Fluid friction arises when two fluid layers move in opposite directions. Viscosity is the term for the internal resistance to flow. The viscosity of a fluid is commonly referred to as its "thickness." All genuine fluids are viscous because they resist shearing in some way. The concept is useful in case of an ideal fluid that gives no resistance to shearing and is thus not viscous.
If there exists a moist area between two thin glass plates, the plates stick together and despite the force of gravity acting on the bottom plate, it does not allow the bottom plate to fall to the ground. This occurs due to the property of surface tension and fluid friction. Fluid friction here acts as an adhesive as the molecular structure grips onto the surface of the glass and binds it relatively close to the other surface, with the absence of air, it forms a vacuum between the layers adding to the adhesive property.
The laws of fluid friction are as follows:
First Law: The fluid friction increases with an increase in the area of contact between the surface and the fluid.
Second Law: The fluid friction increases with an increase in the velocity gradient of the substance.
Third Law: The fluid having a higher coefficient of fluid friction or eta (η) has a higher value of fluid frictional force.
Have you ever wondered why squeezing a tube of toothpaste is so much easier than squeezing honey into your tea? Have you ever tried to put your hand out the window while driving and felt the wind blowback on your hand? Both have to do with fluid friction. It is due to the arrangement of the free-flowing atoms and molecules present in the fluid. Viscosity is a property in which fluid is represented as layers and the layers slip against each other at a certain magnitude, honey has a larger viscosity and thus is comparatively difficult with respect to squeezing a toothpaste. Similarly, the air forms a pressurized pocket against your hand when in motion and this causes the hand to blow back as the force exerted by the air pocket is higher than the mass of the hand.
Fluid friction is the force that opposes motion within the fluid or between the fluid and another medium flowing through it. Internal friction occurs as a result of interactions between fluid molecules, and exterior friction occurs as a result of how a fluid interacts with other things.
Take a look at internal fluid friction first. Fluid seems to be a continuous medium to the naked eye. Fluids, on the other hand, are made up of molecules separated by a significant amount of free space when examined under a high-powered microscope. Squeezing honey through a small hole, for example, requires the molecules to move relative to one another by squeezing past or displacing one another. Internal friction is the cause of a fluid's inability to flow. The more internal friction there is, the more difficult it is to move the molecules and push the fluid to deform. Internal strife, on the other hand, isn't always a bad thing.
For example, without internal friction, you wouldn't be able to drink through a straw because the fluid wouldn't be cohesive enough to allow for such movement.
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