File Name: surface energy and surface tension .zip
Related titles. Download Study Material for preparation of Advanced for free. If instead of a large flat surface we had a long cylinder of small radius, then a lot of water molecules could take advantage of the attraction to the surface and climb higher.
Surface free energy can be considered as the surface tension of a solid. Surface free energy, or SFE for short, arises from the molecular interactions at the air — solid interface. Surface free energy is important in many application areas. It dictates how the solid behaves when put in contact with liquids. The surface free energy of a solid predicts how a material behaves when placed in a human body or how a coating formulation spreads on it.
Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension allows insects e. At liquid—air interfaces, surface tension results from the greater attraction of liquid molecules to each other due to cohesion than to the molecules in the air due to adhesion. There are two primary mechanisms in play. One is an inward force on the surface molecules causing the liquid to contract.
Because of the relatively high attraction of water molecules to each other through a web of hydrogen bonds , water has a higher surface tension Surface tension is an important factor in the phenomenon of capillarity. Surface tension has the dimension of force per unit length , or of energy per unit area.
The two are equivalent, but when referring to energy per unit of area, it is common to use the term surface energy , which is a more general term in the sense that it applies also to solids. In materials science , surface tension is used for either surface stress or surface energy. Due to the cohesive forces a molecule is pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero.
The molecules at the surface do not have the same molecules on all sides of them and therefore are pulled inward. This creates some internal pressure and forces liquid surfaces to contract to the minimum area. There is also a tension parallel to the surface at the liquid-air interface which will resist an external force, due to the cohesive nature of water molecules. The forces of attraction acting between the molecules of same type are called cohesive forces while those acting between the molecules of different types are called adhesive forces.
The balance between the cohesion of the liquid and its adhesion to the material of the container determines the degree of wetting , the contact angle and the shape of meniscus. When cohesion dominates specifically, adhesion energy is less than half of cohesion energy the wetting is low and the meniscus is convex at a vertical wall as for mercury in a glass container.
On the other hand, when adhesion dominates adhesion energy more than half of cohesion energy the wetting is high and the similar meniscus is concave as in water in a glass. 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 imbalance in cohesive forces of the surface layer. In the absence of other forces, drops of virtually all liquids would be approximately spherical.
The spherical shape minimizes the necessary "wall tension" of the surface layer according to Laplace's law. Another way to view surface tension is in terms of energy.
A molecule in contact with a neighbor is in a lower state of energy than if it were alone. The interior molecules have as many neighbors as they can possibly have, but the boundary molecules are missing neighbors compared to interior molecules and therefore have a higher energy. 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. Since any curvature in the surface shape results in greater area, a higher energy will also result. Water striders stay atop the liquid because of surface tension. Lava lamp with interaction between dissimilar liquids: water and liquid wax.
Photo showing the " tears of wine " phenomenon. Surface tension is visible in other common phenomena, especially when surfactants are used to decrease it:. Its SI unit is newton per meter but the cgs unit of dyne per centimeter is also used. For example, . In the illustration on the right, the rectangular frame, composed of three unmovable sides black that form a "U" shape, and a fourth movable side blue that can slide to the right. Surface tension will pull the blue bar to the left; the force F required to hold the movable side is proportional to the length L of the immobile side.
We therefore define the surface tension as. This can be easily related to the previous definition in terms of force:  if F is the force required to stop the side from starting to slide, then this is also the force that would keep the side in the state of sliding at a constant speed by Newton's Second Law.
But if the side is moving to the right in the direction the force is applied , then the surface area of the stretched liquid is increasing while the applied force is doing work on the liquid.
This means that increasing the surface area increases the energy of the film. This work W is, by the usual arguments , interpreted as being stored as potential energy.
Consequently, surface tension can be also measured in SI system as joules per square meter and in the cgs system as ergs per cm 2. 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.
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 : . The quantity in parentheses on the right hand side is in fact twice the mean curvature of the surface depending on normalisation. Solutions to this equation determine the shape of water drops, puddles, menisci, soap bubbles, and all other shapes determined by surface tension such as the shape of the impressions that a water strider's feet make on the surface of a pond.
