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|Contact Line Pinning, Pinning Force, and Slip-Stick Motion|
We know that surfaces with high surface energy will promote wetting which can be observed by low measured contact angles. However, what is less understood is the phenomenon referred to as contact line pinning - that is, when the three-phase line remains static or pinned even when a drop evaporates or is tilted or volume is added to or removed from the drop. In the graphic below, we observe two behaviors. On the left, when a drop evaporates, dewetting occurs and the three-phase line contracts while the contact angle remains relatively static. On the right, the three-phase line remains static while the volume evaporates and the contact angle decreases over time. Our objective is to explore some of the reasons for contact line pinning, what causes it, and how to measure the behavior.
High energy surfaces exhibit strong chemical bonds (e.g., ionic, covalent, or metallic). Thus you would expect that a high-energy surface such as metal would result in complete wetting - or at least a very low contact angle. However, in real world testing, metal surfaces will often produce contact angles approaching 90°.
The first explanation for this behavior is topography - or surface roughness. When a drop is deposited on a surface with any scale of roughness (nanoscale to microscale), the surface promotes pinning. The drop will partially wet but the nooks and crannies at which the three-phase line sits prevent further wetting during drop deposition or dewetting during evaporation. This explains the coffee-ring effect.1 When coffee is deposited on a surface, say from a coffee mug, the liquid remains pinned due to the surface topology. As the coffee evaporates, many of the solid coffee particles migrate to the three-phase line which remains pinned even as the drop volume evaporates. As a result, when the coffee is fully evaporated, the ring on the outside remains darker than the interior of the surface where the drop resided as a result of pinning.
Another cause of contact line pinning is contamination. On a metal surface, for example, organic contaminates can retard wetting and cause pinning to occur. However, even if a metal surface is polished flat (to reduce roughness) and cleaned (to remove organic contaminants), the observed contact angle is typically much higher than expected or even higher than other materials with comparable high surface energy. The proposed explanation for this behavior is contact line pinning.
One way to measure the pinning force is to simply measure the contact angle and drop width as the drop evaporates. Consider surface A and B which both exhibit a static water contact angle of 60°. However, as the drops evaporate, the three-phase line for the drop on surface B remains static (that is the drop width does not change) even as the contact angle reaches 25° before dewetting occurs. The drop in surface A, however, begins to dewet during evaporation when the contact angle reaches 50°. The observed contact angle hysteresis on surface B of 35° indicates a much greater pinning force than that of surface A which has a much lower observed hysteresis of only 10°. We should note that the actual hysteresis is likely larger than the observed hysteresis as additional volume added to both drops would likely result in contact angle values higher than the static contact angles measured. This is explained by contact line pinning and pinning force.
During experiments where volume is added slowly to a sessile drop - for example, when measuring the advancing contact angle using the add/remove volume method - it's often observed that the three-phase line will remain static for a period and then abruptly wet (or dewet if volume is being removed). This cycle repeats and is referred to as slip-stick motion. On polymer surfaces such as PTFE, the slip-stick motion cycle is short as the pinning force is weak as is the surface energy. As a result, the roll-off angle is low indicating a small contact angle hysteresis.
The table below illustrates how it's possible to classify materials in terms of both their surface energy (high or low) and pinning force (strong and weak).
The methods that are most commonly used to measure pinning forces and contact line pinning behavior are:
- Evaporation. This method captures pinning behavior and observed hysteresis.
- Add/Remove Method. Volume is slowly added to a drop until the largest possible contact angle can be measured (advancing) and then volume is removed (or allowed to evaporate) until the smallest (receding) contact angle can be measured.2
- Tilting Base Method. This method will also measure the roll-off angle. A drop is deposited on a surface and while taking a series of contact angle measurements, the surface is tilted.3
Measuring wetting properties through contact angle analysis is the first step to characterizing a solid surface. Careful observance of the contact line pinning, slip-stick behavior, pinning forces, and contact angle hysteresis adds another powerful dimension to understanding the wetting characteristics of a particular solid surface.
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