Viscosity

Definition

Viscosity is a fluid property that describes a fluids internal resistance to flow or deformation, and is one of the most important fluid properties to consider when designing a mixer. There are two common measures of viscosity, dynamic viscosity (µ) and kinematic viscosity (ν). The relationship between dynamic viscosity and kinematic viscosity is shown below:

Latex formula

ν – Kinematic viscosity (cSt)
µ – Dynamic viscosity (cP)
ρ – Density (g/cm3)

Dynamic viscosity is the most relevant to mixing applications as this describes the fluid resistance to flow under shear stress, whereas kinematic viscosity describes the resistance to flow under the pull of gravity.

Newtonian Fluids

In Newtonian fluids the dynamic viscosity is described as the ratio of the shear stress to the velocity gradient in a fluid. The viscosity remains constant through varying shear stress and over time, and the relationship is shown in the equation below:

Latex formula

τ – Shear stress
µ – Dynamic viscosity
du/dy – Velocity gradient

The majority of fluids can be modelled as Newtonian fluids and this is the type of fluid that we come across in the majority of mixing applications.

Non-Newtonian Fluids

Other types of fluid can exhibit non-Newtonian behaviour where the viscosity is not a constant value, and changes in response to some other variable.
Some of these are listed below:

  • Pseudo-plastic or shear-thinning fluids are the most common type of non-Newtonian fluid that we encounter.
    In these fluids the viscosity decreases with an increase in the shear stress applied.
  • Thixotropic fluids behave like pseudo-plastic fluids, except that the viscosity also decreases with time under shear stress.
  • Dilatant and Preoptic fluids are the equivalent of pseudo-plastic and thixotropic fluids respectively,
    except that their viscosity increases with shear rate and time respectively.
  • Viscoelastic fluids exhibit a combination of viscous and elastic properties when stressed.
    This means that they behave partially like an elastic solid and have a tendency to revert back to their original shape after deformation.
  • Bingham plastics have a minimum shear stress required before any flow of the material will take place.

Some common non-Newtonian fluids are water based paints, polymer solutions, and many food and personal care products such as toothpaste and tomato ketchup.

Measuring Viscosity

Viscosity is measured with an instrument known as a viscometer, or rheometer, and there are multiple different types, the most common of which are explained below:

FALLING SPHERE VISCOMETER

Falling Sphere Viscometer

The falling sphere type viscometers consist of a small sphere and a stationary glass tube filled with the fluid to be tested. The sphere is allowed to fall through the fluid and the time taken to pass between two points is measured. Using Stokes’ law the dynamic viscosity can then be calculated using the density of the fluid and sphere.

Viscosity Cups

Viscosity cups are a type of viscosity measuring device consisting of a cup with a precisely sized aperture in the base. The cup is filled with the liquid to be measured and the time taken for the liquid to drain from the cup through the aperture is taken. From this time value, the kinematic viscosity can be calculated using conversion formulas or tables provided by the manufacturer.

Viscosity cups are commonly used in the paints and coatings industries.

ROTATIONAL VISCOMETER

Rotational Viscometer

A more sophisticated type of rheometer is the rotational spindle type viscometer. These instruments work based on the Searle principle, where the torque required to rotate a spindle in each fluid at a certain speed is proportional to the fluid viscosity.

The speed of the rotating spindle can usually be varied which has the effect of varying the shear stress experienced by the fluid. This allows rotational viscometers to be used to measure the viscosity profile of non-Newtonian fluids which exhibit different viscosities at different levels of shear stress.

