The Droplet Lab instrument (reported as a Dropometer by Droplet Smart Tech, with analysis via a sessile-drop app) was used to quantify water contact angles on permanent-marker–treated paper to select ink colors that form robust hydrophobic barriers.

• In Methods 2.6 (page 6), the authors describe applying marker inks (blue, green, red, black) to Whatman Grade 4 filter paper, placing an HPLC-water droplet, capturing an image, and analyzing it with a sessile-drop application.

In Results 3.1 and Figure 3b (pages 7–8), they report high contact angles (e.g., ~151° for blue and ~158° for green at 10 s), supporting the decision to fabricate µPAD barriers with blue/green markers for better confinement and reduced leakage.
Why this matters: The goniometer data directly underpins barrier material selection—a critical factor for utility-field devices where leakage or poor channel definition can invalidate a hardness test.

• Marker-ink hydrophobicity was quantified via contact angle, enabling rational selection of barrier inks (green/blue most hydrophobic; Results §3.1; Figure 3b).
• Leakage testing confirmed that green/blue barriers resisted leakage under dyed-water challenge, while red/black leaked (Figure 4, page 9).
• The µPAD provides WHO-aligned qualitative hardness classification (soft → very hard) by blue-to-pink zone changes in ~3 min (device concept Figures 1 & 5; qualitative results Figure 6a).
• A quantitative route uses the control detection zone and color intensity analysis (scanner/phone + ImageJ) to build a calibration curve (Figure 6b).
• LOD = 0.02 mM, outperforming typical commercial strips and several prior µPAD reports (Results 3.2.3; comparison Figure 7).
• Real tap-water samples from multiple locations showed close agreement with standard EDTA titration (Figure 8).
• The device demonstrates shelf stability (weeks at room temp and 4 °C) and temperature robustness (short exposures up to 100 °C) (Figure 9).

Common ions showed minimal interference under tested conditions (Figure 10).

For utilities, hardness is a practical driver of scaling risk, customer complaints, and treatment decisions (e.g., softening dose, corrosion/incrustation balance). This work demonstrates a field-ready approach that can screen and classify hardness in minutes without pipettes or bulky instrumentation, while also offering a quantitative option when numeric tracking is required (e.g., verifying treatment performance, monitoring district variability). The contact-angle‑guided selection of barrier inks improves device reliability—supporting repeatable point-of-use testing that can inform operational adjustments and maintenance prioritization across distribution networks.