The study used a Droplet Lab Dropometer to quantify the water contact angle (WCA) of the optimized soyhull cellulosic residue (SCR) film, as a direct readout of surface wettability / hydrophobicity:

• Where it’s described: Methods section “2.2.6.5. Water contact angle” (page 4) specifies measuring WCA on the film using a Dropometer (Droplet Lab, Canada), capturing the droplet image via a smartphone interface, and analyzing via sessile drop software.
• Why it mattered in this work: WCA supported how the film surface interacts with water—an important packaging-relevant property tied to moisture interactions, condensation tendency, and practical barrier behavior for fresh produce packaging.

Key quantitative outcome: The optimized film’s WCA was 72.6° (reported and visualized in Fig. 2f and discussed in the wettability section), indicating a moderately hydrophilic surface (< 90°), which aligns with the film acting as a semi-permeable packaging layer rather than a fully moisture-blocking plastic film.

• Optimized formulation (SH12) (0.4 g SCR, 500 mM CaCl₂, 1.5% glycerol) achieved:
  • Tensile strength: 6.3 ± 0.6 MPa
  • Elongation at break: 30.2 ± 0.9%
  • Water vapor permeability: 0.9 ± 0.3 × 10⁻¹⁰ g·m⁻¹·s⁻¹·Pa⁻¹
• Wettability (Droplet Lab measurement): Water contact angle = 72.6°, supporting the film’s moderately hydrophilic surface character (Fig. 2f; discussion in WCA section).
• Functional performance: Film extended raspberry shelf-life by ~6 days at room temperature relative to polystyrene film (shelf-life study results section; Fig. 4).
• Sustainability end-of-life: Film biodegraded within ~25 days at 24% soil moisture (biodegradation section; Fig. 2g).
• Active packaging attributes: Film provided UV protection (transmittance results; Fig. 2c) and measurable antioxidant activity (DPPH IC50 reported in abstract and methods/results).

For packaging developers and fresh-produce packers, this work demonstrates that agricultural byproduct–derived cellulose films can be engineered (via ionic crosslinking + plasticization) to hit a practical balance of mechanical integrity, breathability (moisture control), and functional protection (UV/antioxidant). The contact angle result (72.6°) provides a quick, quantitative surface check that the film is not excessively hydrophobic (which can trap moisture) nor extremely hydrophilic (which can compromise integrity), helping guide material selection, formulation optimization, and QC specifications for produce packaging where condensation management is critical.