Do you remember the first time you used a honey bottle, perhaps to sweeten your green tea, or there are countless other uses? You probably recognized when the honey bottle was nearly empty, that honey did take a long time to reach your mouth. This occurrence of fluid flow is due to a property that is viscous.
It is defined as the property of a liquid by virtue of which, an opposite force (internal friction) arises whenever there is a relative motion between two layers of that liquid.
In effect, viscosity is related to the “thickness” of a liquid. Honey is thicker than water due to honey having a higher viscosity; it is more viscous.
Technically, viscosity can be defined as a measure of resistance to flow caused by shear stress to a liquid.
Viscosity is quantitatively defined in terms of the coefficient of viscosity, η, which is the tangential for a unit velocity gradient (the change in velocity between adjoining layers of the fluid) that occurs in the direction perpendicular to the direction of motion. For small velocity gradients we can mathematically write:
F = -ηA du/dy
Where F is the viscous force acting on area A and du/dy is the velocity (gradient) in the positive y-direction. The negative sign illustrates that the viscous force acts against the velocity gradient. Hence, we note that it takes liquids with greater viscous properties an overall slower rate of flow.
The SI unit for viscosity η is Pascal-second (Pa-s), which equals force (N) per area (m2) divided by a rate of shear (s-1). Since the viscosity of most fluids are usually less than 1 Pa-s , the millepascal-second (mPa-s) will be used.
Every real fluid (other than superfluids) is resistant to some stress, whereas a fluid with no resistance to shear stress is referred to as ideal or inviscid fluid. At a molecular basis, viscosity occurs as a result of the interactions of the various molecules in a fluid.
Similar to its equivalence in friction whenever there is movement between solids, viscosity determines the energy needed to make a fluid flow. The effects of viscosity are, however not limited to liquids, the same concepts apply to gases.
There is a distinction to be made as the viscosity of gases is a result of the random motion of the molecules while having their apparent motion, as opposed to a frictional force between two adjacent layers like in the case of liquids.
Thus, temperatures cause the viscosity to increase in gases and decrease in liquids. Viscosity is a measure of a defining property whenever the application deals with fluid flow. We can consider a few instances of viscosity in our everyday lives.
Examples of Viscosity
1. Honey
Raw honey was one of the sweetest foods humans could indulge in for millennia before cane sugar was available for sweetness and before the synthesizing of corn into high fructose corn syrup.
It has long been recognized for its nutritional and medicinal value and the proof of its extraction is located on rock paintings that are nearly 8000 years old. It is one of the few natural products derived from insects that has industrial, nutritional, and therapeutic value.
More than this, honey is known for its viscosity. The significance of honey’s viscosity is not lost. Sometimes extracting and processing honey is difficult, because it is viscous.
For instance, it is all hard work to extract honey from honeycomb, filtering and pouring honey into jars if the honey is hard and thick.
The viscosity of honey is dependent on the water content and the sugar type and amount. Conversely increased water concentration in honey causes decreased viscosity. Temperature also impacts honey viscosity and in processing honey, heat is often used to reduce viscosity, making the honey easier to process.
2. Engine Oil
You may have heard your mechanic ask you to change the engine oil every time you bring your vehicle in for service! Engine oil has several important functions. But basically, it keeps the engine from wearing out.
Engines are made of many moving parts capable of rubbing against each other, creating friction. The friction can damage the engine parts, and the parts wear out faster. The buildup of friction is a serious problem, as the engine could slow down, become inefficient, and even break down.
While oil is flowing through your engine, the oil coats all of the moving engine parts to prevent grinding friction and reduce wear and tear.
Engine oil also cleans, cools, and protects your engine. Engine oil viscosity tells you how easily the oil pours at a specified temperature!
Oil with low viscosity is thin oil and pours more easily than thicker oil at low temperatures.
Thin oil reduces friction in engines and enables the engine to start quickly in cold weather. Thicker oil creates better film strength and oil pressure at high temperatures and loads.
3. Brake Oil
From a safety standpoint, brakes are the most important component of a motor vehicle. Most of us use faith in circuit of braking mechanisms, Brakes work on the principle of hydraulic system.
Brake Oil (Brake Fluid) is a form of hydraulic fluid that is used in hydraulic brake and hydraulic clutch applications in a vehicle. When you press the brake pedal the brake master cylinder convert electrical input (mechanical force) into hydraulic pressure.
Non-compressible brake fluid is what is used to transfer hydraulic pressure through brake lines and hoses down to the brake caliper and wheel cylinder assemblies.
