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. 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. Mathematically, for a small velocity gradient, we can write:
F = -ηA\frac{du}{dy}
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. The SI unit for viscosity η is the Pascal-second (Pa-s), which corresponds to the force (N) per unit area ({m}^{2}) divided by the rate of shear ({s}^{-1}). However, since the viscosity of most fluids is below 1 Pa-s, the millipascal-second (mPa-s) is often used instead. All real fluids (except superfluids) have some resistance to stress, but a fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. At a molecular level, viscosity is a result of the interactions between the different molecules in a fluid. Just like in the case of friction between moving solids, viscosity determines the energy required to make a fluid flow. The effects of viscosity are not only limited to liquids only but apply to gases also. However, it is important to realize that viscosity in gases arises due to the random motion of the molecules when it is endowed with macroscopic motion rather than a frictional force between any two adjacent layers, as in the case of liquids. Therefore, with the increase in temperature, viscosity increases in gases and decreases in liquids. Viscosity is a key property in the development of any application that involves fluid flow. Let’s discuss a few examples of viscosity in daily life.
1. Honey
Long before cane sugar was available for providing us with a sweet flavor, and before the synthesizing of corn into high-fructose corn syrup, raw honey was one of the sweetest foods humans could enjoy. It has long been acknowledged for its nutritional and medicinal benefits, with evidence of its extraction depicted on various rock paintings that date back almost 8000 years. It is among the few insect-derived natural products that have industrial, nutritional, as well as therapeutic values. Apart from this, honey is also known for its viscosity. Knowing the viscosity of honey is important. Sometimes it becomes difficult to extract and process honey owing to its high viscosity. For example, removing the honey from the honeycomb, filtering, and putting it into jars are all difficult to do if it is too thick and sticky. Honey’s viscosity depends upon the amount of water and the type and amount of sugar it contains. If the concentration of water is increased, honey becomes less viscous. Temperature also changes the viscosity of honey, and heat is often used to make the honey easier to process.
2. Engine Oil
As a vehicle owner, you might have been asked by a mechanic to change the engine oil whenever you take your vehicle for service. Engine oil has a diverse range of vital purposes. However, essentially, it keeps the engine running smoothly. Engines contain many moving parts that have the potential to rub against each other, creating friction. This friction can damage engine parts and cause them to wear faster. If the friction were to build up, the engine would slow down dramatically, making it less efficient and more likely to break down. As oil moves through your engine, it coats the moving engine parts so that they don’t grind against each other and wear down. Engine oil also helps to clean, cool, and protect your engine. Engine oil viscosity refers to how easily oil pours at a specified temperature. Thin oils have lower viscosity and pour more easily at low temperatures than thicker oils, which have a higher viscosity. Thin oils reduce friction in engines and help engines start quickly during cold weather. Whereas, thick oils are better at maintaining film strength and oil pressure at high temperatures and loads.
3. Brake Oil
From a safety point of view, brakes are the most critical component of a vehicle. Most of us put a great deal of trust in the braking mechanism of our vehicle. Brakes work on the principle of the hydraulic system. Brake oil or brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in vehicles. When you press the brake pedal, the brake master cylinder transforms mechanical force into hydraulic pressure. Being non-compressible, brake fluid is responsible for transferring hydraulic pressure through brake lines and hoses down to the brake caliper and wheel cylinder assemblies. For the system to work efficiently, the brake fluid must be able to move easily through the lines and must be able to efficiently transfer the pressure from the master cylinder to the piston. Viscosity is a key characteristic of brake fluid since the system must operate smoothly within a wide range of temperatures. Technologies such as ABS, Stability Control (ESP), and Traction Control, require fast-moving fluid (low viscosity) in order to operate correctly. If the viscosity of the brake fluid is high, then the movement of the fluid through the lines will be sluggish and difficult to control. Therefore, it is important to choose a brake oil that does not give you a hard day during the winter season.
