surface tension is called . Tears of wine is the formation of drops and rivulets on the side of a glass containing an alcoholic beverage. Its cause is a complex interaction between the differing surface tensions of water and ethanol.

It is induced by a combination of surface tension modification of water by ethanol together with ethanol evaporating faster than water. A. Water beading on a leaf.

B. Water dripping from a tap. D. Lava lamp with interaction between dissimilar liquids; water and liquid wax. Surface tension is visible in other common phenomena, especially when surfactants are used to decrease it: Soap bubbles have very large surface areas with very little mass. Bubbles in pure water are unstable. The addition of surfactants, however, can have a stabilizing effect on the bubbles (see Marangoni effect). Notice that surfactants actually reduce the surface tension of water by a factor of three or more.

Emulsions are a type of solution in which surface tension plays a role. Tiny fragments of oil suspended in pure water will spontaneously assemble themselves into much larger masses. But the presence of a surfactant provides a decrease in surface tension, which permits stability of minute droplets of oil in the bulk of water (or vice versa). Its weight, Fw, depresses the surface, and is balanced by the surface tension forces on either side, Fs, which are each parallel to the water's surface at the points where it contacts the needle. Notice that the horizontal components of the two Fs arrows point in opposite directions, so they cancel each other, but the vertical components point in the same direction and therefore add up. One way to picture this is to imagine a flat soap film bounded on one side by a taut thread of length, L. The thread will be pulled toward the interior of the film by a force equal to 2.

Its SI unit is newton per meter but the cgs unit of dyne per cm is also used. As such, in order to increase the surface area of a mass of liquid by an amount, . Consequently surface tension can be also measured in SI system as joules per square meter and in the cgs system as ergs per cm. Since mechanical systems try to find a state of minimum potential energy, a free droplet of liquid naturally assumes a spherical shape, which has the minimum surface area for a given volume. The equivalence of measurement of energy per unit area to force per unit length can be proven by dimensional analysis. Balancing the tension forces with pressure leads to the Young–Laplace equation. If no force acts normal to a tensioned surface, the surface must remain flat.

But if the pressure on one side of the surface differs from pressure on the other side, the pressure difference times surface area results in a normal force. In order for the surface tension forces to cancel the force due to pressure, the surface must be curved. The diagram shows how surface curvature of a tiny patch of surface leads to a net component of surface tension forces acting normal to the center of the patch. When all the forces are balanced, the resulting equation is known as the Young–Laplace equation.

For not very small drops the effect is subtle, but the pressure difference becomes enormous when the drop sizes approach the molecular size. Yet by fashioning the frame out of wire and dipping it in soap- solution, a locally minimal surface will appear in the resulting soap- film within seconds.

For an open soap film, the pressure difference is zero, hence the mean curvature is zero, and minimal surfaces have the property of zero mean curvature. The surface of any liquid is an interface between that liquid and some other medium. Surface tension, then, is not a property of the liquid alone, but a property of the liquid's interface with another medium. If a liquid is in a container, then besides the liquid/air interface at its top surface, there is also an interface between the liquid and the walls of the container. The surface tension between the liquid and air is usually different (greater than) its surface tension with the walls of a container. Where the two surfaces meet, the geometry will balance all forces. The diagram to the right shows two examples.

Tension forces are shown for the liquid- air interface, the liquid- solid interface, and the solid- air interface. The example on the left is where the difference between the liquid- solid and solid- air surface tension, . Out Of Darkness: Cleveland stream online with english subtitles 1280 on this page. The horizontal component of fla. The vertical component of fla.

But in this case we see that because the contact angle is less than 9. Water with specially prepared Teflon approaches this. Which method is optimal depends upon the nature of the liquid being measured, the conditions under which its tension is to be measured, and the stability of its surface when it is deformed.

Du No. Wetting properties of the surface or interface have little influence on this measuring technique. Maximum pull exerted on the ring by the surface is measured. The advantage of this method is that very small sample volumes (down to few tens of microliters) can be measured with very high precision, without the need to correct for buoyancy (for a needle or rather, rod, with proper geometry). Further, the measurement can be performed very quickly, minimally in about 2. First commercial multichannel tensiometers . A vertical plate of known perimeter is attached to a balance, and the force due to wetting is measured. The diameter of a drop within a heavy phase is measured while both are rotated.

Pendant drop method: Surface and interfacial tension can be measured by this technique, even at elevated temperatures and pressures. Geometry of a drop is analyzed optically.

For details, see Drop. Maximum pressure of each bubble is measured. Drop volume method: A method for determining interfacial tension as a function of interface age. Liquid of one density is pumped into a second liquid of a different density and time between drops produced is measured. The height at which the solution reaches inside the capillary is related to the surface tension by the equation discussed below. This value is estimated to be 0.

T = 0. Notice that the mercury level at the center of the tube is higher than at the edges, making the upper surface of the mercury dome- shaped. The center of mass of the entire column of mercury would be slightly lower if the top surface of the mercury were flat over the entire crossection of the tube. But the dome- shaped top gives slightly less surface area to the entire mass of mercury.

Venous Insufficiency: Background, Anatomy, Pathophysiology. Robert Weiss, MD Associate Professor, Department of Dermatology, Johns Hopkins University School of Medicine.

Robert Weiss, MD is a member of the following medical societies: American Academy of Cosmetic Surgery, American Academy of Dermatology, American College of Phlebology, American Society for Dermatologic Surgery, American Society for Laser Medicine and Surgery, Med. Chi The Maryland State Medical Society. Disclosure: Received honoraria from Angiodynamics for speaking and teaching; Received intellectual property rights from Cool.

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