ISO 25549:2026

• Abstract:

  This document specifies a method for evaluating the shear viscosity of ultrafine bubble (UFB) dispersions. The method is based on any of the three measurement devices, namely parallel-plate rotational shear, cone-plate rotational shear and coaxial-cylinder rotational shear. The average effective shear viscosity of the ultrafine bubble dispersion is calculated from the measured stress (or strain rate) and the strain rate (or stress) applied to the sample. There are commercially available instruments based on these measurement methods in the market. This document applies to UFB produced using different methods of production.

• Status: Published
• Publication Date: 17-Aug-2026
• ICS: 07.030 - Physics. Chemistry
• Technical Committee: ISO/TC 281 - Fine bubble technology
• Drafting Committee: ISO/TC 281/WG 2 - Fine bubble characterization and measurement
• Current Stage: 6060 - International Standard published
• Start Date: 18-Aug-2026
• Due Date: 20-Feb-2028
• Completion Date: 18-Aug-2026

ISO 25549:2026 is an International Standard from ISO on how to evaluate the shear viscosity of ultrafine bubble (UFB) dispersions. It specifies a rheometry-based method using parallel-plate rotational shear, cone-plate rotational shear, or coaxial-cylinder rotational shear, and it is meant for users who need comparable viscosity measurements on UFB dispersions made by different production methods.

What does ISO 25549:2026 specify?

ISO 25549:2026 specifies a method for evaluating the shear viscosity of UFB dispersions and shows how to calculate an average effective shear viscosity from measured stress and strain rate, or the reverse. The document is organized into clauses on scope, references, terms, principle, apparatus, procedure, assessment of results, measurement errors, and test report, followed by two informative annexes.

Annex	What it covers
Annex A	Features of the measurement methods, including comparison data for the three rotational shear geometries
Annex B	Influence of the matrix on viscosity

What are the key requirements of ISO 25549:2026?

ISO 25549:2026 focuses on obtaining viscosity values that are comparable across instruments, geometries, and sample histories. In practice, the document ties the result to how the sample is prepared, which device is used, and how the measurement is reported.

• Clause 4 states the principle: shear viscosity is derived from Newton's law of internal friction, using measured stress and shear rate. For the reader, this means the result is not a direct meter reading - it is a calculated rheological value.
• Clause 4 treats the three geometries separately - parallel-plate, cone-plate, and coaxial-cylinder - because each one uses different stress and shear-rate relationships. That matters because results from different geometries should be interpreted with the device type in mind.
• Clause 5 requires the wetted parts of the measuring device to be stainless steel and grounded, and it calls for temperature control with accuracy no worse than 0.5 °C. In practice, this reduces bubble adsorption on walls and keeps the sample state stable during testing.
• Clauses 5.2 to 5.4 set geometry limits linked to bubble size, including minimum clearances of at least 10 times the bubble diameter. This helps prevent bubble-bubble and wall effects from distorting the measured viscosity.
• Clause 6 requires careful preparation and storage in an airtight container without an air-liquid interface, and it records storage time. For users, this is important because UFB content and liquid properties can change between preparation and testing.
• Clause 6.3 forbids pressure-push sampling devices such as injectors and stirrers, and it requires gentle transfer close to the walls. This is meant to avoid introducing large bubbles or disturbing the UFB state.
• Clause 6.4 keeps the shear rate within 0.1 s^-1 to 1 000 s^-1, stabilizes temperature, and requires the matrix viscosity to be measured and subtracted when the UFB effect is being shown. That means the test should isolate the contribution of the bubbles, not just the base liquid.
• Clause 7 requires three repetitions and averaging, and if the standard deviation is more than 5 %, the test is repeated. This gives the user a simple repeatability check before accepting the result.
• Clause 9 specifies what must go into the test report, including the device type, software version, geometry, contact material, storage times, matrix, gas type, bubble size distribution, temperature, torque, shear rate, and the number of measurements. That makes the result traceable and easier to audit.

What terms does ISO 25549:2026 define?

ISO 25549:2026 uses the rheology vocabulary from ISO 3219-1 and ISO 3219-2, and it adds a few terms specific to UFB dispersions.

