Statistical contact angle analyses: ‘slow moving’ drops on inclining flat mono-aminopropylsiloxane surfaces

Abstract

The determination and evaluation of contact angles on solid surfaces is a widely used procedure in industry and scientific research. The reproducibility and comparability of contact angles measurements often suffers from the optical identification of the triple line movements, which is used to define the specific contact angles. Therefore, the analysis of contact angles using high-precision drop shape analysis is an approach to enhance the reproducibility of contact angle measurement, especially for inclined and otherwise non-axisymmetric droplets. In this contribution, we present two recently developed approaches for reliable contact angle determination on inclining high-quality hydrophilic silicon wafers synthesized by straightforward silanization with 3-Aminopropyltriethoxysilane which can be used as coupling agent. Thereby so-called ‘slow moving’ contact angle analyses were performed. On the one hand, the average curve shape of the contact angle relative to the inclination angle is investigated by a sigmoid function. On the other hand, a specific contact angle behavior analysis which is similar to the manually used procedure to determine one advancing or receding angle is automatically performed resulting in specific distributions for θ and φ. Within this contribution the presentation is restricted to the so-called global expectation values for the contact angle and for the inclination angle. Both approaches result in contact angle data that are independent from ‘user-skills’ and subjectivity of the operator to enhance the reliability and comparability of such measurements. Special surface effects by comparing with a surface, prepared after a wet substrate treatment, are also presented and illustrate the potential of the procedure.

Keywords:

• contact angle
• sessile drop
• inclined plate
• wetting
• axisymmetric
• advancing angle
• receding angle
• downhill angle
• uphill angle
• hydrophilic

Acknowledgments

The authors gratefully acknowledge W. Possart for his support.

Notes

1. For the minimal differences between the different pretreatment of the silicon wafer surface compare to the Supplementary Materials.

2. The intinsic base can also catalyse the contensation to the silxane, but most likely due to the absence of the also reactive anion present in the acid route first kinetic studies indicate that with reduced amount of water (form atmosphere) the alkaline route is much slower.

3. For reasons of clarity the results of the second surface are summarized in the Supplementary Materials.