To establish accurate advancing/receding contact angle values, measurements are usually performed at specific rates of motion of the three-phase line. At very low rate of motions, diffusion or spreading of liquid causes the contact angle to be rate dependent. At high rate of motions, the drop becomes deformed due to hydrodynamic forces and contact angle changes. There is a plateau region in between, at which contact angle is constant and independent of the rate of motion of the three-phase line.
Depending on the liquid viscosity, the plateau region may exist from 0.1 mm/min to 10.5 mm/min. This dependence of contact angle on contact line speed is more pronounced for liquids with high viscosity (greater than ~10 cP). At higher rate of motions, advancing contact angle increases and receding contact angle decreases.
Typically contact angle machines enable the users to adjust the injection rate. But the factor affecting the contact angle is not the injection rate but the contact line speed. In other words, depending on the contact angle value, for the same injection rate, different contact line speeds may be observed, e.g. consider two drops of Table 1 where injection rates are equal (1 µl/min) but contact line speeds are very different.
Table 1 A case where injection rates are the same but contact line speeds are different.
|Contact line speed|
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