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Viscoquestions

Is it possible to suspend air bubbles in a tank of liquid, so that they don't rise to the surface?

In order for the air not to rise in a liquid, that liquid would have to have the same density as air, which isn`t possible.

However, you can slow down the rise of air bubbles using very viscous liquid, honey for example. In general, making a liquid cooler also increases its viscosity, e.g. glycerin or gelatin that is liquid when hot and gel-liked when cool. There are also thixotropic liquids, ketchup is an example  that is liquid when moved, but pretty much solid when static. -

What is dynamic viscosity? How is absolute viscosity different from kinematic viscosity?

Dynamic (absolute) Viscosity:

Absolute viscosity or the coefficient of absolute viscosity is a measure of the internal resistance. Dynamic (absolute) viscosity is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid.

The shearing stress between the layers of non turbulent fluid moving in straight parallel lines can be defined for a Newtonian fluid as:

The dynamic or absolute viscosity can be expressed like

τ = μ dc/dy (1)

where

τ = shearing stress

μ = dynamic viscosity

Equation (1) is known as the Newtons Law of Friction.

Kinematic Viscosity:

Kinematic Viscosity is the ratio of absolute or dynamic viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with it`s mass density

ν = μ / ρ (2)

where

ν = kinematic viscosity

μ = absolute or dynamic viscosity

ρ = density

In the SI-system the theoretical unit is m2/s or commonly used Stoke (St)

Is viscosity of liquid related to its density?

Viscosity and density are not related. Liquids with similar densities may have very different viscosities.

Density remains essentially the same regardless of the temperature of a liquid, but viscosity generally changes quite dramatically with temperature.

If you look at different substances/liquids, there is no simple relationship i.e. liquids of very similar density can have widely varying viscosities. Density (p) is an almost static property of a liquid and in most cases the density is similar to that of the solid form of the same substance over the whole liquid range (i.e. from melting to boiling), whereas viscosity (n) is a transport property and can be very sensitive to temperature changes.

However, there are two (very special) exceptions:

- Hildebrand`s relationship 1/ n = B * (V-V0)/V0 for a particular liquid (V is actual volume, V0 is occupied volume, so (V-V0) is free volume, both B and V0 are empirical constants), so density comes indirectly into play as it`s related to volume.
-Polymer solutions have strong dependency between concentration of the macromolecules and viscosity, but again, this applies to one particular solvent only (used in Ostwald or Ubbelohde dilution viscosimeter).

Does the thickness of a liquid affect its absorbency?

Viscosity is a transport property, which only appears when adjacent parts of a liquid are moving at different velocities. While it is true that a more viscous liquid would take longer to get adsorbed *, in the long run other factors, mainly interface tension, which is related to surface tension, determine how much liquid will be adsorbed. Surface tension is an interface phenomenon and is observed on a stationary liquid in contact with another phase (which could be solid, liquid or gaseous). There is no direct relationship between viscosity and surface tension.

Another important factor for the adsorption is the actual size of the molecules of the liquid (or gas).

It requires more pressure to pump very viscous fluids?

Viscosity is the resistance to flow. A more viscous fluid resists flow more so you have to push it harder to get it moving. The pump does this pushing. If the drilling fluid is thick it sticks to the rock cuttings and needs more washing to get it off. It`s like trying to wash a thick, gooey substance off your hands.

How do you increase viscosity of water?

A standard way to increase viscosity is to add long chain molecules (polymers) that are soluble in that liquid.

Organic compounds like Glycerine/Glycol/Glycerol are good viscosity modifiers, environmentally friendly etc.

What temperature (in degree Celsius and Fahrenheit) best used for viscosity experiment?

To convert from Celsius to Fahrenheit: F = C * 9 / 5 + 32
To convert from Fahrenheit to Celsius: C = (F - 32) * 5 / 9

Could you give examples of the coefficient of viscosity of different liquids like glycerine, kerosene, water, etc?

In classical fluid mechanics, coefficient of viscosity is a synonym for dynamic viscosity. It is what`s often just called the viscosity.

Glycerine (pure): 100 cP at 60 degree C
Glycerine (50% solution in water): 2.2 cP at 60 degree C, 6.2 cP at 20 degree C.
Kerosene: 1.1 cP at 60 degree C, 2.4 cP at 20 degree C
Water: 0.5 cP at 60 degree C, 1.0 cP at 20 degree C.
Note: 1cP = 0.001 Pa.s; Pa.s = Pascal-second; cP = centi-Poise

What factors change viscosity, other than temperature?

We can say that viscosity is the resistance a material has to change in form. This property can be thought of as an internal friction.

