ISO 19403-3:2024

Overview

ISO 19403-3:2024 - Paints and varnishes - Wettability - Part 3 specifies an optical test method to determine the surface tension of liquids using the pendant drop method. The standard defines how to form and analyse a pendant drop with a drop contour analysis system and to calculate surface tension from the drop shape via the Young–Laplace equation. It is intended mainly for characterization of liquid coating materials; restrictions can apply when testing non‑Newtonian liquids.

Key topics and technical requirements

• Method principle: Form a pendant drop on a cylindrical needle and analyse its axisymmetric contour to derive surface tension from Young–Laplace.
• Apparatus: State‑of‑the‑art drop contour analysis system (camera, optical system, light source), a dosing unit/micro‑syringe and a needle with outside diameter between 0.5 mm and 2.5 mm (needle choice depends on σ/Δρ).
• Sample handling: Representative sampling per ISO 15528; density of liquid and ambient phase measured per the ISO 2811 series.
• Test conditions: Conduct at (23 ± 2) °C and (50 ± 5) % RH; enter local gravity value in instrument software.
• Image quality & evaluation:
  • Axially symmetric drop contour mandatory.
  • Shape parameter B recommended to be 0.60 ± 0.06 (Annex B).
  • Fit error should be < 1 µm per measuring point; save original images for traceability.
• Precision:
  • Repeatability limit r ≈ 0.7 mN/m.
  • Reproducibility R ≈ 1.6 mN/m for test liquids and ≈ 2.2 mN/m for coating materials.
• Reporting: Test report must reference ISO 19403-3:2024 and include shape parameter B, Worthington number (if applicable), fit error, number of drops, mean surface tension and standard deviation, plus details to identify the test material.

Practical applications and users

• Used by coatings formulators, quality control and testing laboratories, materials scientists and R&D teams for:
  • Characterizing coating liquids, solvents and additives.
  • Monitoring batch‑to‑batch consistency and product quality.
  • Supporting wettability and adhesion assessments (when combined with other measurements).
  • Research on surface energy components (see ISO 19403-4 and ISO 19403-5 for polar/disperse fraction methods).
• Also relevant where accurate surface tension input is needed for process control (spraying, wetting, inkjet, adhesion).

Related standards

• ISO 19403-1 (vocabulary and general principles)
• ISO 19403-4 / ISO 19403-5 (methods to determine surface energy fractions)
• ISO 2811 series (density determination)
• ISO 15528 (sampling)
• ISO 3219-1:2021 (definition of non‑Newtonian behaviour)
• For alternative surface tension methods: EN 14370, ISO 1409

Keywords: ISO 19403-3, pendant drop method, surface tension, wettability, paints and varnishes, drop contour analysis, Young–Laplace, repeatability, reproducibility.