For the system to function efficiently the brake fluid must move easily through the lines and must transfer the pressure from the master cylinder effectively to the piston.
Viscosity is important characteristic of brake fluid as that the system must function well in a wide range of temperatures. Technologies such as ABS, Stability Control (ESP), Traction Control require fast fluidity (low viscosity) to operate accurately.
If the brake fluid has higher viscosity, the liquid which moves through the lines will not react quickly, fluid movement will be sluggish and difficult to control. Hence, it is important to select a brake oil that does not give you a hard day in the winter season.
4. Lubricant
A lubricant is a product that is used to lessen friction and wear amongst surfaces that are in contact. Machinery is very important in simplifying everyday activities. One of the most common causes of machinery failures in the industry is a lack of proper or sufficient lubrication.
All sliding, rolling, and meshing surfaces develop a lot of friction, heat, and wear when lubrication does not exist, resulting in increased noise, accuracy loss, and shortened lifespan.
By the farthest reach of all the chemical and physical properties regarding lubrication the most important one for lubrication is lubricant viscosity.
In bearings or hydraulic systems, viscosity affects friction losses, load bearing capabilities, and thickness of the oil film. It is a physical measure of a fluid’s capability of providing film lubrication under specific operating conditions of speed, load, and temperature.
For example, if the oil is too thick or highly viscous, the machine is required to do more work, which translates to more heat and more energy spent, over time, this will cause unwanted wear and tear on the machine.
On the other hand, if the lubricant is too thin, it may not have adequate film strength, thereby potentially preventing wear from friction, which can also cause unwanted wear and tear on the machine. The viscosity index of lubricants is determined in terms of viscosity given the operating conditions.
5. Cooking Oil
We have a lot of choices for cooking oil when we visit a grocery store, olive oil, mustard oil, avocado oil, coconut oil, etc. All of these can be grouped together as fat, which is a fat that we find in liquid form at room temperature (at least when it isn’t refrigerated).
Oils are generally categorized by their nutritional content, health benefits, and type of cooking that we intend to do.
However, viscosity is another way to distinguish between oils. It is well established that different fluids exhibit viscosity behavior that is affected by temperature.
Therefore, it is advisable to consider the viscosity of oils when selecting cooking oil, because it is an indicator of how dramatically the overall texture of food may change with changes in oil viscosity.
6. Liquid Soap
Liquid soap is a crucial part of every household. It is not a simple substance but a collection of substances that have the same characteristics: they are all water soluble, soapy, and will afford washing of oils, fats and other contaminants from skin, clothing, etc.
Liquid soaps usually come in small, compact squeeze or pump bottles or can be dispensed in small, one-time-use amounts from a soap dispenser.
Viscosity should be considered when choosing a liquid soap refill. Viscosity can have a wide variability between commercially available cream soaps and liquid soaps, typically 1000 -3500 cps (centipoises); viscosity can also be influenced by temperature, evaporation, thickening, etc.
As a general rule of thumb, liquid soaps should flow readily at room temperature. The thicker the soap, the lower the dosing volume per stroke.
7. Printing Ink
Diagram of a typical flexographic printing machine showing the general operational principle
Two of the most talked about topics during the set-up of a printing machine are ink and paint. The principle components of ink are pigment, additives, and solvents.
A small change of the specification or impurity of the raw materials may result in different chemical and physical characteristics of the ink which may affect the printing operation. There are two characteristics of ink that the machine operator needs to control: (1) viscosity and (2) pH.
What is also significant about the viscosity of ink is that it is important to ink manufacturers because this physical characteristic indicates useful information about the performance of the finished product in service. Again, in ink terms, viscosity defines the degree to which ink will resist flowing.
The viscosity will depend on both the printing process and the substrate to which the ink is to be applied.
The viscosity of any specific ink can vary based on the stresses to which it has been applied. The ink viscosity relates to how well the ink will transfer over from the anilox to the plate, and from the plate to the printing substrate.
The viscosity relates to how well the ink will dry on the substrate. Typically, press production speed and print quality are closely related to ink viscosity.
8. Super Glue
Adhesives are an important part of peoples’ lives and at some point will fail you anyway. If you’re careful you still might find yourself breaking something and figuring out how to coerce it back.
Fortunately, super glues can provide a viable option for repairing a variety of material types and applications and are extremely user-friendly.
Super glues are a category of adhesive, also termed cyanoacrylate adhesives, and are not like other adhesives in that they bond based upon what happens when the glue reacts with the moisture in the air and the surfaces of the materials being bonded. Viscosity is a significant difference between grades of super glue.
