Introduction

Applications of fine bubble technologies can be found in cleaning, environmental improvement, the food and drink sector, aeration systems, medicine, water and waste water treatment, as well as agriculture and aquaculture. Developing appropriate terminology for such diverse technologies is therefore critical to business trade or product acceptance by consumers.

Fine bubbles can be present in both liquids and solids. Fine bubbles can contain air or another gas. The bubble can be held in place by surface tension or be surrounded with a coating, e.g. a lipid. Fine bubbles generated for various applications can vary in size, gas content or bubble coating. The generation techniques used are also different.

It should be noted that the motion of bubbles in a medium can be determined by buoyancy forces or randomly and thermally activated processes leading to Brownian motion. For this reason, larger bubbles can display buoyant behaviour (rise upwards) and smaller bubbles remain in the liquid medium displaying random motion. This document focuses on the definitions of such entities.

In recent years, readily available measurement techniques of bubbles have made it possible to characterize microbubbles and ultrafine bubbles. Such techniques have shown that ultrafine bubbles can almost remain as they are for a number of months.

Fine bubble technologies are very new, and their applications are useful in a number of fields today. Developing appropriate terminology for such a diverse area of technology is therefore critical to business trade or product acceptance, in view of the wide range of users of fine bubbles.

For better communication among the users of fine bubbles, this document introduces the quality criteria of a medium such as water, as well as two indices, one for size and the other for number concentration. This document also provides an explanation for classifying fine bubbles by dimensional characteristics and by rise velocity.

It should be noted that the motion of bubbles in a medium can be determined by buoyancy forces or randomly and thermally activated processes leading to Brownian motion. For this reason, larger bubbles can display buoyant behaviour (rise upwards) and smaller bubbles remain in the liquid medium displaying random motion. This document focuses on the definitions of such entities.

Use of the terms “fine bubble” or “ultrafine bubble”, instead of “nanobubble” in ISO/TC 281 documents

 

ISO/TC 281 recognizes that the term “nanobubble” is commonly used to mean “ultrafine bubble”. However, “nanobubble” is not defined clearly, and thus the use of “ultrafine bubble” is strongly preferred.

The measurement and characterization of ultrafine bubbles became possible comparatively recently. Ultrafine bubbles are not visible to the naked eye. ISO/TC 229 defines nano-objects as having a size less than 100 nm. Ultrafine bubbles are defined with the size range of 1 μm to a few nm. The smallest size of an ultrafine bubble depends upon the minimum number of molecules that can contain gas. Figure A.1 shows, with the straight line, the size range of nano-objects (including bubbles) specified in ISO/TS 80004-1. This indicates that their diameters are 100 nm or less.

Recent developments of measurement technology of nano scale have proven that there are very small invisible bubbles of some sizes, which are stationary in liquid for a long time. Such bubbles, which are the major subject of discussion in ISO/TC 281, range approximately from 100 nm to 200 nm in diameter, measured in water.

Although nano-objects are specified as 100 nm or less in ISO/TS 80004-1, there is no scientific or convincing evidence for the time being that there is a difference in physical phenomena of ultrafine bubbles with the dividing line of 100 nm.

Therefore, ISO/TC 281 does not use the term “nanobubble” throughout the fine bubble documents it develops, and instead uses “fine bubble” or “ultrafine bubble” in order to avoid confusion in the industry, the market, and in international standardization activities.

In ISO/TC 281 documents, the term “nanobubble” appears only in this particular annex as an exception for the purpose of explaining why ISO/TC 281 does not use this term.

Fine bubbles dispersed in water may be defined within regions as shown in Figure 1, based on size and number concentration. This type of categorisation can be used to define which areas of the matrix are most appropriate for certain applications.

Key

X  –  size (μm)

Y  –  number concentration (per ml)

 

EXAMPLE Ultrafine bubbles with a size ranging from 0,1 μm to 1 μm and number concentration ranging from 106/ml to 107/ml are expressed by the class indicated by the hatched box.

 

Figure 1 — Fine bubble water dispersion categorization

 

Attributes of fine bubbles by rise velocity

The number concentration of fine bubbles contained in a liquid decreases with time. One reason for this decrease is that a portion of the bubbles rise to the surface due to the buoyant force and disappear in the air.

 

Rise velocity depends on bubble size.

 

Fine bubbles with a very low rise velocity can remain in liquids for a long time. This extended lifetime of fine bubbles may then enable a variety of industrial applications.

 

Fine bubbles with a high rise velocity can also be applicable to a variety of industrial applications. These include:

  • using bubble rise to separate materials,
  • using the gas-liquid surface area of fine bubbles for the mass transport of materials,
  • generating bubble towers through gas-liquid reactions, etc.

 

NOTE 1: It seems to be effective to use fine bubbles for the purpose of floatation separation, where the rise velocity of a group of fine bubbles is 4,0 m/h (about 1,1 mm/s) or higher.

NOTE 2: In the case of water, a threshold bubble terminal rise velocity for effective rise would be about 1 mm/s.

Observation of ULTRAFINE bubble number stability in liquids

 

1 General

This shows examples of bubble number stability of ultrafine bubbles in water, over time. The examples are based on work carried out by two research institutes, Fushiki T. Introduction of Food Flavour to Water by Using Nano-Bubble Generator, Institute of Food Technologists Annual Meeting 2012, Las Vegas, NV, June 25-28 ( 2012), and Terasaka K. “Applications and International Standardization of Fine Bubble Technology”, Proceedings of the 5th International Symposium on Fine Bubble Technology, ( 2014).

2 Example 1

As shown in Figure A.1, for ultrafine bubbles where the initial number concentration index was about (0,8 to 1,4) E+8 bubbles/ml, the number concentration index remained steady for about 3 months.

Key

X  –  time (day)

Y  –  total number of ultrafine bubbles in water per ml

 

Figure A.1 — Example 1: Observation of bubble number stability of ultrafine bubbles in water over time

3 Example 2

Figure A.2 shows the differences in preservation temperatures and in the initial number concentration index of ultrafine bubbles.

As shown in Figure A.1, for ultrafine bubbles where the initial number concentration index was about (0,8–1,4) E+8 bubbles/ml, the number concentration index remained steady for about 3 months.

Key

X  –  time (day)

Y  –  number concentration index (1,0E+6 pcs/ml)

  • water temperature 25 °C; initial number concentration index 13 × 106
  • water temperature 4 °C; initial number concentration index 13 × 106

water temperature 4 °C; initial number concentration index 58 × 106

 

Figure A.2 — Example 2: Observation of bubble number concentration of ultrafine bubbles in water over time

 

NOTE: Samples were left at rest in an isolation bottle.