Paper and board -- Measurement of water contact angle by optical methods

This document specifies the method for optical assessment of the contact angle between water and the surface of paper and board, where the process of droplet formation, application to planar substrates, or measurement of the droplet shape in contact with the solid is performed by automated equipment. The limits of measurement are determined by the capabilities of the instrumentation used. The instrumental capabilities defined by this document use a digital image capturing system operating at a minimum of 50 frames per second and needs the ability to perform the first measurement after no more than 20 ms to 40 ms contact between the droplet and substrate. The test method is applicable to most kinds of paper or board however it cannot be applicable to structured materials.

Papier et carton -- Mesurage de l'angle de contact de l'eau par des méthodes optiques

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Status
Published
Publication Date
10-Jan-2021
Current Stage
5060 - Close of voting Proof returned by Secretariat
Start Date
21-Dec-2020
Completion Date
21-Dec-2020
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TECHNICAL ISO/TS
SPECIFICATION 14778
First edition
2021-01
Paper and board — Measurement of
water contact angle by optical methods
Papier et carton — Mesurage de l'angle de contact de l'eau par des
méthodes optiques
Reference number
ISO/TS 14778:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TS 14778:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TS 14778:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and references ...................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 2

5 Reagents ........................................................................................................................................................................................................................ 3

6 Apparatus ..................................................................................................................................................................................................................... 3

7 Sampling ........................................................................................................................................................................................................................ 5

8 Conditioning .............................................................................................................................................................................................................. 5

9 Preparation of test pieces ........................................................................................................................................................................... 5

10 Procedure..................................................................................................................................................................................................................... 6

10.1 General ........................................................................................................................................................................................................... 6

10.2 Mounting of the test piece ............................................................................................................................................................. 6

10.3 Calibration and adjustment of apparatus ........................................................................................................................ 6

10.4 Method of measurement ................................................................................................................................................................ 7

11 Expression of results ........................................................................................................................................................................................ 8

12 Test report ................................................................................................................................................................................................................... 8

Annex A (informative) Precision ............................................................................................................................................................................... 9

Bibliography .............................................................................................................................................................................................................................12

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TS 14778:2021(E)
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 documents 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).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

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 6, Paper, board and pulps, Subcommittee

SC 2, Test methods and quality specifications for paper and board.

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 © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TS 14778:2021(E)
Introduction

The interaction between the liquid and the solid phases influences the contact angle. Contact angles

above 90° define a situation where the liquid is repelled by the solid; below 90° defines a situation of

attraction where the liquid wets the surface. The magnitude above or below 90° shows the relative

degree of repulsion or attraction between the two phases.

On many surfaces the contact angle varies with duration of contact through a combination of spreading,

penetration (in the case of porous substrates) and evaporation. Both manual and automated apparatus

are available for optical measurement of contact angle, but automated equipment is preferred for

precision and rapid measurement, and because it is often applicable to a wider combination of liquid

and paper or board samples.

Contact angle measurement is used to predict how liquids interact with paper surfaces. This document

describes the most common form of test, using water as probe liquid, and from the data a probable

interaction between the paper surface and another liquid with comparable surface tension and viscosity

characteristics is often inferred. The veracity of this inference should be tested wherever possible.

Notwithstanding the above, contact angle measurement is used widely as a predictive tool in several

industrial settings, for example:

a) for assessing writing, ruling or printing quality with water-based or solvent based inks (e.g. in

inkjet, gravure or flexographic printing);
b) for gluing applications;
c) for wet offset lithographic printing;
d) for hot-foil applications;
e) for barrier or release coatings;
f) for coating applications.

In some cases, measurements of contact angle are used to calculate two or three components of the

surface energy of the paper or board, which requires the use of two or three liquids of known surface

energy, respectively. In such cases, the values calculated for the components of surface energy will be

[1]

related to the liquids chosen for the analysis . This is not covered in this document.

The test method described in this document is sometimes known as ‘static’ or even ‘sessile drop’,

since on many surfaces the droplet remains static and in equilibrium with the paper surface and air.

Yet on paper and board surfaces the droplet often changes its dimensions with time, due to sorption

and wetting phenomena. This has led some instrument manufacturers and researchers to describe the

automated optical technique outlined in this document as measurement of the ‘dynamic’ contact angle.

