Reliable Measurement of the Receding Contact Angle

Surface wettability is usually evaluated by thecontact angle between the perimeter of a water drop and thesurface. However, this single measurement is not enough forproper characterization, and the so-called advancing andreceding contact angles also need to be measured. Measuringthe receding contact angle can be challenging, especially forextremely hydrophobic surfaces. We demonstrate a reliableprocedure by using the common needle-in-the-sessile-dropmethod. Generally, the contact line movement needs to befollowed, and true receding movement has to be distinguishedfrom “pseudo-movement” occurring before the receding angleis reached. Depending on the contact angle hysteresis, theinitial size of the drop may need to be surprisingly large to achieve a reliable result. Although our motivation for this work was thecharacterization of superhydrophobic surfaces, we also show that this method works universally ranging from hydrophilic tosuperhydrophobic surfaces.