• Matrix - the liquid medium that contains the ultrafine bubbles.
• Generation time - the running time of the bubble generator, which is linked to bubble content in the dispersion.
• Storage time - the time between finishing sample preparation and starting the test.
• Ultrafine bubble (UFB) dispersion - the sample under test, consisting of ultrafine bubbles dispersed in a liquid.
• Shear viscosity - the equivalent viscosity obtained from the relationship between stress and shear rate in rotational rheometry.
• Shear stress - the applied stress used together with shear rate to calculate viscosity.
• Shear rate - the rate of deformation imposed on the sample during the measurement.

Who uses ISO 25549:2026?

ISO 25549:2026 is for laboratories, instrument operators, researchers, quality managers, and buyers who need a defined way to measure UFB dispersion viscosity. It is relevant in wastewater and groundwater treatment, froth flotation, drag reduction, agriculture, and healthcare, where UFB dispersions are used and their flow behavior needs to be compared or documented.

Which standards are used with ISO 25549:2026?

ISO 25549:2026 refers to two companion standards for rheometry:

• ISO 3219-1 - provides vocabulary and symbols for rotational and oscillatory rheometry.
• ISO 3219-2 - gives the general principles of rotational and oscillatory rheometry.

These references supply the rheology terms and framework used when interpreting the measurements in ISO 25549:2026.

What does the ISO 25549:2026 document contain?

ISO 25549:2026 contains the calculation principle, device-specific formulae for the three rotational shear methods, apparatus requirements, sampling and testing steps, result assessment, measurement error notes, and reporting items. Annex A compares the methods and includes example results for different generation times and shear rates. Annex B shows how changing the matrix affects the measured viscosity. The document also includes figures, tables, and a bibliography that support method selection and interpretation.

International
Standard
ISO 25549
First edition
Fine bubble technology —
2026-08
Evaluation method for determining
the shear viscosity of ultrafine
bubble dispersions
Technologie des fines bulles — Méthode d'évaluation pour
déterminer la viscosité en cisaillement dans les dispersions de
bulles ultrafines
Reference number
© ISO 2026
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
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or ISO’s member body in the country of the requester.
ISO copyright office
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
4.1 General .2
4.2 Parallel-plate rotational shear method .2
4.3 Cone-plate rotational shear method .3
4.4 Coaxial-cylinder rotational shear method .3
5 Apparatus . 3
5.1 General .3
5.2 Parallel-plate .4
5.3 Cone-plate .4
5.4 Coaxial-cylinder .4
5.5 Measuring device .4
5.6 Sampling devices .4
6 Procedure . 4
6.1 General .4
6.2 Preparation and storage .4
6.3 Sampling .4
6.4 Testing .5
7 Assessment of test results. 5
8 Measurement errors . 5
9 Test report . 6
Annex A (Informative) Feature of measurement methods . 7
Annex B (Informative) Influence of matrix on the viscosity .10
Bibliography .13

