ISO 7392:2024

Fine bubble technology — Evaluation method for determining surface tension of ultrafine bubble dispersions

This document specifies evaluation methods for surface tension of ultrafine bubble (UFB) dispersion in water. Three test methods, Wilhelmy, du Noüy and the pendant drop method, are adopted because of their advantages to detect small change in surface tension by UFB dispersion in water and the high accessibility to commercially available instruments. This document can be used to measure the surface tension of liquid containing UFB dispersion in dilute surfactant water solution such as detergent or machining coolant as well as UFB dispersion in water. NOTE Measurement data of liquid containing UFB dispersion in dilute surfactant water solution are summarized in Annex D.

Titre manque

General Information

• Status: Published
• Publication Date: 08-Apr-2024
• ICS: 07.030 - Physics. Chemistry
• Technical Committee: ISO/TC 281 - Fine bubble technology
• Drafting Committee: ISO/TC 281 - Fine bubble technology
• Current Stage: 6060 - International Standard published
• Start Date: 09-Apr-2024
• Due Date: 22-Apr-2025
• Completion Date: 09-Apr-2024

Standards Content (Sample)

International
Standard
ISO 7392
First edition
Fine bubble technology —
2024-04
Evaluation method for determining
surface tension of ultrafine bubble
dispersions
Reference number
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Apparatus . 2
4.1 General .2
4.2 Wilhelmy method .2
4.3 du Noüy method .2
4.4 Pendant drop method .2
5 Procedure . 2
5.1 Testing environment .2
5.2 Temperature measurement .2
5.3 Handling of the sample . .3
5.4 Cleaning of the measuring unit .3
5.4.1 Cleaning of the plate and ring .3
5.4.2 Cleaning of the measuring cup and syringe .3
5.5 Determination .3
5.6 Measurement of blank water .3
6 Calculation and expression of results . 3
7 Test report . 4
Annex A (informative) Specifications for commercially available measurement instruments . 5
Annex B (informative) Feature of the measurement methods . 6
Annex C (informative) Measurement results under various measuring conditions .12
Annex D (informative) Variation in surface tension of surfactant solution diluted with UFBD.16
Bibliography .20

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 281, Fine bubble technology.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Ultrafine bubble (UFB) dispersion in water has been used to take advantage of UFB’s ability to slip into
narrow space in various technologies such as desalting from the surface of concrete structure, scrubbing
a stain on the floor of restroom, since the early period in the development of UFB technology. Recently
some appliances such as showerhead and washing machine have installed a nozzle to generate UFBs. The
mechanism of removing substances by UFBs is still under investigation, however, squeezing of bubbles
between substrate and substances to be removed is presumed to play an important role. Surface tension is
inferred to be one of the basic characteristics pertinent to such behaviour of UFBs.
The standardization of measuring method for surface tension is necessary to evaluate the performance of
those products using UFBs. The application of measurement technique of surface tension to UFB dispersion
in water requires however, special attention to the fact that the change in surface tension caused by UFBs
is rather small compared to that caused by surface active agent. Furthermore, UFB dispersion in water is
a mixture of water, UFBs and impurities in water, and hence its surface tension can behave in a different
manner compared to that of pure material or homogeneous solution.
This document is intended to specify the evaluation method of surface tension of UFB dispersion in water
using three measurement methods, Wilhelmy, du Noüy and the pendant drop method, which have been
chosen from those widely used in industries. The standardized evaluation method also enables to measure
the surface tension of liquid containing UFB dispersion in dilute surfactant water such as detergent or
machining coolant, leading to an easy choice for surfactant matching to UFBs.

v
International Standard ISO 7392:2024(en)
Fine bubble technology — Evaluation method for determining
surface tension of ultrafine bubble dispersions
1 Scope
This document specifies evaluation methods for surface tension of ultrafine bubble (UFB) dispersion in water.
Three test methods, Wilhelmy, du Noüy and the pendant drop method, are adopted because of their
advantages to detect small change in surface tension by UFB dispersion in water and the high accessibility
to commercially available instruments.
This document can be used to measure the surface tension of liquid containing UFB dispersion in dilute
surfactant water solution such as detergent or machining coolant as well as UFB dispersion in water.
NOTE Measurement data of liquid containing UFB dispersion in dilute surfactant water solution are summarized
in Annex D.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 304:1985, Surface active agents — Determination of surface tension by drawing up liquid films
ISO 19403-3:2017, Paints and varnishes — Wettability — Part 3: Determination of the surface tension of liquids
using the pendant drop method
ISO 20480-1, Fine bubble technology — General principles for usage and measurement of fine bubbles — Part 1:
Terminology
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 20480-1 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
UFB dispersion
UFBD
liquid which contains ultrafine bubbles
[1]
[SOURCE: ISO 21255:2018, 3.2 ]

