Model and Experimental Study of Surfactant Solutions and Pure Liquids Contact Angles on Complex Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 195

angles much better, indicating that surfactant solutions wet the PTFE surface in the
Cassie mode.
It is interesting that surfactant solutions display higher advancing contact angles
on the PTFE surface compared to non-aqueous pure liquids with similar surface
tensions. This supports the hypothesis put forth by Mohammadi et al. [77] that sur-
factant solutions are inhibiting penetration of surfactant solutions into the surface,
either by forming a film over the pores/crevices, or by some other means. The high-
est concentration of MEGA10 (displaying the lowest surface tension) runs counter
to this trend. It may indicate a mode of partial penetration for this solution (since
it still is not matched by the Wenzel prediction). MEGA10 is a non-ionic surfac-
tant, and thus has the lowest affinity for the surface compared to HTAB and SDS.
This suggests that the highest concentration of MEGA10 does not interact with the
surface sufficiently to prevent penetration into the SHS.
That the modified Cassie model predictions (dealing with equilibrium contact
angle) predicts the advancing contact angle so well is an interesting point, and sug-
gests that on SHS, advancing contact angle is a good proxy of equilibrium contact
angle. This is not surprising when CAH is low (e.g., for water), since the high mo-
bility of the drop on the surface means that the advancing and equilibrium contact
angles are similar (i.e., there is a low energy barrier between the advancing and
equilibrium state). As CAH increases however (e.g., as concentration increases and
receding contact angle (discussed below), decreases) the model still predicts ad-
vancing contact angle well. This is a surprising result and suggests that surfactant
solutions, in preventing penetration, maintain the Cassie mode and also preserve
this similarity between the advancing and equilibrium contact angles.
The receding behavior of both surfactant solutions and pure liquids on PTFE is
more complicated than the advancing. For higher surface tensions (lower concen-
trations of surfactants), the receding angle of surfactant solutions is seen to be high.
At lower surface tensions however, the receding angle declines, eventually to zero
degrees (in an abrupt transition). This is similar to the results seen by Shirtcliffe et
al. [26] and counter to the findings of Ferrari [75, 76]. Shirtcliffe et al. proposed
that the low receding angle could be due to stretching of a surfactant film across the
pores/crevices of a SHS. It is also possible that re-organization of the surfactants
across the receding contact line could lead to pinning and decreased receding con-
tact angle [72]. Finally, the mechanism responsible for inhibiting the penetration
of surfactant solutions into the roughness may also inhibit depinning on the recede
at higher concentraiton. Regardless of cause, the result suggests that the effect of
surfactants in increasing advancing contact angles compared to pure liquids is not
necessarily carried over to receding contact angles.
Regardless of the decrease in receding contact angle, surfactant solutions still
show higher receding contact angles than non-aqueous pure liquids of similar sur-
face tension. It is worth noting that the high standard deviation on the receding
contact angle measurement for ethylene glycol results from one high measurement
and two measurements of zero receding angle. This suggests that this surface ten-