# Top-View Method as a Robust Alternative for Contact Angle Measurement

## TL;DR

This paper introduces a top-view method for measuring contact angles that works well for small drops and challenging surfaces.

## Contribution

The top-view method is validated as a robust alternative to side-view techniques for contact angle measurement.

## Key findings

- The top-view method gives contact angles comparable to side-view methods for small drops (1–10 μL).
- A new dimensionless parameter, the drop projection index (DPI), quantifies gravity effects in top-view imaging.

## Abstract

This study validates the top-view method for contact
angle measurement,
based on the spherical cap assumption, as a practical alternative
to conventional side-view techniques. It is particularly useful when
side-view imaging is challenging, such as on rough, deformable, irregular,
or confined areas where baseline visibility, meridian clarity, or
drop symmetry is compromised. To address these challenges, this study
details the simultaneous acquisition of top and side-view images of
sessile drops along with precise drop volume via drop mass measurements.
The contact angles were then deduced from drop volume and contact
area based on the spherical cap assumption. Contact angles of water
and formamide (1–40 μL) on poly­(methyl methacrylate)
(PMMA) and Teflon surfaces, covering both wetting (θ
s
< 90°) and nonwetting (θ
s
> 90°) regimes, were measured. The results
demonstrate that the top-view method yields statistically comparable
contact angle values to side-view methods for small drops (1–10
μL). The study also introduces a drop projection index (DPI),
a dimensionless parameter analogous to the bond number, to quantify
gravity-induced effects in top-view imaging. The results showed that
for θ
s
< 90°, DPI was ∼0.15
for both formamide and water on PMMA, while for θ
s
> 90°, DPI ranged from ∼0.2 (formamide
on Teflon) to ∼0.3 (water on Teflon). Findings in this study
provide a foundation for refining the applicability of the top-view
method based on the spherical cap assumption and adapting it to a
wide range of systems.