Sensor Instruments
Entwicklungs- und Vertriebs GmbH
Schlinding 15
D-94169 Thurmansbang
Telefon +49 8544 9719-0
Telefax +49 8544 9719-13
info@sensorinstruments.de

Translate this page:

What is surface tension?


A certain amount of force must be expended to increase the surface area of a liquid. The surface tension is the ratio resulting from the output required to increase the surface area (increase of surface area of the liquid):

σ = ΔW/ΔA (SI unit: N/m)


The surface tension means that liquids always attempt to reduce their surface area, which results in a spherical surface (a sphere has the lowest surface area of all forms with the same volume).


What is interfacial tension?
All the molecules inside a liquid are equally covered by their neighboring molecules, thereby maintaining balance between the reciprocal attractive forces. A force develops on the edge of the liquid (boundary surface), which due to the absence of a symmetrical division of the reciprocal attractive forces, points to the inside of the liquid.



The surface tension is therefore referred to as the interfacial tension. The term interfacial tension is used for the transition from a liquid to a substrate, for example a metal surface, whilst the transition from the liquid to the air is referred to as the surface tension..

 The term surface tension is used for the transition from air to water
 The processes occurring at the transition from liquid to metal is termed the interfacial tension
Strictly speaking, this transition (liquid / metal) is a transition from a metal oxide layer / an adsorbate layer (carbon dioxide and hydrocarbon on the metal surface) to a liquid

What is the wetting from metals?
A unit for the wetting of a metal surface by the application of a liquid (e.g. as droplets or brushing) is the wetting angle Θ. The wetting of the metal surface increases with a reducing wetting angle Θ.
The wetting process is described by Young's equation:
cos Θ = (σs – σLS)/σL
with
σL = Surface tension of the liquid
σS = Surface tension of the metal
σLS = Interfacial tension between metal and liquid
Θ = Wetting angle (contact angle) of the liquid on the metal



The independent distribution of the liquid on the metal surface (spreading of the brush stroke or the drops, also referred to as spreading) is achieved with a wetting angle of Θ = 0° (accords to cos Θ =1). In this case, Young's equation produces: σL = σS – σLS .
If σLS << σS , the following applies for the liquid which is to wetten a metal surface: σL < σS. This produces the following situation: if the surface tension of the liquid is less than the surface tension of the metal, the metal is wetted by the liquid, but if the surface tension of the liquid is greater than the surface tension of the metal, no wetting is performed.

Determining the surface tension of metals
The surface tension values stated in the specialist literature relate primarily to liquid metals close to their melting temperature. To ascertain the wetting, the surface tension of the metal surfaces covered with oxide and adsorbate layers at room temperature is significant. The surface tension value (free surface energy) can for example, be ascertained using the Mobile Surface Analyzer – MSA from KRÜSS GmbH www.kruss-scientific.com; two drops of the test liquids with different surface tension are applied to the metal to be tested and the surface tension of the metal is ascertained by measuring the wetting angle.



Another provider is SITA Messtechnik GmbH www.sita-process.com. SITA provides a measurement system for the “fast ascertainment of the surface wetting capability in the manufacturing process by measuring the contact angle”. SITA SurfaSpector is a hand-held measuring device for testing the wettability e.g. of metal surfaces before and after surface treatment or cleaning. This method applies a droplet (1µl) of ultrapure water to the surface to be measured via a metering unit integrated in the sensor head. A high-resolution camera in the front end of the sensor is used to ascertain the droplet contour from which the contact angle can be ascertained.



What is the test ink method?
A further method with which to ascertain the surface tension of metals at room temperature is the test ink method. Test ink with a defined surface tension is applied to the metal surface to be investigated using e.g. a brush. If the test ink wettens the metal surface, the surface tension (interfacial tension) of the metal surface corresponds to or even lies over the value of the test ink. If on the other hand, the test ink rolls off the metal surface, the surface tension (interfacial tension) of the metal surface is lower than the surface tension of the test ink.



For example, test inks are available from arcotest GmbH www.arcotest.info. PINK test inks cover a surface tension value of 22mN/m to 60mN/m in 2mN/m steps..



Why is it so important to know about the surface tension of metal surfaces?
For example, before applying color paints to metal, it is necessary to check the surface tension value of the metal surface. If this lies under the surface tension value of the paint, the paint would peel from the metal surface.
The same applies to the application of adhesives. Good adhesion is only possible if the surface tension of the metal surface is greater than the surface tension of the adhesive.

What is the cause of too low a surface tension value for a metal surface?
As already mentioned, the metal surface is really a metal oxide and adsorbate layer. Oxygen atoms and carbon atoms are integrated in the metal surface. Measurements have shown that a chemically degreased metal surface (more precisely, a metal oxide and adsorbate surface) has a surface tension value > 60mN/m. If we consider blanked parts which have been covered with oil during the blanking and bending procedure, and are then subject to cleaning in a conventional cleaning unit, we find a surface tension value of the blanked parts of some 38mN/m and in certain individual cases, 44mN/m (the surface tension value on the metal surface if the blanked parts usually lies under 30mN/m before the cleaning process depending on the punching and drawing oil used). Blanked parts and more importantly, plug contact parts require a high surface tension value (> 38mN/m), as this value points to the amount of residual oil on the metal surface. For example, the residual oil increases the contact resistance, which can reduce output during plugging.
 