iii
Foreword
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iv
Introduction
Ultrafine bubble dispersions are used in a wide range of applications such as wastewater/groundwater
treatment, froth flotation, drag reduction, agriculture, and healthcare. Determining the fluid properties
and dynamic behaviours of ultrafine bubble dispersions can help to achieve precise control of them. In
practice, however, the interaction between the ultrafine bubbles and the liquid can change the flow field,
thus affecting the drag forces. This leads to significant variations in the macroscopic mechanical properties,
especially the shear viscosity, of the ultrafine bubble dispersions. Research has demonstrated that the
[1-3]
presence of ultrafine bubbles can significantly reduce the viscosity of a matrix . In some highly viscous
[3]
matrices, this reduction can be as high as 90 % . Furthermore, the extent of viscosity reduction depends
on many factors such as shear rate, bubble content, and storage time, as detailed in Annex A and References,
[2], [4] and [5].
Conventional shear viscosity measurement standards are not well suited for ultrafine bubble dispersions
due to the special properties of ultrafine bubbles, such as strong hydrogen bonding, negatively charged,
electrical double layers, hydrophobicity, adsorption, etc. Meanwhile, due to the sensitivity of gas solubility
to pressure, the sample extraction and transfer specifications are different from those for pure materials
or other multiphase fluids in general. Commonly used methods such as injection and stirring and mixing
should be avoided as much as possible. Without considering the above issues, the measurement of the shear
viscosity of ultrafine bubble dispersions can be questionable, and the results from different experiments can
be incomparable and inconsistent. An inaccurate viscosity measurement will seriously affect the assessment
of the flow process and flow state, which can involve all aspects of production, transport and application.
Therefore, a standardized method for evaluating the shear viscosity of ultrafine bubble dispersions is of
critical importance.
This document is intended to specify the evaluation method of shear viscosity of ultrafine bubble dispersions:
parallel-plate rotational shear, cone-plate rotational shear, and coaxial-cylinder rotational shear, which have
been selected from the methods most widely used in industry. In general, these rotational shear methods
allow for a wide range of adjustable shear strengths, and they also provide a higher degree of measurement
accuracy. Standardized measurement methods allow the measurement of the shear viscosity of a dispersion
of ultrafine bubbles in different liquids, as well as a variety of shear behaviours such as steady state and
oscillatory.
v
International Standard ISO 25549:2026(en)
Fine bubble technology — Evaluation method for determining
the shear viscosity of ultrafine bubble dispersions
1 Scope
This document specifies a method for evaluating the shear viscosity of ultrafine bubble (UFB) dispersions.
The method is based on any of the three measurement devices, namely parallel-plate rotational shear, cone-
plate rotational shear and coaxial-cylinder rotational shear. The average effective shear viscosity of the
ultrafine bubble dispersion is calculated from the measured stress (or strain rate) and the strain rate (or
stress) applied to the sample. There are commercially available instruments based on these measurement
methods in the market. This document applies to UFB produced using different methods of production.
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 3219-1, Rheology — Part 1: Vocabulary and symbols for rotational and oscillatory rheometry
ISO 3219-2, Rheology — Part 2: General principles of rotational and oscillatory rheometry
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3219-1 and ISO 3219-2 and the
following terms and definitions 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
matrix
liquid media containing ultrafine bubbles
3.2
generation time
running time of the bubble generator, which is positively correlated with the bubble content in the dispersion
3.3
storage time
time interval between the moment when the preparation of ultrafine bubble dispersion is completed and the
moment when the test starts
4 Principle
4.1 General
The effective shear viscosity of the ultrafine bubble dispersion is determined by Newton's law of internal
friction (Formulae (1) and (2)).

  (1)

u

  (2)
h
where
µ is equivalent shear viscosity of the ultrafine bubble dispersion (Pa⋅s ),
τ is the shear stress (Nm/ ),
−1
γ is the shear rate (s ),
∆u is the velocity difference between the fast and slow fluid layers (ms/ ),
∆h is the distance between the fast and slow fluid layers ( m ).
The three devices, parallel-plate, cone-plate and coaxial-cylinder, correspond to different shear styles, as
shown in Figure 1. Therefore, there are different ways of calculating the shear stress τ and shear rate γ for
the three measurement methods. Comparisons of their measurement results are shown in Annex A.
(a) Parallel-Plate (b) Cone-Plate (c) Coaxial-Cylinder
Figure 1 — Different Devices of Shear Viscosity Measurement.
4.2 Parallel-plate rotational shear method
For parallel-plate rotational shear, the shear stress and shear rate are determined by the following
formulae (3) and (4), yielding
2M
  (3)
R
R

  (4)
h
where
M is the torque of the rotor (Nm⋅ ),
R is the maximum radius of the rotor ( m ),
ω is the angular velocity of rotation of the rotor (rads/ ).
4.3 Cone-plate rotational shear method
For cone-plate rotational shear, the shear stress and shear rate are determined by the following formulae (5)
and (6), yielding
3M
  (5)
2R


  (6)

where,β is the angle between the cone of the rotor and the fixed flat plate (rad ).
4.4 Coaxial-cylinder rotational shear method
For coaxial-cylinder rotational shear, the shear stress and shear rate are determined by the following
formulae (7) and (8), yielding
M
  (7)
2LR
b
RR 

cb

  (8)
2 RR

cb
where,
R is the inner radius of the annular gap ( m ),
b
R is the outer radius of the annular gap ( m ),
c
L is the measuring section length of coaxial-cylinder ( m ).
5 Apparatus
5.1 General
Parallel-plate measuring devices are a fixed plate and a circular plate rotor, cone-plate measuring devices
are a fixed plate and a cone plate rotor, and coaxial-cylinder measuring devices are two coaxial cylinders,
one fixed and the other rotating.
Due to the charge and adsorption properties of ultrafine bubbles, all parts of the measuring devices that are
in direct contact
...