When examining fluids you most commonly look at three major aspects -- surface tension, viscosity, and density. It is easy to understand fluids by examining water, one of the most abundant resources around us.
Surface tension in liquids is defined to be the tension at which the liquid is attracted to particles on the surface layer. Surface tension usually results in a minimization of surface area. This can be seen in many everyday situations. One of the most common ways this is obvious is if you were to fill a glass of water to the very top. If you keep adding water, drop by drop, you will notice that it will not immediately overflow. But rather it will keep getting taller and taller, almost creating a bubble. The bond between the atoms in the water are so strong that it causes them to hold onto themselves and the glass (up to a certain point).
The viscosity of a fluid is the measurement of its resistance to change due to the internal friction of the fluid. Viscosity can often be changed depending on the fluid. "For liquids, viscosity also depends strongly on the temperature; for gases, viscosity is approximately proportional to the square root of the absolute temperature (Wilkes)." A prime example of this would be motor oil. Motor oil is sold based on its weight -- or viscosity. Motor oils have viscosity measurements that it is rated for. One is for when the oil is cold and one is when the oil is warmed up or hot. The oil starts thicker when it is cold, but once the car is running and the engine is producing heat, it heats up the oil which in turn causes the oil to get thinner and decreases its viscosity.
Density is how compact a substance is. "Density depends on the mass of an individual molecule and the number of such molecules that occupy a unit of volume. For liquids, density depends primarily on the particular liquid and, to a much smaller extent, on its temperature (Wilkes)". Whereas for gas, density is primarily dependent on the pressure of the system the gas is in. Liquids have different densities based upon what they are made up of.
Image 2: Water Surface Tension (Credit: J. Schmidt, National Park Service )
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