The contact angle of a droplet on a solid surface serves as a fundamental parameter which scientists use to study surfaces. Scientists utilize contact angle measurements to analyze how liquids interact with solids while studying adhesion and contamination. The method serves as a fundamental requirement for multiple industries which include coatings and semiconductors. The traditional process of contact angle measurement needed costly optical equipment which required laboratory settings for operation.
The smartphone-based contact angle instrument from Droplet Lab provides research-level measurement accuracy through its portable design which remains affordable and user-friendly. This article will explain the entire technical process through step-by-step instructions which draw from peer-reviewed studies published in Review of Scientific Instruments.
The contact angle represents the intersection point between the liquid-vapor interface and the solid surface when a liquid drop rests on a solid surface.
Young’s contact angle represents the perfect angle which occurs when a surface has no flaws and maintains uniformity throughout. The actual surfaces exist in a state of roughness or heterogeneity which produces contact angles that fall within the range of two extreme values.
The measurement of these values provides essential information about surface cleanliness, chemical treatment effectiveness and wetting characteristics making them critical for
Traditional contact angle systems include:
The systems provide precise measurements yet their high cost and large size prevent students from accessing them during fieldwork. Smartphones now integrate a camera, processor, display, and storage in a single device.The addition of a modular stage with syringe and backlight components enables smartphones to perform standard laboratory tasks at a fraction of the cost.
The smartphone-based instrument operates through four separate modular components which connect together.
The modular structure enables users to detach components for maintenance and future enhancements and connection to additional systems.
The software contains two main functions which handle image processing and contact angle
measurement
The user starts the process by selecting an estimated contact line through screen tapping. The method provides approximate results which guide the software to the correct area for enhanced false detection prevention. The detection system uses three threshold values which experts established through testing to provide consistent results in various lighting and surface environments.
1. Young–Laplace fitting
2. Polynomial fitting
The measurement of static contact angles requires scientists to place a sessile drop on the surface. Young–Laplace method and polynomial method both apply.
Validation with synthetic drops (2,049 profiles, 10°–162°) showed:
The system delivers measurements which exceed the precision of commercial devices that
operate within a ±1° range.
Summary of the error for synthetic contact angle measurements using both the Young-Laplace and Polynomial fitting methods.
| Fitting method | Average error (%) | Median error (%) | Maximum error (%) |
|---|---|---|---|
| Young-Laplace | 0.01 | 0.01 | 0.09 |
| Polynomial | 0.01 | 0.01 | 0.06 |
● A 50 µL drop is deposited on the test surface.
● Advancing angle (θa): more liquid is added via syringe threads at 3 µL/s.
● Receding angle (θr): liquid is withdrawn at the same rate.
● The needle remains in the droplet, which breaks symmetry.
Because the profile is no longer axisymmetric, only polynomial fitting is suitable. The method uses a local portion of the drop edge, ignoring the distortions introduced by the needle.
Results Compared with Krüss DSA100E
The research tested five different surfaces which included glass and PMMA and PS and Teflon AF and superhydrophobic aluminum. The measurements from the smartphone and Krüss showed identical results for all tested surfaces:
The smartphone system demonstrates its ability to measure all wetting characteristics of surfaces through precise data collection which extends beyond basic static angle measurements.
TABLE III. Comparison between measurement results from commercial and smartphone instruments (advancing and receding contact angle measurement). For each of the surface, three different drops were used. The reported values are the average value of three measurements.
| Surface name | Advancing contact angle (Results in deg) | Receding contact angle (Results in deg) | ||
|---|---|---|---|---|
| Smartphone | Commercial instrument | Smartphone | Commercial instrument | |
| Glass | 46.1 ± 1.5 | 45.1 ± 2.3 | 19.05 ± 1.4 | 19.3 ± 3.0 |
| PMMA | 76.3 ± 1.5 | 76.5 ± 0.7 | 57.6 ± 1.9 | 55.7 ± 1.4 |
| PS | 108.3 ± 1.8 | 107.6 ± 1.1 | 69.0 ± 2.8 | 67.5 ± 1.0 |
| Teflon AF | 124.8 ± 1.2 | 123.1 ± 2.9 | 110.3 ± 2.8 | 110.2 ± 1.5 |
| SHS | 150.12 ± 2.8 | 152.92 ± 1.7 | 148.3 ± 3.1 | 150.1 ± 2.7 |
The researchers selected five different surfaces which represent various points along the wetting scale.
Static measurements were performed simultaneously by smartphone and Krüss from perpendicular angles. The analysis showed that all methods produced similar results within a 1-2 degree range and Young–Laplace fitting demonstrated superior performance because it evaluated the entire droplet profile.
TABLE II. Comparison between measurement results from commercial and smartphone instruments (simultaneous measurements). For each of the surfaces, three different drops were used; note the values shown are for static (or as placed) contact angles.
| Surface / Drop name | Young-Laplace method (Results in deg) | Polynomial method (Results in deg) | ||
|---|---|---|---|---|
| Smartphone | Commercial instrument | Smartphone | Commercial instrument | |
| Glass 1a | 39.3 | 39.5 | 41.7 | 37.4 |
| Glass 2 | 37.1 | 37.8 | 40.3 | 36.6 |
| Glass 3 | 36.9 | 37.7 | 37.2 | 36.8 |
| PMMA 1 | 74.3 | 73.8 | 75.9 | 73.1 |
| PMMA 2 | 72.3 | 73.7 | 75.1 | 72.7 |
| PMMA 3 | 72.7 | 73.1 | 73.0 | 72.3 |
| PS 1 | 95.5 | 95.6 | 92.1 | 92.5 |
| PS 2 | 90.3 | 90.9 | 89.6 | 89.7 |
| PS 3 | 90.0 | 90.8 | 88.1 | 89.7 |
| Teflon AF 1 | 122.0 | 123.3 | 119.5 | 123.2 |
| Teflon AF 2 | 119.7 | 119.9 | 119.4 | 118.8 |
| Teflon AF 3 | 121.8 | 123.5 | 119.3 | 121.7 |
| SHS 1 | 149.3 | 152.4 | 148.2 | 149.8 |
| SHS 2 | 156.5 | 158.8 | 149.3 | 154.3 |
| SHS 3 | 156.4 | 154.4 | 145.5 | 150.5 |
The contact angle between pure water and smooth glass (ideally) should be close to zero. The relatively large value of the contact angle on glass ( nearly 37 degrees) can be caused by the imperfection of the glass surface. The variations of the absolute value of the glass contact angle from the ideal situation do not change the fact that the measurement results from the two instruments match with each other.
The measurement of contact angles serves as the core method to study wetting processes and surface adhesion and treatment effects. The Droplet Lab smartphone instrument delivers 0.01% average error on synthetic drop measurements (2,049 profiles; 10°–162°) according to peer-reviewed research while providing performance that matches the Krüss DSA100E for static and dynamic angle measurements.
The integration of advanced algorithms into a compact smartphone platform has enabled the development of a mobile cost-effective instrument which maintains laboratory-grade scientific accuracy.