VISCOSITY CUPS
CategoryLiquidSpecific gravity @ 16°CAbsolute viscosity (cP)Temperature (°C)Viscosity Type
Industrial ProductsAcetate Glue1200 - 140020T
Industrial ProductsAlcohol, sopropyl2  Room Temp
Industrial ProductsAlkyd resins500 - 3,000   Room Temp
Industrial ProductsAnti - freeze15 Room Temp
Food ProductsBaby Food140093T
Food ProductsBatter2950030T
Food ProductsBeer18   Room Temp
Food ProductsBeet Sauce195076T
Industrial ProductsBenzine5   Room Temp
Food ProductsBiscuit Cream Premix2920018T
PharmaceuticalsBlood10 Room Temp
Fish & Animal OilsBone Oil0,924854N
Food ProductsBrewers Yeast36818T
Food ProductsBroth Mix43018T
Dairy ProductsButter Deodorised4550N
Dairy ProductsButter Fat2065N
Dairy ProductsButter30,000   Room Temp
Dairy ProductsButter cream, sour550   Room Temp
Food ProductsCarob Bean Sauce150030T
Vegetable OilsCastor Oil0,9658027N
Industrial ProductsCaulking compound5,000,000 - 10,000,000 Room Temp
Industrial ProductsCaustic soda 50%45   Room Temp
Dairy ProductsCheese, soft30,000 Room Temp
Vegetable OilsChinawood Oil0,9430021N
Food ProductsChocolate28049T
Food ProductsChocolate syrup2,250   Room Temp
Food ProductsCitrus Fruit Pulp1,2760020T
Industrial ProductsCleaning emulsions1,500   Room Temp
Dairy ProductsCocoa Butter0,925060N
Vegetable OilsCoconut Oil0,935524N
Fish & Animal OilsCod Oil0,933238N
Food ProductsCoffee Liquor 30-4010 - 10020T
Dairy ProductsCondensed Milk40 - 8040-50N
Dairy ProductsCondensed Milk 75 Solids1,3216020T
Vegetable OilsCorn Oil0,922857N
Food ProductsCorn syrup15,000   Room Temp
Dairy ProductsCottage Cheese3000018T
Vegetable OilsCotton Seed Oil0,886224N
Dairy ProductsCream 30 Fat11416N
Dairy ProductsCream 45 Fat0,994816N
Dairy ProductsCream 50 Fat0,9811216N
Dairy ProductsCream 50 Fat5532N
Dairy ProductsCream, cheese100,000   Room Temp
Dairy ProductsCream, clotted20,000 Room Temp
Dairy ProductsCream, double120 Room Temp
Dairy ProductsCream, single50   Room Temp
Dairy ProductsCream, sour15,200 Room Temp
Industrial ProductsCresol Crystals1018T
Food ProductsCustard1,6150085-90T
PharmaceuticalsDetergents147070T
Industrial ProductsDiesel14 Room Temp
Glycol ProductsDiethylene1,123221N
Industrial ProductsDipropyleneglycol107   Room Temp
Food ProductsEdible Oil0,96520N
Industrial ProductsEpoxy resin5,000 - 10,000   Room Temp
Glycol ProductsEthylene1,121821N
Glycol ProductsEthylene glycol16   Room Temp
Food ProductsFruit Juice1,0455-7518N
Fuel & Lubricating OilsFuel oil no 412 Room Temp
Food ProductsGelatine 37 Solids119043T
Food ProductsGelatine1,200   Room Temp
Food ProductsGlucose1,34300 - 860025-30T
Industrial ProductsGlue, gun grade5,000 - 50,000   Room Temp
Industrial ProductsGlue, pourable3,000 - 5000 Room Temp
Industrial ProductsGlycerine 1001.26 at 20°C64820N
PharmaceuticalsGlycerine1,000 - 2,000 Room Temp
Glycol ProductsGlycol20   Room Temp
Food ProductsGravy Slurry111080T
PharmaceuticalsHand cream8,000   Room Temp
PharmaceuticalsHand lotion5,000   Room Temp
Food ProductsHoney1,800 - 3,000 Room Temp
Fuel & Lubricating OilsHydraulic oil30 - 120 Room Temp
Food ProductsIcing10,000   Room Temp
Industrial ProductsInk, printing, liquid550 - 2,200   Room Temp
Industrial ProductsIsopropyl Alcohol1,111,985N
Food ProductsJam Garnish844016T
Food ProductsJam8,500   Room Temp
Industrial ProductsKerosene10 Room Temp
Industrial ProductsLacquer 25 Solids300018T
Paints & VarnishesLacquer, water - based900 Room Temp
Paints & VarnishesLacquers 25% pigments3,000 Room Temp
Fish & Animal OilsLard0,966238N
Fish & Animal OilsLard Oil0.91-0.9340 - 4738N
PharmaceuticalsLatex Emulsion120024T
Paints & VarnishesLatex neoprene5,000   Room TempT
Paints & VarnishesLatex paint750   Room TempT
Vegetable OilsLinseed Oil Raw0.93-0.942938N
Vegetable OilsLinseed oil30- 70 Room Temp
PharmaceuticalsLiquid soap85   Room Temp
Fuel & Lubricating OilsLubricating oil60- 200   Room Temp
Fuel & Lubricating OilsMachine oil, heavy600   Room Temp