To get a good feel for viscosity, laminar flow: If a fluid or gas is flowing over a surface, the molecules next to the surface have zero speed. As we get farther away from the surface the speed increases. This difference in speed is a friction in the fluid or gas. It is the friction of molecules being pushed past each other. You can imagine that the strength with which the molecules cling together will be proportional to the friction. This strength is called viscosity. Thus, viscosity determines the amount of friction, which in turn determines the amount of energy absorbed by the flow.

The viscosity of a fluid is basically a measure of how sticky it is. Water has a fairly low viscosity; things like shampoo or syrup have higher viscosities. Viscosity also depends on temperature - engine oil, for instance, is much less viscous at high temperatures than it is in a cold engine in the middle of winter.

For fluids flowing through pipes, the viscosity produces a resistive force. This resistance can basically be thought of as a frictional force acting between parts of the fluid that are traveling at different speeds. The fluid very close to the pipe walls, for instance, travels more slowly than the fluid in the very center of the pipe.

The viscosity of a pure fluid changes most with temperature. Pressure has a small effect (much less than temperature) on the viscosity of a gas and the effect of pressure on a liquid is extremely small.

Other factors can come into play when considering multiphase liquids, a mixture of liquid, solid and gas. Such mixtures are commonly found in crude oil flowing up an oil well part of the oil often turns to gas as pressure reduces and the mixture might also include water, pieces of rock, wax, and tar. In this case, fraction of each phase will affect the viscosity of the mixture.

Foams - mixtures of gas and liquid - and emulsions - mixtures of liquids - usually have a higher apparent viscosity that the either individual phase on its own.

For polymers (large organic molecule formed by combining many smaller molecules e.g. plastics), viscosity is generally higher for polymers with higher molecular weights - bigger molecules leads to higher viscosity.

A dilute solution of a polymer in a solvent, for example water, can exhibit power-law behavior- it may have high viscosity under low shear, but low viscosity under high shear. The water itself is Newtonian, but the introduction of the polymer in concentrations (by weight) as low as 0.2 % can have a large affect on its rheological behavior.

Change of viscosity depending on the speed or the force you use, it may increase (the faster you move the more viscositiy) or decrease (thixotropic media).

Besides temperature, viscosity of the same fluid may vary with sheer stress and pressure.

Viscosity of different liquids and gases

Viscosity is a property of fluids that indicates their resistance to flow, defined as the ratio of shear stress to shear rate.

Liquids generally flow more easily (become less viscous) when they are heated. Hard materials such as rock can be considered as liquids, because they can flow - although extremely slowly.

Some mixed liquids, such as cooking sauces containing flour, cornstarch or tapioca, thicken as they are heated.

The viscosity of Glycerine is very dependent on temperature. At –40°C it is a viscous as some rocks. At 30°C it is about as viscous as heavy machine oil. The table below lists viscosity (in pascal-seconds) for some materials.

Hydrogen gas 15°K..……..0.0000006
Air 0°C...................……….0.000017
Liquid Hydrogen.................0.00001
Water 100°C............………0.0003
Water 20°C..............………0.0010
Water 0°C...............……….0.0018
Whole blood 37°C....………0.0027
10wt motor oil 30°C...........0.25
Glycerin, 30°C.........……….0.63
Heavy Machine Oil 15°C.…0.66
Glycerin, 20°C.........……….1.5
Honey 20°C......………….....1.6
Basalt Lava..........................1000
Rock Salt.............................1014
Marble.............................…..1016
Glycerin -40°C..………..…....6700
Rhyolite Lava.......................1,000,000
Granite, Quartzite.................10^18 – 10^20
Asthenosphere.....................10^19 – 10^20
Deep Mantle........................10^21 – 10^22
Shallow Mantle.....................10^23 – 10^24

What does temperature physically do to change the viscosity of a liquid and what affect does this have when a solid object passes throughit?

Effects of temperature on the viscosity of fluids in a gas: If the temperature increases, the molecular interchange will increase (because the molecules move faster in higher temperatures).

The viscosity of a gas will, therefore, increase with temperature.

According to the kinetic theory of gases, viscosity should be proportional to the square root of the absolute temperature; in practice,it increases more rapidly.

In a liquid: There will be molecular interchange similar to those developed in a gas, but there are additional substantial attractive, cohesive forces between the molecules of a liquid (which are very much closer together than those of a gas).

Both molecular interchange and cohesion contribute to viscosity in liquids.

The effect of increasing the temperature of a liquid is to reduce the cohesive forces while simultaneously increasing the rate of molecular interchange.

The former effect tends to cause a decrease of shear stress, while the latter causes it to increase.

The net result is that liquids show a reduction in viscosity with increasing temperature.

With increasing temperature, viscosity increases in gases and decreases in liquids, the drag force will do the same.