Using two test inks (38mN/m and 40mN/m) it was possible to demonstrate that the surface tension value of this contact surface lay between 38mN/m and 40mN/m.

Clean wetting is no longer possible with a test ink of 40mN/m; the wetting angle Θ deviates correspondingly strongly from 0°, whilst close to a spread is achieved at 38mN/m.

The following diagram provides a schematic view of how the surface tension of the contact surface reduces with an increasing oil layer thickness (controls with the same test ink are performed):



The wetting of the contact surface reduces with an increasing thickness of the oil layer.

Test inks with various surface tensions are used in sequence to ascertain the surface tension of a contact surface. Should the surface tension of the contact part amount to a minimum of 38mN/m, you can start with a test ink of an identical surface tension value: 38mN/m test ink wetted, 40mN/m test ink not completely wetted, 42mN/m test ink rolls off the contact surface.



The test ink method is a simple and quick control procedure for the random testing of components. It would be preferable to obtain direct information about the residual oil layer thickness, which in the past has proven difficult to perform. The test ink method has established itself as a reliable method for controlling for the presence of thin oil layers in an industrial context.

What is an inline presence control for thin oil layer?
The test ink method is a random test method in which individual plug contacts or components are taken from production and tested as a representative of the quality of an entire batch. A 100% control of the produced components is becoming increasingly desirable, and the focus is increasingly coming to rest on an inline measurement procedure. The inline procedure should be based on the test ink method so that the measured values of both systems – the inline system and the offline system (test ink method) – can be compared easily. The aim is to provide an inline measurement system which displays the surface tension value of the respective component in mN/m.

How does an inline oil film presence control system work?
An inline measurement system depends primarily on the capability of the contact-free measurement. The measuring time required must be able to keep pace with the component feed rate. The measurement system should be equipped with multiple robust and compact interfaces to enable feed of the measurement data to the superordinate control system or a PLC, preferably in real time. Before selecting a suitable measurement procedure, a number of conventional punching and drawing oils were subject to optical spectral analysis, starting in the UVC range (250nm) and going up to the MIR - range (5µm). A measuring window between 3µm and 4µm has established itself as the optimal working spectral range, as it has proven possible to measure all types of punching and drawing oils tested in accordance with this procedure up to this point. The various oils present different absorptions in the respective spectral windows, so that individual calibration using the test ink method is unavoidable, especially as the measured value is to be expressed in mN/m. The comparative (standardized) procedure for two spectral ranges provides measurement results, which enable compensation of the greatest proportion of the reflection characteristics of the contact surface. The following sketches show the principal mode of operation of the MIR sensors:


Wide band MIR transmitters cover the requisite spectral range required for the two measuring windows CH0 and CH1. A part of the light reflected by the metal surface impinges on the two measuring windows; one measuring window (CH0) reacts to the presence of an oil film, whilst the other measuring window (CH1) remains almost unreactive to the presence of an oil film (reference window). A standardized evaluation produces an intensity-independent measured value:

MIR = CH1/(CH0+CH1)



The sketch above does not contain an oil film on the metal surface. Both measuring windows give almost the identical signal level (standardized value = MIRref). In the following sketch on the other hand, the collapse of the signal level from measuring window CH0 results in a change of the standard value MIR.



The thicker the oil layer, the more severe the signal dip in the measuring window CH0:



To ensure that not the standardized value (MIR) but rather the surface tension value of the metal oil layer surface can be transferred to the sensor interface, the relation to the test ink method must first be established (calibration):



To this end, one of the metal parts to be tested is first degreased and then the standardized reference value (MIRref) is ascertained with the MIR sensor. The corresponding surface tension value is then determined in mN/m using the test ink method. As a next step, a degreased metal part is subject to the same procedure with the difference that in each procedure (measured value with oil present: MIR) an increasingly thicker oil layer is applied to the degreased component in a homogeneous fashion (experience shows that the thickness of the oil layer moves between 100 nm and 5µm). The measured values (MIR-MIRref) are then recorded in a table and this is used as a conversion table in a non-volatile sensor memory. Graphical depiction shows the following relationship:


 


NEWS

Participation at trade fairs:

 •   AIMEX Automation World 2024
 •   IFAT Munich 2024
 •   FIP 2024
 •   PRS Europe 2024
 •   SKZ Innovation Day Recycling
 
•   SKZ Network Day
 •   SKZ Technology Day
 •   PRS Middle East & Africa 2024
 •   Plastics Recycling World Expo
     
Europe 2024
 •   Plastics Recycling World Expo
     
North America 2024
 •   PRS Asia 2024
 •   PRS India 2024

more...


Press releases:

Inline color calibration with respect to plastic recyclate
(Inline color measurement systems)

 
Test report on Inline color measurement of recyclates
(Inline color measurement system)

more...


Whitepapers:

Sensor systems for recyclate control in the plastics industry for laboratory and inline use
Checking the plastic type of recyclates and virgin material using NIR technology

more...


A
pplications:


Detection of dents and bumps on metal punching strips
(N° 801)
Differentiation of leather, fabric and synthetic interior components
(N° 802)
Plastic cap color measurement
(N° 803)

more...


WHAT IS ...
... surface tension?
... oil evaporation?
... oil layer thickness
    measurement?

more...


Software news:

SPECTRO3-MSM-DIG-Scope V1.5 (2021.10.20)
V1.5.2

more...




FIND US ON ....

Facebook X (Twitter) Instagram Youtube in

 








TOP