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explain with eg surface tension &viscocity

  • Surface tension:

    Surface tension is a property of the surface of a liquid that allows it to resist an external force.It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders ) to run on the water surface. This property is caused by cohesion of like molecules, and is responsible for many of the behaviors of liquids. Water has adhesion/ cohesion properties (attributed to Van der Waal forces) and these give water its tendency to "clump" together and stick to surfaces. To visualize this, imagine a drop of water rolling down glass. It remains a drop instead of spreading out and it will stick to the glass instead of falling if turning upside down. Similarly, when an object is placed onto water, it behaves to a certain degree as if it were a sheet of elastic material. This is called surface tension and can be readily observed by placing a piece of grass on water. As water tends to stick to itself, it can be thought of as a sheet - when the density is too great the sheet will "snap" (or sink) but otherwise it can support it (float).

 

  • Viscosity :

 

Viscosity is a measure of the resistance to flow that a fluid offers when it is subjected to shear stress. It is commonly perceived as "thickness", or resistance to pouring. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity.

In simple words, it is the resistance of a fluid to flow. For example, water has a low viscosity, when you pour it, it quickly drops into the cup; syrup has a high viscosity; it doesn't pour out nearly as easily as water.

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Surface tension is measured as the energy required to increase the surface area of a liquid by a unit of area. The surface tension of a liquid results from an imbalance of intermolecular attractive forces, thecohesive forces between molecules:

  • A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.
  • A molecule at the surface of a liquid experiences only net inward cohesive forces.

A microscopic view of water illustrates the difference between molecules at the surface of a liquid and water molecules within a liquid.

 

The molecules at the surface of this sample of liquid water are not surrounded by other water molecules. The molecules inside the sample are surrounded by other molecules.

The unbalanced attraction of molecules at the surface of a liquid tends to pull the molecules back into the bulk liquid leaving the minimum number of molecules on the surface. It required energy to increase the surface area of a liquid because a larger surface area contains more molecules in the unbalanced situation.

 

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honeyis "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).[1]

Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. For example, high-viscosity felsic magma will create a tall, steep stratovolcano, because it cannot flow far before it cools, while low-viscosity mafic lava will create a wide, shallow-sloped shield volcano.

With the exception of superfluids, all real fluids have some resistance to stress and therefore are viscous. A fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid. In common usage, a liquid with the viscosity less than water is known as a mobile liquid, while a substance with a viscosity substantially greater than water is simply called a viscousliquid.

 

Viscosity is important in volcanology. The more fluid a magma, the more likely it is to erupt. On the other hand, when more viscous (higher viscosity) lavas do erupt, they usually do so explosively. Viscosity also affects the shapes of lava flows and the mountains they erupt from. The more viscous the magma, the fatter the lava flow. Also, the more viscous the magmas a volcano erupts, the steeper the volcano. Thus, shield volcanoes like we have in Hawai'i have gentle slopes (less than 10 degrees), while stratovolcanoes like the Cascades in the northwestern mainland are much steeper (roughly 25 degrees). As expected, hawaiian volcanoes erupt more fluid lavas (called basalt) than do the Cascade volcanoes, which erupt a lava called andesite.

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