Consequently, the effect of increasing temperature will be to slow down the sphere in gases and to accelerate it in liquids.

Consider a liquid at room temperature.

The molecules are tightly bound together by attractive inter -molecular forces (e.g. Van der Waal forces).

It is these attractive forces that are responsible for the viscosity since it is difficult for individual molecules to move because they are tightly bound to their neighbors.

As the temperature is increased the thermal or kinetic energy of each molecule is increased and the molecules become more mobile.

The attractive binding energy is lessened and therefore the viscosity is reduced. If we continue to heat the liquid the kinetic energy will exceed the binding energy and molecules will escape from the liquid and it can become a vapor.

A solid object falling through a viscous medium experiences a frictional force that is proportional the speed of the object, the viscosity of the medium and the shape and size of the object.

What is the unit of measure of viscosity?

Dynamic (absolute) Viscosity

If  the  measured  values  are  based  on  the  basic  physical  units  of  force  [N],  length  [m] and time [s] 

Dynamic viscosity   = [N/m2] · [s] = force / length2 · time = [Pa] · [s]

The fundamental unit of viscosity measurement is the poise. A material requiring a shear stress of one dyne per square centimeter to produce a shear rate of one reciprocal second has a viscosity of one poise, or 100 centipoise.

You will encounter viscosity measurements expressed in Pascal-seconds (Pa·s) or milli-Pascal-seconds (mPa·s); these are units of the International System and are sometimes used in preference to the Metric designations.

One Pascal-second is equal to ten poise; one milli-Pascal-second is equal to one centipoise.

In the SI system the dynamic viscosity units are N s/m2, Pa.s or kg/m.s where:

1 Pa.s = 1 N s/m2 = 1 kg/m.s

The dynamic viscosity is also often expressed in the metric CGS (centimeter-gram-second) system as g/cm.s, dyne.s/cm2 or poise (p) where:

1 poise = dyne s/cm2 = g/cm.s = 1/10 Pa.s = 1/10 N.s/m2

For practical use the Poise is to large and it`s usual divided by 100 into the smaller unit called the centiPoise (cP) where:

1 p = 100 cP

1 cP = 0.01 poise = 0.01 gram per cm second = 0.001 Pascal second = 0.001 N.s/m2

Water at 68.4oF (20.2oC) has an absolute viscosity of one - 1 - centiPoise.

Kinematic Viscosity

In the SI-system the theoretical unit is m2/s or commonly used Stoke (St) where

1 St = 10-4 m2/s

Since the Stoke is an unpractical large unit, it is usual divided by 100 to give the unit called Centistokes (cSt) where

1 St = 100 cSt

1 cSt = 10-6 m2/s

Since the specific gravity of water at 68.4oF (20.2oC) is almost one (1), the kinematic viscosity of water at 68.4oF is for all practical purposes 1.0 cSt.

 

Saybolt Universal Seconds (or SUS, SSU)

Saybolt Universal Seconds (or SUS) is used to measure viscosity.

The efflux time is Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature and as prescribed by test method ASTM D 88.

This method has largely been replaced by the kinematic viscosity method.

Saybolt Universal Seconds is also called the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol).

Kinematic viscosity versus dynamic or absolute viscosity can be expressed as

ν = 4.63 μ / SG (3)

where

ν = kinematic vicosity (SSU)

μ = dynamic or absolute viscosity (cP)

SG = Specific Gravity

What is the relationship between temperature and viscosity of liquids and gases?

There is an exponential (or logarithmic) dependency between viscosity n and temperature T, recall the formula:

where A and dEvis are constants specific for each liquid and R is the universal gas constant.

Also note, there is a conversion formula v = n/p with v being the kinematic viscosity and n the dynamic viscosity.

So plotting log(n) against 1/T (in Kelvin) gives a straight line, therefore the higher the temperature T, the smaller is 1/T and the lower is viscosity n of the liquid.

Also recall Hildebrand`s formula, that viscosity is related to the ratio between free volume and occupied volume (Vo):

where B is another constant and V is the actual volume.

Note that in latter formula, the temperature is implicitly represented in the volume V as most liquids expand when heated.

What causes viscosity?What does particle size have to do with it?

There is no simple relationship between viscosity and particle size that covers all types of liquids.

Viscosity in a liquid is caused by forces between its particles (also called molecules).

Different liquids have different kind of forces. For examples, water has hydrogen bonds which are quite strong; mercury has metallic interaction between its’ atoms and molecules of fats and oils have so-called "van der Waals" forces, which are fairly weak.