The table below shows how the internal pressure of a water droplet increases with decreasing radius. For not very small drops the effect is subtle, but the pressure difference becomes enormous when the drop sizes approach the molecular size. In the limit of a single molecule the concept becomes meaningless. When an object is placed on a liquid, its weight F w depresses the surface, and if surface tension and downward force becomes equal than is balanced by the surface tension forces on either side F s , which are each parallel to the water's surface at the points where it contacts the object.
Notice that small movement in the body may cause the object to sink. As the angle of contact decreases, surface tension decreases. The horizontal components of the two F s arrows point in opposite directions, so they cancel each other, but the vertical components point in the same direction and therefore add up  to balance F w.
The object's surface must not be wettable for this to happen, and its weight must be low enough for the surface tension to support it. To find the shape of the minimal surface bounded by some arbitrary shaped frame using strictly mathematical means can be a daunting task. 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. The reason for this is that the pressure difference across a fluid interface is proportional to the mean curvature , as seen in the Young—Laplace equation.
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. The surface tension between the liquid and air is usually different greater than its surface tension with the walls of a container.
And where the two surfaces meet, their geometry must be such that all forces balance. Note that the angle is measured through the liquid , as shown in the diagrams above. 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. In the diagram, both the vertical and horizontal forces must cancel exactly at the contact point, known as equilibrium.
The horizontal component of f la is canceled by the adhesive force, f A. The more telling balance of forces, though, is in the vertical direction.
Since the forces are in direct proportion to their respective surface tensions, we also have: . This same relationship exists in the diagram on the right. Water with specially prepared Teflon approaches this.
Because surface tension manifests itself in various effects, it offers a number of paths to its measurement. 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. An instrument that measures surface tension is called tensiometer.
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 cross-section of the tube. But the dome-shaped top gives slightly less surface area to the entire mass of mercury. Again the two effects combine to minimize the total potential energy.
Such a surface shape is known as a convex meniscus. We consider the surface area of the entire mass of mercury, including the part of the surface that is in contact with the glass, because mercury does not adhere to glass at all. 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 was made out of copper, the situation would be very different.
Mercury aggressively adheres to copper. So in a copper tube, the level of mercury at the center of the tube will be lower than at the edges that is, it would be a concave meniscus. In a situation where the liquid adheres to the walls of its container, we consider the part of the fluid's surface area that is in contact with the container to have negative surface tension. The fluid then works to maximize the contact surface area.
So in this case increasing the area in contact with the container decreases rather than increases the potential energy. That decrease is enough to compensate for the increased potential energy associated with lifting the fluid near the walls of the container.
If a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong, surface tension can draw liquid up the tube in a phenomenon known as capillary action. The height to which the column is lifted is given by Jurin's law : . Pouring mercury onto a horizontal flat sheet of glass results in a puddle that has a perceptible thickness. The puddle will spread out only to the point where it is a little under half a centimetre thick, and no thinner.
Again this is due to the action of mercury's strong surface tension. The liquid mass flattens out because that brings as much of the mercury to as low a level as possible, but the surface tension, at the same time, is acting to reduce the total surface area.
In much of the available literature, there is confusion regarding the correct use of the terms surface tension, surface energy and surface free energy. As a result, these three terms have been used interchangeably to describe the same quantity. This problem is particularly serious in the area of solid surface science. Linford has examined and discussed such inconsistencies but failed to differentiate the three quantities clearly. In the present paper, the definitions and the relationships between surface tension, surface energy and surface free energy are examined and their proper usage clarified.
Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension allows insects e. At liquid—air interfaces, surface tension results from the greater attraction of liquid molecules to each other due to cohesion than to the molecules in the air due to adhesion. There are two primary mechanisms in play.
Surface Energy. Surfaces have energy associated with them because work is needed to form them. Surface energy is the work per unit area done by the force that creates the new surface. Material Surface Energy J.
These metrics are regularly updated to reflect usage leading up to the last few days.Kevin A. 07.12.2020 at 15:06
It would seem that interfacial tension can only be employed in such cases as the immediate representative of interfacial energy, as conceived by Gauss.' That a.Spassupholi 09.12.2020 at 00:29
Thus surface tension in the sense of a negative tangential surface stress ofrelatively large magnitude is a property of systems of very many molecules (bulk.Ereracre 14.12.2020 at 12:29
All liquids — water, organic solvents, oils, and so on — have strong intermolecular cohesive forces.Natzari M. 15.12.2020 at 03:42
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