All objects in the world exchange energy. For example, a hand placed on cold metal will heat it gradually, cooling down itself. The same occurs in the micro world: particles (e.g. electrons) exchange energy. This kind of exchange proceeds not gradually but in increments – quanta. For instance, an atom contains several electrons, and each of them possesses a certain amount of energy. If there are more than one electron, they are located "one above the other" (in the different energy levels). Electrons in a higher level have more energy than its lower-level (closer to the nucleus) "neighbors". Assuming we wanted to move an electron in an atom from the first level up to the second level, it would be imparted energy sufficient to make a "leap" to the second level and almost reach the third level. Then the electron would only move to the second level, without a chance of achieving an "intermediate" level. It would still be in the same level even if it gained 99.9999% of the energy needed to transition to another level. It is all or none.
This principle accounts for the luminescence phenomenon.
Luminescence is emission of light by certain materials when they are relatively cool. It is in contrast to light emitted from incandescent bodies, such as burning wood or coal, molten iron, and wire heated by an electric current.
When a substance is given enough energy, some of its electrons move up to a higher level. However, such levels are often unstable and the electrons "drop" back to their ground state. At the same time, these electrons need to give off the excess energy they have gained. They emit energy in the form of photons – particles and waves at the same time. When emitted by myriads of atoms, photons form a flow of light that can be detected by a sensor. Luminescence spectroscopy is based on the following: a special sensor measures the luminescence intensity of photon waves. From the measurement data, the researcher can make conclusions about the molecular and atomic processes taking place in a substance.
This method is the basis for a research study published in the International Journal of Molecular Sciences in 2020. The study compared luminescence intensity of a water sample subjected to mechanical shaking to that measured in water not exposed to any mechanical treatment (control sample). The test sample was found to emit blue spectrum light waves within 40 minutes after exposure, with the intensity of these emissions attenuating over time. The intensity of luminescence in control water was unchanged.
The numerous molecules and atoms present in any test sample always undergo events that result in “background” light, so the value of luminescence intensity for each sample is nonzero at any moment. The greatest intensity of the test sample’s luminescence is observed immediately after shaking. After some time, the recorded signal decays to the level of the control sample (Fig. 1).
Fig.1. Effect of mechanical stress (30 Hz, amplitude of 5 mm, for 5 min) on intrinsic water luminescence.
Several processes are characterized by blue-spectrum light emission following shaking: formation of nanobubbles in the system, as well as radical reactions. Further experiments to examine the mechanism of water luminescence following shaking were carried out.
When seeing a ship’s propeller in a movie, one can notice it leaving behind a multitude of bubbles as it rotates. Different gases, such as carbon dioxide, air oxygen, etc., are able to dissolve in water and, oppositely, "precipitate" as bubbles. For this, gases need to receive some energy (like in the case of water boiling). Gases can also form bubbles if the solution is put to conditions under which the current energy of the gas molecules is sufficient for them to "collapse". A ship’s propeller is what provides such conditions. Mechanical impact creates areas of increased and lowered pressure in water. In the low-pressure areas, the gas molecules can aggregate and form a bubble. This process is called "cavitation".
Shaking can also cause cavitation, which means that there will be fewer free gas molecules dissolved in the liquid as these will be distributed among the bubbles. To verify this hypothesis, the concentration of oxygen molecules was measured in the sample previously exposed to shaking, compared to control water.
Fig.2. The effect of mechanical stress (30 Hz, amplitude of 5 mm) on molecular oxygen concentration in water The insert to figure 2 displays the effect of frequency of mechanical stress on the test parameter (exposure time 5 min). The data are presented as the mean and standard error of the mean for six independent assays. Data differ significantly from control values (0 min) at p <0.05 (*).
The Figure shows that the number of dissolved oxygen molecules decreases in water following shaking. Furthermore, the concentration of free oxygen in the solution declines with higher intensity of shaking. However, this result is inconclusive as oxygen can pass from the solution into the air under the tube cap.
To obtain more data, the following experiment was carried out.
Judgments of the purity of water may be made by holding it up to a light source, rendering any inhomogeneity visible. Bubbles (even nano sized ones) are inhomogeneities too, and sensors can detect them.