4. Lubricant
A lubricant is a substance used to reduce friction and tear among the surfaces that are in contact. Machinery plays a very important role in making our daily life activities easier. Insufficient or improper lubrication is one of the leading causes of equipment failure in the industrial world. Without lubrication, sliding, rolling, or meshing surfaces experience significant friction, heat, and wear, leading to increased noise, loss of accuracy, and reduced equipment life. Considering all the chemical and physical properties, the more important one for lubrication is lubricant viscosity. In bearings or hydraulic systems, viscosity determines friction losses, load capacity, and film thickness. It is a physical measurement of a fluid’s ability to fully provide film lubrication under specific operating conditions in terms of speed, load, and temperature. For instance, if the oil is too thick or highly viscous for the operating conditions, the machine has to work harder, which will result in more heat and more energy consumption. Over time, this will create unnecessary wear and tear on the machine. Conversely, if a lubricant is too thin, the film may not be thick enough to prevent friction. This, too, will create unnecessary wear and tear on the machine. Therefore, depending on the operating conditions, lubricants are determined as per their viscosity indexes.
5. Cooking Oil
Whenever we visit a grocery store, we come across plenty of options for choosing a cooking oil, such as olive oil, mustard oil, avocado oil, coconut oil, etc. Collectively, they can be defined as fats that remain liquid at room temperature. They are usually differentiated based on their nutritious value, health effects, and the type of cooking we are interested in. Nevertheless, you can also spot a difference between oils by observing their viscosity. It is a well-established fact that temperature has a strong influence on the viscosity of fluids. Therefore, viscosity is an important parameter while choosing a cooking oil, since the texture of food can dramatically be impacted by changes in viscosity.
6. Liquid Soap
Liquid soap is one of the most essential household items. It is not a single substance but rather a group of substances with similar properties: they are all soluble in water, soapy, and capable of washing oils, fats, and other contaminants from the skin, clothes, and more. It usually comes in small and handy squeeze or pump bottles or can be dispensed in small, single-use quantities from a soap dispenser. However, viscosity is an important factor to keep in mind while choosing a liquid soap refill. Although the viscosity of commercially available cream soaps and liquid soaps lies in the range of 1000 -3500 cps (centipoises), it may vary according to the temperature, evaporation, thickening, etc. The general rule is that the soap should flow well at room temperature. The thicker the soap, the lower the dosing volume per stroke.
7. Printing Ink
Diagram of a typical flexographic printing apparatus, illustrating the general operating principle
The two topics most discussed during the set-up of a printing machine are probably ink and paint. The main components of the ink are pigment, additives, and solvents. A minor change in the composition or impurity of the raw materials can result in different chemical and physical properties of the ink, which may negatively affect the printing process. There are two properties of ink that the machine operator must control. These are viscosity and pH. Understanding the viscosity of the ink is of great importance to the ink manufacturers also because this physical property provides valuable information on the consistency and longevity of the finished product. In ink terminology, viscosity refers to the extent to which ink will resist flowing. The viscosity of ink will depend on the printing process it is designed for and the nature of the substrate to which it will adhere. The viscosity of a particular ink can vary according to the stresses to which it is subjected. Ink viscosity affects how well the ink can transfer over from the anilox to the plate, and from the plate onto the printing substrate. It affects how well the ink will dry on the substrate. In general, the press production speed and the quality of the print are highly correlated to the ink viscosity.
8. Super Glue
Adhesives play a crucial role in the life of human beings. No matter how careful you are, every now and then you may accidentally break something and find a way to undo it. Thanks to the super glue that offers an effective way of mending various materials while being super easy to handle. Super Glues are a specific type of adhesive, also known as cyanoacrylate adhesives, that bonds by reacting with the moisture in the air and on the surfaces of the materials they are bonding. Viscosity is a major point of difference among grades of super glue. Most of the super glues typically come in liquid or gel forms. Liquids are best for penetrating cracks or fractures and generally dry faster than gels. Therefore, the desirability of a higher or lower viscosity depends on the application. For instance, higher viscosity glues mean less dripping, which offers a more controlled application for projects requiring greater precision or vertical application, like building models. Whereas, low viscosity glues are preferred for working on flat or leveled surfaces. In most applications, medium viscosity type is often used as it keeps the adhesive where we put and prevent it from running off.
