Stood-up Drop

The Stood-up Drop is a dosing and measuring method for the optical determination of the recently receded contact angle (RRCA). This contact angle is formed immediately after the withdrawal of a liquid from a solid and therefore characterizes the dewetting.

How does the Stood-up Drop method work?

In the Stood-up Drop method, the liquid is dispensed at high speed onto the solid surface in a thin jet using pressure. Due to the high dynamics, the liquid initially spreads out flat on the solid (wetting) and then contracts into a drop (dewetting).

Aren't the resulting drop shape and the measured contact angle due to chance?

The forces that draw the droplet together are a combination of the surface tension of the liquid and the interfacial tension between the liquid and the (pre-wetted) solid. The wetted area decreases until an equilibrium is reached between the surface tension and the dewettability-dominating properties (dewetting surface free energy, roughness, etc.) of the prewetted solid. The resulting recently receded contact angle (RRCA) is therefore well-defined.

Is the contact angle independent of how the analyzed drop was formed?

The contact angle that forms during wetting (advancing angle) differs significantly from that during dewetting (receding angle), because the wetting and dewetting properties of a surface are two independent surface characteristics. The advancing angle is never smaller than the receding angle. Apart from that, there is no clear correlation between these contact angles.

The difference between the advancing and receding angles is called contact angle hysteresis, which, as a third, independent parameter, completes the characterization of the behavior of surfaces with respect to liquids.

What properties of the material does the Stood-up Drop characterize?

Since the Stood-up Drop reflects the state of the solid-liquid interface after its reduction, it is particularly suitable for characterizing technical dewetting processes. It has been found, for example, that the results of adhesion tests for pressure sensitive adhesives (PSAs) that come close to a dewetting process often correlate with receding angles.

The Stood-up Drop can also be used to determine the effects of surface chemical or mechanical influences on the dewetting of a material. It has been shown that with many such treatments, e.g. surface activation, the receding angle changes more than the advancing angle (measured in the course of wetting). It can therefore be used as sensitive tool in R&D and troubleshooting activities in addition to the so far established advancing CA measurements.

In many cases, the Stood-up Drop can be quickly integrated into industrial process chains following inexpensive, empirical preliminary investigations. The first step is to investigate whether there is a correlation between the RRCA and a process or QC parameter. This correlation can then be used to reduce the number of complicated quality inspections by means of rapid prescreening or even replace them completely.

Why is the measurement of the RRCA with the Stood-up Drop particularly suitable for quality control?

Quality assurance requires speed and user-independent results. The Stood-up Drop delivers the measured value for the RRCA in less than a second. Thanks to the defined dosing height and the constant, predetermined dosing dynamics, the results are also highly reproducible and particularly reliable.

Withdrawing a dosed drop by suction via a dosing needle takes considerably longer and is also subject to greater influences from user decisions (e.g. volume flow, dosing height).

The Stood-up Drop is also faster and more reliable than the test ink/dyne ink method, which also approximates the dewetting properties of a surface. Unlike the manual and user-dependent ink test, the result is available digitally and is fully documented, including the images saved during the drop shape analysis.

What is the difference between the recently receded contact angle (RRCA) and the dynamic receding contact angle (RCA)?

The dynamic receding angle RCA is measured during the retraction movement of the liquid, whereas the recently receded contact angle RRCA is measured when the three phase contact line of the liquid (baseline) is at rest. With the Stood-up Drop, only the RRCA can be measured, but not the RCA, as the drop shape is largely undefined during the rapid contraction. Both receding angles can be measured with a needle dosing unit, in which the retraction takes place by sucking liquid out of the drop.

RRCA and RCA have the same value for many surfaces, but in some cases, e.g. with inhomogeneous surfaces, the RRCA can be larger than the RCA. This is due to the fact that a force independent of wettability acts on the droplet as a result of suction, which can overcome possible dewetting barriers.

Which liquid is used for the Stood-up Drop?

To date, measurements of the RRCA with the Stood-up Drop are basically carried out with pure water. Avoiding liquids that are harmful to health is another advantage for the QC compared to test inks, which generally use problematic substances. In many studies, the RRCA measured with water was found to be sufficient to determine the surface readiness for further production steps.

Although the dosing of other liquids is conceivable, the satisfactory results to date have given no reason to expand the range of substances. In principle, not all liquids are suitable because the surface tension of the liquid must be sufficiently high for the formation of a well-formed Stood-up Drop, which is the case with water. Density and viscosity also play a role. For Diiodomethane, for example, a standard test liquid for surface free energy measurements, the difference between the RRCA with the Stood-up Drop and the RCA with a needle was often found to be significantly larger than for water. As a consequence, the Stood-up Drop is not used for determining dewetting surface free energy.

A pressure jet dosing unit for contact angles – isn't that what KRÜSS developed the Liquid Needle for?

While the dosing dynamics of the Stood-up Drop are so high that the drop always wets a larger area and then contracts, the Liquid Needle does the exact opposite: it ensures that the drop spreads with low dynamics without prewetting its surroundings.

The contact angles measured with the Liquid Needle are advancing contact angles, more precisely a dynamic advancing contact angle (ACA) during dosing and a recently advanced contact angle (RACA) immediately after dosing. According to our experiments, the RACA with the Liquid Needle and the RRCA with the Stood-up Drop are the most clearly defined drop shapes, both for the advancing and receding process. The two methods combined provide a full picture of the wetting/dewetting behavior.