More about Dynamic Viscosity
Dynamic viscosity of water at 20 °C is about 0.001 Pa•s. Hilton Falls. Southern Ontario, Canada.
Newtonian and Non-Newtonian Fluids
Viscosity in Daily Life
In Food Preparation
In Health Sciences
Pouring viscous cream into a cup of non-viscous coffee
Viscosity is a property of a fluid to resist the force that pushes the fluid to flow. Dynamic or absolute viscosity measures the resistance inside of the fluid and is independent from the density of this fluid. Viscosity is often talked about in the context of shear stress, where shear stress is defined as stress that results from a force that acts parallel to the cross section of the object. For example, imagine a substance so viscous, that it can retain its shape, say a cube, for several minutes almost without changing — like a very viscous fruit jam. If you put this jam cube onto a plate, then touch the top of the cube with your hand, and pull your hand parallel to the surface of the cube in any direction — you will be acting on the jam cube with a force that causes shear stress. The jam will follow your hand and the cube will be distorted. The property of the jam to follow your hand is viscosity — the jam does not flow, even though you push it, but follows your hand instead.
Solids that undergo small continuous stress deform similarly to liquids and could be described in terms of viscosity, but this property is most often used in relation to liquids and gases. High viscosity refers to high resistance to shear stress.
Viscosity of many substances is temperature-dependent, therefore it is important to specify temperature when providing viscosity information. For example, warm honey is easy to stir but is more viscous when colder, and so are some oils. Olive oil is not viscous at room temperature, but it becomes viscous when refrigerated.
Water is a newtonian fluid
Newtonian and Non-Newtonian Fluids
It is important to note the difference between Newtonian and non-Newtonian fluids when talking about viscosity. Newtonian fluids do not change the degree of viscosity based on the force that acts upon them. Non-Newtonian fluids become either more viscous or less viscous depending on how the force is applied to them. Cream is a good example of a non-Newtonian fluid because it is not very viscous under normal conditions or if some small amount of force is applied to it, but becomes very viscous if the force is intensified. When you stir cream with a spoon, its viscosity is low, but when you beat it with a mixer, its viscosity increases gradually, and eventually it gets so viscous that it becomes stiff enough to keep its shape.
Viscosity in Daily Life
Understanding viscosity and how to control it is very important in a range of industries. Controlling it saves lives, helps generate lucrative profits to the manufacturers of cosmetics, and makes cooking easier.
Cosmetic products, such as liquid foundation, lip gloss, liquid eyeliner, mascara, nail polish, and various lotions, to name just a few, use viscosity to ensure that the products stay on the skin. Lip gloss manufacturers, for example, always experiment to find a good balance between a viscous product that stays on the lips well, or a less viscous gloss that has to be reapplied often, but does not feel too sticky, as the viscous one does. People who make their own cosmetics control viscosity of the final project by combining the oils and waxes until the desired viscosity is reached. Substances called viscosity modifiers are also commercially available.
Viscosity of body wash also needs to be balanced well to make sure that it stays on the body long enough to cleanse it, but not too long that the bather feels dirty. Viscosity is artificially enhanced to ensure the properties above.
Lotions, creams, and ointments, whether medicated or not, are distinguished by their viscosity. All three are emulsions of water and oils, and higher viscosity is a result of higher oil content. Emulsions are mixtures of several substances that do not mix to form a homogeneous solution, such as oil and water. Additional substances called emulsifiers are often used to stabilize the emulsion — this is commonly the case in cosmetic products. Some examples are emulsifying wax, which is wax-treated with a detergent-like solution, and ceteareth, a substance that is a combination of saturated fatty alcohols. Some lotions remain as a partitioned mix of two oil-based and water-based halves. They stay separated in the bottle, and before use one shakes them to make a short-lived emulsion. It is often the case that in those substances the water-based liquid is less viscous than the oil-based one, and when mixed the viscosity also becomes higher than that of the water based liquid but lower than the oil-based one.
Ointments generally have the highest viscosity, followed by creams and then lotions. Because of their viscosity, lotions feel lighter and more refreshing on the skin. They feel pleasant in hot weather but do not stay on the skin as easily and need to be rubbed in well. Lotions do not cling to hair as much because of lower viscosity, and thus are easier to use on the scalp and other areas covered with hair. Creams and ointments are heavier and feel more moisturizing. This is especially desirable in cold dry weather. A good example of such cream is body butter. Ointments do not get absorbed by the skin as fast as creams and feel greasy, but due to high viscosity they stay on the applied area better than creams. This property makes them popular in medicine.
