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
J. Drelich et al., 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, Journal of colloid and interface science, 179(1), 1996, pp. 37-50
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 wa
s found that for each technique used, sessile-drop and captive-bubble,
different advancing contact angles and different receding contact ang
les were frequently obtained for nonideal systems with rough and heter
ogeneous solid surfaces, The disagreement between contact angles, as m
easured with the two different techniques, increased with increasing i
mperfection of the solid surface. Also, it was observed that solid sur
face 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 d
iameter) was observed when smooth and homogeneous solid surfaces were
well prepared. It is possible that metastable states, which are respon
sible for the contact angle hysteresis, also affect the contact angle/
drop (bubble) size relationship. These three-phase systems with sessil
e drop and captive bubble at heterogeneous and/or rough solid surfaces
are complex because solid surface heterogeneity and roughness cause c
ontortions 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 (b
ubble) diameter does not guarantee a high-quality surface state. Measu
rements 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. (C) 1996 Academic Press,
Inc.