posted on 2024-02-26, 16:08authored byOwen M. Johnson, Filippo Mangolini
Measuring
the contact angle at the solid/liquid/vapor triple point
in sessile drop experiments is one of the most popular and simple
ways to quantify the wettability of surfaces and determine the surface
free energy. Despite decades of technical advancements in contact
angle measurements, which allowed for improving the precision of sessile
drop measurements below ±1°, an often overlooked source
of experimental error in these measurements originates from the camera’s
parallax angle (PA) – the angle between the camera optical
axis and the sample stage surface. Here, we quantified the systematic
errors in the measurement of contact angles due to the acquisition
of drop images at finite PA values by simulating sessile drop experiments
in which synthetic drops were created using the Young–Laplace
equation. The absolute contact angle error induced by imaging drops
at nonzero PAs was found to increase as the true contact angle (TCA)
deviates from 90° and resulted in an overestimation (underestimation)
of the contact angle for drops having TCAs lower (higher) than 90°.
The computed absolute contact angle error reaches values as high as
−20° (+12.2°) for drops having a TCA of 175°
(5°) when imaged with a PA of 10°, thus indicating the importance
of considering the PA when accurately quantifying contact angles in
sessile drop experiments. The shape and, by extension, volume of the
sessile drop was also found to affect the magnitude of the absolute
contact angle error as sessile drops with higher apex curvatures exhibited
lower absolute error than those with lower curvatures at any given
PA. The outcomes of this work provide guidelines for minimizing systematic
errors in sessile drop measurements due to the collection of drop
images at nonzero PAs.