9. Paints
Choosing the right color for your room or home is undoubtedly a demanding task. While customers often choose paint solely on its color, there are several other important factors one should consider if he is going to paint the room himself. One of those factors is the viscosity of the paint. It is a measure of how resistant a paint is to spreading. Therefore, viscosity determines how much paint should be applied to a brush or roller, and how much paint is necessary to cover a given area. Furthermore, the viscosity of a paint or coating will determine the appropriate calibration for sprayers and airbrushes. For example, while planning on how much paint will be needed to finish a job, it is important to know that more viscous paint will cover more area than less viscous paint. Therefore, often the viscosity of the coating or paint is altered prior to the job. While air-brushing or using a sprayer, it is important to understand that thicker paints will require more air pressure to distribute evenly on the surface. Finally, it is also important to realize that different temperatures of both the paint and the surface to which it is being applied can affect the viscosity of the paint.
10. Syrups
The syrup is another great example to understand viscosity. For instance, the large chains of carbohydrates in syrup slide past one another with great difficulty than the minuscule molecules of water. The ungainly shapes of constituent molecules create greater friction as they slide past one another. In cooking, almost any thick or highly viscous sweet liquid is generally known as syrup. The viscosity arises from the multiple hydrogen bonds between the dissolved sugar, which has many hydroxyl (OH) groups. There is a range of syrups used in food production, for example, glucose syrup, maple syrup, corn syrup, golden syrup, cane syrup, and agave syrup. Most of them are made by reducing (the process of thickening via boiling) naturally sweet juices from cane, sorghum, maple sap, or agave nectar. However, corn syrup is made from corn starch using an enzymatic process that converts it to sugars.
11. Bitumen
We depend vividly on roads to get us from one place to another. In fact, there are 33 billion meters of roads on Earth. The durability and the long term satisfactory performance of roads are always influenced and affected to a greater extent by the employed ingredient materials and their inherent properties. Bitumen, otherwise known as Asphalt, is a thick-gooey substance, usually black or dark brown in color, derived as a natural by-product of the distillation of crude oil. Although its chemical composition is highly complex, bitumen consists of roughly eighty percent carbon, ten percent hydrogen, six percent sulfur, one percent oxygen, and one percent nitrogen. The upper layers of a road structure are vital in taking care of load/stress alleviation and protecting the structure. Therefore, the viscosity of the bitumen plays a very important role in road construction. Depending upon the location, bitumen for road construction is selected on the basis of its viscosity grading. For instance, viscosity grade bitumens have a thermoplastic property, which causes the material to soften at high temperatures and to harden at lower temperatures. This unique temperature-viscosity relationship is important when determining the performance parameters such as the adhesion, rheology, durability, and application temperatures of bitumen. In the Viscosity Graded Bitumen specifications, further emphasis is placed on the Bitumen ductility.
12. Blood
Blood is, in fact, the most vital fluid in our body that is responsible for various crucial functions taking place in our body. To the naked eye, it may seem to be made of uniform red liquid, similar to food coloring or paint. However, if you were to look under a microscope, you would see that it has four main components: plasma, red blood cells, white blood cells, and platelets. The old saying “Blood is thicker than water” makes sense for family ties. But for the heart and circulatory system, though, thinner, more watery blood might be better. It is important to understand the role of blood viscosity as a clinical marker. The flow of blood through the vessels is described as laminar flow, i.e., the blood forms layers (lamina) that slide easily over each other. Looking at the blood vessel from the side, we would see the fastest flowing blood in the center layers, with slow-moving blood in the outer layers near the wall of the vessel. Highly viscous blood does not slide as smoothly as less viscous blood, leading to turbulence that can cause damage to the delicate structure of the blood vessels. On the other hand, overly-thin blood can also lead to severe health complications such as no clotting if you get bruised or cut, internal bleeding, and heavier-than-normal menstrual periods.
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