While It was found that the diameter of bubbles produced was almost unaffected by shaking (with the average size being identical to that in control water), the number of bubbles was significantly increased following exposure. Since only micro- and nano-sized bubbles can "resist" the Archimedes force, which expels them out of the water, and given that their size was not reduced by shaking, it can be suggested that most of the dissolved molecular gas assembled into large bubbles, which then floated up to the surface. Accordingly, the hypothesis that the shift in luminescence towards the blue region is caused by bubble formation following shaking proved unfounded. Next, the radical reaction hypothesis was tested.
Free radicals can be called unstable molecule fragments. They are characterized by having unpaired electrons. With one electron in a pair missing, they have a lot of energy, which is unfavorable. So they "seek" the electron and can even "grab" one from another molecule.
Electrons do not move around independently in water where an electric current has been generated but use ions as "transport" – positively or negatively charged parts of a compound.
Since radical reactions may change the electrical conductivity of water, this parameter was selected to answer the question as to how radical reactions contribute to the luminescence intensity of water exposed to "stress". Being very unstable, free radicals have an extremely short lifetime. For this reason, their concentration in the system was evaluated indirectly – from hydrogen peroxide concentration.
Fig.3. The effect of mechanical stress on hydrogen peroxide concentration in water. (A) The effect of frequency of mechanical stress on hydrogen peroxide concentration (amplitude of 5 mm, exposure time of 5 min). The insert displays the effect of exposure time on hydrogen peroxide concentration (30 Hz, amplitude of 5 mm). (B) Changes in hydrogen peroxide concentration following multiple mechanical stresses. The time of a single exposure was 5 min, with the frequency and amplitude of 30 Hz and 5 mm, respectively. The time of a single exposure was 5 min, with the frequency and amplitude of 30 Hz and 5 mm, respectively. The data are presented as the mean and standard error of the mean for six independent assays. Data differ significantly from control values at p < 0.05 (*).
The rate of hydrogen peroxide formation was found to increase specifically during shaking, and it was observed to increase with higher intensity of shaking. It is known that hydrogen peroxide can generate hydroxyl radicals. Therefore, their concentration in test samples was further measured.
Figure 4. Hydroxyl radical generation in water under mechanical stress (30 Hz, amplitude of 5 mm). (A) The effect of exposure time on hydroxyl radical concentration. The insert displays the effect of frequency of mechanical stress on hydroxyl radical concentration (amplitude of 5 mm, exposure time of 5 min). (B) Changes in hydrogen peroxide concentration following multiple mechanical stresses. The time of a single exposure was 5 min, with the frequency and amplitude of 30 Hz and 5 mm, respectively. The time of a single exposure was 5 min, with the frequency and amplitude of 30 Hz and 5 mm, respectively. The data are presented as the mean and standard error of the mean for six independent assays.
As seen from the diagrams above, the concentration of hydroxyl radicals increases during mechanical stress and when its intensity is increased.
Looking back at what we started with, what is the exact process that underlies the luminescence of water exposed to mechanical stress? The scientists have found that cavitation does not significantly contribute to luminescence while the concentration of hydroxyl radicals increases under mechanical stress. Blue-spectrum emissions are the characteristic of radical reactions.
But what substance enables the formation of hydrogen peroxide? The researchers have suggested that oxygen plays an important role in this process. Dissolution of oxygen in water intensifies oxygen peroxide formation in it. If half of the oxygen is displaced by argon, an inert (slightly reactive) gas, the formation of hydrogen peroxide becomes less apparent.
The hydrodynamic theory contains the terms "laminar flow" and "turbulent flow". A turbulent flow, as may be suggested by its name, is chaotic: mechanical stresses, for instance, create turbulent, chaotic flows in a vessel. A laminar flow is ordered: there are a great number of photos and videos on the Internet featuring water jets seeming "frozen", not changing in time. These jets are examples of a laminar (highly organized) flow. The experiment examined changes in physicochemical properties of water exposed to turbulent flow mixing. However, do these properties change with laminar flow mixing?
Indeed, these characteristics were different between intact water and water exposed to mixing in laminar flow. Interestingly, when one part of "laminar" water was mixed with 99 parts of intact water, the resulting mixture was similar in its physicochemical properties to "laminar" water despite its concentration in the resultant liquid being only 1%. This means that additivity of properties does not apply to the components mixed. Namely, water exposed to mechanical impact can change the properties of intact water. This phenomenon needs further investigation.
In summary, physical impact (shaking) influences the physicochemical properties of water, i.e. its electrical conductivity and the intensity of its luminescence. These findings may be important to the pharmaceutical industry, where much focus is placed on quality control of products, which are often available in the form of aqueous solutions of biologically active substances.