Typically super glues come in either liquid form or gel. Liquids work best when penetrating cracks or fractures and additionally usually dry faster than gels. Therefore, whether you would want/need a low or high viscosity is application driven.
To clarify reasonings, higher viscosity glues mean less drippage, and when the application is subject to more precision or vertical application like model building, provide for a more controlled application.
Whereas low viscosity glues are preferred for flat or leveled surfaces. In most applications, a medium viscosity type is most often used, as it keeps the adhesive at the desired placement, preventing it from running off the application surface.
9. Paints
In choosing the color for your room or home can be a difficult task, and while many customers only consider the paint based on the color, there are a number of other considerations if you plan to paint the room yourself.
One of these considerations is the viscosity of the paint. Viscosity is how viscous a paint is to spreading.
Thus, viscosity dictates how much paint can be loaded onto a brush or roller, and how much paint is needed to cover an area.
Moreover, the viscosity of the paint or coating determines the proper calibration for the spraying equipment, and airbrushes.
For example, it is important to remember when planning how much paint is needed to complete a job, that more viscous paint will cover more area than less viscous paint.
Also, in planning sometimes the viscosity of the coating or paint will be altered before a job. With air-brushing or using a sprayer one must realize that thicker paints will require more air pressure for proper distribution on the surface.
Lastly, one must also think about different temperatures of the paint and the surface to which it is being applied. The temperature can alter the viscosity of the paint.
10. Syrups
Syrup is another excellent example for understanding viscosity. For example, the large chains of carbohydrates in syrup slide more than the minuscule water molecules, but with great difficulty.
The often ungainly shapes of the constituent molecules create more friction as they move past each other. In culinary usage, almost any thick or highly viscous liquid, sweet or otherwise, is referred to as syrup.
The viscosity occurs because of the multiple hydrogen bonds between the dissolved sugars that have many hydroxyl (OH) groups in them.
There are different syrups used for food production: glucose syrup, maple syrup, corn syrup, golden syrup, cane syrup, and agave syrup.
Most syrups are produced from naturally sweet juices by the process of reducing (thickening via boiling) at the production stage: cane (sugar) syrup, sorghum syrup, maple sap syrup, agave nectar syrup, and so forth.
Corn syrup syrup is produced from corn starch, which undergoes a process of enzymatic conversion to sugars to produce corn syrup.
11. Bitumen
Roads connect distinct points across the globe. In actuality, there are 33 billion meters of roads on Earth.
The longevity and performance of roads is influenced, in much greater detail, of the ingredients materials used and their properties.
Bitumen, or Asphalt as it is often referred to, is a gooey, thick, large-colored, commonly black or dark brown, naturally occurring material that is produced as a common by-product from crude oil distillation.
Bitumen is very complex, and is composed of approximately 80% carbon, 10% hydrogen, 6% sulphur, 1% oxygen and 1% nitrogen.
The upper layers of a road structure are also useful in performing load/stress relief and shielding the road structure. Accordingly, the viscosity of the bitumen is an important part of road construction.
In different locations, viscosity graded bituminous is used for road construction, according to viscosity grading. Importantly, viscosity graded bitumens have thermoplastic characteristics. That is it softens at high temperature and hardens at lower temperatures.
The temperature/viscosity profile of the bitumen is a key means to evaluate important performance parameters such as adhesion, rheology, durability and application temperatures for the bitumen use.
In addition to viscosity graded bitumen specifications we also stress ductility of the bitumen.
12. Blood
Blood is considered the most powerful liquid in our body and allows many important things to happen in our body. NOTE: To the naked eye, blood appears to be one red liquid similar to the consistency of food coloring, or paint.
However, under the microscope, blood has three main components: plasma, red blood cells (RBCs), and white blood cells (WBCs) and platelets. The old saying, “Blood is thicker than water,” is appropriate for family ties.
But we would not want thicker blood, just thinner, watery blood, for the heart and circulatory system. Blood viscosity is a significant indicator that warrants further investigation.
Blood flow through the vessels is laminar flow, meaning that the blood is made up of layers (lamina) that flow easily on top of each other.
If you looked at the blood vessel from the side, you would see the fastest moving blood in the centre layer and slower moving in the outer layer near the vessel wall.
Blood that is thicker due to higher viscosity properties will NOT flow as freely as blood with lower viscosity properties, increasing turbulence and potentially damaging the fine structures of the blood vessels.
Too thin blood can lead to complications regarding internal bleeding, the absence of clotting if bruised or cut, and heavier than normal menstrual periods.