This document does not use the term ‘dynamic’ because this nomenclature confuses a measurement

changing with respect to time with one that changes due to a plane of shear. For example, the Willhelmy

plate method of contact angle measurement, which measures the force required to push a solid material

[2]
into and then pull it out from a liquid reservoir, is a true ‘dynamic’ method .

Similarly, this document does not cover the situation where a droplet is placed on a horizontal surface

that is subsequently tilted so that gravity causes the droplet to assume an asymmetric shape, then to

commence movement.
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
TECHNICAL SPECIFICATION ISO/TS 14778:2021(E)
Paper and board — Measurement of water contact angle by
optical methods
1 Scope

This document specifies the method for optical assessment of the contact angle between water and the

surface of paper and board, where the process of droplet formation, application to planar substrates, or

measurement of the droplet shape in contact with the solid is performed by automated equipment.

The limits of measurement are determined by the capabilities of the instrumentation used. The

instrumental capabilities defined by this document use a digital image capturing system operating at

a minimum of 50 frames per second and needs the ability to perform the first measurement after no

more than 20 ms to 40 ms contact between the droplet and substrate.

The test method is applicable to most kinds of paper or board however it cannot be applicable to

structured materials.
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 186, Paper and board — Sampling to determine average quality

ISO 187, Paper, board and pulps — Standard atmosphere for conditioning and testing and procedure for

monitoring the atmosphere and conditioning of samples

ISO 13530, Water quality – Guidance on analytical quality control for chemical and physicochemical water

analysis
3 Terms and references
For the purposes of this document, the following terms and definitions apply:
3.1
baseline

line of contact between the water droplet and paper or board surface, determined by optical means and

taken as the plane from which the contact angle is measured

Note 1 to entry: Precise measurement of contact angle requires precise assessment of the baseline. For the

optical system to achieve the best estimate of the baseline, it may be necessary to tilt the camera so that it views

the droplet slightly from above, at a shallow angle (typically 0-3° relative to the horizontal), rather than directly

from the side (0°). Experience shows this small change in viewing angle does not affect the measured contact

[3]
angle value but can enhance detection of the baseline .
3.2
contact angle

angle to a baseline (3.1), formed by means of a tangent on the droplet contour through one of the three-

phase points at the specified contact time
Note 1 to entry: The contact angle is expressed in degrees (°).
© ISO 2021 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/TS 14778:2021(E)
3.3
contact time

duration of contact(s) between the water droplet and the test piece surface, as measured automatically

from the instant at which the timing mechanism is triggered to the instant of measurement

3.4
droplet height

distance from the top of the droplet in contact with the test piece surface as measured from the baseline

(3.1) at the specified contact time (3.3)
Note 1 to entry: The droplet height is expressed in millimetres.
3.5
droplet base diameter

maximum width of the droplet base in contact with the test piece surface, as measured along the

baseline (3.1) at the specified contact time (3.3)
Note 1 to entry: The droplet base diameter is expressed in millimetres.
3.6
three-phase point

point of intersection of the liquid/solid, liquid/gaseous and solid/gaseous boundary lines

4 Principle

A water droplet is applied to the horizontal planar surface of paper or board, the angle of contact it

makes with the material is assessed by optical digital imaging.
Figure 1 — Principle of measurement
2 © ISO 2021 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/TS 14778:2021(E)
Key
1 substrate
D droplet base diameter
H droplet height
Θ contact angle
5 Reagents

5.1 Distilled water, with a surface tension of at least 72 mN/m at 23 °C, as specified in ISO 13530.

NOTE 1 This method can be used with other liquids providing they can be delivered in the form of a droplet

by the liquid delivery system (6.1.3) described in this document, and providing they do not cause substantial

swelling of the substrate over the time scale of the measurements such that the baseline can no longer be defined.

Also, for liquids with high vapour pressure, the rate of evaporation may be too fast to allow the technique to be

used, unless performed in a vapour-saturated atmosphere (e.g. a closed glass cuvette containing a few drops of

solvent). Contact angle values produced using other liquids are not comparable with those produced using water.

NOTE 2 Some contact angle measuring apparatus (6.1) have the facility to heat liquids prior to droplet

formation. Heating alters both surface tension and viscosity, both of which can affect contact angle. If the liquid is

heated, this is a deviation from the standard.

