yogurt), while for others the viscosity increases with increasing shear rate (curve 3 | e.g. The flow behavior of non-Newtonian liquids can be far more complex than these basic examples. To learn more consult “ The Rheology Handbook”. Still, shear rate is not the only influencer. Three factors determine a substance’s flow behavior. Normally, an increase in pressure causes a fluid’s viscosity to increase, too.
However, fluids are not dramatically affected if the applied pressure is low or medium: liquids are almost non-compressible in this pressure range. Most liquids react to a significantly altered pressure (from 0.1 MPa to 30 MPa) with a viscosity change of about 10 %. For comparison: This same change in viscosity is usually provoked by a minor temperature change of 1 K (1 ☌). In case the pressure goes up from 0.1 MPa to 200 MPa, the viscosity can rise to 3 to 7 times the original value.
This applies to most low-molecular liquids. Highly viscous mineral oils react with a viscosity increase of times 20000 under identical circumstances.įor synthetic oil this pressure change can even result in a viscosity increase by a factor of up to 8 million. For example, lubricants in cogwheels or gears can be submitted to pressures of 1 GPa and higher.Įquation 4.
VISCOSITY DEFINITION CHEMISTRY FREE
In most liquids, pressure reduces the free volume in the internal structure, and thus limits the movability of molecules. Consequently, internal friction and viscosity increase.Do you remember the first time you used a honey bottle, perhaps for green tea, or something else? You must have noticed when the honey bottle is nearly empty, and it takes very long for honey to reach the mouth of the bottle. Such behavior of a liquid in the flow is described by an intrinsic property called viscosity. It is defined as the property of a liquid by virtue of which an opposite force (internal friction) comes into play between different layers whenever there is a relative motion between these layers of the liquid. In other words, viscosity corresponds to the “thickness” of a liquid. For instance, honey is thicker than water because it is more viscous. In technical terms, viscosity is a measure of the resistance to the flow that a liquid offers when it is subjected to shear stress. Mathematically, for a small velocity gradient, we can write: It is quantitatively expressed in terms of the coefficient of viscosity, η, which is defined as the tangential for a unit velocity gradient (the difference in velocity between adjacent layers of the fluid) that exists in the direction perpendicular to the direction of the motion. Here, F denotes the viscous force acting on area A and du/dy is the velocity gradient along the positive y-direction. The negative sign signifies that the viscous force is directed against the velocity gradient. Therefore, more viscous liquids have a lower rate of flow.