If we consider only one kind of interaction, larger intermolecular forces mean a more viscous liquid. Some examples of different types of forces and viscosity in millipascals (mPa) at 25 deg C are:

Hydrogen Bonds Water (H2O) 0.890
Metallic interaction Mercury (Hg) 1.526
Van der Waals forces Tetrachloromethane (CCl4) 0.908
Polar forces Tetrachlorosilane (SiCl4) 99.4

In the case of van der Waals forces, the forces grow with particle size.

In hydrocarbons, there is an almost linear increase of viscosity from C1 (methanol) to C10 (decanol).

This is due to the increasing length of the linear hydrocarbon chain. Vegetable oil has long molecules, so the van der Waals forces are large and hence the viscosity is high.

In the case of hydrogen bonds, the number of bonds that a molecule can form has a major effect on its’ viscosity.

Consider three simple liquids, all of very similar molecular size, with one, two and three hydrogen bond forming groups respectively. The viscosity in millipascals (mPa) at 25 deg C is:

·Propanol 2
·Propandiol 40
·Propantriol (glycerol) 934

The reason for this dramatic increase is that more hydrogen bonds per molecule enable strong 3 dimensional networks between the molecules in the liquids, while single hydrogen bonds can only form into linear chains, or at best, rings.

Viscosity change with flow rate

Fluids which do not change viscosity with flow rate are called Newtonian fluids.Water is an example of a Newtonian fluid.

Fluids whose viscosity changes with flow rate are very common.

In these fluids, the change in flow rate causes changes in the relative speeds of neighboring fluid "particles".

These "Non-Newtonian" fluids may display either an increase or a decrease of viscosity when flow rate (or shear rate) increases. Some examples:

1. Paint is a classic example. Paint has low viscosity during brushing but high viscosity for zero or very small flow, which is why paint stays on a vertical wall.

2. A suspension of clay in water can display a decrease of viscosity with flow.

At rest, the suspension looks like a weak solid, a gel, with some elasticity. When sheared, it becomes fluid. This is "shear-thinning" behavior. Ketchup can display similar behavior.

3. A suspension of corn flour (cornstarch) in water displays the opposite behavior.

At rest it looks like water but when it is strongly sheared it becomes solid. This is shear-thickening behavior.

Highest viscosity in the world

What material has the highest viscosity in the world?

A noted, viscosity is a measure of a liquid`s resistance to flowing. Thin (low viscosity) liquids flow easily. Thick (high viscosity) liquids flow more slowly or need the application of shear stress to induce flow.

All solids will eventually flow in response to shear stress, so solids can, in principle, be considered as liquids with a very high viscosity.

The question "what is the highest viscosity in the world" depends upon your opinion of the dividing line, if any, between liquids and solids. The dividing line is around 1012 Pa"s.

If we accept that all solids are liquids, then diamond probably has the highest viscosity.

Does temperature affect the viscosity of motor oil, olive oil, and salt water?

Temperature affects the viscosity of these liquids, but changes in the viscosity of salt water are quite small and would be difficult to observe.

In general, as the temperature of a liquid increases, the viscosity decreases and the liquid becomes more easy to pour. This is true of most liquids.

However, some kinds of motor oil do not act that way.

Ideally one would want the viscosity of the oil to remain the same when the engine is hot as when it is cold; so, motor oil usually includes additives that are designed to lessen changes in viscosity due to temperature.

Some motor oils are especially designed to be less viscous (thinner) when cold and more viscous (thicker) when hot.

For example, multi-viscosity or multi-grade motor oils exhibit a low viscosity at low temperatures and a higher viscosity at high temperatures.

The idea behind this is to provide thin oil when the motor is started (usually at a cooler temperature) and to provide an oil of the proper thickness at the operating temperature.

These multi-viscosity or multi-grade motor oils can be identified by designations such as 10W-30.

These numbers refer to specifications developed by the Society of Automotive Engineers (SAE).

The "10W" in this example corresponds to the lower oil viscosity when the engine is cold and not running. The "30" refers to the higher oil viscosity when the engine is hot and running normally.

How can a gas have viscosity?

For matter, in a gaseous state at rest, all molecules are in continuous random motion.

This is why it is difficult to imagine how a gas can be viscous. However, when the gas is made to flow, for example as natural gas is made to flow through a pipeline, the bulk motion is superimposed on the random motion for all molecules and then distributed throughout the gas by molecular collisions.

The molecular collisions act as a resistance to flow, and are responsible for the appearance of viscosity in a flowing gas.

As the temperature of a gas is raised, the random motion of the molecules becomes stronger, which increases the resistance to flowing.

In particular, the viscosity of a gas has been observed to increase in proportion to the square root of the absolute temperature.

This behavior is different from that of a liquid, which generally exhibits a lower viscosity at higher temperatures.