Viscosity of a cosmetic product may be the determining factor of whether the consumer buys this product again. Because of this manufacturers first design a product by carefully finding the optimal viscosity. They then strictly adhere to the production guidelines and measure the viscosity of the product during the manufacturing, to ensure that it meets the viscosity standards.
In Food Preparation
Viscosity is commonly used in cooking to enhance food presentation and the ease of consumption. Viscous food is also easier to spread and can be used to hold foods together. Mayonnaise, for example, holds ingredients in a salad or on a sandwich, due to its high viscosity. If olive oil is used instead, the ingredients do not stay together as well as with mayonnaise. Many people prefer higher viscosity in salads. Those who do not eat mayonnaise due to health concerns often substitute it with yogurt, to preserve the viscosity, and mix it with olive oil to enhance the flavor or the oil content.
Foods high in viscosity are also often used for decoration because they keep their shape. They also allow one to place other foods on top of them in a decorative manner like you can see in the pictures of mayonnaise and yogurt.
This is also why creamy pasta sauces are popular — they hold the pasta and the toppings together. Cream and butter, when cooked, become viscous, and are then used as a base for cream sauces. Some recipes substitute milk for cream and butter to make it low-fat. When milk is cooked, it does not become viscous. Therefore, to increase viscosity, starch or flour is added to the milk in these recipes. Either of them thickens the liquid and makes it more viscous.
To increase viscosity, oils from plants are often hydrogenated. Margarine is created in this manner. This makes the oil easier to control during cooking, so that it can stay on the surface when spread. Margarine has been popular until recently because of its high viscosity but low price, compared to butter. In the past, it has been linked with health problems due to the high content of saturated and trans fats, that are thought to increase the blood cholesterol level. Manufacturers are responding by lowering the trans fat levels in margarine. Before buying margarine it is a good idea to check its label for the trans fat content.
A rescue operation in cold water. Mississauga, Ontario, Canada.
In Health Sciences
Blood viscosity is extremely important in medicine because high blood viscosity can lead to a range of medical problems. Blood with high viscosity does not travel as well and as fast through the body as blood with normal viscosity, and this may prevent proper nutrition and oxygen flow to the tissue and organs, and even the brain. Inadequate supplies can result in the death of some tissue and can subsequently damage limbs or internal organs. For example if a person is subject to hypothermia, blood becomes more viscous in the cold, it does not carry enough oxygen to the limbs, and tissue in fingers or toes may die as a result. In this case they would often be amputated.
Some causes of high viscosity other than cold temperatures include hereditary or anomalous conditions of too many blood cells and too little plasma in the blood, as well as high cholesterol levels. Treatments include gradually warming up the body in the case of hypothermia, and thinning the blood by adding plasma, among other solutions, in other cases.
Mañana Island with two craters at the eastern end of the Island of Oʻahu in the Hawaiian Islands
Viscosity of magma affects how violent a volcano eruption will be. When magma is low in viscosity, it will be pushed out of the crater with less pressure, and it will flow along the mountain. Magma from the Hawaiian volcanoes is an example of this. Such magma can be moved easily, so the eruption is more likely.
If the magma is more viscous, it will require more pressure to move it up, and the magma will be pushed out with forceful explosions instead of flowing smoothly. The explosions are caused by gas that is captured in bubbles inside the magma. Such explosions are more dangerous because they are spontaneous. The famous volcano eruption near Pompeii was such a kind.
Preparing semolina breakfast pudding
It is easy to watch a similar phenomenon in the kitchen when heating up soup in a pot on the stove. If a broth that has low viscosity is left boiling on the stove with no lid, it will keep boiling until the liquid evaporates, most likely without creating a mess on the stove. If a viscous soup like a potage or a cream soup is boiled, it will bubble violently, and will more likely explode over the stove or the kitchen counter in the process. Air bubbles in the soup will act the same as gas bubbles in magma, causing the mini eruption. The boiling of semolina in the picture is very similar in nature.
The viscosity of magma depends on its temperature, but also on its chemical properties. Higher concentration on silica results in more viscous magma due to the structure of the silica molecules.