Advancing/receding contact angles drop method

Advancing/receding contact angle. Advancing and receding contact angle should be determined with the drop expansion/contraction method. Typically, the measurement starts with a small sessile droplet ( 2 pL), and small amount of test liquid is added at a very slow rate, e.g., 0.2 pL/s. The drop profile is captured as the drop is... 

Figure 9.2 Determination of contact angle from the sliding drop on an inclined plate method. The advancing contact angle, 9 , is the measured angle at the downhill edge of the drop, the receding contact angle, 0r is measured at the uphill edge, and 0, is the tilt angle.

The advancing and receding contact angle values may decrease with decreaseing drop size (or in the captive bubble method, with the size of the bubble). This decrease is more pronounced in 0, values than in 0a. 

Advancing and receding contact angle have been known for more than a century to be the maximum and minimum angle that can be measured reliably and reproducibly when liquid is added to or withdrawn from a liquid droplet at a very slow rate [4,5]. Most of the current goniometer is equipped with an automated dosing system, where drop volume and rate of addition/withdraw can be controlled very precisely. The needle-embedded sessile drop method is the most frequently used method and is schematically shown in Fig. 2.15. 

Advancing and receding contact angle should be determined by the drop expansion/ contraction technique. The tilting plate method has significant shortfall particularly with high hysteresis samples and should be avoided if possible. 

---