NOTE 3 Many contact angles measuring apparatus (6.1) are also capable of measuring liquid surface tension

and can be used to examine water surface tension. It is recommended to periodically check that the water has not

been contaminated as it would have a direct effect on the contact angle readings.

6 Apparatus
6.1 Contact angle measuring apparatus.
Key
1 platform
2 light source
3 droplet
4 pump
5 liquid reservoir
6 droplet application system
7 video camera
8 substrate
Figure 2 — General schematic of automated contact angle apparatus
© ISO 2021 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/TS 14778:2021(E)

For the optical determination of contact angles on a horizontal, planar substrate, any type of apparatus

having the general geometry shown in Figure 2 and further conforming to the requirements in the

following sub clauses is used.

NOTE This schematic is not intended to describe all possible instrumental arrangements. Accordingly,

instruments without a platform, with separated liquid container, with different kind of construction or those

using different viewing arrangements, are also permitted.

6.1.1 Platform with a holder mechanism (key item 1), to secure the test piece, capable of maintaining

flatness of the test piece over the duration of the test. The platform shall be horizontal.

NOTE The “platform” can be a table-top and the holder can be the housing of the device.

6.1.2 Light source (key item 2), capable of illuminating the droplet, with suitable heat filters, if

needed to eliminate heating of the test piece or water droplet. The illumination system shall produce

homogeneous lighting over the entire field of view.

6.1.3 Liquid delivery system (key item 5), consisting of a water reservoir and a dosing mechanism,

capable of producing a water droplet of known volume to a tolerance of +/- 10 %. If the droplet volume is

determined by the pumped volume, are shall be taken that no air bubbles are present as they will cause

significant volume error.

NOTE 1 The volume relates to the droplet produced at the end of a capillary, not necessarily to the volume

applied to the test piece surface. After release of the droplet there will always be a small portion of water

remaining on the capillary, the amount of which will relate to the water, capillary diameter, capillary type and

material, and droplet application technique.

NOTE 2 The dispensing volume and reproducibility of dosing system can be checked using an impermeable

reference surface.

NOTE 3 Some contact angle measuring apparatus enable the volume calculation directly from pendant droplet

in which case air bubbles won’t affect the final droplet volume.

6.1.4 Droplet application system (key item 6), allowing a water droplet of specified volume to be

suspended at the end of a capillary before being applied to a paper or board surface under controlled

conditions. The diameter and material of the capillary shall be chosen for application o

...

TECHNICAL ISO/TS
SPECIFICATION 14778
First edition
Paper and board — Measurement of
water contact angle by optical methods
Papier et carton — Mesurage de l'angle de contact de l'eau par des
méthodes optiques
PROOF/ÉPREUVE
Reference number
ISO/TS 14778:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TS 14778:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

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: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TS 14778:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and references ...................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 2

5 Reagents ........................................................................................................................................................................................................................ 3

6 Apparatus ..................................................................................................................................................................................................................... 3

7 Sampling ........................................................................................................................................................................................................................ 5

8 Conditioning .............................................................................................................................................................................................................. 5

9 Preparation of test pieces ........................................................................................................................................................................... 5

10 Procedure..................................................................................................................................................................................................................... 6

10.1 General ........................................................................................................................................................................................................... 6

10.2 Mounting of the test piece ............................................................................................................................................................. 6

10.3 Calibration and adjustment of apparatus ........................................................................................................................ 6

10.4 Method of measurement ................................................................................................................................................................ 7

11 Expression of results ........................................................................................................................................................................................ 8

12 Test report ................................................................................................................................................................................................................... 8

Annex A (informative) Precision ............................................................................................................................................................................... 9

Bibliography .............................................................................................................................................................................................................................12

© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO/TS 14778:2020(E)
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 documents 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).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

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 6, Paper, board and pulps, Subcommittee

SC 2, Test methods and quality specifications for paper and board.

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 PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TS 14778:2020(E)
Introduction

The interaction between the liquid and the solid phases influences the contact angle. Contact angles

above 90° define a situation where the liquid is repelled by the solid; below 90° defines a situation of

attraction where the liquid wets the surface. The magnitude above or below 90° shows the relative

degree of repulsion or attraction between the two phases.

