It may be possible to compensate for this by increasing the fluid pressure as increased pressure will reduce the droplet size, but spraying at higher pressures also increases the flow rates. This would mean the spray would have 2.6 times lower surface area to volume ratio making it less likely to adhere to a surface and form an even layer. For example, double cream (Cps 120) sprayed would form droplets about 2.6 times the size of that of water sprayed from the same nozzle. This effect is relatively small for fluids with viscosities below 10cps but becomes more pronounced with higher cps. As such, finer sprays are better able to spread out on their target surface. This is because sprays consisting of smaller droplets have a much larger surface area per volume than those made up of larger droplets. It may also affect how well the coating adheres to the target. For very viscous fluids this will have a detrimental effect on coating applications as small droplet sizes help to ensure an even coating. water has a Cp of about 1 at room temperature.Īs the viscosity of a fluid increases, it will tend to form larger droplets when sprayed. This is most commonly redefined as the unit Centipoise (cps) with 1 cps being equivalent to 1 millipascal second, i.e. Viscosity is measured in pascal seconds with water at 20 degrees C having a viscosity of 1.002 mPa.s. It is an approximate measure of the 'thickness' of a liquid. Viscosity is a measure of a fluid's resistance to shear stress. In the food industry there are many viscous fluids that need to be sprayed so clearly this a key concern in this sector. Consequently, internal friction and viscosity increase.The spraying of viscous fluids can represent some challenges. In most liquids, pressure reduces the free volume in the internal structure, and thus limits the movability of molecules. For example, lubricants in cogwheels or gears can be submitted to pressures of 1 GPa and higher.Įquation 4. 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. This applies to most low-molecular liquids. 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. For comparison: This same change in viscosity is usually provoked by a minor temperature change of 1 K (1 ☌). Most liquids react to a significantly altered pressure (from 0.1 MPa to 30 MPa) with a viscosity change of about 10 %. However, fluids are not dramatically affected if the applied pressure is low or medium: liquids are almost non-compressible in this pressure range. Normally, an increase in pressure causes a fluid’s viscosity to increase, too. Three factors determine a substance’s flow behavior. Still, shear rate is not the only influencer. To learn more consult “ The Rheology Handbook”. The flow behavior of non-Newtonian liquids can be far more complex than these basic examples. yogurt), while for others the viscosity increases with increasing shear rate (curve 3 | e.g. If its viscosity changes with the shear rate, a liquid is non-Newtonian and – for exact definition – one has to specify the apparent viscosity.ĭifferent shear-dependent fluids behave differently: For some, their viscosity decreases when the shear rate increases (curve 2 | e.g. Typical Newtonian liquids are water or salad oil. For a Newtonian liquid, this function is a straight line (curve 1) see figure 6. Such fluids are named ‘Newtonian liquids’ after Sir Isaac Newton.Ī viscosity function shows the viscosity over the shear rate. the shear rate – acting upon the fluid, it is ideally viscous. If a fluid’s internal flow resistance is independent of the external force – i.e. High-viscosity fluids resist deformation. Fluids which flow easily show a low resistance to deformation. viscous fluids that contain an elastic portion. The specific field of viscometry covers ideally viscous fluids, and – considering certain restrictions – also viscoelastic liquids, i.e. sweet jelly) or as a viscoelastic liquid (like e.g. According to a material’s properties, we either classify it as a viscoelastic solid (like e.g. That is, substances which are neither completely elastic, nor entirely viscous. In everyday life, we mostly come across viscoelastic materials. Scientists specify solid materials as being elastic and liquids as being viscous. Imagine all materials as classified on a virtual scale from solid to liquid. Rheology deals with the flow behavior and deformation of materials. In order to determine a fluid’s viscosity, you have to enter the field of viscometry, a subject area of a wider science called rheology.
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