4 Apparatus
4.1 General
Measuring instruments which are designed for the following established measuring methods, Wilhelmy,
du Noüy and the pendant drop methods, are used with ordinary laboratory apparatus. The instrument
for Wilhelmy and du Noüy method and that for the pendant drop method, which are usually commercially
available, shall be in accordance with ISO 304:1985, Clause 5 and ISO 19403-3:2017, Clause 5, respectively.
In most cases, the instruments are computerized, and measurement and data analysis software are installed.
Measurement is conducted with measuring unit, a rectangular plate in Wilhelmy, a ring in du Noüy, and a
needle in the pendant drop method. The measuring units should be chosen from those recommended by
manufacturers of instruments.
A measuring cup for the Wilhelmy and du Noüy methods such as a petri dish and a syringe for the pendant
drop method, which hold the sample liquid to be tested, should be made of glass to avoid adsorption of UFBs
[1],[2]
on the wall thereof.
Special attention should be paid to the cleanliness of apparatus to be in contact with the sample liquid
because the change in surface tension by the presence of UFBs is expected to be small in the measurement.
NOTE Specifications for commercially available measurement instruments are provided in Annex A.
4.2 Wilhelmy method
When a plate for measurement is distorted, it should be turned back to normal shape or replaced with a
brand-new one.
4.3 du Noüy method
When a ring for measurement is distorted, it should be turned back to normal shape or replaced with a
brand-new one.
A special jig to restore the original shape can be provided from the manufacturer of tensiometer.
4.4 Pendant drop method
Outer diameter of needle for the pendant drop method d should be chosen based on the manufacturer’s
o
instruction. The relationship between d and σ/ρ the ratio of surface tension σ and sample density ρ
o
illustrated in ISO 19403-3:2017, Clause 5, Figure 2, is an informative guide to find an appropriate needle size.
5 Procedure
5.1 Testing environment
Measurement should be carried out in an air-conditioned room.
The use of a heat regulating unit for a measuring instrument to keep the temperature of sample liquid
constant is more desirable.
5.2 Temperature measurement
The temperature of the sample shall be determined immediately before the measurement.
When the measurement is replicated for the same sample liquid, the temperature of the sample should be
monitored during the measurement.
Fluctuation of the sample temperature should be kept within 2 °C.

To prevent the contamination during the temperature measurement, attention shall be paid to the cleanliness
of the sensor surface of the thermometer.
Temperature should be read with an accuracy of 0,1 °C.
5.3 Handling of the sample
A sample should be kept in an airtight container without air-liquid interface under constant temperature
[1],[2]
until immediately before testing.
To avoid the increase and decrease in the number of ultrafine bubbles during sampling, the following
practical measures are recommended: pouring a sample liquid gently into a measuring cup and syringing a
sample liquid slowly.
To keep the homogeneity of the sample liquid, gentle stirring with a motor-driven drum roller illustrated in
[3]
ISO 20298-1:2018, Annex A, is recommended.
5.4 Cleaning of the measuring unit
5.4.1 Cleaning of the plate and ring
Before every measurement, immersion cleaning with ethanol followed by heating to red heat with the
alcohol lamp should be carried out.
A plate and a ring shall be kept in ambient temperature after heating to avoid deformation by quenching
with the sample liquid.
5.4.2 Cleaning of the measuring cup and syringe
Immersion cleaning with neutral detergent followed by rinsing well with purified or distilled water should
be carried out before measurement.
5.5 Determination
The horizontality of instrument shall be confirmed before determination.
As for the pendant drop method, vibrations and air flow shall be minimized. Special attention should be paid
to the reduction in vibration of droplet during measurement when a small size of needle is chosen. Intense
exposure to light from outside shall be avoided to keep the precision in optical profilometry of droplet.
Carry out the determination on the sample to be analysed according to the manufacturer’s instructions for
each instrument.
5.6 Measurement of blank water
To explicitly show the influence of UFBs on surface tension, the surface tension of blank water shall be
determined with the methods described in 5.5 and subtract data thereof from those of the sample to be
analysed.
NOTE Raw water for fine bubble generation can be used as blank water.
6 Calculation and expression of results
For the practical procedure, it is recommended to use the software supplied by the manufacturers of the
instruments.
Alternatively, for Wilhelmy and du Noüy methods, calculation methods are specified in ISO 304:1985,
Clause 7.
NOTE 1 The pendant drop method assumes the usage of software for optical profilometry of water drop and
calculation of surface tension.
NOTE 2 The feature of the three measurement methods is demonstrated in Annex B.
NOTE 3 The influence of measuring condition on measurement results is demonstrated in Annex C.
7 Test report
The test report shall include the following information.
a) the test method used, together with a reference to this document, i.e. ISO 7392:2024 and information
about the measuring instrument used:
— the name of the instrument and its manufacture’s name, version of software;
— the specification of the needle used in the pendant drop method;
b) the nature of the water used:
— property of the water sample such as pH, electroconductivity, etc.;
c) the measuring conditions:
— the temperature of the water when the measurement was carried out;
— the retention time of a droplet in the measurement by the pendant drop method;
d) th
...