Fuel & Lubricating OilsMachine oil, light150   Room Temp
Food ProductsMalt Extract1,4300060T
Food ProductsMalt Extract 80950018T
Food ProductsMaple syrup150 - 300 Room Temp
Food ProductsMayonnaise5,000 - 20,000 Room Temp
Industrial ProductsMethyl ethyl ketone4   Room Temp
Dairy ProductsMilk1.02-1.05218N
Dairy ProductsMilk Whey 48 Sugar800 - 150040T
Dairy ProductsMilk condensed6,000   Room Temp
Dairy ProductsMilk evaporated6,100   Room Temp
Food ProductsMincemeat10000030T
Food ProductsMolasses5,000 - 10,000 Room Temp
Fuel & Lubricating OilsMotor oil SAE 1050 Room Temp
Fuel & Lubricating OilsMotor oil SAE 20125   Room Temp
Fuel & Lubricating OilsMotor oil SAE 30200 Room Temp
Fuel & Lubricating OilsMotor oil SAE 40320 Room Temp
Fuel & Lubricating OilsMotor oil SAE 50540 Room Temp
Fuel & Lubricating OilsMotor oil SAE 601,000 Room Temp
Fuel & Lubricating OilsMotor oil SAE 701,600 Room Temp
Food ProductsMousse Mix12005T
Industrial ProductsNaOH 201,22118N
Industrial ProductsNaOH 301,33118N
Industrial ProductsNaOH 401,432018N
Vegetable OilsOleic acid40   Room Temp
Vegetable OilsOlive Oil0,914038N
Food ProductsOrange Juice Concentrate30 Brix63020N
Paints & VarnishesPaint, water based2,400   Room Temp
Vegetable OilsPalm Oil0,924338N
Food ProductsPancake batter2,200   Room Temp
PharmaceuticalsParaffin Emulsion1,2300018T
Industrial ProductsParaffin10 Room Temp
Vegetable OilsPeanut Oil0,923838N
Food ProductsPeanut butter150,000 - 250,000 Room Temp
Vegetable OilsPeanut oil100   Room Temp
Food ProductsPectin30038N
Industrial ProductsPetrol, gasoline8   Room Temp
PharmaceuticalsPetroleum jelly64,000   Room Temp
Industrial ProductsPlastisol2,52800018T
Industrial ProductsPolyester1.1 at 30300030T
Industrial ProductsPolyester resin3,000   Room Temp
Industrial ProductsPolyglycerine caprinate6,000 - 7,000   Room Temp
Industrial ProductsPolyisobutylene1.09 at 85°1250085T
Industrial ProductsPolymer solution20,000   Room TempT
Industrial ProductsPolyol, non - pigmented500 - 5,000   Room Temp
Industrial ProductsPolypropylene24000050T
Industrial ProductsPotassium  hydroxide67   Room Temp
Industrial ProductsPrinters Ink550 -220038T
Dairy ProductsProcess Cheese3000018T
Glycol ProductsPropylene1,045221N
Industrial ProductsPutty100,000,000 Room Temp
Industrial ProductsPVA resin65,000   Room Temp
Industrial ProductsResin Solution88024T
Industrial ProductsResin pourable880 - 975 Room Temp
Industrial ProductsResin solution7,100   Room Temp
Food ProductsRice Pudding10000100T
Food ProductsSalad Cream1300 -260018T
Food ProductsSauce - Apple1,150080T
Vegetable OilsSesame oil94   Room Temp
PharmaceuticalsShampoo3,000 - 5,000 Room Temp
PharmaceuticalsSoap Arylan1.0 at 40°C63060T
PharmaceuticalsSoap Solution1.03 at 60°C8260T
Food ProductsSorbitol1,2920020N
Vegetable OilsSoya Bean Oil0,936024N
Food ProductsSoya Bean Slurry5000 -10,00050-90T
Food ProductsSpaghetti sauce1,000   Room Temp
Fish & Animal OilsSperm Oil0,882438N
Food ProductsStarch solution 25%300   Room Temp
Food ProductsSugar syrup, sucrose1,210   Room Temp
Industrial ProductsSulphonic Acid1,0412530T
Industrial ProductsTitanium dioxide5,000   Room Temp
Industrial ProductsToluol6   Room Temp
Food ProductsTomato Ketchup100030T
Food ProductsTomato Paste 3019518T
Food ProductsTomato juice180   Room Temp
Food ProductsTomato ketchup1,000   Room Temp
Food ProductsTomato puree50,000   Room Temp
Food ProductsTomato sauce2,600   Room Temp
PharmaceuticalsToothpaste70,000 - 100,00018T
Industrial ProductsTriacetate Dope48,000 / 60,00040T
Glycol ProductsTriethylene1,124021N
Vegetable OilsTurpentine0,86216N
Paints & VarnishesVarnish320   Room Temp
Vegetable OilsVegetable oil40   Room Temp
Food ProductsVinegar12 - 1520N
PharmaceuticalsWax0,950093T
Fish & Animal OilsWhale Oil0,9325 - 3938N
Dairy ProductsWhey800 - 1,500 Room Temp
Dairy ProductsWhole Egg1504,5T
Food ProductsYeast Surry2018T
Dairy ProductsYoghurt3,000 - 8,000 Room Temp
DO NOT DELETE THIS SECTION


