Digital Dyne

Digital Dyne^® is a measurement and evaluation method developed by KRÜSS for the automatic wettability testing of activated materials based on the receding contact angle of water. The contact angle result is converted into a dyne value, which corresponds to the result of a manual test of the same sample using test inks or dyne pens. This method enables a transition from test ink testing to contact angle measurements without having to change the existing quality limits for pass/fail evaluation.

Digital Dyne enables a simple transition from manual testing with test inks or dyne pens. The method is based on measuring the receding angle of water, from which a Dyne value comparable to that of test inks is determined using reference curves. The method offers a high correlation with traditional ink tests but operates with significantly greater reproducibility and is independent of the user. Additional advantages include the digital recording and archiving of measurement data, as well as the use of pure water instead of test inks that are harmful to health. Thus, Digital Dyne represents a modern, safe, and efficient alternative for industrial quality assurance.

What is the principle behind Digital Dyne?

To obtain a Digital Dyne result, the receding angle of water is measured – a contact angle that describes the process of dewetting rather than wetting behavior. This measured value correlates excellently with a wide range of empirical testing methods for surfaces – particularly with the results of ink tests on differently pretreated plastics. To determine the Digital Dyne value, a reference curve is first used to establish the correlation between water contact angles and the test ink results for surfaces with varying degrees of activation. Based on this relationship, a Digital Dyne value corresponding to the subsequently measured contact angles can then be determined.

The established relationship is not universally applicable but applies individually for specific combinations of material (e.g., PE or PP) and test ink type (formamide-, methanol-, or ethanol-based). However, since in practice the same plastic is usually tested with the same testing equipment, it is not necessary to constantly create new reference curves. The KRÜSS ADVANCE software already contains the reference curve for certain combinations, so the Digital Dyne value can be output directly.

The receding angle is measured using the KRÜSS Stood-up Drop^®, which enables reliable, user-independent measurements in a matter of seconds. Conventional methods, such as measuring the volume reduction of dispensed drops, could in principle also yield Digital Dyne values, but they are too slow and labor-intensive for routine quality control.

What accounts for the strong correlation between Digital Dyne and test ink results?

In the ink test, the user forcibly wets the test surface and then examines the coalescence of the applied line. This is therefore a dewetting process. In the Stood-up Drop method for Digital Dyne, the drop –dosed at high speed under pressure – also wets a larger area before contracting into a defined drop. Consequently, a measurement is also taken after forced prewetting, except that – unlike the ink test – this process is fully controlled. The strong correlation is not only plausible but has been substantiated by a large number of comparative studies between ink tests and contact angle results.

What are the advantages of switching from test inks to Digital Dyne?

Ink tests allow for simple, quick wetting tests, but they come with drawbacks that many quality managers are no longer willing to accept and for which Digital Dyne offers a solution:

Electronically recorded measurement data: One of the advantages of the new method is evident from its name: The process provides measurement values that are output directly into software, archived, and exportable, eliminating the need for manual documentation. The test results are thus completely transparent – for example, for quality audit purposes – and are free from errors caused by incorrectly entered values.
User-independent measurement results: The measurement method itself – the determination of receding angles using the Stood-up Drop – follows a standardized procedure in which parameters such as dosing distance and speed cannot be altered by user decisions. The same reproducibility applies to the video analysis of the doses of water drop, from which the contact angle is derived.

In tests using test inks, countless factors can influence the result. The tester must visually assess whether and to what extent the liquid has retreated or whether the ink’s behavior can be evaluated as complete wetting. The dynamics of the application and the tested area can also influence the result. When applying the ink with a brush, the brush may pick up contaminants from the sample, thereby contaminating the ink supply and altering the surface tension. Evaporation of the more volatile components of the mixture - for example, if the bottle is left open for too long – also affects the result.
Tests using pure water instead of toxic substances: Most common test ink mixtures contain toxic substances – primarily formamide, which is harmful if inhaled or in contact with the skin and is even classified as toxic to reproduction. Since users are typically exposed to these substances daily and possibly for years, the health risk should not be underestimated. Digital Dyne eliminates this problem: It uses pure water as its sole test liquid.

How are Digital Dyne and surface free energy related?

Since the Digital Dyne method provides values analogous to those of test inks, this question relates to the interpretation of test ink results.

Many users of test inks assume that they are using them to determine the surface free energy (SFE) of materials, which in this context is often referred to as the surface tension of the solid. This assumption stems from the concept of “critical surface tension” according to Zisman. This model states that the lowest surface tension of a liquid at which a surface is just completely wetted (contact angle = 0°) corresponds to the SFE.

In fact, it has long been recognized that the degree of wetting depends on the formation of polar and dispersive interactions and must therefore take into account the polarities of liquids and solids – which is precisely what current models for determining SFE do. KRÜSS therefore does not refer to the measured value for Digital Dyne as the SFE of the solid.

▶ Read more on this topic in our application report AR272: Why Test Inks Cannot Tell the Whole Truth About Surface Free Energy.

Why is KRÜSS developing a method based on ink tests with Digital Dyne despite doubts about them?

The conversion from receding angle to a Digital Dyne value is not intended to provide a scientific analysis of the surface but rather follows an empirical approach. The method paves the way for users who have established test ink results in QC and have had positive experiences with them to easily transition to automated contact angle technology—with the aforementioned advantages over manual testing with problematic substances.

The quality limits determined for test inks for a “pass/fail” evaluation of the test results can be retained, thereby avoiding time-consuming preliminary investigations required for the introduction of a new method. Digital Dyne therefore does not replace the scientific determination of the SFE but serves in quality assurance as a modern technical alternative – and successor – to test inks.

Apart from that, there are users who fundamentally distrust the ink test, for example because defects occur in production despite “QC-passed” results. In this case, switching to Digital Dyne does not provide a solution; instead, a switch to SFE determinations using contact angles of various test liquids should be considered.

Where does the name “Digital Dyne” come from?

In addition to the SI unit mN/m, the analog CGS unit dyn/cm (dynes per centimeter) is also commonly used to measure surface tension. In practice, the terms dynes or dyne level is often used as a shorthand to refer to a test result determined using test inks.