Substances behave differently

Water does not always remain in a liquid state: it changes to a gaseous and ice-like solid state. This is a chemical change that happens when you add or take away heat, which is known to everyone. However, I think it is not well known that substances change by themselves even if they are not treated.

For example, do you know about the pitch drop experiment? It is an experiment where a mass of resin is put in a funnel and left as it is. The experiment started in 1927, and nine drips from the mass, which looks like a solid, have been observed over the 87 years to 2014.

This is a phenomenon called a flow of complex fluids, and depending on the length of the observation period, the phenomenon of a solid behaving like a liquid can be observed.

In contrast, I think many people know of some liquid behaviors that change instantaneously.

For example, when you slowly go into water in a pool, the water does not change at all; but if you jump from a diving board, you will feel pain when you hit the water’s surface.

In other words, it feels like the water has instantaneously behaved like a solid. In fact, it is said that the surface of water is the same as concrete when you jump from a high place.

As shown above, substances that appear to remain unchanged at a glance actually show various behaviors.

One of the factors that determine fluid behaviors is viscosity. It represents the characteristics of a thin (low-viscous) or thick (high-viscous) state. We have successfully applied the change in viscosity of liquids to tools and other items that we use in daily life.

For instance, extremely thick paint is difficult to apply, and extremely thin paint may drip when you apply it to a wall and the like. Therefore, paints have been developed based on calculations so that they will have user-friendly viscosity.

The degree of viscosity is expressed in units such as cP (centipoise) and Pa⋅s (pascal second), where the viscosity of water at 20°C is 1 cP.

Common methods for measuring viscosity are the capillary method and the rotational-viscometer method.

In the capillary method, the liquid to be measured is placed in a narrow glass tube, and its viscosity is determined by the period of time the liquid takes to flow down. It is a very simple classical method but still the most accurate measurement method.

On the other hand, in the rotational-viscometer method, the rotor is immersed in a container containing a sample liquid, and the rotor is turned. The resistance that the rotor receives is weak if the liquid is thin and strong if it is thick. The rotational-viscometer method is used to measure the resistance.

These viscometers are essential items for developing daily products around us.

EMS viscosity-measuring system that surpasses conventional measurement methods

Indeed, these two methods for measuring viscosity have advantages and disadvantages. For example, while the capillary method features high measurement accuracy, it takes time and labor to clean and maintain the capillary, and it is difficult to measure the change in viscosity caused by the difference in shear rate.

Shear rate is, in simple terms, the speed of flow. For example, when paint is applied with a moving brush (the shear rate is high), it becomes less viscous and can be applied more easily; after the application (the shear rate is zero), it becomes more viscous and can be fixed more easily.

The same thing happens to various products around us. When you scoop hand cream from a container (the shear rate is low), if the viscosity is not high, the cream will drip, making it difficult to scoop. However, it is hard to rub cream with high viscosity against your hand (the shear rate is high) because it is thick.

The property by which a liquid changes in viscosity depending on the shear rate as shown above is called non-Newtonian property. It is difficult to examine the non-Newtonian using the property of the capillary method.

The rotational-viscometer method has the advantage of reproducing such a change in shear rate by changing the rotation speed of the rotor.

However, mechanical contact by the motor or shaft for rotating the rotor causes friction, which may affect the measured value. In particular, the effect on the measured value of a thin liquid is relatively greater, resulting in a larger error.

Recently, we have been developing the EMS viscosity-measuring system.

Let me explain it simply. First, magnets are placed facing each other. This generates a magnetic field in the middle. When a metal piece or the like is placed there, induced current flows through the metal.

Here, when the magnets are moved along the circumference of the magnetic field, the force that is generated by the orthogonally crossing current and magnetic field follows the movement of the magnetic field. This makes the metal piece rotate in the same position synchronized with the circumferential movement of the magnets.

During this movement, the magnets are mechanically rotated, and the metal piece is a rotor without mechanical contact.

Then, after putting this metal piece in a container containing a sample liquid, when the container is placed in the center of the magnetic field, the only resistance to the rotation of the metal piece is the viscosity of the sample. This means that viscosity can be measured error-free.

As explained, we have developed the EMS system to overcome the disadvantages of rotational viscometers, but another advantage of the system is attracting attention. This system allows the sealing of a container containing a sample and metal piece.

Advantages of the EMS viscosity-measuring system highlighted by the medical fields

In recent years, we have often heard the expression that blood is thin or thick. It is because blood viscosity has been found to be related to various diseases.

However, probably no one has so far collected a large amount of data on blood viscosity. I think this is because using blood as a sample requires very careful handling.

For example, it is necessary to completely sterilize and disinfect measuring instruments and avoid exposing blood to outside air during measurement. When performing methods using a capillary and a rotational viscometer, it was very time- and labor-consuming and difficult.

However, the EMS system allows measurement of sample blood in sealed containers.

Also, blood comes in contact with the container and the metal piece of the rotor contained in it, but they now can be manufactured at the same cost as disposable injection needles; therefore, it is possible to produce disposable containers and metal pieces.

In addition, the required amount of blood is about 1 mL at most. If we could receive only some of the blood collected in a medical checkup, we would be able to easily collect more and more data.

In fact, the viscosity of blood changes greatly owing to the difference in the flow conditions (speed of flow) between when it flows through a wide blood vessel in the body and when it flows through a narrow blood vessel.

Namely, thin blood is not always good: the higher the ability to become thick according to the situation, the healthier the blood may be.

In order to elucidate such a mechanism of blood, it is meaningful to collect viscosity data of blood, and we believe that the EMS system can do it.

Here is another piece of research that makes use of the advantages of the EMS system. It is an attempt to use viscosity to quantify evaluations of alcoholic beverages, which have been performed based on feelings such as mouth feel and satisfaction.

At present, we have been working on fermented liquor such as sake and wine, and we have found that the viscosity of sake varies depending on the raw materials and the polishing rate of rice.

It would be interesting if we could elucidate how the sense of umami relates to the viscosity when alcohol is rolled over the tongue (the shear rate is low) and when it passes through the throat (the shear rate is high) at some time in the future.

Actually, the EMS system, which allows measurement of substances in sealed containers, is the very system that enables us to highly accurately measure the viscosity of a liquid including an evaporative ingredient, namely alcohol, such as alcoholic drinks.

There are a lot of products that make good use of the viscosity of liquids around us.

This is true not only for paints and hand creams I mentioned earlier but also for dentifrices that come mainly in a paste form. When you squeeze a tube with a force strong enough, the content will be pushed out, but if the toothpaste remains on a toothbrush, the shape will not change.

Like these, the wisdom of physics is utilized for daily products that we usually use, and they are controlled so that they will be user-friendly.

I suppose that if you look at the daily supplies from such viewpoints, it may be interesting because you will wonder how it is possible, or when you give a lot of thought to it, it looks strange. Probably they are an accumulation of wisdom in physics.

Measurement of the viscosity of blood and alcohol that we are currently researching may be utilized for medical care and product development in the future. I would like to actively deliver such information and draw your interest and attention.

* The information contained herein is current as of October 2022.
* The contents of articles on are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.

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