Needle Matters: Addressing the Effect of the Needle on Contact Angle Hysteresis Surface Characterization

Reliable measurements of Contact Angle Hysteresis (CAH) are crucial for understanding surface wettability and droplet mobility. Although sessile drop goniometry - where droplets are inflated and deflated using a syringe needle - is widely used, systematic errors can occur due to variations in needle properties and dosing flow rates. These influences are not well quantified, which limits the reproducibility and comparability of CAH data across studies. Additionally, CAH measurements can be highly sensitive to the specifics of the experimental setup, a sensitivity that remains to be fully established. In this work, we systematically investigate the effects of needle size, material, and dosing flow rate on CAH measurements. We assess needle-droplet interactions at the primary surface-droplet and at the secondary needle-droplet triple-phase contact line (TPCL) and how these interactions affect droplet shape during CAH characterization, impacting the baseline contact angle and contact line dynamics. Our findings demonstrate that needle size is the primary factor significantly affecting the accuracy of advancing and receding contact angle measurements. The needle material has a secondary influence, especially on hydrophobic surfaces with high hysteresis. In contrast, hydrophilic surfaces are less affected by needle properties due to strong adhesion and high pinning forces, while low-hysteresis surfaces are also less affected owed to their low resistance to droplet movement. Although dosing flow rate significantly impacts contact angle measurements, particularly on hydrophobic surfaces, the effects of needle size and material are more pronounced. A flow rate of 10 μL/min, combined with recommended needle properties depending on the surface characterized, is suggested to facilitate quasi-steady droplet volume changes and minimize dynamic effects. These findings illuminate the intricate relationship between surface wettability, needle characteristics, and droplet mobility, providing quantitative criteria for optimizing experimental conditions and enhancing measurement accuracy.