On many surfaces the contact angle varies with duration of contact through a combination of spreading,

penetration (in the case of porous substrates) and evaporation. Both manual and automated apparatus

are available for optical measurement of contact angle, but automated equipment is preferred for

precision and rapid measurement, and because it is often applicable to a wider combination of liquid

and paper or board samples.

Contact angle measurement is used to predict how liquids interact with paper surfaces. This document

describes the most common form of test, using water as probe liquid, and from the data a probable

interaction between the paper surface and another liquid with comparable surface tension and viscosity

characteristics is often inferred. The veracity of this inference should be tested wherever possible.

Notwithstanding the above, contact angle measurement is used widely as a predictive tool in several

industrial settings, for example:

a) for assessing writing, ruling or printing quality with water-based or solvent based inks (e.g. in

inkjet, gravure or flexographic printing);
b) for gluing applications;
c) for wet offset lithographic printing;
d) for hot-foil applications;
e) for barrier or release coatings;
f) for coating applications.

In some cases, measurements of contact angle are used to calculate two or three components of the

surface energy of the paper or board, which requires the use of two or three liquids of known surface

energy, respectively. In such cases, the values calculated for the components of surface energy will be

[1]

related to the liquids chosen for the analysis . This is not covered in this document.

The test method described in this document is sometimes known as ‘static’ or even ‘sessile drop’,

since on many surfaces the droplet remains static and in equilibrium with the paper surface and air.

Yet on paper and board surfaces the droplet often changes its dimensions with time, due to sorption

and wetting phenomena. This has led some instrument manufacturers and researchers to describe the

automated optical technique outlined in this document as measurement of the ‘dynamic’ contact angle.

This document does not use the term ‘dynamic’ because this nomenclature confuses a measurement

changing with respect to time with one that changes due to a plane of shear. For example, the Willhelmy

plate method of contact angle measurement, which measures the force required to push a solid material

[2]
into and then pull it out from a liquid reservoir, is a true ‘dynamic’ method .

Similarly, this document does not cover the situation where a droplet is placed on a horizontal surface

that is subsequently tilted so that gravity causes the droplet to assume an asymmetric shape, then to

commence movement.
© ISO 2020 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
TECHNICAL SPECIFICATION ISO/TS 14778:2020(E)
Paper and board — Measurement of water contact angle by
optical methods
1 Scope

This document specifies the method for optical assessment of the contact angle between water and the

surface of paper and board, where the process of droplet formation, application to planar substrates, or

measurement of the droplet shape in contact with the solid is performed by automated equipment.

The limits of measurement are determined by the capabilities of the instrumentation used. The

instrumental capabilities defined by this document use a digital image capturing system operating at

a minimum of 50 frames per second and needs the ability to perform the first measurement after no

more than 20 ms to 40 ms contact between the droplet and substrate.

The test method is applicable to most kinds of paper or board however it cannot be applicable to

structured materials.
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 186, Paper and board — Sampling to determine average quality

ISO 187, Paper, board and pulps — Standard atmosphere for conditioning and testing and procedure for

monitoring the atmosphere and conditioning of samples

ISO 13530, Water quality – Guidance on analytical quality control for chemical and physicochemical water

analysis
3 Terms and references
For the purposes of this document, the following terms and definitions apply:
3.1
baseline

line of contact between the water droplet and paper or board surface, determined by optical means and

taken as the plane from which the contact angle is measured

Note 1 to entry: Precise measurement of contact angle requires precise assessment of the baseline. For the

optical system to achieve the best estimate of the baseline, it may be necessary to tilt the camera so that it views

the droplet slightly from above, at a shallow angle (typically 0-3° relative to the horizontal), rather than directly

from the side (0°). Experience shows this small change in viewing angle does not affect the measured contact

[3]
angle value but can enhance detection of the baseline .
3.2
contact angle

angle to a baseline (3.1), formed by means of a tangent on the droplet contour through one of the three-

phase points at the specified contact time
Note 1 to entry: The contact angle is expressed in degrees (°).
© ISO 2020 – All rights reserved PROOF/ÉPREUVE 1
---------------------- Page: 6 ----------------------
ISO/TS 14778:2020(E)
3.3
contact time

duration of contact(s) between the water droplet and the test piece surface, as measured automatically

from the instant at which the timing mechanism is triggered to the instant of measurement

