Dynamic Contact Angles, DCA

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Definition:

The Dynamic Contact Angles are divided into Advancing and Receding contact angles, which are defined as the contact angles measured when the three phase line is in controlled movement by wetting the solid by the liquid or by withdrawing the liquid over a pre-wetted surface, respectively.

Summary/Description:

Dynamic Contact Angles is the contact angles when the three phase line is in controlled motion. Can be divided into Advancing and Receding contact angles. The advancing angle is the contact angle when the three phase line is moving over and wetting the surface or "pushing" away the gas phase, while the receding angle is the contact angle when the three phase line is withdrawn over a pre-wetted surface or "pushing" away the liquid phase. The Dynamic Contact angles can be determined in several ways. The best technique is perhaps the Wilhelmy plate technique performed with a Tensiometer where a solid sample is immersed and withdrawn into and out from a liquid while simultaneously measuring the Force acting on the solid sample. The Advancing and Receding contact angles can then be determined from the obtained force curve. The main drawback of this technique is that the sample has to be symmetrical and has a regular shape (rod, cube, round rod, rectangle, wire etc.)

Another ways to determine the Advancing and Receding angles are by using a Goniometer in which one is looking on a small liquid drop placed on the solid sample and measure the contact angles while the drop size is increased or decreased, or alternatively tilt the sample stage and put the drop in movement.

Often the Advancing and Receding contact angles are mixed with the so called 'Advanced' and 'Receded' angles. The production of drops with advanced and receded edges involves one of two strategies. Drops can be made to have advanced edges by addition of liquid. Receded edges may be produced by allowing sufficient evaporation or by withdrawing liquid from the drop. Alternately, both advanced and receded edges are produced when the stage on which the solid is held is tilted to the point of incipient motion.

The difference between the Advanced/Receded and Advancing/Receding is that in the static case motion is incipient and in the dynamic case motion is actual. Dynamic contact angles can easily be assayed at various rates of speed. Often it is found that there exists a simple relationship between Dynamic Contact Angles measured at low velocities with properly measured static angles.