We all know that temperature change is the main effect on viscosity since it is easy to observe results of changing of viscosity regarding to temperature change. Imagine it is a happy weekend day breakfast and you are one of them who keep cream chocolate out of fridge, yes Nutella (Some of keep it in the fridge). Having forgotten Nutella in the fridge the day before, the resistance of chocolate against flow would increases and you should have to wait until it gets warm. Surveillance is great! Temperature is one of main actors as to change viscosity. What about the others ? How do viscosity occurs? Every now and then, you would consider the other parameters if you have not a exact knowledge about them. We ought to examine the factors whereas viscosity changes regarding to changes these ones. Apart from temperature, some of properties are partially effective on viscosity. These are Molecular Structure, Shear Rate, Ambient Conditions and Flow Conditions. Also, those factors could effect each other as to cause various physical phenomenon. Molecular structure: Assume that molecular bonds of ingredients in honey structure probably might be higher than the ones. The tighter the molecules are linked, the more the substance will resist deformation. Shear rate: Newtonian liquids are independent of the external force. The vital question is whether a change of shear rate does or does not change a fluid's viscosity. This question draws the line between Newtonian and non-Newtonian fluids. If a fluid's internal flow resistance is independent of the external force – i.e. the shear rate - acting upon the fluid, it is ideally viscous. Such fluids are named Newtonian liquids after Sir Isaac Newton, who discovered the mathematical relation between viscosity and the external force acting upon a fluid. For non-Newtonian Liquids, If a substance is not ideally viscous, its viscosity changes with the shear rate. Their viscosity decreases when the shear rate increases. For other substances the viscosity increases with increasing shear rate (i.e. yogurt, body gel, starch) The ambient conditions: Apart from temperature, the pressure could be effective on viscosity within particular conditions. In most cases, a fluid's viscosity increases with increasing pressure. Compared to the temperature influence, liquids are influenced very little by the applied pressure. The reason is that liquids (other than gases) are almost non-compressible at low or medium pressures. For most liquids, a considerable change in pressure from 0.1 to 30 MPa causes about the same change in viscosity as a temperature change of about 1 K (1°C). Consequently, the molecules can move less freely and the internal friction forces increase. The result is an increased flow resistance that might be ignored. The Flow Behavior of Water under Pressure: The anomaly that water has its maximum density at +4°C is widely known. Such an anomaly can also be observed for the flow behavior of water under pressure. For temperatures >+32°C, water behaves like other liquids. Its viscosity increases with increasing pressure. Below +32°C and under pressures of up to 20 MPa, the water's viscosity decreases with increase. Flow Conditions: For testing a fluid's viscosity, defined flow conditions are essential. The fluid has to develop laminar flow. With laminar flow, the substance moves in imaginary thin layers in which molecules do not change from one layer to another. The flow has an orderly structure. In turbulent flow, on the other hand, no recognizable structure or layers can be observed. Molecules move freely. The fluid forms vortices. If testing a fluid under turbulent flow conditions, the results will give a falsely higher viscosity. (The turbulent movement of the molecules will be misinterpreted - so to speak - as higher flow resistance by a measuring instrument). Here is a short video related to viscosity and how to measure it. Enjoy your watch:
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