3.4
droplet height

distance from the top of the droplet in contact with the test piece surface as measured from the baseline

(3.1) at the specified contact time (3.3)
Note 1 to entry: The droplet height is expressed in millimetres.
3.5
droplet base diameter

maximum width of the droplet base in contact with the test piece surface, as measured along the

baseline (3.1) at the specified contact time (3.3)
Note 1 to entry: The droplet base diameter is expressed in millimetres.
3.6
three-phase point

point of intersection of the liquid/solid, liquid/gaseous and solid/gaseous boundary lines

4 Principle

A water droplet is applied to the horizontal planar surface of paper or board, the angle of contact it

makes with the material is assessed by optical digital imaging.
Figure 1 — Principle of measurement
2 PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/TS 14778:2020(E)
Key
1 substrate
D droplet base diameter
H droplet height
Θ contact angle
5 Reagents

5.1 Distilled water, with a surface tension of at least 72 mN/m at 23 °C, as specified in ISO 13530.

NOTE 1 This method can be used with other liquids providing they can be delivered in the form of a droplet

by the liquid delivery system (6.1.3) described in this document, and providing they do not cause substantial

swelling of the substrate over the time scale of the measurements such that the baseline can no longer be defined.

Also, for liquids with high vapour pressure, the rate of evaporation may be too fast to allow the technique to be

used, unless performed in a vapour-saturated atmosphere (e.g. a closed glass cuvette containing a few drops of

solvent). Contact angle values produced using other liquids are not comparable with those produced using water.

NOTE 2 Some contact angle measuring apparatus (6.1) have the facility to heat liquids prior to droplet

formation. Heating alters both surface tension and viscosity, both of which can affect contact angle. If the liquid is

heated, this is a deviation from the standard.

NOTE 3 Many contact angles measuring apparatus (6.1) are also capable of measuring liquid surface tension

and can be used to examine water surface tension. It is recommended to periodically check that the water has not

been contaminated as it would have a direct effect on the contact angle readings.

6 Apparatus
6.1 Contact angle measuring apparatus.
Key
1 platform
2 light source
3 droplet
4 pump
5 liquid reservoir
6 droplet application system
7 video camera
8 substrate
Figure 2 — General schematic of automated contact angle apparatus
© ISO 2020 – All rights reserved PROOF/ÉPREUVE 3
---------------------- Page: 8 ----------------------
ISO/TS 14778:2020(E)

For the optical determination of contact angles on a horizontal, planar substrate, any type of apparatus

having the general geometry shown in Figure 2 and further conforming to the requirements in the

following sub clauses is used.

NOTE This schematic is not intended to describe all possible instrumental arrangements. Accordingly,

instruments without a platform, with separated liquid container, with different kind of construction or those

using different viewing arrangements, are also permitted.

6.1.1 Platform with a holder mechanism (key item 1), to secure the test piece, capable of maintaining

flatness of the test piece over the duration of the test. The platform shall be horizontal.

NOTE The “platform” can be a table-top and the holder can be the housing of the device.

6.1.2 Light source (key item 2), capable of illuminating the droplet, with suitable heat filters, if

needed to eliminate heating of the test piece or water droplet. The illumination system shall produce

homogeneous lighting over the entire field of view.

6.1.3 Liquid delivery system (key item 5), consisting of a water reservoir and a dosing mechanism,

capable of producing a water droplet of known volume to a tolerance of +/- 10 %. If the droplet volume is

determined by the pumped volume, are shall be taken that no air bubbles are present as they will cause

significant volume error.

NOTE 1 The volume relates to the droplet produced at the end of a capillary, not necessarily to the volume

applied to the test piece surface. After release of the droplet there will always be a small portion of water

remaining on the capillary, the amount of which will relate to the water, capillary diameter, capillary type and

material, and droplet application technique.

NOTE 2 The dispensing volume and reproducibility of dosing system can be checked using an impermeable

reference surface.

NOTE 3 Some contact angle measuring apparatus enable the volume calculation directly from pendant droplet

in which case air bubbles won’t affect the final droplet volume.

6.1.4 Droplet application system (key item 6), allowing a water droplet of specified volume to be

suspended at the end of a capillary before being applied to a paper or board surface under controlled

conditions. The diameter and material of the capillary shall be chosen for application of water in

accordan
...

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