High-precision drop shape analysis on inclining flat surfaces: Introduction and comparison of this special method with commercial contact angle analysis

Abstract

Drop shape analysis is one of the most important and frequently used methods to characterise surfaces in the scientific and industrial communities. An especially large number of studies, which use contact angle measurements to analyse surfaces, are characterised by incorrect or misdirected conclusions such as the determination of surface energies from poorly performed contact angle determinations. In particular, the characterisation of surfaces, which leads to correlations between the contact angle and other effects, must be critically validated for some publications. A large number of works exist concerning the theoretical and thermodynamic aspects of two- and tri-phase boundaries. The linkage between theory and experiment is generally performed by an axisymmetric drop shape analysis, that is, simulations of the theoretical drop profiles by numerical integration onto a number of points of the drop meniscus (approximately 20). These methods work very well for axisymmetric profiles such as those obtained by pendant drop measurements, but in the case of a sessile drop onto real surfaces, additional unknown and misunderstood effects on the dependence of the surface must be considered. We present a special experimental and practical investigation as another way to transition from experiment to theory. This procedure was developed to be especially sensitive to small variations in the dependence of the dynamic contact angle on the surface; as a result, this procedure will allow the properties of the surface to be monitored with a higher precession and sensitivity. In this context, water drops onto a 111 silicon wafer are dynamically measured by video recording and by inclining the surface, which results in a sequence of non-axisymmetric drops. The drop profiles are analysed by commercial software and by the developed and presented high-precision drop shape analysis. In addition to the enhanced sensitivity for contact angle determination, this analysis technique, in combination with innovative fit algorithms and data presentations, can result in enhanced reproducibility and comparability of the contact angle measurements in terms of the material characterisation in a comprehensible way.

Statistical Contact Angle Analyses with the High-Precision Drop Shape Analysis (HPDSA) Approach: Basic Principles and Applications

“…In this regard, we want to encourage not just the small community of specialists but especially the multiple users of sessile drop experiments to perform meaningful contact angle measurements and analyses instead of just measuring some angles in-between the advancing and receding angle. Especially, naming and storing of the experimental conditions such as drop volume, inclination rate, vapor atmosphere, contact angle definition, frame rate, etc., as illustrated in our investigations [53,54,64,[71][72][73][74][75][76][77][78], are mandatory to ensure reproducibility and comparability of sessile drop experiments.…”

“…Detailed information about the experimental procedures and analyses can be found in the corresponding publications concerning contact angle measurements [64,[71][72][73][74][75][76][77][78], the surface preparation by straight-forward gas phase silanization [71,[73][74][75][76]78], AFM analyses [71,[73][74][75][76]78], FTIR analyses [78] and spectroscopic ellipsometry analyses [75]. In the following, only the general procedure of contact angle measurements will be described.…”

“…The basic information about the HPDSA procedure can be found in publication [64]. To avoid confusion, a summary of all used symbols, variables and parameters within this contribution can be found in the Abbreviations Section.…”

“…This fact was recently highlighted by Volpe et al [63]. In this regard, we developed a special calculation and evaluation strategy for contact angle measurements and wetting experiments, which is named high-precision drop shape analysis (HPDSA) [64]. An intention of the complete procedure (the combination of HDPSA and CA analyses) was allowing the user to determine valid and reproducible contact angles, which are free of any subjectivity of the operator.…”

Statistical contact angle analyses; “slow moving” drops on a horizontal silicon-oxide surface

“…High-precision drop shape analysis (HPDSA) [9] is designed to analyse drop shapes independent on the symmetry of the droplet, so that our recent investigations were focused on dynamic inclining plate measurements. The contact angle hysteresis Dh [10,11] existing on every real surface is an additional aspect which complicates contact angle measurements and interpretations.…”