The Effect of Drop (Bubble) Size on Advancing and Receding Contact Angles for Heterogeneous and Rough Solid Surfaces as Observed with Sessile-Drop and Captive-Bubble Techniques

Abstract

Sessile-drop and captive-bubble techniques were used for contact angle measurements. The advancing and receding contact angles were measured for water and ethylene glycol at self-assembled monolayer surfaces of dodecanethiol, for water at methylated quartz surfaces, and for water at roughened polyethylene and polytetrafluoroethylene surfaces. It was found that for each technique used, sessile-drop and captive-bubble, different advancing contact angles and different receding contact angles were frequently obtained for nonideal systems with rough and heterogeneous solid surfaces. The disagreement between contact angles, as measured with the two different techniques, increased with increasing imperfection of the solid surface. Also, it was observed that solid surface roughness and heterogeneity affected a variation of the advancing and receding contact angles with drop (bubble) size. No contact angle change with respect to drop (bubble) size (in the range 1–7 mm base diameter) was observed when smooth and homogeneous solid surfaces were well prepared. It is possible that metastable states, which are responsible for the contact angle hysteresis, also affect the contact angle/drop (bubble) size relationship. These three-phase systems with sessile drop and captive bubble at heterogeneous and/or rough solid surfaces are complex because solid surface heterogeneity and roughness cause contortions in the shape of the three-phase contact line and the drop (bubble) surface in the vicinity of the three-phase contact line. These contortions may affect a variation of the internal free energy of the liquid drop (gas bubble). It is shown that a slight variation in the advancing contact angle value over a few millimeters change in drop (bubble) diameter does not guarantee a high-quality surface state. Measurements of the receding contact angles provide more information on the quality of the solid surface and they should always be included with the measurements of advancing contact angles.