Kinematic
Viscosity
(cSt)


Cup Type

12347.5101520253040506070809010012515017520022525027530032535037540050060070080090010001250
‘Ultra Low’ Cup536884
Shell Cup #11721.526354657
Shell Cup #218.32230.4394756
Shell Cup #2.519253035.646.657
Shell Cup #318.62228.635424855
Shell Cup #3.517.52024.629.23438.44347.859.4
Shell Cup #4182124273037455260
Shell Cup #516202428323640444852566064
Shell Cup #617.51920.522.5242531.537.5445056.56378
Zahn Cup #130.53235384245526068
Zahn Cup #218192022.5252830343843536372
Zahn Cup #31719.52224.527303235.53841434548.55870
Zahn Cup #4171819.52122.52426283032384551546570
Zahn Cup #51718.5202122.527313540454960
Ford Cup #3323740435363738393103113120130
Ford Cup #42022252835404855627076829098103
ISO Cup #4263339465360
DIN Cup #420222426283035404650566171
AFNOR Cup #420222426283035404650566171
Norcross Cup #1313640537087
Norcross Cup #21923283445556776
Norcross Cup #31720253035404550556879
Norcross Cup #41822273236404448535761656885
Norcross Cup #5202226313641455062


Kinematic
Viscosity
(cSt)


Cup Type

12347.5101520253040506070809010012515017520022525027530032535037540050060070080090010001250
‘Ultra Low’ Cup536884
Shell Cup #11721.526354657
Shell Cup #218.32230.4394756
Shell Cup #2.519253035.646.657
Shell Cup #318.62228.635424855
Shell Cup #3.517.52024.629.23438.44347.859.4
Shell Cup #4182124273037455260
Shell Cup #516202428323640444852566064
Shell Cup #617.51920.522.5242531.537.5445056.56378
Zahn Cup #130.53235384245526068
Zahn Cup #218192022.5252830343843536372
Zahn Cup #31719.52224.527303235.53841434548.55870
Zahn Cup #4171819.52122.52426283032384551546570
Zahn Cup #51718.5202122.527313540454960
Ford Cup #3323740435363738393103113120130
Ford Cup #42022252835404855627076829098103
ISO Cup #4263339465360
DIN Cup #420222426283035404650566171
AFNOR Cup #420222426283035404650566171
Norcross Cup #1313640537087
Norcross Cup #21923283445556776
Norcross Cup #31720253035404550556879
Norcross Cup #41822273236404448535761656885
Norcross Cup #5202226313641455062

Effect on Mixing

A fluids viscosity is one of the most important considerations when designing a mixer for a given application.

The greater a fluids viscosity, the more energy input is required to mix the fluid, and this means larger impellers, and more powerful motors to turn them. For highly viscous materials it may be necessary to use a close clearance type impeller, such as a gate or spiral type impeller, to achieve adequate mixing across the full tank volume.

Laminar & Turbulent Mixing

Mixing at low viscosity almost always takes place under turbulent conditions.
This means that mixing is controlled by inertial forces rather than by molecular diffusion, and mixing is achieved by the many eddies and vortices created by the mixer impeller.

Impellers such as the hydrofoil and pitched blade turbine type impellers are used for low viscosity mixing as they provide large pumping capacities and create large amounts of turbulence in the mixing vessel.

On the other hand, mixing at high viscosities takes place mainly in the laminar flow regime where the viscous forces dominate, damping down any turbulence, and molecular diffusion is the main source of mixing. To aid the molecular diffusion the aim of mixing in the laminar regime is to “cut and fold” the fluid so that there is greater interfacial area between the different fluids for the diffusion to take place.

Here, gate or screw type impellers are used as the large surface area
presented to the fluid helps to increase the “cutting and folding” action
across the entire tank volume.

The below plot shows the suitable viscosity ranges for various types of standard impellers.

Plot of Impeller Types vs Their Suitable Operating Viscosities
PLOT OF IMPELLER TYPES VS THEIR SUITABLE OPERATING VISCOSITIES

Non-Newtonian Mixing

Achieving adequate mixing in non-Newtonian fluids can be challenging. If you have a non-Newtonian mixing application and you require assistance in selecting a suitable agitator, please get in touch and one of our application engineers will be happy to help.

Static Mixers

STATIC MIXER ELEMENT

The effect of viscosity on mixing in an inline static mixer is like the effects on stirred tank mixers. For lower viscosity applications the mixing is dominated by turbulent eddies, whereas higher viscosity applications are still dominated by molecular diffusion and rely on the same “cut and fold” method of mixing as stirred tank mixers described above.

STATIC MIXER ELEMENT

Different types of mixing element are used for low and high viscosity fluids and generally follow the same principles as stirred tank impellers. The higher the viscosity, the larger and more complex the mixer elements placed in the pipe to ensure mixing across the entire pipe diameter, and this increases the pressure drop across the mixer. This increases the power required from the pump so, as in stirred tank mixers, more power is required for mixing high viscosity fluids.

John Whittle MEng (Hons)

John Whittle MEng (Hons)

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